Hi everyone.

I have a 25.6V 110Ah LiFePO4 battery connected to a 3kW non-hybrid Axpert inverter.
I've been exclusively using it in "line mode". (the inverter prioritises drawing power from the grid and resorts to solar and battery backup only when grid power is unavailable.)
Recently, I decided to test the inverter in "battery mode". In this mode, the inverter is supposed to prioritise the solar panels and the battery to supply power, reverting to the grid only if the battery reaches a set voltage threshold or if no power is generated by the solar panels.

In "battery mode" I've noticed that the inverter is constantly charging and discharging the battery. For instance, when the house's power consumption is at 300W, easily manageable by the solar panels alone, but as soon as the power draw increases, even if it's by just a couple of watts, the inverter indicates that the battery is discharging. Basically assisting the solar panels to supply power. I presume this occurs because the inverter cannot instantly match the load required, by only using the solar panels, leading it to utilise the battery to bridge the gap?
Whilst the battery is "filling in" the extra load needed, the inverter slowly raises the output from the panels to match the new load and the moment it achieves that, it starts charging the battery again to replenish the little that it used.
The process obviously keeps going back and forth as the load fluctuates from appliances that are being switched on and off during the course of the day

I'm just curious how this would affect the life of the battery, by being constantly charged and discharged?
Will it be better for this type of inverter to be left in "line mode", and only use it as backup instead? I'm just getting the idea that this is not really the optimal way to use this type of inverter. I guess the bottom line is that I'm trying to use it as a hybrid inverter which it's not.

 

3 hours ago, Gerrit84 said:

The process obviously keeps going back and forth as the load fluctuates from appliances that are being switched on and off during the course of the day

I'm just curious how this would affect the life of the battery, by being constantly charged and discharged?

I'm pretty sure that this has only a fairly small effect on battery life. Personally, I would not go to a utility first priority mode just to save that little bit of wear on the battery. The battery has a calendar life as well as a cycles life, so you may as well use it and save the cost of the utility power.

  

1 hour ago, Coulomb said:

I'm pretty sure that this has only a fairly small effect on battery life. Personally, I would not go to a utility first priority mode just to save that little bit of wear on the battery. The battery has a calendar life as well as a cycles life, so you may as well use it and save the cost of the utility power.

 

Thank you very much for your feedback. I'm going to leave it in "battery mode" to see how much it affects my utility bill.
If you don't mind sharing your thoughts as well on the following. I don't use all that much power at once. In my case it's either the dishwasher, microwave or the wife's hairdryer that uses the most power. I make sure not to use these at the same time, since neither the inverter or battery will be able to cope with more than any one of these loads at the same time.
I've been doing al lot of reading regarding Lithium batteries and all the do's and don't. I know that Lithium batteries don't like a high discharge rate, the lower the better. During load shedding I avoid using anything that has a high power draw, but it often happens that we might have load shedding during 18:00-20:00 when my wife needs to use the microwave. (I obviously don't want to have my dinner cold so I'm not going to stop her from using the microwave. Hahaha!)
It seems to use about 1500W according to the inverter, with the rest of the lights in the house and TV it ads up to about 1800-2100W or around 80-90A. 
Normally she would use the microwave for only 10min max to quickly warm something. Having such a high load on the battery isn't great, but if I keep it to short periods like 10 min or less shouldn't be a problem right?

1 hour ago, Coulomb said:

I'm pretty sure that this has only a fairly small effect on battery life. Personally, I would not go to a utility first priority mode just to save that little bit of wear on the battery. The battery has a calendar life as well as a cycles life, so you may as well use it and save the cost of the utility power.

Thank you very much for your feedback. I'm going to leave it in "battery mode" to see how much it affects my utility bill.
If you don't mind sharing your thoughts as well on the following. I don't use all that much power at once. In my case it's either the dishwasher, microwave or the wife's hairdryer that uses the most power. I make sure not to use these at the same time, since neither the inverter or battery will be able to cope with more than any one of these loads at the same time.
I've been doing al lot of reading regarding Lithium batteries and all the do's and don't. I know that Lithium batteries don't like a high discharge rate, the lower the better. During load shedding I avoid using anything that has a high power draw, but it often happens that we might have load shedding during 18:00-20:00 when my wife needs to use the microwave. (I obviously don't want to have my dinner cold so I'm not going to stop her from using the microwave. Hahaha!)
It seems to use about 1500W according to the inverter, with the rest of the lights in the house and TV it ads up to about 1800-2100W or around 80-90A. 
Normally she would use the microwave for only 10min max to quickly warm something. Having such a high load on the battery isn't great, but if I keep it to short periods like 10 min or less shouldn't be a problem right?

6 hours ago, Gerrit84 said:

Thank you very much for your feedback. I'm going to leave it in "battery mode" to see how much it affects my utility bill.
If you don't mind sharing your thoughts as well on the following. I don't use all that much power at once. In my case it's either the dishwasher, microwave or the wife's hairdryer that uses the most power. I make sure not to use these at the same time, since neither the inverter or battery will be able to cope with more than any one of these loads at the same time.
I've been doing al lot of reading regarding Lithium batteries and all the do's and don't. I know that Lithium batteries don't like a high discharge rate, the lower the better. During load shedding I avoid using anything that has a high power draw, but it often happens that we might have load shedding during 18:00-20:00 when my wife needs to use the microwave. (I obviously don't want to have my dinner cold so I'm not going to stop her from using the microwave. Hahaha!)
It seems to use about 1500W according to the inverter, with the rest of the lights in the house and TV it ads up to about 1800-2100W or around 80-90A. 
Normally she would use the microwave for only 10min max to quickly warm something. Having such a high load on the battery isn't great, but if I keep it to short periods like 10 min or less shouldn't be a problem right?

I have the same size Axpert and mine behaves differently. Using PV 1st and during good sun all loads up to about 1000W is fed from PV and on the left display one can just see less amps being used from PV to charge the battery. 

I also use it for any load below about 1800W. When using my micro it might be for 2min with a 45sec pause before the next 2min to heat the 2nd plate of food. Using it for 10min from battery only after sunset already uses about 18% of your total battery power. 

I never murder the inverter to use a 2000W induction cooker although this cooker is normally used at about 50% power. 

2 hours ago, Scorp007 said:

I have the same size Axpert and mine behaves differently. Using PV 1st and during good sun all loads up to about 1000W is fed from PV and on the left display one can just see less amps being used from PV to charge the battery. 

I also use it for any load below about 1800W. When using my micro it might be for 2min with a 45sec pause before the next 2min to heat the 2nd plate of food. Using it for 10min from battery only after sunset already uses about 18% of your total battery power. 

I never murder the inverter to use a 2000W induction cooker although this cooker is normally used at about 50% power. 

Thanks for your reply. Mine also behaves exactly as you described, although I have never seen it get to 1000W. Max for me was 850W and around 28A from the PV.

What I was trying to point out is that when the load on the inverter increases, it takes a couple of seconds for the inverter to raise the output from the PV to match the required load. When this happens whilst the battery is charging, I can see that less amps are going to the battery.

My concern about battery life is when I'm running the inverter in battery mode and the battery is already fully charged, the PV might be taking care of the load at any given moment, but when the load suddenly goes up the PV can't adapt quick enough to match it and it starts using battery to make up the shortfall. When the PV eventually does match the load it immediately starts charging the battery irrespective of how much capacity was used. 

It's this constant charging and discharging of the battery that has me concerned that it might accelerate the ageing of the battery significantly.

4 minutes ago, Gerrit84 said:

Thanks for your reply. Mine also behaves exactly as you described, although I have never seen it get to 1000W. Max for me was 850W and around 28A from the PV.

What I was trying to point out is that when the load on the inverter increases, it takes a couple of seconds for the inverter to raise the output from the PV to match the required load. When this happens whilst the battery is charging, I can see that less amps are going to the battery.

My concern about battery life is when I'm running the inverter in battery mode and the battery is already fully charged, the PV might be taking care of the load at any given moment, but when the load suddenly goes up the PV can't adapt quick enough to match it and it starts using battery to make up the shortfall. When the PV eventually does match the load it immediately starts charging the battery irrespective of how much capacity was used. 

It's this constant charging and discharging of the battery that has me concerned that it might accelerate the ageing of the battery significantly.

OK I do see you got an answer on the life of the batteries. 

You don't quote the time the PV takes to ramp up. Once mine has reached its MPP if more load is switched on it takes about 3 sec to adjust to the new load drawing from PV. 

Have you set your setting 02 high enough to use more power from PV? 

4 minutes ago, Scorp007 said:

OK I do see you got an answer on the life of the batteries. 

You don't quote the time the PV takes to ramp up. Once mine has reached its MPP if more load is switched on it takes about 3 sec to adjust to the new load drawing from PV. 

Have you set your setting 02 high enough to use more power from PV? 

Mine is definitely not that quick. Maybe 10 seconds before it matches the the new load.

Now that you mention it. I never thought of it. I've got setting 02 at 30A. Maybe that's why it never reaches 1000W.

Also, the fuse for the PV is 30A (I popped the 20A that was in there before when I raised setting 02 from 20A to 30A without realising what I was doing) so I won't be able to go higher unless I swop them out for higher capacity ones.

32 minutes ago, Gerrit84 said:

Mine is definitely not that quick. Maybe 10 seconds before it matches the the new load.

Now that you mention it. I never thought of it. I've got setting 02 at 30A. Maybe that's why it never reaches 1000W.

Also, the fuse for the PV is 30A (I popped the 20A that was in there before when I raised setting 02 from 20A to 30A without realising what I was doing) so I won't be able to go higher unless I swop them out for higher capacity ones.

At 30A it might not go over 800W.

13 hours ago, Gerrit84 said:

Having such a high load on the battery isn't great, but if I keep it to short periods like 10 min or less shouldn't be a problem right?

It depends on the battery's rating, which depends on its detailed chemistry.

Usually, house batteries are LFP (a few are NMC), and LFP seems to mainly come in C/2 and 1C variants. For example, if it's a 110 Ah battery (C=110), then if it's a C/2 battery, its recommended maximum charge and discharge current will be 110/2 = 55 A, or about 1.4 kW for a nominally 24 V battery. But if it happens to be a 1C battery type, or if you had 220 Ah of C/2 battery installed, then the battery will be comfortable with 2.8 kW continuously. The rating for your battery should be written on it somewhere, or in a specification somewhere.

1.5 kW (AC in, not microwave out) is a fairly modest microwave size, but it likely suits your modest system. Of course, you have to run it longer to get the same cooking effect as a 1.8 kW microwave, but it's much friendlier to your battery.

As for whether 10 minutes can be considered short enough that you can overload it for that length of time, it's hard to say, but I would think that 2 minutes is more the time that overloads can be considered basically harmless to the battery life.

If your battery turns out to be the more common C/2 type, then  ideally, you would double the battery capacity, but I understand that this may not be possible at least for a while. While 10 minutes of significant overload is past my personal comfort point, it's below my "never exceed" point of double the recommended discharge current (again, assuming C/2, i.e. recommended maximum discharge current of about 55 A). I'd say the risk of damaging the battery is quite low compared to the discomfort of cold dinners.

 

An interesting thing is a few months ago we replaced our over 10yr old micro that used about 1550W input from AC with a smaller 1100W input unit. This new one actually cooks faster while using just over 35% less power. 

Thus we find the tech on micros seem to have improved as is the efficiency for most gadgets as we move along in time. 

I am using 3 x 1C battery banks for the inverter so maximum reliable current use should see my batteries not being stressed. 

4 hours ago, Coulomb said:

It depends on the battery's rating, which depends on its detailed chemistry.

Usually, house batteries are LFP (a few are NMC), and LFP seems to mainly come in C/2 and 1C variants. For example, if it's a 110 Ah battery (C=110), then if it's a C/2 battery, its recommended maximum charge and discharge current will be 110/2 = 55 A, or about 1.4 kW for a nominally 24 V battery. But if it happens to be a 1C battery type, or if you had 220 Ah of C/2 battery installed, then the battery will be comfortable with 2.8 kW continuously. The rating for your battery should be written on it somewhere, or in a specification somewhere.

1.5 kW (AC in, not microwave out) is a fairly modest microwave size, but it likely suits your modest system. Of course, you have to run it longer to get the same cooking effect as a 1.8 kW microwave, but it's much friendlier to your battery.

As for whether 10 minutes can be considered short enough that you can overload it for that length of time, it's hard to say, but I would think that 2 minutes is more the time that overloads can be considered basically harmless to the battery life.

If your battery turns out to be the more common C/2 type, then  ideally, you would double the battery capacity, but I understand that this may not be possible at least for a while. While 10 minutes of significant overload is past my personal comfort point, it's below my "never exceed" point of double the recommended discharge current (again, assuming C/2, i.e. recommended maximum discharge current of about 55 A). I'd say the risk of damaging the battery is quite low compared to the discomfort of cold dinners.

 

Wow! That's a lot of great info. Thank you very much for the great explanation. Just shows me how much I still need to learn. Like you mentioned. I do have a very modest system, but I'm SO impressed with how far it has taken me and if you take care of it how much it's actually capable of. Especially after I swopped out the Lead Acid batteries for a Lithium.

Unfortunately I'm not sure whether the battery is 1C or not. I'll attach a picture of it. If you are able to tell me from the picture I'd really appreciate it. My knowledge is quite limited.

17 minutes ago, Gerrit84 said:

Wow! That's a lot of great info. Thank you very much for the great explanation. Just shows me how much I still need to learn. Like you mentioned. I do have a very modest system, but I'm SO impressed with how far it has taken me and if you take care of it how much it's actually capable of. Especially after I swopped out the Lead Acid batteries for a Lithium.

Unfortunately I'm not sure whether the battery is 1C or not. I'll attach a picture of it. If you are able to tell me from the picture I'd really appreciate it. My knowledge is quite limited.

100a continuous,so 1c (reportedly)

Maybe worth mentioning that keeping the battery fully charged at 100% without cycling is not good for battery lifespan.

A little cycling may keep your battery healthier than no cycling at all and full charge. 

Edited February 10, 20242 yr by Mauritius B

1 hour ago, Mauritius B said:

Maybe worth mentioning that keeping the battery fully charged at 100% without cycling in not good for battery lifespan.

A little cycling may keep your battery healthier than no cycling at all and full charge. 

That's a very good point. I wish that I had a propper way of preventing it from being fully charged all the time, but maybe there is some hope. From what I could find on the net is that 24V (25.6V) Lithium batteries are made up of 8 cells. So at full charge it's 3.7V per cell or 29.6V for the entire batttery.
My battery states that I should keep bulk and float charge from the inverter at 28,4V wich means that the battery is never completely full, but only at 90-95% state of charge.

So if my inverter is feeding the battery only 28.4V it's not being fully charged?
Another issue is that my battery isn't "smart", so it can't communicate with the inverter on what state of charge it is at. It has it's own built in BMS that I also have no control over.
I can only assume that this is a "cheap" way of the battery manufaturer to ensure that if the battery doesn't get cycled enough it's not sitting at 100% charge all the time?
 

2 hours ago, Gerrit84 said:

From what I could find on the net is that 24V (25.6V) Lithium batteries are made up of 8 cells. So at full charge it's 3.7V per cell or 29.6V for the entire battery.

LFP should NEVER be charged beyond 3.65 VPC (Volts Per Cell) average. Personally, I think of 3.6 V as the "never exceed, prefer never to reach" voltage. Your battery mentions 28.4 V for the  maximum (and float!!) voltage, which is 3.55 VPC. Personally, I don't run my own prismatic LFP cells beyond 3.45 VPC. That would be 27.6 V for you.

Battery manufacturers have this nasty habit of quoting the float voltage as the same as the bulk charge voltage. This inflates the battery's apparent run-time, at the expense of lifetime (which they have no real incentive to maximise). I personally run mine at 54.0 V (formerly 53.7 V, but that turned out to cause problems with equalisation, at least now that they are 15 years from manufacture, and almost 12 years of operation. That's 27.0 and 26.9 V for your system, and 3.38 VPC or 3.36 V respectively. For a chemistry like LFP that has a notoriously flat voltage versus SoC charge curve, there is a huge difference between 3.38 and 3.55 VPC.

2 hours ago, Gerrit84 said:

So if my inverter is feeding the battery only 28.4V it's not being fully charged?

The fully charged voltage of an LFP cell is actually a matter of opinion; there is no real standard, believe it or not. The higher you go in voltage, the last few tenths of a percent of storage you get, at the expense of some nasty chemical reactions that are far more likely at higher temperatures and higher cell voltages (both of these give electrons the energy they need to overcome some barriers to causing permanent harm). That's why heat is the enemy of all batteries, and LFP's advantage over NMC could be largely due to the lower cell voltage (3.6 max versus 4.2 max). It will be interesting to see what happens with LMFP batteries (Lithium Manganese Ferrous Phosphate) chemistry, when they finally make it to the market. They claim the longer life of LFP coupled with the higher voltage (and therefore energy density) of NMC, but if it's the lower voltage that makes them last longer, then maybe they're just a cheaper form of NMC (manganese is a lot cheaper than nickel).

In  summary: at 28.4 V, your battery is getting very very close to as full as you can get it, and pushing it higher will give you minuscule longer run time, at significantly lower expected life. I actually encourage you to reduce your charge voltage, as this will cost you still very little in run-time, and should extend life considerably. Especially considering how the less expensive inverters like the Axperts tend to overshoot their voltage targets, often for tens of seconds at a time, due to poor PID control algorithms.

2 hours ago, Gerrit84 said:

Another issue is that my battery isn't "smart", so it can't communicate with the inverter on what state of charge it is at.

Many 24 V inverters can't talk to smart batteries anyway, though there are more and more that can these days.

Some would say that there is an advantage not letting the battery take charge (so to speak). For people like me that don't like to see 3.55 VPC running all day, having manual control of the the bulk and float voltages is a good thing. But you do lose the ability of the battery to tell the charger (in the inverter) to back off when one or a few cells gets to a dangerously high voltage; with the manual set-up, you only see the total battery voltage.

2 hours ago, Gerrit84 said:

I can only assume that this is a "cheap" way of the battery manufacturer to ensure that if the battery doesn't get cycled enough it's not sitting at 100% charge all the time?

Can you rephrase, please? I'm not following this question.

Thanks again for your very detailed explanation and advice. I can't tell you how much I appreciate it.

Oh wow! OK! And here I was thinking that I’m not over charging the battery and that the manufacturer was actually trying to be wise enough about it and help me as customer to get the best performance out of their product.
After reading your post I immediately changed the “bulk” and “float” charge down to 27.6V. I have had the battery for about a year. I just hope that I haven’t degraded it too much by charging it to 28.4V all the time.

You mentioned that Axpert inverters tend to overshoot their voltage targets. I have witnessed this myself. Especially when the battery is charging from the solar panels and not the grid. It always goes over the 28.4V to at least 28.6V before it backs off, sometimes even 29.2 for a brief moment.

I completely agree with what you said about keeping the voltage at 28.4V can cause other issues apart from just accelerating the aging process.
My thinking was as simple as, the more voltage the “fuller” the battery will get. Obviously it doesn’t work like that.
I also understand that if I drop the voltage just a bit to say 27.6V like you mentioned. I won’t loose much capacity but would increase the life of the battery.

You asked to rephrase the sentence where I said: “I can only assume that this is a "cheap" way of the battery manufacturer to ensure that if the battery doesn't get cycled enough it's not sitting at 100% charge all the time? “
English is not my first language so I understand that I could have come across unclear. Let me try again.
What I was trying to say is that, after reading online that a LFP battery is “full” at 29.6V. It gave me the impression that the manufacturer of my battery was being modest in the sense that they don’t “want” me to “fully” charge the battery to 29.6V. They are actually trying to help me take care of the battery by suggesting to not charge it to 29.6V (what I thought was 100%) but to “only” 28.4V.
I was completely wrong here.
 

The questions I have are:
1. Should I keep the “bulk” and “float” charge setting at 27.6V or should they be different, meaning that the “float” should be lower than the “bulk” setting?
2. How much capacity do you think I will lose if I only charge to 27.6V?
3. I was under the impression that if I set the charge voltage lower than what the battery states it requires, it won’t be able to fully charge, that somehow it could possibly “confuse” the built in BMS of the battery? Maybe I just don’t fully understand how a battery actually absorbs charge?

4. Do I need to equalize the battery? I see that some sites suggest 29.2V, and that if you don't do it often enough the cells in the battery will become unbalanced. How will this work, won't the BMS  equalize the cells automatically?
5. What would you suggest as a safe cut-off voltage? At the moment mine is at 23.2V. Not sure if that was the default setting on the inverter.

Edited February 9, 20242 yr by Gerrit84

18 hours ago, Gerrit84 said:

Thanks again for your very detailed explanation and advice. I can't tell you how much I appreciate it.

Oh wow! OK! And here I was thinking that I’m not over charging the battery and that the manufacturer was actually trying to be wise enough about it and help me as customer to get the best performance out of their product.
After reading your post I immediately changed the “bulk” and “float” charge down to 27.6V. I have had the battery for about a year. I just hope that I haven’t degraded it too much by charging it to 28.4V all the time.

You mentioned that Axpert inverters tend to overshoot their voltage targets. I have witnessed this myself. Especially when the battery is charging from the solar panels and not the grid. It always goes over the 28.4V to at least 28.6V before it backs off, sometimes even 29.2 for a brief moment.

I completely agree with what you said about keeping the voltage at 28.4V can cause other issues apart from just accelerating the aging process.
My thinking was as simple as, the more voltage the “fuller” the battery will get. Obviously it doesn’t work like that.
I also understand that if I drop the voltage just a bit to say 27.6V like you mentioned. I won’t loose much capacity but would increase the life of the battery.

You asked to rephrase the sentence where I said: “I can only assume that this is a "cheap" way of the battery manufacturer to ensure that if the battery doesn't get cycled enough it's not sitting at 100% charge all the time? “
English is not my first language so I understand that I could have come across unclear. Let me try again.
What I was trying to say is that, after reading online that a LFP battery is “full” at 29.6V. It gave me the impression that the manufacturer of my battery was being modest in the sense that they don’t “want” me to “fully” charge the battery to 29.6V. They are actually trying to help me take care of the battery by suggesting to not charge it to 29.6V (what I thought was 100%) but to “only” 28.4V.
I was completely wrong here.
 

The questions I have are:
1. Should I keep the “bulk” and “float” charge setting at 27.6V or should they be different, meaning that the “float” should be lower than the “bulk” setting?
2. How much capacity do you think I will lose if I only charge to 27.6V?
3. I was under the impression that if I set the charge voltage lower than what the battery states it requires, it won’t be able to fully charge, that somehow it could possibly “confuse” the built in BMS of the battery? Maybe I just don’t fully understand how a battery actually absorbs charge?

4. Do I need to equalize the battery? I see that some sites suggest 29.2V, and that if you don't do it often enough the cells in the battery will become unbalanced. How will this work, won't the BMS  equalize the cells automatically?
5. What would you suggest as a safe cut-off voltage? At the moment mine is at 23.2V. Not sure if that was the default setting on the inverter.

Perhaps not the best input but I have my float just 0.4V lower than bulk. I am happy using 27.8V for bulk. 

Equalisation is off and not needed. As you said going up to above 28.5V for long enough is only to allow balancing of cells and not to get the battery fully charged. 

Your cut out is fine. I use 24V in order not to discharge very low as you gain very little in capacity by going down to 21V. Falling off the cliff for lithium. 

My S-100 drop inn's are doing very well after over 2 years. 

Edited February 10, 20242 yr by Scorp007
Spelling

10 hours ago, Gerrit84 said:

Should I keep the “bulk” and “float” charge setting at 27.6V or should they be different, meaning that the “float” should be lower than the “bulk” setting?

For longevity, make the float voltage less than the bulk voltage. 

10 hours ago, Gerrit84 said:

How much capacity do you think I will lose if I only charge to 27.6V

About 0.1%.

10 hours ago, Gerrit84 said:

I was under the impression that if I set the charge voltage lower than what the battery states it requires, it won’t be able to fully charge, that somehow it could possibly “confuse” the built in BMS of the battery?

All the BMS needs to not be confused is to see a high enough voltage that it csn reset its internal SoC estimate to 100%. Hopefully one of the overshoots will get it there. Usually the BMS doesn't have to see that voltage for long. But the exact details of BMS working are rarely revealed. You might need a regular (say once per week) "equalise" stage for say 1 minute, if it doesn't seem to be resetting to 100%.

10 hours ago, Gerrit84 said:

Do I need to equalize the battery?

As Scorp007 already answered, no, with the possible exception above, and that's not real equalisation. 

10 hours ago, Gerrit84 said:

What would you suggest as a safe cut-off voltage?

I suggest 24.0V, which is probably as high as you can go. That's 3.0 VPC, and there is litte energy down there anyway. Very low cell voltages are also harmful, for different reasons. 

6 hours ago, Coulomb said:

For longevity, make the float voltage less than the bulk voltage. 

How much less than the bulk charge would you suggest. I have it at 27.2V at the moment.

6 hours ago, Coulomb said:

About 0.1%.

Wow! That is nothing!

6 hours ago, Coulomb said:

All the BMS needs to not be confused is to see a high enough voltage that it csn reset its internal SoC estimate to 100%. Hopefully one of the overshoots will get it there. Usually the BMS doesn't have to see that voltage for long. But the exact details of BMS working are rarely revealed. You might need a regular (say once per week) "equalise" stage for say 1 minute, if it doesn't seem to be resetting to 100%.

OK. I'll activate the float setting at 28.4V and have run once a week for 10 min?

6 hours ago, Coulomb said:

As Scorp007 already answered, no, with the possible exception above, and that's not real equalisation. 

I suggest 24.0V, which is probably as high as you can go. That's 3.0 VPC, and there is litte energy down there anyway. Very low cell voltages are also harmful, for different reasons. 

You are correct. The lowest I can go is 24V. I'll keep it at that.

20 hours ago, Gerrit84 said:

How much less than the bulk charge would you suggest. I have it at 27.2V at the moment.

Well, the "received wisdom" for 15S Pylontechs is 51.8 V, so that would be 51.8 / 15 * 8 = 27.6 V for you. I run 54.0 V on my 16S LFP battery, so that would be 54.0/2 = 27.0 V for your 8S battery. Anywhere between 27.0 and 27.6 is probably fine; lower is slightly better for battery life, higher is slightly better for run-time. 27.2 is probably a good figure 🙂

20 hours ago, Gerrit84 said:

OK. I'll activate the float setting at 28.4V and have run once a week for 10 min?

I think you meant "activate the equalisation cycle". I would hold off to see if it's necessary. You should look for discrepancy between the reported SoC and  what you expect from its performance. There is a quite good chance that you won't need it. But it probably won't hurt much to try it.

20 hours ago, Gerrit84 said:

The lowest I can go is 24V. I'll keep it at that.

I think you mean the highest. 24.0 is a good cut-off value when you don't have fully patched firmware. I would never let my 16S LFP get as low as 48.0 V, for example, but I have the  luxury of being able to patch my own firmware.

3 hours ago, Coulomb said:

Well, the "received wisdom" for 15S Pylontechs is 51.8 V, so that would be 51.8 / 15 * 8 = 27.6 V for you. I run 54.0 V on my 16S LFP battery, so that would be 54.0/2 = 27.0 V for your 8S battery. Anywhere between 27.0 and 27.6 is probably fine; lower is slightly better for battery life, higher is slightly better for run-time. 27.2 is probably a good figure 🙂

OK. That sounds good. Think I'll keep it at 27.2 for now and see how it affects runtime, if at all.

3 hours ago, Coulomb said:

I think you meant "activate the equalisation cycle". I would hold off to see if it's necessary. You should look for discrepancy between the reported SoC and  what you expect from its performance. There is a quite good chance that you won't need it. But it probably won't hurt much to try it.

Yes, that's what I meant. Sorry about that.
OK, I'll leave it off for now and see how the battery holds up. The last year of use has given me a pretty good indication of how long it can last, so it should be easy to see if it's not lasting that long after dropping the voltage. Just wish their was a more accurate way of telling the actual state of the battery. The inverter obviously only shows the voltage and that is not accurate in any way.
The only other way I know is to test the voltage directly on the battery by completely disconnecting it from the inverter, which is quite the hassle.

3 hours ago, Coulomb said:

I think you mean the highest. 24.0 is a good cut-off value when you don't have fully patched firmware. I would never let my 16S LFP get as low as 48.0 V, for example, but I have the  luxury of being able to patch my own firmware.

Sorry and yes, again. That's what I meant.
I see that in other posts there is custom software you can run of PC or a raspberry pie to have better control of the inverter. Maybe I could try that?

14 hours ago, Gerrit84 said:

I see that in other posts there is custom software you can run of PC or a raspberry pie to have better control of the inverter. Maybe I could try that?

Yes, but that won't allow you to run a higher cutoff voltage; only fully patched firmware can do that. The problem is that the values are crammed into 8-byte CAN bus packets that are sent from one machine to the others, so the master can see what the slaves are doing, and you can send a command sent to any inverter's RS-232 port to find out data of any paralleled machine.

14 hours ago, Gerrit84 said:

The only other way I know is to test the voltage directly on the battery by completely disconnecting it from the inverter, which is quite the hassle.

This is still nowhere near accurate with an LFP battery, because the voltage changes so little with change of state of charge. The BMS will have a decent estimate of the SoC, but unless it shows this on an LC Display or similar, or you can ask the SoC via a command, then this is no help.

I think I should look at at getting an upgrade in the near future. More capacity and a more capable inverter.
I've learned a lot in the past year. I'll put that knowledge and all the advice you've given me the last couple of days to good use.
Thank you very much for taking the time to patiently educate me on all of the question I had.
I highly appreciate it.