Hi everyone,

I started finding a couple of weeks ago that my batteries were getting quite low overnight. Each battery is 102Ah 12V lead acid deep cycle with the full bank 408Ah 24V.

I took the following readings from them at 11pm one night and assigned a letter to the top left of each battery. The L voltages are while under a light 2amp load and the U voltages were after I disconnected and waited a couple of minutes. You can also see the load in amps across the pairs (measured with clamp meter) and see that the B-F pair isn't doing much:

This resulted in me removing F and D batteries and moving H to where F was.

F and D seemed to be the weaker batteries, but I'm not sure I'm entirely correct and I'm still finding that my batteries don't fully charge even on a sunny day. They always seem about 10-20Ah below charge and I turn on my Mecer/Axpert to charge them from the mains for 30-45 minutes each night.

With regard to my Mecer/Axpert settings I have a 28.8V bulk charging voltage and a 27.6V float and my maximum charging current is 40A.

In addition I notice a slight bulge in D and E and some slight moisture by what looks like a vent, however I wouldn't think it'd ever be overcharged and I've checked their temperatures while charging and they seem ok.

 

So I guess I just want to know if this is normal for roughly 3 year old deep-cycle batteries, where I have pulled on the longest nights 40Ah overnight? 

I'll admit that I did not buy all the batteries at exactly the same time, so there may be some additional ageing from that.

Should I try put the weaker batteries in their own pair and back into the battery bank? Should I reorganise the ordering of all the batteries in the bank?

 

Thanks in advance for any responses.

Regards,

Lindsay

 

On 2020/06/08 at 8:50 PM, Lindsay said:

They always seem about 10-20Ah below charge and I turn on my Mecer/Axpert to charge them from the mains for 30-45 minutes each night.

That sounds like the premature float bug. There is no patched firmware for 24 V models, so if you have timed absorb or equalise settings, you could try getting more solar charge energy in that way (minimising the need to mains charge them at night).

Edited June 9, 20206 yr by Coulomb
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3 hours ago, Coulomb said:

That sounds like the premature float bug. There is no patched firmware for 24 V models, so if you have timed absorb or equalise settings, you could try getting more solar charge energy in that way (minimising the need to mains charge them at night).

Thanks for the suggestion. Last winter I probably mains charged my battery only a couple of times, when it was very rainy/cloudy, surely I would have noticed the issue then or sometime in the past or is this much more noticeable as the batteries age?

I know I can set a timed equalise, could I set this for every day though at 28.8V?

Do you think I should put the two weaker batteries back in the bank as a series pair?

8 hours ago, Lindsay said:

I know I can set a timed equalise, could I set this for every day though at 28.8V?

Exactly, yes.

8 hours ago, Lindsay said:

Do you think I should put the two weaker batteries back in the bank as a series pair?

At least that way they do something, and don't hinder two other modules from performing well. I'm very uncomfortable with two battery modules in series with very different voltages.

9 hours ago, Coulomb said:

Exactly, yes.

At least that way they do something, and don't hinder two other modules from performing well. I'm very uncomfortable with two battery modules in series with very different voltages.

I log the data from the inverter, so I'll take a look and see how long it sits in absorb before being dropped to float. If I have to, how long, considering my size battery bank, do you think I should set it at 28.8V?

I'll reintroduce the two weaker batteries in their own series-pair back into the battery bank and see how that goes. Should I worry about the slight bulge and moisture by the pressure vent on two of the batteries?

3 hours ago, Lindsay said:

I'll take a look and see how long it sits in absorb before being dropped to float.

Also check what voltage they typically were at just before they dropped to float.

2 hours ago, Lindsay said:

If I have to, how long, considering my size battery bank, do you think I should set it at 28.8V?

I think you'll have to experiment. Try 1 hour to start with. You might want to set the "equalise" voltage lower than 28.8 V, which is the start of gassing at 25°C, especially in summer. Perhaps 28.4 V.

2 hours ago, Lindsay said:

Should I worry about the slight bulge and moisture by the pressure vent on two of the batteries?

I don't know. The bulge and moisture could be signs of age. If the battery was chronically under-charged, you did well to get 3 years out of it. Different age battery modules may have been a factor. I'm no lead acid expert.

You mentioned a 40 Ah discharge on the longest nights; is that for the whole battery (4 strings), or per string (so 160 Ah total)?

 

1 hour ago, Coulomb said:

You mentioned a 40 Ah discharge on the longest nights; is that for the whole battery (4 strings), or per string (so 160 Ah total)?

I just checked and last night it was 47Ah, that's across all 4 strings, so about 12Ah per string. I have a fridge, freezer, router, some security cameras, LED lights and chargers for phones and laptop that run off it, so not a heavy load. I have a slow cooker, induction stove and mini-oven, but only use those in the day when there's plenty of sun. I should probably look back properly through my logs and see how it ran last year this time to do a proper comparison.

I do notice that sometimes the battery voltage reported by the inverter goes a bit over 30V (pretty sure it has done this since I got the inverter). I think this is brief and just when it's switching modes (absorb -> float) etc so I haven't worried about it.

I gather when I find the battery bank is really struggling I should switch out the whole battery bank in one go. I seem to remember the manual saying you shouldn't use any battery type other than lead-acid. However I gather if I get a bms and adjust the absorb and float voltages would it be ok to use a different battery type (lithium etc)?

Thanks for all the help, I'll let you know what I find!

12 hours ago, Lindsay said:

I seem to remember the manual saying you shouldn't use any battery type other than lead-acid.

? I've not seen that anywhere. It might be a 24 V specific thing, but I doubt it.

12 hours ago, Lindsay said:

However I gather if I get a bms and adjust the absorb and float voltages would it be ok to use a different battery type (lithium etc)?

Certainly. I would guess that lithium based batteries overtook lead acid some years ago, in home energy systems connected to Axpert inverters. Most lithium batteries, unless you build them yourself, come with a BMS in the box. They cost a bit more initially, but are cheaper in the long run, and you have less hassle with them.

Disclaimer: other than reading Axpert manuals and lurking on forums when doing pre-purchase homework I do not know much about the Axperts and their various incarnations

On 2020/06/08 at 2:09 PM, Coulomb said:

That sounds like the premature float bug. There is no patched firmware for 24 V models, so if you have timed absorb or equalise settings, you could try getting more solar charge energy in that way (minimising the need to mains charge them at night).

 

15 hours ago, Lindsay said:

I have a fridge, freezer, router, some security cameras, LED lights and chargers for phones and laptop that run off it, so not a heavy load. I have a slow cooker, induction stove and mini-oven, but only use those in the day when there's plenty of sun.

Wonder if the batteries ended up working harder/more often than thought or often(ish) do not complete a charge cycle. With the base load going and using something like an induction hob (1000-2000W?) even with good sunshine the batteries may get tapped unless "output source priority" is set to "utl"? Slow cooker adding 200W-400W(?) for couple of hours to base load...

 

3 hours ago, Coulomb said:

? I've not seen that anywhere. It might be a 24 V specific thing

Even in the King manual 😉

15 hours ago, Lindsay said:

I gather if I get a bms and adjust the absorb and float voltages would it be ok to use a different battery type (lithium etc)?

The "dumb"/drop-in replacement type lithiums generally should be okay with any inverter/charger if you can set the Bulk charge/float voltage and  low cut-off to what they need (I call these batteries "dumb" because they have a bms - it just does not interact with the inverter like the pylontechs etc). On the Axpert you should be able to set a user defined battery that meet these requirements.  Lithiums do not need equalise (CV setting to 0min?). Since you will not have complex bms interaction make sure to set the maximum charge current to within spec for the battery. One issue might be to accurately determine/display state of charge/discharge? (no idea how the axperts determine state of charge/discharge but voltage alone is fairly useless for this on lithiums). Your choices for 24V lithium are a bit more limited (locally I can think of pylontech and then the freedomwons and bluenovas). Sticking 2 "dumb" types in series will probably be taking more money out of your pocket than one of the fancier pre-built 24V types, unless you go the second life battery route. DIY is also an option.

I have no idea how well the pylontechs/BYD/Shoto/Narada etc batteries function without the comms cable and also do not know if the 12/24V axperts have any comms capability.

If potentially looking at lithium type money to replace batteries I think I would do some serious calculations (of the accounting type) looking at potentially upgrading to 48V. Don't know how it will impact your solar though..

 

 

17 hours ago, Lindsay said:

I seem to remember the manual saying you shouldn't use any battery type other than lead-acid.

23 minutes ago, introverter said:

Even in the King manual 😉

I think that the above can be read as "If you choose to use a lead-acid battery, make sure it's a deep cycle one [ , not a starter or marine type ]. Also don't try to charge a non-rechargeable battery." I'm sure that the marketing somewhere says that they (certainly Kings) are lithium battery compatible. Indeed, the latest Kings have a "PyL" battery type setting (Pylontechs are all lithium iron phosphate batteries, as far as I know). But I'll agree that the above does seem to suggest not to use a lithium battery.

Just now, Coulomb said:

I think that the above can be read as "If you choose to use a lead-acid battery, make sure it's a deep cycle one [ , not a starter or marine type ]. Also don't try to charge a non-rechargeable battery."

100% agree. When reading through the thread I just remembered that I came across what seemed like a lead-acid only warning somewhere. Think those of us more cautious in nature will quite easily draw the "do not risk lithium" conclusion.

2 hours ago, introverter said:

Wonder if the batteries ended up working harder/more often than thought or often(ish) do not complete a charge cycle. With the base load going and using something like an induction hob (1000-2000W?) even with good sunshine the batteries may get tapped unless "output source priority" is set to "utl"? Slow cooker adding 200W-400W(?) for couple of hours to base load...

I only run the induction cooker or slow cooker on the weekend and around lunch time. The induction cooker only uses 700W as I avoid running it to the full 1400W. It doesn't at any time pull more than 10amps out the battery as the coil switches on and off in the lowest power mode and when the coil is momentarily off then I can see amps going into the batteries. I've only once in the last couple of months briefly (for less than a minute) pulled over 30amps from the batteries, but that's not normal, I think the inverter was busy switching modes and briefly internally disconnected the solar panels.

1 hour ago, Coulomb said:

I think that the above can be read as "If you choose to use a lead-acid battery, make sure it's a deep cycle one [ , not a starter or marine type ]. Also don't try to charge a non-rechargeable battery." I'm sure that the marketing somewhere says that they (certainly Kings) are lithium battery compatible. Indeed, the latest Kings have a "PyL" battery type setting (Pylontechs are all lithium iron phosphate batteries, as far as I know). But I'll agree that the above does seem to suggest not to use a lithium battery.

I guess they probably just trying to cover themselves incase someone drops in lithium batteries without having a bms and destroys something 

2 hours ago, introverter said:

If potentially looking at lithium type money to replace batteries I think I would do some serious calculations (of the accounting type) looking at potentially upgrading to 48V. Don't know how it will impact your solar though..

Thanks for the info on the other battery types. There is comms from the 24V Mecer/Axpert, via USB, which is how I'm logging data, but the inverter has no idea what the charge state of even lead-acid batteries are. I'll have a look at what lithium batteries are available and how best they would fit into the system. Unfortunately I've built a lot of the devices in my house around the solar delivering 24V DC, which makes it harder to consider an upgrade to 48V 😐

 

I took a look at my logs last night, so can now give some further info on the voltages in my setup:

18 hours ago, Lindsay said:

I do notice that sometimes the battery voltage reported by the inverter goes a bit over 30V (pretty sure it has done this since I got the inverter). I think this is brief and just when it's switching modes (absorb -> float) etc so I haven't worried about it.

The voltage very briefly shoots above 30V sometimes, mostly on days where it is not very sunny, so sometimes the batteries are below absorb then 30V, then it settles on absorb, then the fridge/freezer kick in and the voltage drops again, then they switch off and it goes over absorb briefly again, back to absorb etc. Seems to happen for only a few seconds at a time. This doesn't seem to happen on sunny days when I'm not home, so I don't think I need to worry about this.

On 2020/06/08 at 2:09 PM, Coulomb said:

That sounds like the premature float bug. There is no patched firmware for 24 V models, so if you have timed absorb or equalise settings, you could try getting more solar charge energy in that way (minimising the need to mains charge them at night).

I see voltages above 28.7V (absorb) for 2.5-3hrs on a sunny day, after this it drops to 27.4-27.6V (float), so it doesn't sound like the premature float bug. However, on a very cloudy day it looks like it might only hit absorb voltages for around 10 minutes  

Since you were suggesting an absorb for an hour as a starting point, doesn't 3 hours on a sunny day sound too long for my battery bank? Is it maybe being overcharged on those days?

Thanks again!

 

 

4 minutes ago, Lindsay said:

I see voltages above 28.7V (absorb) for 2.5-3hrs on a sunny day, after this it drops to 27.4-27.6V (float), so it doesn't sound like the premature float bug. However, on a very cloudy day it looks like it might only hit absorb voltages for around 10 minutes  

This is the problem. A cloud comes along, the battery charge current goes low as a result, the inverter checks that the battery voltage is higher then the float voltage setting (less half a volt), and says yep, my job here is done. Off to float we go. Of course, if you only get 10 minutes of absorb because by then the sun has nearly set, not much can be done about that.

When you have solid sun, then the current never falls low, and the premature transition to float never gets triggered.

9 minutes ago, Lindsay said:

Since you were suggesting an absorb for an hour as a starting point, doesn't 3 hours on a sunny day sound too long for my battery bank?

It's at the long end, but it depends on your loads, how much PV you have installed, and the battery capacity. 3 hours doesn't sound too long, as long as the battery current is still tapering. On a good day, if the loads are steady, you should be able to see the PV power tapering towards the load level, almost a straight line. Or the net battery current should be similar, it should be straight line tapering towards a low value (say 5 A).

Thanks Coulomb, so do you suggest then that I use the "equalise" setting to force absorb on cloudy days for a couple of hours?

1 hour ago, Lindsay said:

 do you suggest then that I use the "equalise" setting to force absorb on cloudy days for a couple of hours?

Yes, as long as you're convinced your battery is being undercharged. Maybe have everything set up but disabled, and use the "equalise now" 'setting' on cloudy days. You'd want to avoid 2 extra hours of absorb after it already received 3 hours.

Does your model have timed absorb? That might work out better, with less intervention required, and less chance of overcharging when unattended. 

3 hours ago, Coulomb said:

Yes, as long as you're convinced your battery is being undercharged. Maybe have everything set up but disabled, and use the "equalise now" 'setting' on cloudy days. You'd want to avoid 2 extra hours of absorb after it already received 3 hours.

Does your model have timed absorb? That might work out better, with less intervention required, and less chance of overcharging when unattended. 

Definitely no timed absorb. I would hope that if you set it to equalise then it wouldn't do the absorb cycle the same day, so that there would be no chance of overcharging.

I'll try set it up for the weekend when I'm home so I can keep an eye on it.

Thanks for all the help

 

On 2020/06/10 at 12:30 PM, Lindsay said:

I only run the induction cooker or slow cooker on the weekend and around lunch time. The induction cooker only uses 700W as I avoid running it to the full 1400W. It doesn't at any time pull more than 10amps out the battery as the coil switches on and off in the lowest power mode and when the coil is momentarily off then I can see amps going into the batteries.

What I was hinting at (very badly) is that if (some of) the batteries are a bit knackered it is not necessarily strange as they went through more charge/discharge than just the "over night consumption" (even if the rate of discharge was low). Different age batteries and the premature float issue probably had bit of slow decay that you may not have become aware of seeing how little of the overall capacity you tend to use? Any how let me get out of the way of trouble shooting the existing battery bank and remind you of solutions to problems you do not yet have. 😉..😈.. 

On 2020/06/10 at 12:30 PM, Lindsay said:

 I've built a lot of the devices in my house around the solar delivering 24V DC, which makes it harder to consider an upgrade to 48V 😐

DC-DC converter?

On 2020/06/10 at 12:30 PM, Lindsay said:

Thanks for the info on the other battery types. There is comms from the 24V Mecer/Axpert, via USB, which is how I'm logging data, but the inverter has no idea what the charge state of even lead-acid batteries are. I'll have a look at what lithium batteries are available and how best they would fit into the system.

Seems like the "interactive" comms between even the pylontech type lithiums and the inverter are not mandatory . Have a look here . While refering to 48V the following could give you something to work from. I do think that if very accurate display  of SOC is important you will probably benefit from some form of BMV/shunt. If just wanting to make sure that the battery is charged after and ready for the nightly discharge (sheesh that sounds wrong) then experimenting with especially the float setting should get you there. The advantage of the lithiums here are that you do not have to really fear straying over the 80%..50% DOD line (with a 24V pylontech, based on what looks like a fairly predictable 40-50Ah capacity requirement you should be hitting about 50% DOD).

On 2020/06/13 at 11:55 AM, introverter said:

Different age batteries and the premature float issue probably had bit of slow decay that you may not have become aware of seeing how little of the overall capacity you tend to use?

Ok, yeah, I guess there's not much I can do about it now, other than change those equalise settings and attempt to do that everyday. I did set it up on Friday night to equalise at 28.5V every day for 2 hours and on Saturday I noticed it was displaying E9 which I believe means it was in equalise. However it didn't display this the next day, so I'll have to pull the logs and see what happened. 

On 2020/06/13 at 11:55 AM, introverter said:

DC-DC converter?

Yup, 24V down to 12V for lights, 24V to 19V for laptop etc

On 2020/06/13 at 11:55 AM, introverter said:

I do think that if very accurate display  of SOC is important you will probably benefit from some form of BMV/shunt. If just wanting to make sure that the battery is charged after and ready for the nightly discharge (sheesh that sounds wrong) then experimenting with especially the float setting should get you there.

 Yeah, I have a shunt between the inverter/charger and the batteries as well as between the batteries and the DC loads, monitored with an ADC and Raspberry PI so this is how I know they have not been charging fully and have been 10-20Ah short every day. The voltage being 0.5V lower in the middle of the night, than a couple of weeks before was also a giveaway.

Are you suggesting I change the float voltage as well? At the moment it is 27.6V?

So far I have not seen much change in the charging with the daily equalise, it's at least 10-15ah short, but I need to download those logs....

20 hours ago, Lindsay said:

Are you suggesting I change the float voltage as well?

No.. I was in the future where you upgraded to lithiums 🙂. I come from the perspective of battery fully charged as backup supply being the priority. With the voltage discharge curve of lithium being so flat if the charger thinks the battery is still fairly full (based solely on voltage) it does not inniate a bulk charge (think most inverters work on battery @ float voltage minus X to decide to start bulk). So if you do not have a way for the inverter to accurately know SOC and no other interactive BMS integration but you want to make sure a bulk charge is started every day then if you have a fairly predicable load you can use the float voltage setting to achieve that. But if your (lithium) battery is quite large and you use only 10-15% during the night the voltage could still be quite high so a smaller battery where you end up at 50% DOD or more could make life simpler (and unlike the lead acids the 50%....80% DOD line is not that anxiety provoking).