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Battery Charging Concerns


raymondtrueman
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I have a new 3KW clone inverter produced by Must Power with a 24V Battery bank consisting of 2 brand new 100Ah AGM batteries. It is not connected to solar and charges from the grid. I noticed its charging light flashes orange 24/7 non-stop. Usually with these Axpert-type inverters when it finishes charging it should enter floating charge mode and the light should stop flashing and turns a solid orange. This is not happening and I am a little worried that my batteries might get damaged due to a constant high charge current being applied to it. It may be because of the infamous charge/float bugs on some of these clones. 

Another thing I am not sure about is when the grid is cut-off the battery voltage takes big drop from around 27.4V to 26.2V in only a couple of minutes. Is this normal? Same thing happens when the grid power comes back on - it increases almost instantly to 26V.

I have some charts below of the whole Battery cycle during 2 recent load-shedding events if that helps. Unfortunately the monitoring software does not monitor the grid charging current. Thanks for your help

Sept 1 Load-shed - Click here for interactive chart

Load Shed Sept 1 - Battery Voltage

Sept 2 Load-shed - Click here for interactive chart

Sept 2 - Load Shed

Edited by raymondtrueman
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On 2020/09/03 at 8:27 AM, raymondtrueman said:

I noticed its charging light flashes orange 24/7 non-stop.

Even during load shed? If so, are you sure it's the charge light, not the AC/INV light?

Of course, being a clone, who knows what the behaviour should be.

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It may be because of the infamous charge/float bugs on some of these clones. 

It seems to return to about 27.4 V after a load shed; that's a float voltage. It should be going to about 28.2 -28.8 V to recharge the battery fully.

What are your absorb and float voltage settings?

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Another thing I am not sure about is when the grid is cut-off the battery voltage takes big drop from around 27.4V to 26.2V in only a couple of minutes. Is this normal?

Yes, that's normal battery behaviour. 27.4 V (13.7 V per 12 V module) takes a little charge current to keep it there. When this charge current is removed, the battery drops almost immediately to around 13.0 V (13.1 V in your case), and should decrease slowly from there.

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Same thing happens when the grid power comes back on - it increases almost instantly to 26V.

That's slightly more concerning; it indicates a high internal resistance. That might be because they are full, or it might be because they are not getting charged fully.

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I have some charts below of the whole Battery cycle during 2 recent load-shedding events if that helps.

Yes, very helpful, thanks.

Unfortunately the monitoring software does not monitor the grid charging current.

OK, but it would be good to get a few manual charge current readings: soon after the charging starts, and after some hours. Hopefully one of your clone's displays shows charge current.

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Even during load shed? If so, are you sure it's the charge light, not the AC/INV light?

No, the light is off during the load-shed - sorry I should have mentioned that. I meant while the grid is on - the light always flashes orange, it never goes to a solid orange

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What are your absorb and float voltage settings?

Settings are as follows (all left as default from factory):

Float: 27V

Absorption: 28.2V

Battery high: 30V

Battery Low: 20.4V

Grid Max Charger Current: 20A

Battery Stop Discharging: 23V

Battery Stop Charging: 27V

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It seems to return to about 27.4 V after a load shed; that's a float voltage. It should be going to about 28.2 -28.8 V to recharge the battery fully.

Maybe why the charging light keeps flashing...?

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That's slightly more concerning; it indicates a high internal resistance. That might be because they are full, or it might be because they are not getting charged fully.

"Instantly" according to the chart is around 2 minutes - so not really that instant. Is 2 min acceptable to make that climb?

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OK, but it would be good to get a few manual charge current readings

There are a few current readings from the monitoring software but they all go to 0A as soon as the grid power is back on or not relevant to charging. There is a "battery charger current" but I am pretty sure it only indicates values for solar charging because its always 0. On the screen itself there is no charge current reading. I have a multi-meter but to connect that in series will be a bit of a mission

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Hi, I have the same Must 3Kva, 24v inverter and the charge light also flashes continuously if it has a charge source. I have since added a bmv and that has been useful for seeing what is happening. The inverter uses a constant 30w from the batteries regardless of use priority, so the inverter is constantly pushing power to the batteries to keep them at the float voltage. 

I am not sure if this is doing any harm to the batteries, however it doesn't seem to be in my observation. 

The Must I have has some quirks that don't seem to affect the axpert guys from what I've seen. 

 

 

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9 hours ago, raymondtrueman said:

I have a hunch that the “Battery Stop Charging” setting might be to blame here...

I assumed that this is the Must equivalent of the Axpert "back to discharge" or "back to battery" setting (setting 13 on an Axpert). But it may not be.

My other though is that for Axperts to return to bulk charging from float, either the battery voltage has to go quite low instantaneously (like 2.0 V lower than the float setting, for a 24 V model), or it has to be at least 0.1 V (on a 47 V model, so 0.5 V in a 24 V model) lower than the float setting for 30 minutes. Musts may have similar preconditions.

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I scanned the manual and attached it here. When you look at it on page 18, the setting I am talking about is number 21 called "Battery stop charging voltage when grid is available". Now that, to me, seems like it is going to stop (absorption maybe?) charging when the voltage reaches 27V seeing that I am only charging from the grid and don't have solar. So charging starts from 25V after a load-shedding session with absorption, and then reaches 27V and stops due to this setting, ignoring the absorption setting of 28.8V. And then either it keeps charging and stopping to remain at 27V (hence orange light persistent flashing) OR it enters float charging mode, which also has a setting of 27V immediately after it stops at 27V. This theory also matches what we see in the graph I posted. But I might be wrong. I am so confused even after reading the manual. On page 26 they show how the charging cycle works but seems like the absorption voltages are wrong in that table at 25V (typos?). Anyway, please have a look through this manual and let me know your thoughts. Thanks

Must 1-3kW Inverters PDF

P.S. It is the "MPPT-80A" version when the manual refers to a model

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16 hours ago, raymondtrueman said:

I scanned the manual and attached it here. When you look at it on page 18, the setting I am talking about is number 21 called "Battery stop charging voltage when grid is available". Now that, to me, seems like it is going to stop (absorption maybe?) charging

No, it sounds like settings 20 and 21 are meant to be like settings 12 and 13 on Axperts, i.e. the back to grid battery voltage, and the back to battery (back to potentially discharging) battery voltage. So when mains is available, it should switch to line mode (which I note is missing from the manual, at first glance at least) to prevent the battery from discharging much further. If charge sources are available (e.g. utility charging is enabled), then the battery should charge, while the mains powers all the loads. When the battery voltage reaches the value in setting 21, the inverter should switch back to battery mode. Whether the battery will be net charging at that point depends on how much charging power is available and enabled in other settings, and the load at any point. I'll assume that charging power is fixed, and the load is variable as you turn things on and off. So it might be charging one moment, and discharging the next, depending on whether the loads are higher or lower than the charging power.

16 hours ago, raymondtrueman said:

when the voltage reaches 27V seeing that I am only charging from the grid and don't have solar. So charging starts from 25V after a load-shedding session with absorption,

That's actually called bulk stage; it's not absorbing until it reaches the CV or absorb voltage. Your problem is that you never get there.

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and then reaches 27V and stops due to this setting, ignoring the absorption setting of 28.8V. 

It could be that when the inverter switches to battery mode, your loads are high enough that the utility charger just can't push the battery (now carrying all the loads) past 27 V. What maximum utility and total charging limits do you have in your settings?

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I am so confused even after reading the manual. On page 26 they show how the charging cycle works but seems like the absorption voltages are wrong in that table at 25V (typos?). 

Yeah, that's really confused/confusing. I think 27.4 V is meant to be the float voltage, 25 V the point where it restarts a bulk charge, and 28.8 V or 28.4 V is the absorb voltage.

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6 hours ago, Coulomb said:

That's actually called bulk stage

Sorry That was a mistake, I meant to say bulk.

 

6 hours ago, Coulomb said:

Your problem is that you never get there.

Yes since I got it, the mains went off 5 times and not once did it charge above 27.4V. My total load is small - max 350W but more like 200W average. Maybe the battery does not drain enough to trigger a full 4 stage charge cycle. The lowest was around 25V. Next time I will keep the mains off until <24V and then see what it does.

I have added a the load to my previous chart if you want to see it: Sept 1 - Batt Voltage and Load

 

7 hours ago, Coulomb said:

What maximum utility and total charging limits do you have in your settings?

I have all settings on the default settings shown in the manual. I don't really see any settings for utility and total charging limits other than the ones we discussed here

Thanks for your help. I will report back when I have some more data

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On 2020/09/04 at 9:22 PM, Dougiedanger said:

I have since added a bmv and that has been useful for seeing what is happening. The inverter uses a constant 30w from the batteries regardless of use priority, so the inverter is constantly pushing power to the batteries to keep them at the float voltage. 

Would you mind posting the data from one of your charge cycles? Not from the MUST though but from the BMV if that is captured...? I am curious to see what it looks like

I will also order a BMV and a balancer - seems like handy investments

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3 hours ago, raymondtrueman said:

I have added a the load to my previous chart if you want to see it: Sept 1 - Batt Voltage and Load

I was going to say that your loads are very modest, but your charging power is only 500 W at present.

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Grid Max Charger Current: 20A

Try increasing that to the max of 30 A. That's still only 750 W, but it may be enough more than your loads to get the battery better charged.

3 hours ago, raymondtrueman said:

Maybe the battery does not drain enough to trigger a full 4 stage charge cycle.

Yes. I'm thinking now that your main problem is not starting a bulk charge. That coupled with more charge power might be enough to get you beyond 27.4 V, and save your battery.

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On 2020/09/06 at 11:41 PM, raymondtrueman said:

Would you mind posting the data from one of your charge cycles? Not from the MUST though but from the BMV if that is captured...? I am curious to see what it looks like

Hi, sorry I am actually not sure how to get data off the BMV in order to send it. I use it through the app but it disconnects every time you do something else on the phone so not sure how to send a whole charge cycle?

Maybe someone else could let us know?

On 2020/09/06 at 11:41 PM, raymondtrueman said:

I will also order a BMV and a balancer - seems like handy investments

Yes absolutely, some of the best advice I got from this forum and well worth the extra expense.

Are you in Zim by any chance?

 

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2 hours ago, Dougiedanger said:

Hi, sorry I am actually not sure how to get data off the BMV in order to send it. I use it through the app but it disconnects every time you do something else on the phone so not sure how to send a whole charge cycle?

Maybe someone else could let us know?

Thanks, I actually know what you need because I had done some research while looking at the BMV. You need to connect the BMV to one of the below devices. They connect the BMV via WiFi/Ethernet to the internet and upload your readings to the cloud constantly, and then you can login on to their portal or app to view all the data and charts:

https://www.victronenergy.com/live/venus-os:start

Pretty cool and probably worthwhile if you want to monitor everything and have history data to work with

I live in Johannesburg btw

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