BMV- 712 shows 81% capacity remaining but voltage is reading 10.66 & 10.70 on my 2 lead acid batteries (110 ah capacity ea.) after what’s indicated as a 55.4 ah consumed. I’m also unable at this point to use the batteries to crank my diesel generator. Previous setup with only 2 75 ah batteries allowed me to run them down to =/- 35% and still be able to start the generator. My gut feeling is that these newer batteries are lacking in capacity (due to bad cell or cells). Any advice appreciated.
It seems that the battery monitor may not be correctly set up for the higher capacity batteries and also that they are not being properly charged (unless they are actually faulty).
We will need more detail to assist further:
- A screenshot of the BMV712 settings,
- details of the exact model and type of the new batteries,
- how are the batteries charged and do you have voltages readings or settings for the charging system?
- are these batteries connected in parallel or series?
There is a sticky post at the top of he DIY Category with lots of FAQs in, perhaps you missed this. One of the topics has a title that describes your problem.
See
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This should be screenshots of the settings and the battery label. The bank is wired in parallel. Charger is a Noco 2 bank charger (10 amp per). It seems to be functioning correctly but to give you exact numbers I’ll have to go back out and plug it in. I’ve still been watching the discharge with the data given above to see what it does.
When I swapped to these larger Capacity batteries last summer I didn’t change anything other than the setting making it 200 amp capacity from 150. Previously my setup had worked perfectly for almost 5 years. Initially these newer batts. seemed work fine, just recently been seeing the issue.
Thanks for the reply.
Sorry, I did miss that sticky. Lots of info on this page and I get bogged down reading stuff that’s irrelevant because I find it interesting! I’ll give that post a read. Thanks
If you read the sticky it tells you that your charged voltage setting is wrong and why, swap 13.2V for 0.2V below whatever the absorption setting the chargers use.
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So to verify I’m wrapping my head around all this correctly, with my current settings if the monitor sees 13.2 volts and 8 amps or less current for 3 minutes while charging. then it believes the bank to be charged and syncs the SOC to 100%. When in actuality the bank is being undercharged. Sound right?
Yes, that is probably what is happening. Serial under charging results in sulphation and capacity loss so the situation just gets worse.
I put them in last August so makes sense that’s they’ve slowly been losing capacity from undercharging. I know I didn’t purposly change any settings other than the AH capacity. Would my current settings have been sufficient for my older setup of 2 75 ah batts?
FYI… charging now at 13.56 v @13.74 a. I think I’m going to pull each battery and try a separate charger to de-sulphate them then reinstall with revised settings on the monitor.
There are two separate problems here.
- The battery monitor is perceiving and reporting 100% SoC before the battery is fully charged.
- A separate battery charger that is not controlled by the BMV seems to be undercharging the batteries (or both batteries are faulty).
The BMV settings can be adjusted to better synchronise the BMV to the actual SoC. This solves problem 1 provided the batteries are healthy.
Problem 2:
If the charger is being manually connected and disconnected on the basis of the reported SoC then fixing problem 1 will solve the undercharging issue.
However, if the charger is permanently connected then either the load on the system exceeds the charging capacity, the charger is faulty, the charging profile does not suit these AGM batteries or the batteries are faulty. To diagnose these options:
- following charging and with no load on the system remove the caps on the battery cells one by one if possible. If any cell gives off a hydrogen sulphide (bad egg) odour the battery is faulty. Since hydrogen could also be present in healthy cells, ensure no ignition or spark potential nearby and the area is well ventilated. Wear eye protection. Alternatively…if the voltage drops rapidly to below 12V after a full charge cycle and with no load on the system then the batteries are faulty.
- Make sure your Noco charger is set to use its AGM charging mode. During normal charging use a voltmeter or voltage display of the BMV to check that charging voltage is within the ranges listed on the batteries during bulk, absorption and float stages. If lower the batteries will be progressively undercharged and if higher they will be damaged over time.
- the BMV can also display the current (amps) passing into or out of the battery. If there is continuous draw on the system make sure the charger has enough output to supply the load and charge the batteries simultaneously.
Thanks for the troubleshooting tips. The charger is always connected and monitored so I feel that your problem #1 is the case. Apparently my settings whether changed somehow or not, are not sufficient thus leading to bad info being fed into the monitor thus leading to chronic undercharging over several months.
I suppose the charger could be faulty but it is indeed charging but you might be on to something regarding the correct profile if something has gone wrong internally. All these things offer these days is a red or green LED. Hard to discern a lot sometimes from those. 
However if any part of this were faulty I’d lean towards the batteries. It wouldn’t be the first time I’ve had Interstate batteries fail early in their life. On the other hand, my last pair lasted for 5 years and were actually still going good except for a decrease in capacity.
Thanks again for your inisght!
In your system the monitor and the charger operate independently. Inappropriate monitor settings will not influence the charger’s behaviour.
From your description it seems most likely that the batteries have failed. If they are still within the warranty period get them checked and hopefully replaced.