Lead Acids low voltage despite fully charged

The past few days I have noted a very low voltage reading from my batteries even when fully charged.

Why could this be? They are 4 x12V AGMs (2x 200Ah & 2x 190Ah) wired in serie.

Out of balance cells maybe?

They are now approximately a year and a half old and they have never done this before.

Volt reading is 51.29 but they have been set to float at 54V

Do you have a temperature sensor attached to them?

No but I have been meaning to do that. I got this one with the inverter when I bought it. At least I think it is temp sensor. The number on the crimping reads E203950 (if this means anything)

I suppose the lug would go to the positive terminal of a battery on one end and the other two wires somewhere to the Venus?

No, I was just asking about a temperature sensor since a reduced charge voltage could be the result of a high battery temperature. But then we know it is not that.

I the picture you posted, I see the battery is slowly discharging. So it is hard to say that anything is wrong. Even a full battery that is being discharged at 15W over a prolonged period will drop to around 12.8V or so (per 12V increment). That’s 51.2V. So what I see there is not too concerning. Even it seems lower than what you are used to… it could just be that the batteries aren’t new anymore.

Measure the voltage of each battery (cell) to check if they are all the same…

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The reason why I’m kinda ignoring the suggestion of an imbalance… is that that will not cause a lower voltage while charging. It will cause one of the batteries to overcharge (sit at a higher voltage than the rest).

So the proper way… set GX to “Keep batteries charged”. Then when it reaches Absorption, measure each battery and make sure the voltage is the same, or within 2% at the very least.

But as I understand the original post: He is concerned about a lower voltage than he is used to at 100% SoC… but because that battery is actually discharging slowly… it really isn’t possible to say whether anything is wrong. I still see a good 12.8V per battery. That’s plenty good for lead acid…

Thanks Plonkster, I hadn’t thought about is that way. They’re still 12V batteries and I agree that 12,8 is pretty good.

That said, I set the system to keep batteries charged last light night like you and Richard suggested, and left it there overnight. (it was already basically full when I did this)

I measured this morning and these are the results:

1st battery (which is the + of the 48V) = 13.25V (190Ah)

2nd battery = 13.30V (190Ah)

3rd battery = 13.68V (200Ah)

4th battery (which is the - of the 48V) = 13.60V (200Ah)

Me thinks this is still very good.

The moment I switched back to optimized with battery life, the volts dropped immediately to 52 something as per below screenshot. I supposed I didn’t think a small load like this would make the volt reading drop that far. I think it is still remnants of the olden days when I started playing with batteries and dual battery systems when most of us thought volts and SOC is the same thing.

OK, now that may be a reason for concern. You have a 13.68/13.25 ~= 3% imbalance between the highest and lowest. At 13.2V the battery doesn’t get enough charge to fully counter self-discharge, while at almost 13.7V that battery is high enough to off-gas a little (even at float). This is usually the point where people start to look at a battery balancer.

Also, you indicate that some of the batteries are 10Ah less than the others? Are the batteries not identical? They should be (in series). In parallel, a small discrepancy would not matter too much.

Edit: Also, it might be interesting to pull the voltage down a bit to trigger an absorption cycle, and then repeat the voltage tests. Normally one wants your 58V charge voltage to divide evenly over the 4 batteries, giving each one around 14.5V. But once there is an imbalance like this, you may well find that some push to 14.8V or even 15V while the lower ones struggle to exceed 14.2V. And at 14.8V an AGM battery tends to off-gas more than it should…

I initially went with a 24V system and bought two 200Ah AGMs

3 months later I saw the light and went 48V but despite trying hard, I couldn’t get more 200Ah units at the time, and thus went for 190Ah.

Ok so how do I practically do this? Drain the batteries to say 95% SOC and then measure again? Should there be a load on them? Must I disconnect the solar array?

If you have a meneer of a charger for regular 12V batts I would simply connect it to the batteries that are low in comparison to the others and manually balance them.
If this helps then look at an automatic balancing cct.
(In the bad old days you would ‘equalize’ the batteries by cooking them at high voltage & current)

Not a meneer. I have Benton BX2 that does 7 amps and has served me well over the past 8 years.

I don’t suppose I can charge them individually whilst still connected in serie? Or can I?

The Multi goes back into Bulk at Vfloat - 1.3V (or 12.9V, whatever is higher). Times 4 for 48V. So you need to do something to pull the voltage down below 51.6V. You only need to do this for a minute or so, you don’t have to go all the way down to 95% SoC.

Then immediately put it back to “keep charged” and wait for it to reach absorption. Measure the individual batteries once this happens, to get an idea of how bad the imbalance is at the top.

You will only have a short time space to do this. The Multi stays in absorb for only a short time, because it times absorb based on how long Bulk was… which is not going to be very long because you didn’t take it down very low. But that is fine… you just want to do a quick test…

That is fine. You just want to boost the low ones a bit with some extra charge. You don’t need a big charger for that.

Sorry Plonkter, these two statements confuses me a little. They appear contradictory in my layman’s mind.

So do I have to measure during the absorption cycle? Or only once it finishes the absorption cycle?

But can I do this whilst they are still in serie? And I suppose I have to remove all loads and switch over to the grid for this? Maybe also disconnect the PV?

Dude… you want to trick the Multi into pushing the batteries to 58V (which I assume is your absorption charge voltage). To do that, you need to pull the voltage down low enough to bump the Multi out of Float mode. Once you’ve done that, the Multi will take it to 58V, but only for a short while before it goes back to float.

What you want to do is within this window where the batteries are held at 58V… you want to measure your individual batteries to check the balance.


Yes. You can do this with the batteries in circuit and with the solar chargers still charging. You can also leave the Multi on, but I would probably not let it discharge the batteries, ie set it to Charge-Only (you can do that on the GX device), or at least set the ESS mode to keep charged.

What will happen is that the solar chargers/Multi will still aim for a total of 54V (at float), but because you are pulling those individual batteries up with extra charge current, the ones who sit at 13.7V right now will naturally come down a bit.

Ok thanks. I will do this as soon as the batteries reaches 100% today. There seems to be something wrong with the VRM portal though.

It has been over two hours since it updated and I cant get it to refresh. Nothing happening or moving on it.

Also cant get into Remote Console

That is just on your side. Works fine here.
Move away from the Dashboard, and then back. You can also try F5 to refresh to hole website.