question

sampeett avatar image

Battery voltage high & drops low under load!!

Hi all,


I’ve just installed a 300w solar panel and 100/20 MPPT smart to my van with a 12v 95ah class A AGM battery. I have, I guess two problems, one being the voltage seems very high (13.77) when not in use? See photos.

Second, when I first installed the system the battery seemed to sit at 12.8 but after loading the system for 30mins it dropped to 12.6 then by one hour 12.40 to which I turned the load off! X3 2w led lights and a fan vent. It said the current was 1.9amps be pulled. I turned them off as I heard dropping before 12.30 could damage the battery.

Not sure what I have done wrong or if I need to set the app up correctly?

I plan to add a second battery in parallel but don’t want to do so until this issue is sorted.


Kind regards




battery system voltage
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5 Answers
justme avatar image
justme answered ·

It looks like your battery has lost a serious amount of capacity to drop that much voltage with a load that small.

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Justin Cook - Bay Marine Supply USA avatar image
Justin Cook - Bay Marine Supply USA answered ·

@Sampeett I agree that your battery resting voltage appears high; normal resting voltage of an AGM battery is typically around ~12.8v. I suggest reaching out to your battery manufacturer regarding this, or taking the battery and having it tested.

Dropping under load, however, is exactly how it works... when you apply a load to a battery, the voltage will drop. This behavior is significantly less when using an LFP battery, but still present - it's simply how a battery behaves.

In your case, you have a very small battery (95Ah = ~47Ah usable) so the voltage will drop rapidly even under relatively low load, so this behavior is as expected. I certainly recommend that you upgrade your battery bank; this may end up being required if, after a battery test, you discover that you need a new battery.

So, in summary: Battery voltage dropping under load is normal and expected. Your high battery resting voltage is probably not normal, so please check with your battery manufacturer regarding the expected resting voltage of your battery, and then -unless they say that ~13.7v is normal- go have that battery checked.

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It's worth questioning if "when not in use" means resting though or just not pulling a load with the controller applied. With a float voltage of 13.8v that reading of 13.77v where ever it was taken is very close to the target float voltage.

If it is without the controller applied then the question should be how long has the battery rested before taking that reading. It can take a little while for the voltage to decay to the resting voltage.



sampeett avatar image
sampeett answered ·

Thank you for all this information. It is very strange as the battery is brand new, any ideas as to why is drops so fast under load? If I had left the load on surly it would have gone past 12v which could damage the battery? I worked out it should last days if not weeks running just lights and a fan before dropping to

Well I presume it was at rest, there was not sun out for the solar to take. I don’t really understand how the MPPT works. On the app the load was set to always off so I turned it on to always on was this a mistake? I did read an article saying 13.8 was the float state of charge?

sorry for all these questions, I am very new to this.

kind regards



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JohnC avatar image
JohnC answered ·

Hi @Sampeett

If your battery is 'brand new', it likely just needs some work to fire it up. Many batt makers actually publish info saying that full rated batt capacity isn't developed until subjected to quite a few cycles.

Akin to an athlete running a race, but limbering-up beforehand. Yours is just rolling out of bed.. :)

And your tiny Wh history isn't helping do that. That's nothing, and your athlete needs at least a couple of stretches before breakfast. Don't get excited about V at this point, just try to remain aware of how much you're taking from the batt. The rating of that batt suggests > 1000 Wh capacity, and to draw it down 300 Wh/d would be a very safe level once it's fully broken in. Fooling about at < 20Wh/d won't get you there.

And oh, I note you have User-defined your V charge settings to higher levels. I'd revert to one of the lower stock algorithms, regardless of what you read of lab figures from the batt maker. Long life, eh..

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Daniel avatar image
Daniel answered ·

Hey, 13.8V at idle of a completely new battery is fine. I recently bought a new battery (200Ah) Wich I have cycled a couple of times now and when I have no load and fully charged it sits around 13.1V.

I have a 15Ah, too. I cycled it around 8 times with 7A because it sitting in the shelf a lot and now it stays idle 13.5V. That's normal at AGM. You can safely discharge it down under 12V. But don't go beyond 10.5V. Since 10.5V is the lowest the cells could handle before taking damage (Discharged my little one to 9V accidentally but charged it directly).

The best is, wait for a sunny day and then pull some amps to discharge and let it charge up again. Do this 3 to 4 times and your battery will get better.



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