Checking state of charge - voltage

[Mirage35]

Have been studying batteries and charging systems because ... well, I'm about to spend the money for new AGMs but don't want to do it again any time soon.

Looking for confirmation on one detail: I understand that the best way to check a battery's SoC is to disconnect it, wait an appropriate time, then measure voltage. If it shows what you expect for the chemistry and number of cells, it's charged.

But: My understanding is that this just shows it's charged to its current capacity. If it has been abused or is just old, it may not provide its nominal Ah rating, but this will still show it as fully charged. Right?

I'm getting up to speed, and I've read Mainesail's stuff on the subject (many, many thanks), am just looking for a sanity check on this detail. Thanks.

[Dsanduril]

For the most part, yes. A battery can be fully charged - i.e. all of the chemical reaction that can occur has occurred - even if it has reduced capacity. However, that provides no information about any material that is no longer available for reaction (it has reacted in an irreversible way). Only (practical) way to get that information is a load test.

[sailorboy1]

Basically yes.

Long wandering “help” will soon show up for you to retype all you have properly read elsewhere

[s/v Jedi]

Correct. To find current battery capacity you need to do something like the 20h test

[Mirage35]

Originally Posted by Dsanduril

... all of the chemical reaction that can occur has occurred...

Wonderful - this is the technical detail that explains it for me. Got it.

[Mirage35]

And thanks for the other confirmations as well.

[john61ct]

The capacity remaining later on, compared to the peak experienced within the first few months when coddled, is SoH% or State of Health.

State of Charge is the relative level within that maximum capacity.

Voltage **can** give a general indication of SoC, but various factors keep that an approximation only.

Same is true of SoC battery monitors, but some are pretty accurate most of the time without going to too much ongoing trouble.

The really important 100% Full point,

before which your charge sources should not drop to Float in order to get good bank longevity,

is best judged by trailing current at Absorb voltage, measured with an ammeter.

At which point resetting your BM may be needed to keep its SoC% reading relatively accurate for the next few cycles.

As well as adjusting SoH downward over time, and maybe tweaking CEF and Peukert value.

The Merlin / Balmar SmartGauge is a bit simpler than the Ah-counting models in this regard, and for most banks remains one of the most accurate, but it does not show Ah consumed, just its SoC estimate for House and voltage for House plus Starter.

IMO better than using voltage alone. Only works with lead chemistries, not LFP.

Many use both SmartGauge and the Victron BMV-712,

or Link-PRO (aka E-Xpert Pro, originally from TBS) for Ah-counting.

There are many differences of opinion in this topic area.

To review Maine-Sail's excellent articles, https://www.google.com/search?q=site...attery+monitor

A*knowledgeable person willing to go to the trouble, certainly can coddle their big expensive bank without any SoC meter.

The question as to whether it is worth paying for one is of course up to each of us to decide, just owning one certainly is no magic bullet.

And most owners, who only spend a few hundred on their bank, and don't mind replacing it every few years, would certainly not see a quick ROI from a strictly economic POV.

While others are willing to spend their money on electrickery gadgets just for the fun of it.

[Larimar450]

I just replaced my generator battery this week, it tested fine on the volt meter but under load not so much.
If you can get them tested under load I find this is the best way to determine the health of the battery.
all I got ! I find two years seems to be the limit on batteries for me