Battery Monitor vs Voltage

[Beeb]

Hey all,

I have 3 of these batteries. They're 125Ah each, 12v, AGM deep cycle. So, that puts me at 375Ah. I have 500W of solar on the roof, and everything has been going quite smoothly since installed.

However, this has been our first week of nothin' but clouds... ugh. We're on day 5, and I finally shut the power off (we don't have a secondary charging method yet). But I've noticed something else that I have a question about...

When I got the batteries, it came with a slip of paper that said 13v or more equals fully charged, 12.4v equals 50% charged, 12.2v equals 20% charged, and 12v equals 0% charged.

However, I've noticed that my battery monitor's percentage doesn't match this. The 3 batteries were reading at 12.4v when the battery monitor was still saying 75% charge. And now the battery monitor is saying 60% charge, but the batteries are reading at 12.1v. Is something wrong? Or do I just not understand how batteries work?

 

[john61ct-SKO]

Those are not what I would call deep cycling. Have you got their avg-DoD vs life cycles chart?

Voltage has very little correspondence to SoC if you need accuracy.

Most charge sources get nowhere close to 100% Full at default profile settings, you need to adjust / verify using an ammeter to measure endAmps, stop dropping to Float too early.

In many conditions relying too much on solar inputs, can just set Float to the Absorb V. Just remember to put it back when insolation conditions improve.

That is a great BM, but needs manual 100% resets at verified endAmps Full, as close to every cycle as possible. All of that type drift **a lot** after 3-4 cycles, especially if you never get to Full, and 6-10% accuracy is as good as they get anyway.

Merlin SmartGauge is easier and more accurate.

Balmar's new SG-200 may combine best of both types, even claims to give a SoH reading, but just out of beta so too early to know for sure.

 

[Beeb]

Those are not what I would call deep cycling. Have you got their avg-DoD vs life cycles chart?

Um... Where would I find that?

Voltage has very little correspondence to SoC if you need accuracy.

Most charge sources get nowhere close to 100% Full at default profile settings, you need to adjust / verify using an ammeter to measure endAmps, stop dropping to Float too early.

In many conditions relying too much on solar inputs, can just set Float to the Absorb V. Just remember to put it back when insolation conditions improve.

That is a great BM, but needs manual 100% resets at verified endAmps Full, as close to every cycle as possible. All of that type drift **a lot** after 3-4 cycles, especially if you never get to Full, and 6-10% accuracy is as good as they get anyway.

Merlin SmartGauge is easier and more accurate.

Balmar's new SG-200 may combine best of both types, even claims to give a SoH reading, but just out of beta so too early to know for sure.

sigh... okay. So I need an ammeter to measure endAmps (I assume I can find what that means and how to do that online somewhere), and then when the endAmps are verified as full, I need to manually reset the BM?

 

[john61ct-SKO]

Um... Where would I find that?

Check the maker's web site, google, contact tech support of the maker?

FFR if you can't easily get such basic specs when you're shopping, good reason to strike that brand off your list.

Heavy batteries should only be bought locally, usually from specialist sellers recommended by the maker.

Buying online, shipping costs eat up too much of their value.

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> endAmps (I assume I can find what that means and how to do that online somewhere)

Once the charge circuit gets to your target Absorb setpoint, check - at the batt - every hour or so how many amps are flowing as that "trails down".

If the BM's shunt is wired properly, you may be able to trust its reading, but verify periodically of a known-good ammeter, even the priciest gear needs occasional calibrating.

If you can get the mfg spec for endAmps, use that. Probably cannot, then use .005C, or .5A per 100Ah capacity.

The charge sources should all be adjusted so that Absorb stage is held, IOW do not drop to Float V, until after that endAmps 100% Full point is reached.

Then tell the BMV to reset its Ah-counter, and for at least the next few cycles it should be pretty accurate.

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For more details, go to http://marinehowto.com, and CTRL-F search for three articles with "battery monitor" in the title. Also AGM. Also Deep Cycling.

Everything to do with electrickery at that site is pure gold, worth repeated close reading.

There's a PP Donate button there, I chip in very regularly. He also sells great gear, best butyl tape on the planet, can order most anything, well worth supporting for all the work he donates to the community.