Measuring State of Charge (SOC) at almost full troubles

[john61ct]

Originally Posted by Jerry Woodward

How to reliably tell 100% SOC of your bank. I would like to know that as well. Assume we're talking about a lead acid bank, 400ah golf cart batteries in my case.

the canonical measure is endAmps, all you need is a decent ammeter . After the CC(Bulk) to CV(Absorb) transition, trailing amps keeps dropping, maybe 4-6 hours later it hits the specified endAmps level.

Often .005C is a good default, so 2A for your bank when new.

As they age, .02C may be good, or 8A, or just use

Current change over 1 hour period of less than 0.1A

Note you don't have to hit this point every cycle, for longevity, but most cycles would be great, and less than 20-30% of the time, you're verging on light PSOC abuse and cycle lifespan will take a hit.

Most owners off grid hardly ever get there, extreme PSOC is the norm, many don't care and more aren't even conscious of the issue.

[john61ct]

Originally Posted by Jerry Woodward

clamp-on ammeter. Are these for permanent installations

No but once you've calibrated your SC's Hold Absorb time so you hit 100% most cycles, you don't need to verify too often, unless your usage cycle changes.

An AH counter may be inline, shunt included, but then you have to go to the bank to read it.

> To use this effectively, I presume one would turn off all loads

Not at all. As long as you're measuring Amps **into the bank** that all you care about with SoC.

You can put an ammeter at different lications to measure the total for specific loads or sources.

> this would depend on the voltage the various charge controllers are putting out. So would it be better to say, 100% SOC = 1% amps going in AT A GIVEN CHARGE VOLTAGE?

All your sources should have Absorb set within the narrow range spec'd by the mfg of your House bank.

They ideally each will hold Absorb until endAmps, only drop to Float after 100% is reached.

what specific devices would you recommend for charging amp measurements

A minimum clamp ammeter is $40-$80.

Can get a cheap AH counter for about the same.

For better SoC accuracy a good AH counting BM is I think around $300? Victron's 712-BMV is my reco.

Note the shunt must be sized 30-50% larger than the maximum current it will see.

[john61ct]

SmartGauge is best for SoC in general, easiest to use, helps prevent dropping too low at the bottom, say 50%.

but no amps measurement, so not to be used for calibrating end of charge cycle. Also lead chemistries only.

Many owners combine SG and the BMV, but a pricey combo.

[Jerry Woodward]

Originally Posted by john61ct

> To use this effectively, I presume one would turn off all loads

Not at all. As long as you're measuring Amps **into the bank** that all you care about with SoC.

You can put an ammeter at different lications to measure the total for specific loads or sources.

John
Thanks for your explanation. But I don't understand your answer above. Say my bank is accepting 6A from the alternator with no other loads on. In this case, I would say that my bank is pretty well fully charged. However if I took the measurement with my refrigerator on drawing 6A. Wouldn't I now see 12A going into the bank on an ammeter? What am I missing here? Not trying to be argumentative, just trying to understand

[john61ct]

All depends on **where** you're measuring.

Say you have a 500A charge source running, but your 1500AH lead bank is getting near 100%, thus only accepting 40A. No other loads.

An ammeter A with its shunt placed at the bank displays +40A.

Now a 100A watermaker comes online.

Ammeter B shunt at the alt shows -140A, ammeter C with its shunt at the watermaker shows +100A, and ammeter A is unchanged.

Hope that helps.

[Jerry Woodward]

Got it. Thanks!