SmartShunt, Weird SOC and not sure I understand the SOC calculations and Peukert

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cybermaus asked Feb 5, '24

SmartShunt, Weird SOC and not sure I understand the SOC calculations and Peukert

So I bought a new Lead-Acid, a Q-batt SEM 105 with 95Ah at C20 and a Smart-Shunt. My solar topped it off to float yesterday, validating the 100%, and I left it overnight with a small 0.7A load.

This morning it sat at 10.6Ah used with 93% SoC.
At first glance, those numbers do not match, -10.6Ah should lead to 88% SoC

Reading up, using this http://www.smartgauge.co.uk/peukert3.html
I realize Victron likely uses for C*(C/R)^1.25 formula and assumes my battery to be a 140Ah Peukert 1A capacity. Or at least, then the 93% would match.

Is this correct? Should I leave the default 1.25 factor as reasonable? Obviously I do not have such detailed specs for my battery. Should I change anything?

I lost one expensive branded Bosch AGM battery to over-discharge already, do not want to loose another, even if it is a cheaper one. BTW: I bought a cheaper one as I consider it a stop-gap. I want to install Sodium or Lithium, but need to plan for the electrics and the space, it is a very tiny camper, and I would need to reshuffle some of the guts around. Probably a project for next autumn.

SmartShunt

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Answer 1 · 2024-02-05T08:55:42Z

pwfarnell answered · Feb 05 at 08:55 AM

The SmartShunt manual has a chapter (8) about Peukert coefficient and battery capacity and your understanding is correct. Just keep an eye on the SOC and keep it above 50% or so.

As you have solar charging, it is a good idea to increase the Charged Voltage setting on the SmartShunt to 0.2V below the absorption voltage on your solar controller. If you have 14.4V absorption set the SmartShunt charged voltage to 14.2. This ensures that your SOC resets to 100% only when fully charged.

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Answer 2 · 2024-02-05T09:30:30Z

cybermaus answered · Feb 05 at 09:30 AM

Thanks. So no problem with the 1.25 default/example factor then?

Also, you say 50%, and I realize this is normal for common starter batteries.

But this Q-Batt is sold as flooded Semi-Traction, they list 75% DoD (with 300+ cycles). I was thinking putting it at a conservative (relative to 75%) 35%.

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Alexandra ♦ commented · Feb 05 at 09:41 AM

The 1.25 default is about correct for FLA battery types. In theory if you are discharging lower than the Crating for the battery you have more capacity.

DOD is your choice as you know that affects life span.

As does making sure they charge fully everyday (through absorption back to float every day).

So only discharge as much as you are able to charge up fully everyday.

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