question

stevecounsell avatar image
stevecounsell asked

State of Charge voltages for 200ah Battery

I have a fairly new installation (July 2019) of a 200ah battery and a BMV 712.

I’ve the last week or two I am noticing that the % reason on the BMV seems off from the actual voltage reading.

EG SoC is 51% and Voltage is 13.26

I know my fully charged voltage (SoC 100%) is 14.4 volts but can’t find any list of voltages for lower SoC.

Is there a way, list, or calculation that will show me the _real_ SoC for the voltage at the battery.

My system is:

Multiplus 3000

200amh Victron Lithium battery

Victron bms

Battery protect X 2

Sterling Power Dc charger.

All fitted in my Motorhome where we live full time.

TIA


BMV Battery MonitorSOC
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4 Answers
wkirby avatar image
wkirby answered ·

It is not possible to get an accurate SOC from battery Voltage. There are other factors that affect the battery Voltage other than SOC. Charge or discharge current and temperature are factors that affect battery terminal Voltage independant of SOC..
If you applied a 100A load to your fully charged 200Ah battery, the Voltage would drop significantly but the battery is still pretty much full right?

The BMV calculated SOC by measuring the charge and discharge energy going into and out of the battery including losses.
If you applied a 2A load to your 200Ah battery for 10 hours (20Ah), then the BMV would read 90% (losses are minimal at low discharge rates).
Try not to rely on battery Voltage as a SOC indicator. If the BMV is set up right then it will track SOC much more accurately by counting energy in and out.


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stevecounsell avatar image stevecounsell commented ·

So, is it true / accurate that the battery is fully charged at 14.4 volts?

If that is accurate what would be the fully discharged voltage?

OR have I got this completely wrong?

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Matthias Lange - DE avatar image Matthias Lange - DE ♦ stevecounsell commented ·

1. 14.4V is the absorption voltage for lead batteries

2. The absorption voltage for a 12V (12.8V) lithium battery is 14.2V

3. The standby voltage (no charge or discharge) of a fully charged 12.8V lithium battery is about 13.2-13.3V

4. Especially with lithium batteries you can't say "voltage x = SOC y". Lithium batteries have a very constant voltage from 100% to about 15-10% SOC. Below 10% SOC the voltage will drop faster.

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Justin Cook avatar image
Justin Cook answered ·

@SteveCounsell, I'm curious to see pictures of your installation, and particularly your batteries / BMV shunt connections. When customers report excessive variance from expected SOC shown on the BMV, the fault is usually found in a load or charge source bypassing the shunt, which means the shunt can't "see" the charge going in (or load out) and will therefore report a different SOC than might actually be the case.

It's absolutely critical that the only thing connected to your battery NEG terminal is the "battery" side of the shunt, and that all other NEG connections, including loads, chargers, chassis grounds, and etc, are connected to the "loads and chargers" side of the shunt.

That's not to say that your BMV actually is reporting incorrect data... it's perfectly possible for your battery voltage to show 13.26 while your SOC is 51%... but since an incorrectly-connected shunt is the cause of 98% of the calls I receive regarding unexpected BMV readings, I am biased toward suspecting that there's something bypassing yours.


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stevecounsell avatar image stevecounsell commented ·

@justincook thanks for the detailed response. I’m pretty sure that the shunt is connected correctly. I’ve attached a picture.

The only connections on the battery are the POS and NEG cables.

My BMV reading as I type this is 28% and 13.29 volts

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Justin Cook avatar image Justin Cook ♦♦ stevecounsell commented ·

@SteveCounsell, interesting, that does appear to be connected correctly... and that NEG cable on the left goes to your battery and is the only NEG cable attached to the battery, correct?

I suppose the other thing I'd immediately check is that you've correctly entered your total Ah capacity into the BMV? Should be 200Ah, obviously, and charge efficiency should be set to 99% and Peukert exponent should be set to 1.05% for the Victron LFPs.

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rickp avatar image
rickp answered ·

This may not be an answer, but I’ll pass it on as a suggestion. You can manually synchronize the BMV-712 when your battery bank is at a known 100% of charge in the settings via VictronConnect. You can try that and see if it tracks reliably from there.

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brandon avatar image brandon commented ·

Thanks you, Rick. I have tried that and it tends to just drain and the charge percentage doesn't come back up, despite the voltage showing that it is charged.

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nickdb avatar image
nickdb answered ·

You need to check what current the batteries are drawing under charge, that will give you an idea of how charged they actually are.

Shunts need to be set correctly so the SOC synchronises reliably.

The manual gives some tips for charge voltages but the duration and tail current need adjusting. The default 4% is way to high for lead acid batteries.

On a 4-battery string I have found that 1.8/1.9% with a duration of 8 - 10 minutes seems to do the trick.

The manual advises the charge voltage be either just under float (by 0.3V) or just under absorption for solar installs. On advice I have set my solar installs to midway between float and absorb.

The best place to start with these issues is to set the BMV/shunt correctly then do a proper discharge, and monitor how it behaves.

This process has worked well now for me.

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