My understanding is, one is the opposite of the other, but yet my battery’s can be showing depth of discharge at say 24.2 volts, and yet my state of charge is showing above 75%. Which one do I go by?
asked
State of charge versus depth of discharge
Battery voltage vs state of charge tables assume a "resting" state for the battery. That is, no load or charing for some period of time, usually 10s of minute to hours. It's really not possible to relate battery voltage to state of charge under charging or loaded conditions. Typically both charging and some load are always present in a working system so you need a battery monitor that works on current in and out of the battery to calculate the actual state of charge. This is what SmartShunt or the BMV series do. These units need to be told when the battery is at its 100% SOC. This is done by looking at charging voltage and current during charging.
The default setting for the SmartShunt and BMV is to reset to 100% SOC when powered up. This option should be turned off so that the battery must reach a full charge before the reset to 100% occurs. Otherwise the SOC reading could be way off.
Some people use "depth of discharge" rather than state of charge. They are opposite indicators:
SOC = 100% - DOD
Some people use "consumed amp-hours" as in indication of state of charge:
DOD = consumed amp-hours / battery capacity (in amp-hours) * 100%
And unfortunately people keep asking why the Ah drawn doesn't match the SOC. It's not designed to, the Peukert Exponent sees to that. If you type "Ah SOC" into the searchbar here you'll likely get a few hits on just this issue.
Thanks Kev, last night I turned off that default setting and ran my generator and charger to ensure the battery’s had got to 100% and been in absorption for about 4 hours, they didnt go into float but I’m presuming that’s because I had about 6 amps going out whilst charging? this morning the shunt said 93% soc and 15.2 amps consumed. The most amps consumed before recharging I’ve seen was 60 amps, so I’m well within the 160 amp battery’s capability. From now on I will trust that soc reading, and keep one eye on the amps consumed.
You can not compare SOC @ voltage on one hand, vs SOC @ BMV (?) on the other hand.
The absolute first thing I learnt about my LifePo4 battery was the voltage was super stable at varying SOC.
Hence I bought a BMV and dialed it in to suit my battery.
Hi, thanks for replying,I’m reading it on the smartshunt Bluetooth app, is the Bmv much better? The shunt will show me that I’ve fully recharged the lead carbon 160 amp batteries, and then the next day show me I’ve used say 40 amps since last full charge and it says soc is still above 80% but my battery’s show low 24volts. I will take a screen shot tomorrow. I have set the shunt to 160 from the factory 200 setting,and the setup is in a camper van that has solar and if needed a generator to run the 24 volt 12 amp victron charger, which when I have used it has definitely put the batteries into absorption and is showing 100%, thanks again
For measurements, BMV/Smartshunt are equivalent. Both need setting up correctly.
