24V Series Battery Voltage SOC question

[hongning]

I just changed from a 12V to a 24V battery system.

I have (2) 100AH LifePo4 Batteries that I changed from parallel to series to make 24 V

I need to set a voltage level for a MOES auto transfer switch, so it will automatically switch to grid given a low voltage level of battery,

So my question my 12V battery has a voltage of 12.9V for 20% SOC

in a 24V series should the voltage be 25.8V (12.9x2) for 20% SOC?

Just asking as my Victron BMV-700 is showing about 70% when it reaches 25.8V.

 

[rmaddy]

SOC voltage for a 24V battery is exactly double that of a 12V battery. For LiFePO₄, 20% would be 12.8V on a 12V battery or 25.6V on a 24V battery. Note that voltage is a terrible indication of SOC for LiFePO₄ which is why you have the BMV-700.

If your shunt is showing 70% at 25.8V (really 30%) then your shunt hasn't be properly synchronized yet.

In order to ensure your 12V batteries work well together in series as a 24V battery you should do the following before going any further.

Fully charge each 12V battery, one at a time, with a 12V LiFePO₄ friendly charger. Once both have been fully charged, wire the two batteries together in parallel (not series). Hook up the charger so one lead is on one battery and the other lead is on the other battery. Fully charge them together. When the charge is complete, disconnect the charger but leave the two batteries connected in parallel overnight. Now you can separate the two batteries and connect them in series into your system.

With the shunt installed, reset it to 100%. Make sure its settings have been properly updated with your new battery voltage and capacity. It's now a 24V 100Ah battery bank, no longer a 12V 200Ah battery bank. Make sure the "charged voltage" is set to 0.1V less than the absorption charge setting you use for your 24V battery (28.2V - 29.2V).

 

[hongning]

Thanks let me try that.

 

[StuartV]

I too have 2 x 100ah batteries together in series. My `resting' voltage is 26.2, seems to stay on that for ages too. Then, when it gets below that 26.2V, it quickly discharges and I'm left with no free electricity ☹ ( i say "quickly" as in several hours or overnight)

I read a post last week saying that the float charge should be 27.5V for a 24V system. So I changed the settings on my MPPT AIO. Maybe its just the batteries I have, but they didn't stay at 27.5V for long, went down to the usual 26.2ish V and stayed there for most of the evening.

I've attached my battery specs ?