I want to make a circuit that cuts off a mains power inverter connected to a vehicle's battery.

I have found, however, SLI batteries come with very sparse specifications. For example, the Varta E43 72Ah.

I wanted to cut off the inverter when the battery approached 40-50% state of charge. Firstly to avoid damaging the battery, and hopefully to leave enough juice for the starter.

I was going to determine the approximate state of charge a point in time by measuring the current draw by the inverter, the voltage across the battery, and the temperature in its vicinity.

But given the lack of battery specs, this is a problem. Given this, I don't even know how to determine the state of charge manually. (So I could then therefore take the battery down to 40% and then profile the battery for V/I characteristics at different temperatures while at this 40% state of charge).

AFAIK, the cells can't be accessed, so I can't even use a hydrometer, not that the specs even give the specific gravity for any state of charge.

Any advice would be great.

Edit: Note to self: calculating the SoC would need to incorporate hysteresis - see image - for example, it is often said that open-circuit voltage is best only used for calculation of SoC if the battery has been at rest for 6-8 hours - "polarisation". enter image description here


1 Answer 1


It's still a lead acid battery and has the following typical characteristic of state of charge under load conditions: -

enter image description here

It shows several discharge curves and typically C/100 means that if the battery has a capacity of 650 Ah, C/100 means a discharge current of 6.5 amps.

Picture taken from here, a very informative document.

  • \$\begingroup\$ Great, thanks. I've actually used that reference a lot. However, it does say "The data presented here on the graphs was generated from our set of Trojan L-16W deep cycle lead-acid batteries". And then even says: "Other types of lead acid cells, like car batteries, lead-calcium cells, and “RV deep cycle” batteries will have different charge/discharge curves" \$\endgroup\$
    – CL22
    Mar 29, 2015 at 11:00

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