Help me to understand Voltage drop ( decrease ) while under load vs at rest and state of charge

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dallas asked Oct 1, '20

Help me to understand Voltage drop ( decrease ) while under load vs at rest and state of charge

I was wondering what is the true reading of a AGM battery under a heavy load ( 150 watts)

The manufacture of my battery says the halfway mark is 12.4 volts ( 50% ) When I power on my television, I keep it on until it reaches 12.4 but then the voltage increases back up to 12.65 after it’s turned off

I guess my question would be this- would the 12.4 constitute a true halfway mark when running the television because of voltage decrease under a load ?
Or is it safe to run it lower than 12,4 if it bounces back to 12.65 ?
I have noticed that Victron records the lowest number under a load even after it bounces back up.

voltage battery system voltage

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Answer 1 · 2020-10-01T12:31:31Z

benl answered · Oct 01, 2020 at 12:31 PM

The battery has internal resistance - this means a voltage drop *within the battery* when you're pulling current. You can imagine a real life battery as a perfect ideal voltage source with a resistor in series. The voltage lost across the internal resistance will result in a lower voltage at the terminals, with the voltage drop proportional to current draw.

As to whether you should cut power at the under-load or no-load terminal voltage: I'd expect the simple answer is "whichever is smaller", which would be the under-load voltage. i.e. if the battery manufacturer recommends you don't go under 12.4 V, then don't go under 12.4V. When your heavy load causes the voltage to drop to 12.4, you could probably disconnect that and continue with smaller loads until eventually they too result in a terminal voltage under the limit.

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dallas commented · Oct 01, 2020 at 03:48 PM

Thank you BenL for taking the time to answer my question. I have been using the TV until 12.4 then disconnecting TV and using battery until it reaches 12.4 under small load. I need to replace this TV with a 25watt that way the battery will not have a lot of resistance compared.

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spirou dallas commented · Oct 01, 2020 at 06:24 PM

To deduce SOC from voltage it should be measured under no load (or charge), after about an hour or more at rest (although about 15min rest will give you a decent idea). Obviously you can consider the smaller voltage under load as a kind of a buffer or reminder before you actually get to any limits. Possibly more problematic is higher voltage when charging as people often think a battery is charged when its nowhere near full.

A substantial drop under small loads on the other hand is a good indicator of a bad battery.

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