Discharging agm

[Gerard Fivewin]

If there is still a load of a tv, approx 55watt, the voltmeter gives 11.8volt. If i turn it off thus NO load anymore it shows 12.2 to 12.4.

Ive read this here on this forum:
"discharging any 12volt battery to 10.8v is a death sentence. You really should never go below 12volt since they usually start at approximately 12.8v at 100% SOC."

perhaps a stupid question but i ask anyway:
Never should go below 12v with or without load??

[Wy_White_Wolf]

What does your battery manufacturer recommend?

WWW

[NEOH]

If there is still a load of a tv, approx 55watt, the voltmeter gives 11.8volt. If i turn it off thus NO load anymore it shows 12.2 to 12.4.

Ive read this here on this forum:
"discharging any 12volt battery to 10.8v is a death sentence. You really should never go below 12volt since they usually start at approximately 12.8v at 100% SOC."

perhaps a stupid question but i ask anyway:
Never should go below 12v with or without load??

The Resting Voltage, with no load connected for several hours, is useful as a State of Charge measurement.
The voltage of a Battery UNDER-LOAD will vary tremendously (see image below)
You can see in the chart below that a light load, of 0.05C amps, drops the voltage very little.
While a heavy load of 3C amps, drops the battery voltage immediately to ~11.6 Volts.
11.6 Volts with a heavy 3C amps load = normal.
11.6 Volts with a light 0.05C amps load = Nearly Dead
Huge difference.

https://i.stack.imgur.com/uTlqV.jpg

[NEOH]

Without load. If your 55 W TV is pulling your battery down 0.5 V it suggests an internal resistance of (0.5^2)/55 = 4.5 mohm.

I do not understand your Internal Resistance formula.
I agree that the Load is dissipating 55 watts
But you are saying, the Internal Resistance is also dissipating 55 Watts per your formula ...
Internal R = ( Edrop x Edrop) / Watts of the load

I have always used ...

Internal Resistance = ( Voc - Vloaded ) / Amps

111 milliohms = ( 12.3v - 11.8v ) / 4.5 Amps

[sensij]

I have always used ...

Internal Resistance = ( Voc - Vloaded ) / Amps

111 milliohms = ( 12.3v - 11.8v ) / 4.5 Amps

Thanks for the correction! Mental note... No more math in the middle of the night.

That is, however, a very high IR for AGM. I think there is more background on this system in another thread.

[PNjunction]

Generally, when you go below 10.8v under a constant load,(beyond 100% capacity) the chemical reactions damage the battery to a degree. It isn't an immediate death-sentence, provided you can recharge right away, (which involves a "slow start" procedure) but you haven't done the battery any favors. Perhaps taken many future cycles from it in addition to the 100% discharge already.

Much like eating a triple-bacon-guacamole-swiss cheesburger. If you get up, do some exercise and burn some off, then once in awhile won't kill you. Still, you haven't done yourself any favors.


Neoh's chart is pretty useful. Also, The general rule of thumb for charge/discharge max current for AGM, is that you don't want to exceed 0.25C discharge, even if you can. Just like charging an agm - usually no more than 0.25C is recommended to keep the chemical reactions acting normally. Same with discharge. You can exceed these rates at a cost, depending on application.

Long story short - don't go below 10.8v under a normal constant load. In fact, it's better to limit it to 12v under load max. 10.8v is a dead-man-switch kind of catch you see on ac inverters. Keep your charge/discharge current at 0.25C or less, unless you are running high-end $$ pure-lead types.

[ToyHauler]

Thanks