Battery question

[Top Hat]

It's generally accepted that to look after your battery it should not be allowed to go below 50% discharge or 12.2v at rest.
Under a heavy load a full battery (12.7v) voltage could approch 12.2, which got me thinking if I start with a battery at rest at 12.3 under a heavy load down to say 11.9v does this harm the battery at aĺl if at rest it's still in the 50% reigon.
Hope this makes sense

Charlie

 

[Zigisla]

If you start at 12.2V at rest, your battery is not 100% charged as it should be in the region of 12.6V. Therefore you are harming the battery even more than it is already.

 

[two]

The '50% mark' is a bit like a 'sell-by date'. It's rather rough & ready. There's nothing magical about that value, other than one to try and avoid. It's a guideline, not to go below it but that doesn't mean you can safely go up to it either. Visit it occasionally, perhaps, but not routinely.
The longer you can keep away from that region, the longer your battery is likely to last and, when you do discharge one, you should recharge it asap. Batteries left at a low state of charge will deteriorate and may never recover. If you have a battery whose Voltage drops quickly, it is probably because it's old and has lost capacity. It's condition is likely to deteriorate until unusable or you decide to replace it.

 

[autorouter]

if I start with a battery at rest at 12.3 under a heavy load down to say 11.9v does this harm the battery at aĺl if at rest it's still in the 50% reigon.

No it doesn't harm the battery. The voltage at any instant is not a good indicator of state of charge. Only if you disconnect the battery from all loads for an hour or so does the voltage indicate the state of charge.

Think of it this way. A proper battery monitor will measure the current at each instant in time, and keep a tally of all the discharges. It works out the state of charge, showing a continuous and accurate display.

If you have a 100Ah battery that is 60% charged is contains 60Ah of charge. If you discharge it by 5Ah (say 20 amps for 15 min) the voltage will drop quite a lot, but you'll still end up with 55Ah of charge, ie 55% of a 100Ah battery.

I think some people are a bit over-protective of batteries. 500 cycles to 50% gives 250 times the charge capacity. 1000 cycles to 75% also gives 250 times the charge capacity.

It's true that once you get to 20% or below, there's a risk of unbalanced cells causing problems and even reversing, but as long as you stay above that, you can just discharge the amount of energy you need.

 

[pappajohn]

A battery is said to be permanently beyond help at 10v.

Ive seen my engine battery as low as 9.5v in winter trying to start my car (knackered glow plugs, prolonged cranking) on more occasions than I remember.

It's been doing this for the last two winters but once the weather warms a bit its fine.

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[tonyidle]

The voltage under a heavy load will sag dramatically due to the internal resistance of the battery. It is of no relevance when trying to determine the state of charge by measuring resting voltage - which should only be checked when the battery has been disconnected from anything for 2 hours or so.