How much discharge Pb 12V cell

[jiri]

Hi,
I just got my first Pb cell on 12V and 7Ah for my
charger. I was wondering how I know that the cell
is empty to charge it over night? What is the minimum
voltage it can reach without damage (without load)?

Many thanks,
Jiri

[rdn1153]

There is very little benefit from cycling a lead acid battery. If you do discharge a Pb battery, use a discharge cutoff voltage of 1.8V per cell and don't leave it in a discharged state for very long. Pb batteries us the same charge structure as Lithium batteries, that being constant current / constant voltage. The max voltage you should not exceed is 14.7V for a 12V battery. As the battery reaches a full charge it will resist additional charge current. If you want to slow charge it, I'd use C/20, or 350mAh for your 7A battery, and don't leave it charging for more than 20 hrs.

[jiri]

Thanks a lot.
I do not want to cycle the Pb cell, I just wonder how
do I know when I should charge it again. What is the
voltage when I should put it on the charger again?
Should I charge only if it is almost discharged or can
I just charge it after every use like NiMHs (e.g. when
it still has 4/5 of its capacity)?

Jiri

[Hooloovoo]

I just work it out from the capacity.....

e.g. I can charge my 2100mAh packs five times from my 12Ah Pb pack, since 12/2.1=5.71 ie after five charges there wont be enough left, just enough for 71% of the sixth charge.

In reality I just take three or four charges off it and then recharge it ready for the next day.

Most chargers will tell you when the battery is flat - my Orbit gives me an audible warning shortly beforehand, then stops charging.

[Hooloovoo]

Yes apparently my new SuperNova will do that [img]/ubbthreads/images/graemlins/grin.gif[/img] With my old one the charge current just gradually got less and less... it would still try and charge but would only output a few hundred milliamps.

[jiri]

Well, I am not sure about my Hobicco Quick Field Charger.

But the question is still whether I can charge the Pb
cell even if it is almost full or should I wait until
it gets discharged? Does it behave like NiMH (charge
anytime you want)?

Jiri

[Hooloovoo]

[ QUOTE ]
Does it behave like NiMH (charge
anytime you want)?

[/ QUOTE ]

Yes.

Think of a car battery..... you take a little out of it when you start the car and then it instantly starts charging again when the car is running.

Just top it up when you've taken a few packs from it.

[jiri]

Thanks,
that's what I wanted to hear!

Jiri

Pb batteries lifespan will be shortened if you leave the battery in a discharged state, or repeated cycling below 50% (deep cycling). It will not hurt one bit to "top off" the battery, in fact, it's best.

Because a battery is labeled "deep cycle", doesn't mean it can tolerate deep cycles better, it's about the plate construction which gives the battery more AH, but less capacity for high amperage discharge rates (like turning a cold engine).

Size and weight considerations aside, calculate the number of AH you need between charges, and buy a battery with a capacity twice your need.

Some newer, VERY expensive batteries are more tolerant of deep cycles, but regular old Pb "Marine Deep Cycle" batteries are the cheapest AH for your buck (maybe pound too, I can't speak for prices in the UK).

Greg