Hi all,

how many amps can be drawn from a battery without damaging it?

I have a battery for a bosch drill (18v 1.5A/hr) that i occasionally wish to discharge.

Many thanks.

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Chris

 

Discharge it fairly slowly, certainly 1A to 1.5A would be fine - but don't discharge it too far, or you will damage it.

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hi nigel,

thanks but what do you mean too far?

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Chris

 

If you discharge it below a certain voltage, you won't be able to recharge it.

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Surely, this depends on the type of battery. AFAIAA, Nicads should be completely discharged to avoid the memory effect, or am I missing something?

Mike.

 

Of course it does, a car battery might be able to take hundereds of Amps but a tiny button cell would strugle providing a few ten thousenths of an Amp.

You should never fully discharge a car battery.

Most modern electric drills use NiMH cells which don't suffer from the memory effect.

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As far as lead acid is concerned I totally agree, never discharge them. But, why would discharging NiMH cause them to be unchargable?

Mike.

 

I don't know the technical reason but it's happened to me before and the same is even true with NiCad cells.

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You should never fully discharge a battery that has cells in series.
One cell will have less capacity than the others and it will be completely discharged first, then the current from the remaining discharging cells charges it in reverse which is very bad for it.
The discharge should be stopped when the average voltage of each cell is 0.9V with load.

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Uncle $crooge

 

Surely, modern batteries are fairly well matched and this isn't a problem. I thought that the biggest problem with Nicads was whiskers growing on the electrodes that could only be removed by completely discharging them. The complete discharge placing the metal back into solution.

Mike.

 

Energizer, Sanyo and Panasonic say not to fully discharge their battery cells in series unless the current is low. The Battery University also explains what happens to them.

I think discharging cells in series makes "whiskers" on the weakest one that gets charged in reverse. The "whiskers" short the cell.

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Uncle $crooge

 

I thought that the metal got deposited when the battery was charged and therefore whiskers could only grow if you partially discharged them before recharging.

Mike.

 

I've heard that it's possible to blast the whiskers away with a huge current pulse of up to 10C for under a second.

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Yes, you can blast whiskers.

Do you have any link about your earlier statement that you shouldn't discharge below a certain voltage.

Mike.

 

In The Battery University, the author says that over-discharging series cells damages the weakest one which causes it to short with "whiskers" that penetrate the separator. He says you can blast the whisker away with a high current but the damage to the separator remains. The cell is not matched to the others so the problem frequently re-occurs.

In Energizer's Ni-MH Applications Manual they talk about the damage that occurs to a cell that is reversed-charged due to over-discharging of series cells. They say that Ni-Cads are worse than Ni-MH cells.

I read an article somewhere where they dicussed the formation of smooth small particles or large jagged ones (whiskers) but I don't remember the details.

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