Discharging Ni-Cad drill batteries.

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389poncho

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Hello,
Outdated title I know, but I bought & stored a number of new Ni-Cad 18 volt drills some time ago and know they are good drills and batteries if cared for properly [the "memory" thing 'ya know].

I don't want to waste good drill motors by clamping the trigger just to discharge batteries for recharging. For long bulb life, I want to connect a 24 volt incandescent bulb to them to discharge them gently. My two options are a 6.5 watt bulb or a 50 watt bulb.

Any thoughts on this would be greatly appreciated.
T'care,
Mike
 
Everything I've read suggests that it's a bad idea. A Ni-Cad battery should not be deep-discharged. It should be charged once it drops enough to need charging.
 
As I understand, the modern NiCd batteries don't have much, if any, memory effect. I'd just start using them; if you're really concerned, discharge at maybe 10% (of capacity) for a couple of hours butdo not do a full discharge. Or buy a battery conditioner to do the job.
 
The thing which severely damages NiCd cells is reverse charging. The individual cells can be safely discharged to 0V.

If you discharge a battery consisting of cells in series, then one of those cells will likely discharge first. Continue to discharge the battery and that cell will get reversed.

If you have an NiCd battery that has been in storage, your best bet is a slow equalizing charge (C/20 or less) for long enough that all cells reach a full charge state. Then use as normal. The first couple of cycles will probably be at reduced capacity.

Jon
 
NiCds were the best we had in the last decade of the 20th century, but the myth of the battery memory, in actuality, was just what happened when the battery wore out.

I remember guys putting c clamps on the triggers of cordless drills to make sure the batteries were fully discharged. Ridiculous idea. I also remember guys saying "Don't unplug a battery charger if the battery hasn't fully charged". Equally ridiculous.
 
If you have a multiple cell series string of NiCad batteries, you may benefit from rebalancing it when performance drops. Shunt each cell individually after pack has been discharged. Then recharge normally.

In the early days of NiCad batteries they had a substantial excess of one electrode material to prevent outgasssing on recharging.
In one experiment the miltitary overchanged a 1AH D cell at C/2 for several weeks. At the end of the test they had a 10AH cell. :)
 
zbang said:
As I understand, the modern NiCd batteries don't have much, if any, memory effect. I'd just start using them; if you're really concerned, discharge at maybe 10% (of capacity) for a couple of hours butdo not do a full discharge. Or buy a battery conditioner to do the job.
Click to expand...
I could misremember, but more than 15 years ago I recall reading that the memory effect was a myth.
 
From what I recall about the 'memory effect':

In a very specific situation with precisely controlled charge/discharge cycles, if you kept discharging to the same depth of discharge the cells would 'remember' that and lose capacity. But this was only in situations with very specifically controlled charge/discharge cycles.

There is separate issue that can be confused with memory effect, and which is real.

If a cell 'cooks' on a charger, the electrode materials can anneal slightly to a lower energy state. This reduces the output voltage and increases cell impedance. Importantly discharging and re-charging the cell cleans up this issue. But apparently if you have cells that have become voltage depressed and you partially discharge them you 'see' the voltage depression again when you reach the same state of discharge. The sudden change in voltage can look like end of discharge on the discharge curve, and thus look like 'memory'. The difference from true memory effect is that you can safely continue the discharge past this point.

Again this can be fixed by a deep discharge, the danger being actually damaging a cell if you deeply discharge the _battery_ and reverse a cell.

-Jon
 
zbang said:
As I understand, the modern NiCd batteries don't have much, if any, memory effect. I'd just start using them; if you're really concerned, discharge at maybe 10% (of capacity) for a couple of hours butdo not do a full discharge. Or buy a battery conditioner to do the job.
Click to expand...
Thank you zbang. Is the battery condition you mentioned similar, in theory, to the battery tender I use on my stored car?
 
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