Zener Diode Question - Discharging NiMh Batteries

[safrole]

Has anyone done this? I guess you are not supposed to completely discharge nickel metal hydride batteries. One 7.2v set I have says to "discharge after use to 5.4 volts." Nicads I just flatten out, but could I use a 5.1v zener diode and a 12v light bulb to discharge NiMh packs down to the desired level?

I'm pretty sure I could, but I also wondered about voltage oscillation around the cutoff voltage, if it exists and could it be bad for the batteries.

[carcharadon]

I believe a 5.1v zener

I believe a 5.1v zener diode would limit voltage greater than it's stated value, not lesser than.

[JWLaRue]

Jason,

....not quite a direct answer,

Jason,

....not quite a direct answer, but I use a programmable charger that handles the discharge and charge cycles for my R/C sub batteries. I use gel cells, niCads, and NiMH....with the NiMH being the ones that I have bought/used for my more recent boats.

These things take all the work and worry out of the care and feeding of the various battery types.

There are several on the market, I happen to use one sold by FMA Direct.

-Jeff

[safrole]

I got the poor man's

I got the poor man's version, a Duratrax for about $50 with a lot of programmable features for NiCd/NiMh, but no auto-discharge.

Am I wrong about zeners in general? I thought they flow freely one direction like any other diode, but when you run them "backward" they at first resist (like any other diode) but when the voltage rises to a certain point (like 5.1v here) then it "breaks" and allows current to flow. So I thought using it in series with the light bulb would flow ~7.2v until the batteries dropped to 5.1v and then it would stop the flow. I could be wrong. I only know enough to be dangerous, but it seems like there is the potential to have a couple of $5 circuits to drain all my packs overnight, unattended.

[Guest]

You could use a standard

You could use a standard diode in forward bias, together with a 4.7v zener, so 0.6v forward + 4.7v reverse bias would give a voltage of 5.3v, not including tolerances.

[safrole]

So I can consider the

[color=#000000]So I can consider the 4.7v zener as a 4.7 volt drop? Sorry to sound ignorant, but I am. Then I get a .6v drop from the regular diode, too, almost as if it were a resistor?

PS]



Edited By safrole on 1128792016

[Guest]

Why do you want to

Why do you want to discharge NiMH batteries? I thought that, because there is no memory problem, you just fully charge them after using them, without regard to the degree of discharge. Am I missing something?

[safrole]

I've heard both sides. Both

I've heard both sides. Both sides agree that deep cycling is bad for them. I find it difficult to believe there is no memory effect whatsoever, but that could just be some residual NiCad paranoia.

Maybe I'm misunderstanding the label on the battery which says in part "After use, the battery pack should be discharged to no less than 5.4 volts."

Maybe they don't mean 'After use, you should then discharge it to no less than 5.4v', but 'In the course of using it, don't discharge it below 5.4v'.

[Guest]

The latter would be my

The latter would be my interpretation of the instructions, according to what I have read about NiMH batteries.
I have been using AA NiMH battery packs for my transmitters for a while, and recently started using waterproofed 12V, 2600Mah packs in my new Teskey Flying sub/Whipray, with good results. See pictures on my web site- My Webpage



Edited By Art Broder on 1128978142