Info on discharging batteries

[CDN_Stalker]

Just seeing what people find works well (doesn't include chargers with discharging feature, or commercially made discharging units) as far as homemade units goes.

I made one last year that I found took forever to discharge, had a current sink of an LED and a DC fan, but recently I thought about vacuum tubes since I have a lot of expereince with them, have many types kicking around as well as the sockets and such. 12A*7 types, when wired in parallel, the heaters draw 300mA at 6.3VAC/DC. Would this be relatively decent to safely discharge an 8.4 3300mAh battery with a suitable current buffering resistor? Or a suitable resistor hooked up to a 6V6 (900mA) or 6L6 (1800mA) be better/worse? While being relatively familiar with electronics, discharging batteries and the formulas/ratings for such eludes me.

[Mantelope]

This will unfortunately add nothing to your discussion, but hopefully prevent some dangerous misinformation: NiMH owners, remember that your batteries do not need to be discharged. There is no advantage to discharging NiMH batteries, and you risk damaging them.

[MadMax]

If you're already electronics savvy, try regulating the voltage down to say 5 volts with a DC voltage regulator (LM7905?) and running it through a 15ohm power resistor (~330mA). A 8.4v battery would draw down to the regulator cutoff voltage and the regulator would automatically stop the discharge cycle. A diode could be put in series with the battery to apply a 0.7v voltage drop if you wanted to discharge a 9.6v pack too.

[CDN_Stalker]

Both are good info, particularly the NiMH not needing discharging. Max, you'd agree with that? I was under the impression all batteries would benefit from the occasional complete discharge then recharge.

[MadMax]

I can't say for certain. I though NiMh did suffer from memory effect, but not as badly as NiCd. In any case, very deep discharging damages pretty much any pack. You end up pushing one cell into reversal.

[Gryphon]

Consumer nickel-based batteries don't get memory effect. The only time that's been witnessed is in satellites that had extremely rigorous charge-discharge cycles in very specific conditions. What people think is "memory" is in fact voltage depression or as Max said, cell reversal.

NiMHs benefit from a discharge every so often to break up dendrites and keep the electrolyte from crystallizing too heavily, but they don't need it every time. NiCds usually ought to be discharged each cycle but it's not really necessary either. The problem is most chargers aren't intelligent enough to know when to stop in either direction and that kills your packs. Get something that you can change the settings on like the Intellipeak ICE and your packs will last a lot longer because you'll know exactly what's going on with them.