Subject:
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Re: Pure Energy & the RCX
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Newsgroups:
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lugnet.robotics
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Date:
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Thu, 18 Nov 1999 22:57:45 GMT
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Viewed:
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725 times
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In lugnet.robotics, Robert Munafo writes:
> It's strange to see you describe the "how to find the bad cell" problem with
> reference to Alkaline cells, then go right on and suggest the use of Ni-Cad
> battery packs!
>
> Ni-Cad battery packs have the same problem that any other type of serial
> multi-cell battery pack has: there's no way to find or eliminate one bad cell.
Actually, finding a bad NiCd is easy - split the battery into its component
cells, and individually charge/discharge them (or even just check their voltage
and polarity if they're REALLY shot) to rate their characteristics. Dedicated
racers, and profitable companies, match new cells this way - and double the
price of a pack in the process!
It's the alkaline rechargeables that are the problem.. I've found that fully
discharging one of these cells kills it completely - ruining it, venting its
internals. This makes finding a bad one rather difficult. Someone mentioned
earlier in the thread that the charger finds bad cells by not charging them -
well, I can't say I've had that happen. Even an unvented but toasted cell
allows the illusion that it has taken a charge. But that's just my own
experience.
> Ni-Cad batteries have to be discharged fully before recharging, for best
> results. (Unlike alkaline and lead-acid cells, Ni-Cads develop "memory"
> problems from being less-than-fully discharged). So, naturally, users of NiCad
> packs will try to use the pack until it's dead, before recharging it.
>
> When this is done, after 10 to 20 discharge cycles you reach the point where
> one cell in a 6-cell pack gets down to 0 volts while the other 5 cells are
> still at near-nominal voltage and still have a fair amount of stored energy.
> The result is that the weak cell will then begin to be *reverse* charged by
> the other 5 cells in the pack. Reverse charging is even worse than the memory
> effect.
One more caveat here - NiCd's must never be discharged past 0.9 volts per cell.
Not in regular use, anyway. Hardcore R/C racers will discharge batteries, then
dead-short them for storage. (not recommended for casual users or the faint of
heart!) If you don't REALLY know what you're doing, you risk the same damage
Rob lists above - reverse-charging a cell in a pack, or worse.
While we're on the subject of charging, no battery should ever be overcharged.
Timer chargers should never be left unattended - always watch 'em like a hawk.
As one racer in my R/C club can attest, an overcharged NiCd makes a lousy
rocket engine....
Be safe and have fun!
K.M.
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Message is in Reply To:
 | | Re: Pure Energy & the RCX
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| It's strange to see you describe the "how to find the bad cell" problem with reference to Alkaline cells, then go right on and suggest the use of Ni-Cad battery packs! Ni-Cad battery packs have the same problem that any other type of serial (...) (25 years ago, 18-Nov-99, to lugnet.robotics)
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