Re: Nickel-Metal Hydride Message #5 Posted by Glynn on 9 Sept 2000, 9:19 p.m., in response to message #1 by Frank B. (Germany)
More than you ever wanted to know about NiMH:
It's a flavor of battery that replaces nasty heavy-metal Cadmium with a compound that actually has better energy storage characteristics. A NiMH AA can hold 40% more energy than a NiCad, at similar voltages and discharge rates.
It is a bit more sensitive to heat (so it is not as good a choice for a device stored in your car during summer). It also self-discharges slightly faster than NiCad, so taking the batteries out, when you don't intend to use the device for a few weeks, is best.
But charging is the REAL rub. It has much less "memory" effects than NiCad, and it can have a longer overall lifetime than NiCad, but ONLY if it is charged under pretty strict controls to keep the chemistry in the battery as intended.
You MIGHT be able to limp by with NiMH in a NiCad application, IF you are careful (and maybe use a stopwatch with the charger), but because of charging differences, you'll never get the full benefits of the extra energy capacity and longer life of NiMH unless you redesign the charger. In fact, you'll probably find that a NiMH put through several NiCad charger cycles will die pretty quickly; not a cheap thing to have to throw out.
NiCad is generally charged at a set rate until topped out, then a trickle charge rate is applied; but NiMH has a much higher (10x) beginning charge rate, switched at about 30-40% of full capacity to the NiCad rate, and then, BEFORE totally topped out, switched to a much smaller trickle charge than NiCad.
While NiCad can be monitored for these conditions by VOLTAGE, thus easily, NiMH is not easily monitored this way, so a "smart" charger design looks instead at the change in temperature of the battery occurring during the charge cycle. The voltage off a NiMH battery doesn't "curve" the way a NiCad's does, until you are already way overcharging it. Temperature is really the only fairly reliable way of knowing what is going on within the cell.
So you could substitute NiMH for NiCad without changes, but have slightly less life, more heat and venting, and none of the advantage of the increased energy capacity. Obviously, that would make little economic sense.
The possible venting of hydrogen (and later, oxygen, if way overcharged) means you might need an escape hole or two to reduce exposure of circuits to environmentally-induced acids or oxides.
Okay, all this could be overcome. A battery pack using NiMH, incorporating thermistors and a thermal fuse, and a vent, that is all possible; and a charger replacement for the usual wall-charger, with a sensor circuit to monitor the thermistors and switch from Fast to Slow to Trickle-- hey, it could be done. Then you'd get all the advantage. Hey Designers?!!
But, at least for now, it is easier to take the batteries OUT when discharged or not using them for awhile, and put in charged ones when you will be using the calc, and put the old ones back into a NiMH-capable charger. Always having a spare set charging, you could easily go 2000 cycles or more... and longer calc use times, no "memory" problem even if you only partially discharge them. Cool!!!
If you put NiMH in your calculator, a great idea, but just put a "cork" of some sort in the socket where the charger plugs in, just to remind yourself not to charge these in the unit itself.
I have been using NiMH (Energizer) in a Sony world-band receiver, and they are wonderful-- I run it about two weeks before changing batteries, switching between two sets of them, keeping one set in the charger while I use the other. Goodbye NiCads, I don't miss you at all!!!
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