HP Forum Archive 01

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Re: Classic Battery Packs and NiMH Message #1 Posted by Reinhard Hawel on 3 June 1999, 7:38 p.m. I don't know why they shouldn't fit. NiMHs are mostly equivalent to NiCds besides the higher capacity. You would have to load them longer (twice the capacity=twice the load time). The HP charging circuits load with some kind of more or less constant current (in fact they load via a resistor coming from a constant voltage source (7806 IC)). This is the same method used in the cheapest external charging devices. I mean the chargers you get for some $ everywhere. I don't see a difference. When it is cold, they lose capacity, which maybe can be a problem in winter time. The benefit is, that they don't have any memory effect. They should also not be left on the charger for infinite time like much users treat the NiCds. This is also no good practice for the NiCds, but probably the NiMHs react bad. (get hot, I think) I'm not a battery expert, but when used the right way, I don't think they can do any harm to the calculators. I think, I have to check this. Anybody else out there (some electronics engineers, I know you're here, guys) who knows more ?

Re: Classic Battery Packs and NiMH Message #2 Posted by Steve on 3 June 1999, 8:10 p.m., in response to message #1 by Reinhard Hawel In terms of power, discharge curve, voltage, etc, NiMH fall between NiCad and a typical Alkaline cell. Thus, the voltage of the NiMH cell is a little higher than NiCad (But not as high as Alkaline), the impedance (equivalent series resistance) is a little higher than NiCad (but not as high as Alkaline). If Alkaline cells can be used (and I hear that they CANNOT) then NiMH would seem a reasonable replacement for NiCad. Inserting an additional series diode inside the calculator will cause a voltage drop of approx 0.6 V which may allow for the use of slightly higher voltage batteries. Note: I have not tried this procedure!

Re: Classic Battery Packs and NiMH Message #3 Posted by John on 3 June 1999, 11:35 p.m., in response to message #2 by Steve Thanks for the info guys - John

Re: Classic Battery Packs and NiMH Message #4 Posted by tom Lianza on 9 June 1999, 2:59 p.m., in response to message #3 by John Do Not, I repeat, do not blindly substitute a NiMh battery for a NiCad. Under conditions of trickle charge, they are very similar, but there is a danger of explosion at charging rates greater than .2C (that means charging greater than 20 percent of the battery capacity) . Almost all commercial NiMh packs include either temperature sensors and/or presettable fuses. Apple computer made this mistake in some power books and some early machines caught fire.....Charging through a resistor as one poster noted is not a constant current charge, it does limit the charging current. Under conditions of deep discharge, the required current might be quite high. This will cause a bit of heat to be dissipated in the charger. You may damage the charger under such conditions. Be careful and make some measurements.... Good Luck

Re: Classic Battery Packs and NiMH Message #5 Posted by Reinhard Hawel on 9 June 1999, 7:32 p.m., in response to message #4 by tom Lianza Yes ... this was my post. I know, that a voltage regulator and a resistor is not a constant current source, but I didn't know these dangers in charging NiMHs. I said it before, I'm not a battery expert. I think the voltage regulator IC used in the chargers has a thermal shutdown (and current limit, hopefully), so that's not the problem. I discussed this theme with somebody else and he said the NiMHs would not charge well with the low charging currents delivered by the calculators. Unfortunately I didn't find the value of the resistor (I remember the voltage source was an 7805 or 7806), so I can't determine the current. Anyway, there are high capacity NiCads, that avoid all of these problems and don't have very much lower capacity the NiMHs. I'd try some of these, because they _do_ have the same characteristics like the originals. If I find something, I'll post these information. Thanks, Tom, for the information

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