HP Forum Archive 05

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Re: Dead pack and NiMH cells Message #1 Posted by Mark Sims on 28 Feb 2001, 5:10 p.m. I've only used the NiMH cells in classic models and HP-25's. If your charge time is specified at eight hours or longer (i.e. trickle charge) you are probably safe with NiMH cells, if it is a quick charge (4 hrs or less) definitely stick with nicads. Dumb charge circuits are generally more compatible with NiMH retrofits than smart circuits. My '25 generates quite a bit of heat when charging batteries, but this is the internal current limiting resistor. I tested the battery pack away from the calculator by using clip leads and the batteries themselves were not heating up. My HP97 nicads get quite warm when left on the charger but the NiMH cell pack does not. An interesing side effect of NiMH cells in the HP's is caused by their discharge curve. Nicad voltage falls off quite fast when they are dead, NiMHs fall off much slower. As a result, nicads give about 5-7 minutes of run time once the low battery indicator lights, NiMHS give over 30 minutes.

Re: Dead pack and NiMH cells Message #2 Posted by Steve (Australia) on 28 Feb 2001, 6:04 p.m., in response to message #1 by Mark Sims > I've only used the NiMH cells in classic models and > HP-25's. If your charge time is specified at eight > hours or longer (i.e. trickle charge) you are probably > safe with NiMH cells, if it is a quick charge (4 hrs > or less) definitely stick with nicads. Dumb charge > circuits are generally more compatible with NiMH > retrofits than smart circuits. Beware of what HP say. For example, the HP41 nicad pack specifies 12 to 16 hours for a full charge (suggesting C/10 or thereabouts), however the charge current is 16mA (this is from memory, I gave a more detailed answer when I sat down with the packs in front of me several months ago) for a 55 mAHr battery, which is C/3.5. Whilst measuring the internal temperature during charge of a pack inside an hp41 tape drive (could have been a printer) I observes a rapid temperature rise after about 6 hours (again from memory -- check my posting). Both of these suggest a charge rate in excess of C/10. Oh, and the charger circuit for the 82120A (both of them) have a full wave rectifier, and some filtration. Whilst the re is also going to be some variation in voltage during charging, it will be far less than the older diode/resistor combination, and in the case of the newer version, far less. I also had a concern that I believed that NiMh didn't like to be completely discharged, which is why many devides that use them (e.g. mobile phones) turn themselves off when they detect low battery. I am also uncertain of the typical failure mode of NiMH if overcharged. I do know that they can get rather hot (even with correct charging). I'd *love* to use NiMH but I would want to be assured that i'm not going to have my calculator explode in front of me whilst on charge :-) I have found it quite difficult to get good information on NiMH cells, and without such information I have tended to steer away from them. About the limit of such information appears to be "NiMH require a special charger -- do not use a NiCad charge unless it is designed to charge NiMH". What I want is charts, technical information, information about the charging end point, etc, etc. Why do manufacturers seem to think this is proprietry -- or have I been looking in the wrong place.

Re: Dead pack and NiMH cells Message #3 Posted by Mark Sims on 28 Feb 2001, 10:20 p.m., in response to message #1 by Mark Sims I know the NiMHs work quite well in the classic models, HP-25's, and HP97's (the sub-C NiMH cells are quite difficult to find). I have a portable frequency counter that uses 4 AA cells. The standard nicads always got VERY warm charging and would last about a year. I've had NiMHs in it for going on three years of continuous charge and they are still going and barely get above room temp. The criterion I use for determining weather the charger is going to work is how long they charge... overnight is a good thing. Also I see how warm the cells get... a little temp rise is OK. I've never had a failure in these circumstances. I generally try to start charging when the device goes into low battery mode. NiMH cells seem do this before nicads do and so would not be as fully discharged as nicads. I have depleted them fully numerous times though with no noticeable effect. Any rechargeable pack should probably be replaced every three-four years regardless. I've never has a NiMH cell leak yet when following these criterion and have used about 100 cells in the last three years. I've had one cell go bad early. It was in an ICOM R-10 receiver and after two years the radio started going out after two hours instead of eight. I repaced all the cells (best not to mix old and new cells in a pack) and used the three remaining good ones in a GPS network time module where they are still going strong. The warnings about using nicad chargers applies to those quick charge units. A trickle charge circuit just applies a low current to the cells with no end-of-charge detection, etc. As long as the charge current is less than C/10 things should be OK. I really prefer around C/20 for things that get left on charge all the time. Rates less than C/50 are not recommended because cell to cell variations within a pack can make charging unreliable.

Re: Dead pack and NiMH cells Message #4 Posted by Mark Sims on 1 Mar 2001, 4:50 p.m., in response to message #1 by Mark Sims Steve, when you measured your charge current was it when the cells were low of fully charged? A simple resistor charger will supply a higher charge current to a discharged cell. The current will drop as the cells charge and their voltage rises. As an example I have an 8 cell, 2400maH pack that has a discharged voltage of 8 volts and a charged voltage of 11.2 volts. I charge it from a 14 volt source through a 20 ohm resistor. The current starts out at 300 ma (C/8) and drops to a steady state trickle charge of 140mA (C/15). It is the current into a fully charged pack that you want to keep limited to that C/15 - C/20 area if you are going to keep the pack on charge of extended periods.

Re: Dead pack and NiMH cells Message #5 Posted by Steve (Australia) on 1 Mar 2001, 11:48 p.m., in response to message #4 by Mark Sims Steve, when you measured your charge current was it when the cells were low of fully charged? A simple resistor charger will supply a higher charge current to a discharged cell. The current will drop as the cells charge and their voltage rises. There are two types of 82120A charger. One uses the rectified nominal 8V RMS to charge the pack, the other uses a 12 volt regulated rail from which to charge. In the former case a 580 ohm resistor is used from a DC rail that can easily reach 20VDC (Due partially to poor load regulation on small transformers, but also to line voltage variations and other considerations). Assuming the same 6V end point on the nicads, a 0.6v drop across the diode and a more typical 15V rail (under load), the charge current is about 15mA which is C/3.7 In the latter case a 370 ohm resistor is used from the 12v rail. Allowing for a 0.6v drio across a diode, and a "fully charged" voltage across the pack of 4 cells being 6V, this is a shade over 14mA, which is C/3.9. Both designs have approximatly the same charge current, which is far higher than I would expect given HP's suggested 12 to 16 hour charge period.

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