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Nickel Cadmiun vs Nickel Methal



#16

To rebuild HP: battery packs can I use Nickel Metal batteries instead Nickel Cadmium?
I understand that NiMe are better than NiCa. 1.2V AA


#17

I have rebuilt numerous HP classic and Woodstock packs with metal hydride cells and a couple of HP97 packs. They work great, run 2-3 times as long as the standard batteries (also take 2-3 times as long to charge).

They do have a different voltage discharge curve which may affect when the battery low indicator turns on. In my '67 I get around a 10 hour run time with 1650mAH AA NiMH cells. The low battery LED turns on with about an hour of life left.

#18

If you use Nickel Metal, charge them in an external, specific charger. While the voltage is almost the same, the charging methods are quite different. Improper charging may damage the batteries, and there is the risk of an explosion (true!)


#19

As long as the charger is a trickle type charger the metal hydride and nicads can be charged with the same circuit. This applies to all the HP calculators where the charge time is something over eight hours. You can trickle chrage metal hydrides at a rate of C/10 continuously, where C is the capacity of the cell in maH (I got this directly from a field applications engineer from Sanyo and verified it with the the pack in my cell phone). Also since the metal hydrides have at least twice the capacity of the nicads the HP chargers are charging them at a rate of between C/15 for the HP97 to C/30 for the HP35, etc. Charge rates below C/50 may not fully charge the batteries.


#20

Hello:

I have read the same thing about charging rates, but I'm not so sure about the continuous charging formula. I use Sanyo 1600 mAh AAs, and my charging circuit supplies about 165-175 mA. I assumed this meant that the batteries would be fully charged in about 12 hours (just a little more current than the safe continuous amount). But, I found that 12-13 hours of this charging current resulted in warm batteries. These batteries were then shown to have decreased capacity.

Therefore, I would be very leery of C/10 charging for more than 4 to 6 hours. The classic chargers seem to supply about 55 mA (about C/30 for 1600s), which seems to fully charge the battery (without overcharging) in 36-48 hours.

Bruce.


#21

Yes, I much prefer to trickle charge hydride cells in the C/20 - C/30 range. My cell phone does a 2 hour fast charge and then drops into a C/13 trickle (100mA)... seems excessive to the task. Packs last about 2 years in it, I wind up chaging it 2-3 times a month.

The best indication of a good trickle charge is how warm the batteries get. I don't like how warm the HP97 hots up its standard pack, but they do seem to last. HP does specify that they only use "fast charge" cells in HP97 packs even though they set the charge current for a 7 hour charge. My 3200maH hydride pack barely rises above room temp in a '97... I think the charge circuit makes most of the heat.

I don't mind the 30+ hour recharge time of the 1650mAH classic packs so I have not attempted to boost their charge current. Same for the Woodstock packs. I have hydride packs that have been in daily use for over three years in both types of machines... no explosions yet.


#22

can someone clarify this for a dummy in relation to 41 bat pacs? nimh n cells are available.i guess they can be used. is the standard charger ok to use? if not what is near perfect and for how long?


#23

The 41C battery pack is a little problematic. It uses 1/2N cells (also called N75 cells). They quit making them in NiCads a couple of years back and they are rather difficult to find today.

The HP charger circuit tends to want to dump around 15mA into the cells. This works out to a C/5 charge rate. One would think that this would tend to overcharge the cells, but (according to a battery engineer) in relation to their mass 15mA does not cause much of a temp rise (at least in NiCd cells) and and SHOULD be OK.

I doubt that you can find 1/2N size NiMH cells (I haven't), but you may be able to find something a little smaller that would fit if you used something to act as a space filler behind (and mayby between) the cells. Try to find as large a capacity as possible that will fit in the holder. I would monitor them fairly closely on charge to make sure that they did not overheat and/or overcharge. Perhaps put the charge plug on a timer. Your charge time should run about 1.2 * (cell capacity in maH / 15) for a fully discharged pack.


#24

david - i saw nimh n cells at the radio shack here in river city. thanks for the formula. - d

#25

DigiKey (www.digikey.com) has 1/3AA NiCd batteries; they are a couple of millimeters wider and taller than the 1/2N cells in the 82120.

of course, you can always an ordinary battery holder and put NiCd or NiMH cells instead of alkaline cells, and charge them in a proper charger like you would charge any batteries.

/ji


#26

The 1/3-AA cells won't fit in the 82120A (I've tried), but 1/3-AAA cells will. However, Tom Drewski has located a source for those (no longer made) 1/2 N cells. See my just updated article (on this site) for more information on all this.


#27

I just talked with a local battery supplier and he claims that GP Batteries has the 1/2N cells in production. They apparently picked up where Sanyo left off.

Also it looks like you can get 150mAH NiMH cells that are almost identical in size to the 1/2N cells. See Katies' piece in the Articles forum (the same supplier as the 1/2N packs has them). They would charge at a C/10 rate in an unmodified HP41 rechargeable pack. This should be OK but I would probably increase the value of the charge current limit resistor around 50% if I was to use them.


#28

Yesterday I rebuilt my pack for hp65/67...Nickel Cadmiun.

It is working nice. I rebuilt in 2 hours.
I had the luck to obtain a pack of 3 batteries soldered, 3.6V and insulated (US$10). I only soldered the metal tabs in the pack to the battery cables.
No Nickel Methal...these machines are priceless.

Steve Loboyko's article was the clue.

#29

Am trying to find a battery pack for a HP-31E calculator I received as a gift. The mannul says nickel-cadmium battery pack. Does that mean I can use 2 nickel-cadmium batteries? If so what size? Thanks, C


#30

Batteries are AA size. Please DO NOT USE your calculator connected to an AC adapter if you don't have good batteries and with clean contacts. The batteries act as a voltage regulator, being in fact part of the power supply circuit, and a AC adapter may damage your calculator (very likely) if the batteries are not filling such function.

Just for a test, you may want to try with 2 AA regular alkaline batteries. The voltage is greater than NiCd, but probably be within tolerance (I am saying so because of my own experience with an HP25 calculator, I have no experience with the 31E and so I just offer these suggestions WITHOUT ANY WARRANTY).

If you rebuild your pack with Nickel Metal Hydride (NiMH) batteries, you will obtain more operating time between recharges, but in such case please see that, while the voltage and size may be the same, the recharge cycle of NiMH is different, so it may be safer to recharge them in a specific charger outside the calculator.

Recommended: See previous postings here (specially those from Glynn and from Katie Wasserman among many others) about battery types, rebuilding packs, etc.


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