The Museum of HP Calculators

HP Forum Archive 02

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HP 25 calculator
Message #1 Posted by Armando Someillan on 14 Mar 2000, 9:16 p.m.

I've got an HP25 in mint condition that my wife bought for me recently. The battery pack seems to be in excellent condition. However I do not know how long I'll be able to preserve it like that. According to the manual one is NOT to use the calculator on AC power without the battery pack in. If and when the day comes that my battery pack goes bad, does anyone know if I can safely use it on AC power even though the battery pack in it is no longer good? Also if anybody out there knows where I can get a replacement pack, I'd appreciate the information. Thanks.

Re: HP 25 calculator
Message #2 Posted by Larry M. on 14 Mar 2000, 10:35 p.m.,
in response to message #1 by Armando Someillan

Exercise the batteries by turning on the calculator for at least ten minutes every week, whether you need to use the calculator or not. Do not recharge the batteries until they are almost completely discharged. Then, immediately recharge the batteries for 8 to 10 hours, but no more than that. By following this routine I have kept the same rechargeable batteries in my HP-25 since 1991. They still work perfectly.

Replacement rechargeable batteries can probably be obtained by calling Agilent at 1-877-447-7278.

Re: HP 25 calculator and batteries
Message #3 Posted by Andrés C. Rodríguez on 15 Mar 2000, 7:16 a.m.,
in response to message #1 by Armando Someillan

Please NEVER use your HP25's AC adapter without a rechargeable battery pack inside. The rechargeable batteries serve as both a filter and a regulator for the AC adapter. Without the rechargeable batteries, excessive voltages will be applied to the calculator, causing severe and irrecoverable damage.

Some people wonder if capacitors may be used on the batteries' place with the AC adapter. The answer is ABSOLUTELY NO, as some visitors of this museum had sadly found.

Just as an emergency option, you may use two standard alkaline AA cells as a replacemnent for the rechargeables, but NEVER use the AC adapter with alkalines, since a charging current will be applied to the alkalines with concrete risks (even explosion).

Whenever possible, stay using with the original packs, or rebuild them with new batteries (be careful about polarity!)

Re: HP 25 calculator
Message #4 Posted by Joe on 15 Mar 2000, 8:57 a.m.,
in response to message #1 by Armando Someillan


None of the LED models will work properly without the battteries except the classic series that have the three conductor charger cords. Those will work fine on the charger alone since the third wire is used solely to supply REGULATED power to operate the calculator. I strongly recommend against even trying to run the other models without a battery since the battery is designed to provide a load on the UNREGULATED charger and it prevents excessive output voltage. Operating a 2xC model without a charger will burn up the memorys in it instantly. It doesn't appear to do any immediate harm to the other models but why take a chance?

The batteries used in the HP 25 battery pack can easily be replaced by someone that knows how to do it and that has the right equipment. They're a standard size battery cell so you should always be able to get replacements. I strongly recommend that you get the batteries from someone that can weld the straps on them and not buy off the shelf batteries and simply solder a strap to them. The NiCad batteries have a vent in them that is sealed with a plastic ball. Soldering them can easily overheat them and melt the ball and cause it to plug the vent. If that happens then the batteries can explode if they are over charged. OR the ball can melt and leave the vent open in which case the battery electrolyte can leak into the calcualtor and cause corrosion. I can rebuild any of the HP batteries. Contact me directly if you need any fixed.


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