Chargers for HP 25 calculator



#12

I have an HP25 calculator that is in excellent working order. I have misplaced the charger. I need to be able to charge the batteries and I am wondering if I can use a charger for cell phones. I have determined the polarity of the charger connections and have used pieces of bent stainless steel wire inserted into the leads to serve as connection points for clip on leads. What I don't know is what voltage to look for in a cell phone charger? I can easily determine the polarity of the charger and solder on spring loaded clips to the leads of the cell phone charger to attach to the stainless wire clips I have rig up. Does anyone know what voltage I should be looking for in the discarded cell phone chargers that would work. I don't want to arbitrarily use any charger for fear of damaging the calculator. The two rechargeable batteries only need 2.5 to 3.0 volts but the calculator needs more to operate. I have determined this by using an adjustable DC power supply. I understand that the original chargers were unregulated and I am not sure if the cell phone chargers will work. Does anyone have any info that would help me out??

Edited: 4 Jan 2006, 5:37 p.m.


#13

Do NOT charge batteries in the calculator. It's too easy to fry the electronics. I remove the center plastic strip in the battery pack and insert (with a bit of force) AA alkalines. Works just fine for my 21,22,25C,and 29C.

-John

#14

I have done the same with my 25 - replaced my HP rechageables with AA's. Calculator lasts a very long time as a result - and no charger to worry about.

There is a number of comments about same here at the museum.

http://www.hpmuseum.org/batts/battery.htm

Try this link first. Hopefully it will help you out.

#15

I have an extra Woodstock charger I can sell you. I just did inventory and have an extra one. BTW, I thought that several on this forum warned against using AA batteries because their extra length applies too much pressure against the contacts of your Woodstock and can easily break them. I am considering going to Radio Shack and buying the shorter "AA" rechargeable batteries and stand-alone charger so a charger never needs to be connected to the calculator.


#16

The standard AA cells with the button + end issues are in reference to Spices, not Woodstocks.

Woodstocks don't care so much about length but are incredibly sensitive to both bad connections and batteries when connected to a charger. Woodstock safety tip #0: Never charge the batteries in the calculator using the HP wall wart. Never. To quote the Drink Driving adverts: "You can't afford it".


#17

Thanks for the clarification regarding the use of standard alkaline "AA" batteries in the Woodstocks.

I'm a little confused, then, about how to recharge the battery pack using the HP charger. (Of course, I should just go read my owner's manual!,... but I don't really use mine very much having recently acquired a 22 & 25.) Wouldn't you simply hook up the HP charger with the battery pack in the calc and turned off?


#18

Hi Steve,
As I recall, the output of the woodstock AC adapter is on the order of 12vac. It's then rectified in the calculator, and regulated by the battery pack. Hence the danger...if the battery pack develops an open for any reason (bad contact, bad cell, etc.) you will apply overvoltage (around 10-12vdc) to the calc. The only safe way you can use an HP ac adapter is to acquire one of those woodstock external chargers (they look like a miniature calc with no keys or display, and the battery pack goes where the keyboard would be). These external chargers have rectifier circuitry in them, and the ac adapter connects up to them in the same fashion as to a calculator.

In my 29c I just use NiMH rechargables from radio shack, and charge them externally (with a radio shack wall charger). They last a long time and work great. You just have to cut the center rail out of the battery pack to get the cells in and out.

You raise a good point, however, about charging with the HP ac adapter connected to the calculator and just leaving the calculator off. I don't know if this is safe or not.
Can anyone else address this issue...can you protect a woodstock from unsafe voltages from the ac adapter by simply keeping it switched off while the adapter is connected?
Thanks and best regards, Hal


#19

I'll read my Woodstock manual tonight to verify what HP says was the proper way for recharging (back then, at least). However, I'm growing more and more interested in the idea of just having one or two sets of Radio Shack rechargeable and external charging system. Since most of my HPs are for collecting, I'll just insert the Radio Shack batteries whenever I feel like rotating the calc in/out of rotation for use. How does this sound?


#20

The manual will say that the battery pack must be in the calculator when the ac adapter is connected, and that you can use the calc while the battery is charging. This is not the safe way to do it however, twenty years later if you want to maximize the life of your machine.
Regards, Hal


#21

Thanks for the advice! This sure makes it easier to negotiate the HP charger / battery pack issue.


#22

My pleasure.
And please forgive my arithmatic mistake in my last post...let's see...2006 Enter 1976 subtract puts 30 in the X register doesn't it (not 20). My how the years fly by!
Hal :)


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