HP Forum Archive 08

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Short Battery Life 41CX Message #1 Posted by T Smith on 2 July 2002, 9:06 p.m. I searched the archives, but didn't see anything related to my question, so apologies if this has already been discussed. My 41CX has started eating batteries every few months, even when the calculator is not turned on! I read an earlier post in which someone said it could be the main power supply capacitor, but I'm thinking that would be the case, only when the power is turned on. I measured the current draw with the calculator both off and on. When on, it varies, but seems to be a little over .001A. When off, the current is .00002A (measured with an HP multimeter, of course:-)). Now assuming an N size battery gives 50mAH (I have no idea what the actual number is), this would yield only about 3 months of battery life when off! This is fairly close to what I'm getting. And I killed 2 sets of batteries, so it's unlikely that I got a bad set of batteries. Has anyone experienced this before or have any ideas what it could be? Is there a schematic available, or is it possible to remove the keyboard to see if there is junk underneath that might be causing leakage in the off-state. Thanks for any suggestions. Tom

Re: Short Battery Life 41CX Message #2 Posted by David Smith on 2 July 2002, 10:12 p.m., in response to message #1 by T Smith "N" cells produce a lot more than 50maH. The average power consumption of a turned off 41CX is around 5-10uA. Many of mine will keep their memory active for up to a month with no batteries.

Re: Short Battery Life 41CX Message #3 Posted by Ellis Easley on 8 July 2002, 8:29 a.m., in response to message #2 by David Smith I was going to say that non-rechargeable batteries aren't rated in A-H, since the last time I looked at the Eveready website, the individual data sheets for Energizer cells only gave the capacity in terms of hours connected to a standard load (I think it was 100 ohms) until the voltage fell below a certain value. But when I looked just now (http://data.energizer.com/) there is a chart with a summary of specs for all Energizer sizes and it has a column for capacity in mA-H - it says N cells have 1000 mA-H!

Re: Short Battery Life 41CX Message #4 Posted by Tom (UK) on 3 July 2002, 7:13 a.m., in response to message #1 by T Smith I don't know what the circuit is, but from the memory retention ability of the HP41 when the batteries are removed I'd guess the main power supply capacitor is on the 'battery' side of the on/off control. So if the capacitor was draining the batteries it would not matter if the calculator was turned on or off. Electrolytic capacitors are famous for getting worse with age so I'd replace this as a first fix.

Re: Short Battery Life 41CX Message #5 Posted by T Smith on 3 July 2002, 3:37 p.m., in response to message #4 by Tom (UK) Thanks David and Tom. Since my 41CX is drawing 20uA, and a standard CX should draw 5-10uA, then that doesn't seem like enough of a difference to drain the batteries that fast (assuming my meter is correct at that low range). I guess it's possible I got 2 sets of bad batteries. Anyway, I will change the main capacitor and see if that reduces the "off" current. Thanks for the help! Tom in Houston

Re: Short Battery Life 41CX Message #6 Posted by David Smith on 3 July 2002, 5:10 p.m., in response to message #5 by T Smith Well actually, 20uA vs 5 uA is a 4 times reduction in battery life. If you can expect a year of life from an idle set of batteries, then that is pretty close to what you are seeing. A lot of 41 series machines only seem to get around six months or so out of their batteries anyway.

Re: Short Battery Life 41CX Message #7 Posted by Dave Shaffer on 4 July 2002, 1:56 p.m., in response to message #6 by David Smith Maybe I've been lucky, but it seems to me that you should get a lot more than a year from a set of '41 batteries. I've had a 41CX since the early 80's, and to the best of my recollection, it is on its third set of batteries, and I think I put those in when I went on a foreign trip where it was absolutely necessary that the calculator be operable. (i.e. there was unknown life left in battery set #2). I do not use this machine very much, and never with any power draining accessories. My even older '11C is also on its third set of batteries. It gets somewhat more everyday use, but also not a lot. Based on these experiences, a '41 just sitting there, turned off, should get much more than a year from a set of batteries. Change that capacitor!

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