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Recently I decided to replace my Classic series NiCd battery packs with non-rechargeable batteries. Since I wanted to easily swap out the disposable batteries, I purchased AAA battery holders that use 3 cells. These battery holders are somewhat smaller than the classic series battery cavity opening, so I padded the extra space with closed cell foam. I then attached copper tape to the underside of the battery packs, and soldered the connections. The new holders work fine.

My question for the forum members is regarding the voltage tolerance of the Classic series. I chose to install Lithium batteries for the usual reasons, energy density, shelf life, etc, but I didn't give much advance thought to the overall battery pack voltage. However, as I was verifying the battery pack polarity, I noticed that the cumulative voltage displayed on my DMM was about 5.3V. This elevated voltage should have been an obvious consideration prior to building the battery packs. Now I am wondering if I should replace the 3 cell holders with 2 cell holders to drop the voltage down to the range of the original 3 cell NiCd packs, 3.6V.

I haven't looked at a Classic power supply circuit for awhile, so I can't recall if HP used a Zener reference to set the internal voltage levels. I am curious if the internal power supplies are regulated and if the over voltage due to the Lithium batteries is a concern? I should mention that my first test unit is an HP-67, with a rebuilt card reader.

Hi Robert,

I regularly run classic series from 3 x Alkaline AA batteries with no adverse affects. With all "classics", (and even though the 67 is not really a classic, I think it's still the same), the battery voltage goes only to a DC-DC converted inside the calc which generates the three separate voltages the calc needs. And yes, the internal DC-DC converter output voltages are voltage limited by a zener diode.

For more information, take a look at Jacques LaPorte page on DC-DC converters in HP classics