Classic Battery Adapter

[Sadsilence]

(01-21-2016 07:27 PM)rprosperi Wrote:  Can AA NiMH cells (mentioned on your site) be charged in a Classic HP Calculator safely?

Agree with Katie. I have been using Texcus 3AA NiMH 2100 mAh packs with low self discharge characteristics within Classic calculators for a long time now and charge them with orginal ac adapters. Checked packs externally regularly at the beginning. No desease of pack after 10 - 20 cycles. LSD cells have the great advantage, that they hold charge for 1 year and more with only 20% loss, compared to normal NiCD/NiMH cells who loose 100% after 3 - 6 months (and leak after some more time).

Charge current is around 60 mA with classics AC adapter so modern NiMH cells with capacity C >2000 mAh simply cannot be overcharged no matter how long you charge them. Typical datasheet information of cells states no damage of cell when charged with C/10 for one year. C/10 would be 210mA in my example.

Senseful charging time for low charge currents (C/10 or less) can be selected with following rule for empty NiMH/NiCD cells:
Take REAL capacity of cells * 1.6 / low charge current.

e.g. 2100 mAh * 1.6 / 60 mA = 56 h

To be on the safe side (printed capacity often exceeds real capacity) I would choose 48h. It is easy to remember and NiMH/NiCD cells need a lot of time to get in last few percents. I checked and get 95% of real capacity out of cells.

Anyway that is a lot of charge time but 40 years old clip holders of battery doors for classics are a weak point of case and I try move them as little as possible. Of course it is possible to charge an empty pack outside with around C/2 charge current for 135 min using a matching charger.

To be honest there is another drawback. Charging modern NiMH cells with such low currents all the time let vanish their high current capabilities. No real problem, as 100 - 200 mA supply current for classics do not count as high current ;-).

Only one more hint: Within AC adapters of classics are some electronic parts that convert AC into DC (later AC adapters for woodstock/spice/HP-41 and peripherals have an AC only output). One key element for conversion is a 400uF capacitor which more often than not is dead to due age. Other components like diodes and transformer are normally made for eternity. As a result both adapter outputs have a more or less ac ripple which reduces charge current for battery pack even more and can influence calculator functionality when it is switched on at the same time. AC adapter supplies calculator completely in that case. A typcial sign are fllickering LED digits at start until AC adapter reaches operating temperature. Replace mentioned capacitor with a modern 470uF/25V type and be happy again ;-).