The HP Woodstock family of calculators includes HP-21, HP-22, HP-25, HP-25c, HP-27 and HP-29c.
The battery for a Woodstock calculator contains two rechargeable AA flat top cells. The batteries were produced by HP until sometimes in 1980s so by the year 2021 the youngest one is well past its prime at a venerable 40+ years of age.
HP Woodstock Battery
How does a battery look like? Here it is (rebuilt). Note the model number (82019).
Can a Woodstock battery be refurbished?
NiCd batteries must be used for optimum performance. In an unexpected search for cost reduction, HP has decided to use the batteries with dual scope. They are used both as source of power when no AC adapter is used, and voltage limiter / regulator when an AC adapter is used. This greatly simplifies the power circuits both in the AC adapter and the calculator BUT it has a serious drawback. In the absence of the battery, it subjects the calculator electronics to the direct output of the AC adapter (well, almost), and that in turn is known to damage some sensitive circuitry in some models.
The models affected the most are the continuous memory models (HP-25c and HP-29c) which are almost guaranteed to fry when an AC adapter is connected without a good battery installed.
However if you did try this and your calculator was not damaged, do not try it again – why take chances?
Using NiCd batteries to rebuild the Woodstock battery block has disadvantages. The NiCd batteries may leak and damage your machine. I recommend never leave the batteries in the calculator unless you plan to use it immediately. There is no drawback in taking the battery out, it is easy to install when necessary.
Another recommendation is to use a charging station to charge the battery, if possible. A charging station looks like this:
Using a charging station will protect your precious calculator in the event that the battery is bad, or simply does not make good contact in the machine.
Newer NiMh batteries can be used. However the NiMh batteries need a special charging profile which of course the Woodstock calculators know nothing about. So while it is possible to charge a NiMh battery, the charging must be timed manually in such a way not to damage the battery.
If you have old batteries that need to be refurbished, or simply no longer need them, please contact me using the form below. If you do have extra batteries, you will make a number of people VERY happy! Also, if you have any questions, I will try to answer as soon as I can. Usually I have a very limited number of batteries that I have refurbished already so if you need a new battery urgently I may (or not, depends) have one available immediately.