I’ve always been a fan of rechargeable batteries because batteries, as a one-time-use product, simply don’t make sense. The ability to recharge the same batteries, hundreds of times, means less waste and more value for money. A downside to NiMH and NiCad batteries is that they output a lower voltage: 1.2V instead of the 1.5V provided by alkaline batteries, which can result in some devices displaying incorrect battery levels or shutting down too soon.
Another major downside is price, and shady marketing. Several years ago a 4 pack of rechargeable NiMH AAs, at 2500mAh, would cost about $12. It was a premium over alkaline batteries, but it was great value considering the many additional uses. However, these days I can’t seem to find the same type and capacity of batteries for under $17 at retail stores. Those batteries that are under $17 often do not have their capacity marked anywhere on the packaging or themselves. After some research, I discovered that these [brand withheld] batteries are only 1200mAh – less than half the standard for NiMH AAs.
It is unfortunate that a product that was once so economical and convenient was turned into nothing more than a cash grab. “Save the environment” slogans leading consumers to buy products at marked up prices. Supply and demand fails in this system. But I digress.
At the beginning of the semester, I turned on my calculator to find the “low battery” nag screen, followed by an immediate shut down. They were depleted. Being curious, and a little cheap, I decided to act upon an article I had read years ago. The article describes how, with patience and constant attention (if you wish to avoid acid explosions), you can recharge “non-rechargeable” alkaline batteries.
When charging any type of battery, there are two principal factors to consider with regards to the charging source: the voltage and current of the source.
According to the above article, “Afroman’s guide to recharging alkaline batteries”, 65mA is a “safe” amount of current for charging an alkaline battery. Although it is a relatively small amount of current for charging rechargeable batteries (my NiMH charger puts out 190mA), the chemistry of an alkaline battery was not designed to be charged. The risk of having too much current charging the batteries is that they will heat up until they burst. At that point, the battery is useless, and you’ve got an acid mess to clean up. For that reason, I actually used even less current (see below).
For voltage, you want the source to be slightly higher than the battery’s rated voltage, but not too high. A NiMH charger will almost always output 1.4V to charge 1.2V batteries. 12V car batteries are often charged using 13.5-13.8V. Using this logic, I decided 1.7V per 1.5V cell should be a reasonable middle ground.
WOAH, hang on a second!
Sorry for the very exaggerated interruption, however my (spoiler: successful) process wasn’t a simple “set it and forget it” affair. Being this was during class hours, and I would be beside my bench power supply all day, I did things gingerly.
The combined voltage of the four cells, in series, was closer to 5V, not 6V as they should be. Because I didn’t want to make a mess (I was using my calculator as the battery holder, after all) I started charging at below 6V.
I didn’t want to rush into it and end up with a mess to clean, so I started it off very lightly. Almost no current and less than the nominal voltage to charge. When the batteries were initially connected, the displayed voltage dropped significantly, but on the “precision” multimeter I could see it slowly climbing. So I left it for a while, perhaps 45 minutes.
At this point I was more confident, however I wasn’t ready to open the taps yet since I hadn’t really tested the waters much. At 6.3V, I was now above nominal voltage, and this would be the first test of charging to capacity. The current is increased to speed up the voltage climb, also a test of how much current the batteries could handle without getting hot. It was left here for around 2 hours.
No heat, no fuss, that batteries were perfectly content at the previous level. However, the voltage was no longer climbing, and the battery was simply absorbing current without great benefit. The calculator was now able to run on its own, but with time to spare I wanted to ensure they would last me more than a day of usage.
At this final charging voltage, I left the batteries for an additional 2 hours, periodically checking their temperature. Although some warmth could be felt, it was nothing compared to human body temperature.
Finally, as I finished my final class, I unplugged the charging apparatus and let them sit before performing a final voltage measurement. I measured above 1.5V on all cells, and considered my operation a success. I had done the unthinkable – I’d recharged non-rechargeable batteries.
This was great fun, not only from an electrical aspect, but also in terms of entertainment (nothing like laughing irresponsibly around a potentially dangerous project). In all seriousness though, it can be done if the right amount of patience and care is put into it.
I did this project early September, and only two weeks ago I used this calculator, with the same batteries, for my final exams. No additional top-ups.