Tuesday, 2 July 2013


I'm sure you've seen it on the periodic table, way up at the top, straight after hydrogen and helium. The most minimal of metals. There's also a lump in your phone.

And I have several cells charging the thermometer data loggers in my house. When I saw that these rechargeable batteries were cheaper than the disposal ones from the local electric shop, I pounced on them. What did I have to lose?

I despaired a little at the first two I got when they didn't seem to work at all. They came together with a charger, and I since bought several Ultrafire batteries, which seem to work OK. Then I realised I hadn't unpacked the first batteries, and they were still shrunk wrapped in clear plastic. They worked fine after that. In fact they were ready-charged. All rechargeable batteries should be ready-charged, and now many are. All batteries should probably be rechargeable, but that's another story.

I have 16 data loggers in the house, and the first set of batteries I got were only available with the charger. I only needed one charger, so I would have got the Ultrafire anyway.

So a couple of months later the new batteries start to need replacing. I find this out when I do the monthly download of data from each logger to the recorder. When it announces a communication error I know that the battery is flat, so I replace it. I've then been recharging the batteries that come out, and trying to put them in another data logger the next time a battery runs out.

And sometimes they don't work.

My initial reaction, of course, is that I now have an explanation for the price being lower than the disposable batteries.

Then I started investigating the charging technique of lithium ion batteries. Here's a good website called batteryuniversity.com on how to prolong them, from which I stole this graph demonstating the loss in capacity as their life goes on.

There seems to be a two- or three-step process: applying a high voltage, then balancing the voltage, then applying the target voltage.

Now that I've started watching the charger charge, I notice it goes from a red light when you put the battery in, to a flashing red light an hour later, presumably when it's balancing, to a flashing green light when it's ready.

The charger has two slots, and I've been charging batteries in pairs. This seems an obvious thing to do, and is sensible when the batteries are new and empty, or if the batteries had been together in the same appliance, discharging their power in lock-step.

As it is they have been on their own, discharging at slighly different rates over slightly different periods. We usually think of batteries being full or empty, as if they are tanks of fuel, but actually they don't work that way. As they lose charge their voltage gradually falls. If it's a battery connected to a bulb, then the current will keep flowing as the voltage gets lower until there is no current left. If they are driving something electronic, At some point the voltage will drop below a useful level for those transistors. The device will notice this and start taking action, primarily giving an alert that the battery needs charging, and switching off non-essential activities that use a lot of power. Like transmittting a couple of months of data. The data loggers will keep logging data for a while after telling us that the battery is fading, and I think only once or twice in two years have I lost data from a battery going below a level at which it can log data. 

So I need to recharge the batteries separately, or the charging cycle will start balancing before the batteries are ready, or after it's too late, and perhaps neither battery will be fully charged.

I should have done so from the very beginning, but since I got the rechargeables, I've started writing a note of when I changed the battery on each data logger. Perhaps I should be writing on each battery though.