Battery Rehab

I wrote all of this up based on reading up on the Maha C9000 charger, but before I got it. Once I start getting some results, I will post that later. I have a collection of 40 NiMH batteries ranging in age from 9 years to just a couple of weeks. They all take a charge on my old charger, but some are pretty weak. A couple of sets that Susan used in a quick charger are pretty damaged and don’t hold a charge for very long at all. I was interested to see what kind of results I could get in reviving them with my new charger, the Maha MH-C9000.

If a battery hasn’t been used for 3 months or has just been purchased, Maha recommends starting out with break-in cycle. This cycle is based on some international standard used to measure the capacity of batteries.

First I have to cover a basic thing. The capacity of a fully charged battery is measured in milliamp-hours. This means it can produce some number of milliamps (electrical current) for some number of hours. If it can produce 800 milliamps for 2 hours, then it is 800×2=1600 milliamp-hours, or mah. This is the capacity or just C. This charger can apply a break-in cycle based on that capacity. A high-capacity battery gets a higher charge than a low-capacity battery. It can make a lot of difference since my batteries run from 1450 to 2900 mah (and the AAA batteries are even less: 650-800 mah). So you might apply a charge of 0.5C. For the 1600 mah battery, that would be 800 milliamps (the time part gets left off). Ideally a completely drained 1600 mah battery could be charged fully in 2 hours (2 hours at 800 milliamps = 1600 mah), but realistically it will take a little more than that. The discharge rate is usually done at half of the charging rate. So it might be 0.25C. This means you should be able to discharge the battery in 4 hours and charge it in a little over 2. However you don’t drain it completely. NiMH batteries have a nominal voltage of 1.2 volts, though you can measure about 1.4 volts when they are freshly charged. The discharge cycle takes them down to 1.0 volt before stopping. If you go way down you can damage the battery. Also NiMH batteries hold their charge pretty well and by the time they get to 1.0 volt they are mostly done anyway. After that voltage would probably drop off pretty quickly.

So back to this break-in charge. It consists of applying a charge of 0.1C for 16 hours. This not only charges the battery, it actually overcharges it to the point that it changes some of the battery chemistry (hopefully for the better, but after you get the battery to its peak performance, further “forming charges” like this can damage it). Then it rests for 1 hour. After that it discharges the cell at 0.2C which should take 5 hours. It rests for another hour and then does another 16-hour charge at 0.1C. So it takes at least 39 hours to run a break-in cycle.

If the battery is performing poorly (like mine) the manual recommends doing a Refresh and Analyze cycle. This charges the battery to full, then discharges it to 1 volt, then charges it again. The default is 1000 ma charging and 500 ma discharging (though you can pick any rate you want). It should take 6 hours or so. Maha recommends doing this every 10 charges and doing it up to 3 times in a row for damaged batteries. For some batteries the charger just refuses to charge them at all.

So I want to see what kind of improvement my batteries can show. The charger can also run a straight discharge cycle to 1 volt and tell me the capacity at the end with the battery essentially dead. So I figure I want to do that first to see what I’m working with, but only after charging the battery in my old charger first. If the battery continues to behave poorly the manual recommends doing a break-in charge on it up to 3 times if it is showing improvement (39 hours times 3!). If not, it should be thrown out (recycled). One guy at Candlepower says he throws out batteries if they can’t get to 80% of their original capacity (which might be only 90% of the capacity on the label, so figure 70-75%).

For my brand new 2900 mah batteries that were included as a bonus with my flashlight, I should have waited and done a break-in charge, but I didn’t know I’d be getting this charger. So they have already been through a couple of cycles of charging and then putting them in the Archos until it cuts off (only a few hours, but the second time was definitely longer). I’m also ordering some brand new Eneloop batteries and because those come pre-charged, I don’t think it is necessary to break them in. Some people do but it doesn’t seem to bring the capacity up. I will probably do a Refresh/Analyze cycle on them and if the actual capacity comes up close to the nominal capacity, I’ll be done.

One of the things I would like to test is whether the break-in charge really works, or if several discharge cycles will do just as much to bring a battery to its full capacity. The other charger I had considered (the Lacrosse BC-900) can’t do a break-in. Since the break-in wasn’t established for the purpose of breaking in batteries but as a standardized way to measure battery capacity, I’m not sure if it is really being used correctly. But I have no idea. Maybe the intentional slow overcharging really does improve the battery chemistry.

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