Dealing with battery self-discharge

2009-04-01

in Geek stuff, Photography

The most annoying thing about rechargeable batteries is the way in which they ‘self discharge’ – slowly tracking towards dead even when nothing is drawing power from them. It’s actually a characteristic that grows more annoying the more batteries you have. Now, due to all the photographic gear I lug around, I need at least twenty AAs to ensure that I back a backup set for all devices. There are few things more frustrating than to spend a couple of weeks carrying around a big sack of batteries, only to discover that they are all dead when you need some. This happens especially often in cold weather.

The solution I have devised is based on grouping batteries into sets. I have devices that require 2, 3, or 4 batteries at a time, so I have made sets of those sizes. The batteries in each set are marked with the same letter, and held together with an elastic band when not in use. Obviously, any set can be used with any device that requires that number of batteries.

If one battery you put in is weaker than the others, the whole device will fail when it does. As such, it is important to charge the whole set at the same time. In addition to that, it is vital to remember to top up the sets regularly, rather than forgetting them long enough to kill them. Finally, to maximize their lifespan, you should use a charger that can put in energy slowly, preventing overheating. I use a La Crosse BC-900 and am very happy with it. It charges batteries at 200mA, by default, thus prolonging their lives. It can also ‘re-condition’ batteries that have lost most of their capacity. Given the number of batteries my growing collection of flashes makes me juggle, I may even end up getting a second one, so I can charge them eight at a time.

I hope somebody eventually comes up with affordable AA and AAA cells that you can charge, leave in a backpack for a few months, and still count on being able to use.

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{ 12 comments… read them below or add one }

R.K. April 2, 2009 at 2:50 pm

For devices that draw down batteries quickly, rechargeables are fine. For things like flashlights I seldom use, I find that alkalines are a lot better, since they hold their charge in storage.

. April 2, 2009 at 2:55 pm

Nickel-metal hydride battery
From Wikipedia, the free encyclopedia

NiMH cells historically had a somewhat higher self-discharge rate (equivalent to internal leakage) than NiCd cells. The self-discharge is 5-10% on the first day, and stabilizes around 0.5-1% per day at room temperature. This is not a problem in the short term, but makes them unsuitable for many light-duty uses, such as clocks, remote controls, or safety devices, where the battery would normally be expected to last many months or years. The rate is strongly affected by the temperature at which the batteries are stored with cooler storage temperatures leading to slower discharge rate and longer battery life. The highest capacity cells on the market (> 8000mAh) are reported to have the highest self-discharge rates.

Matt April 2, 2009 at 3:36 pm

I think you have a units problem here:

“It charges batteries at 200mA per hour, by default, thus prolonging their lives.”

Amps being current, are already a rate (a coulomb per second). So saying 200mA per hour is the same as saying 200 millicoulombs per second per hour which you can see doesn’t make sense. Perhaps you mean simply “charges batteries at a rate of 200mA” or maybe you mean “charges at a rate of 200mAh per hour.” The latter doesn’t seem plausible to me, though as all batteries would differ.

Milan April 2, 2009 at 4:21 pm

Corrected.

My rudimentary impression was that amps are a measure of the total capacity of a battery, like the amount of water in a tank, and that volts were akin to the amount of pressure the water is under.

Milan April 2, 2009 at 4:35 pm

I am confused now.

Why is battery capacity expressed in units of current? When they say ‘2500mA’ are they just abbreviating ‘2500 milliamp-hours?’

Milan April 2, 2009 at 4:49 pm

I understand now.

The ‘2500’ refers to milliamp-hours, not milliamps. As such, a 2500mAh cell can provide something like 2.5 amps of current for an hour (the exact amount varies by discharge time and temperature). My charger uses a current of 200mA to add 200mAh per hour to the batteries. It takes a long time to charge them, but doing it this way extends their lives.

Matt April 2, 2009 at 5:17 pm

Water is a good analogy for electricity because it can be visualized. Amps would be an analogous to volume amount of water flowing past a certain point, litres per second for instance, but not the volume of the reservoir.

Battery capacity is measured in milliamp-hours as you surmised above. When you take into account the battery voltage as well, you can calculate the total energy the battery is able to produce. A 9V battery rated at 1000mAh will produce 9 watt hours of energy, where as a 1.2V battery rated at 1000mAH will only produce 1.2 watt hours of energy.

For reference:
Joules: Base unit of energy, a newton meter.
Coulomb: Unit of charge, meaning a specific number of electrons
Amps: Base unit meaning coulombs per second, how many electrons are moving past a certain point per second.
Volts: Base unit of potential (pressure) in Joules per coulomb.
Watts: Unit of power or rate at which work can be done, Amps x Volts giving Joules per second.
Watt hour: equivalent to 3600 joules of energy. (Note it’s not a rate).

My feeling is batteries should be rated in watt hours, not milliamp hours because a 9volt battery and a 1.2 volt battery both rated at 1 watt hour could provide the same amount of total energy. Basically, this unit is normalized for all voltages.

Matt April 3, 2009 at 5:01 pm

“My charger uses a current of 200mA to add 200mAh per hour to the batteries.”

Just one point on this: The charger is nowhere near 100% efficient. you can still run a charging current through batteries when they are 100% full, for example.

Milan April 3, 2009 at 5:14 pm

True enough. At least the batteries don’t get hot, like they do in the 15-minute chargers the battery companies sell.

It’s no surprise the companies like them: they are convenient for the user, and ensure that they will need to buy replacement batteries relatively soon.

Tristan April 3, 2009 at 6:02 pm

What would it mean for a charger to be 100% efficient? Doesn’t Milan’s fancy one turn off when the batteries get fully charged?

Milan April 3, 2009 at 6:06 pm

Presumably, a 100% efficient charger would require exactly 2500mA to charge a 2500mAh cell in one hour.

Mine actually keeps charged batteries on a ‘trickle’ charge, to prevent self-discharging. That way, they should be at 100% whenever you take them out.

. September 14, 2010 at 10:05 am

“AA Ni-MH cells got into a claimed-capacity war about 10 years ago. Starting at a perfectly reasonable 1,200 mAh, which was double that of Ni-Cd, makers got clever and used thinner and thinner internal insulators to get more metal, and thus more capacity, in each cell.

What they also did by using thinner insulators between the rolled-up layers of conductors is to increase leakage currents, so many of these cells with preposterously high claimed capacities self-discharged (ran down) within days.

Yes, maybe you’d get 2,500 mAh, but only if you ran them down immediately. Wait a week, and maybe only 1,400 mAh was left. Wait three weeks, and the spare set of cells you carried in your bag was already dead!”

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