A Hairy Guide to Batteries

A Hairy Guide to Batteries (Extracted from Windsock Feb 2003)

There I was, half way round a UK record triangle and the GPS died...

Batteries cost us all, not just in price and inconvenience, but also because they all contain chemicals that we really shouldn’t be disposing of by burying them in the ground. I hope this will help you make the most of them.
The base-line battery is the trusty Duracell type alkaline cell. These are great, cheap-ish, last well, have a long shelf life and are availab le in every shop worldwide. Just the thing for a vario that uses very little power, where a set will last a year or more. GPS and radio users will be all to familiar with a much greater rate of battery consumption and will know that re-chargeable are the thing for the job, either as a battery pack or separate cells.

It is worth noting that the while NiCd and Alkaline cells lose significant power in the cold, Lithium Ion types are little affected. They also have a capacity even greater than alkaline cells. Unfortunately they are very expensive and require extremely accurate charging. Most such cells and chargers are a matched set.
However, they could be worth considering if planning a trip somewhere seriously cold (or use underwear
mounted instruments.)

Re-chargeable batteries

These include NiCd (Nickel Cadmium) and NiMH (Nickel Metal Hydride), either as separate AA cells or as
a pack. They have a lower voltage than alkalines, so your torch will be dimmer and your vario battery
indicator will be reading flat even with a fully charged NiMH. That’s why in some instruments you can tell
the software what type of batteries you're using. NiCd cells are now largely displaced by NiMH, which
behave in much the same way but have significantly greater capacities. A typ ical AA NiCd is 700 mAh,
where 1600 mAh NiMH are now common. For the uninitiated, consider mAh to be a volume – the chemists
have squeezed over twice as much electricity in.

Both kinds suffer from memory effect and early death from abuse, although they’re getting better all the
time. The solution is simply to flatten them after use before giving them a full charge. Treated well, they
should last for hundreds of cycles with negligible reduction in performance. They should never be allowed to go completely flat, so it’s best to flatten using an electronic device designed for use with re-chargeable which switches itself off at the correct level of discharge (e.g., GPS, radios). If they’re going to be out of use for a while, they should be stored flat. They will self-discharge in a few months anyway. That fact makes them unsuited to use in a vario, because they will have gone flat of their own accord long before all the power is used. It’s best to keep batteries in sets to ensure they are matched. If a set seems to have lost performance, test each one in turn in a single-battery powered toy and bin the offender.

The correct way to charge an AA cell of either kind is to give it trickle charge of about 150% of the rated
capacity, for example:

A Jessops NiMH has a capacity of 1600 mAh written on the side. That means it can give out 1600 milliAmps
for 1 hour. (Think of mA as being flow rate, pints per hour.)
So to charge to 150%, the required charge is 1.5 ∗ 1600 = 2400 mAh

A good charge rate is 10% or less of rated capacity (that makes 160 mA for my NiMH). More than 30% is
only for special cells, at risk of explosion! I have an old NiCd charger that says it gives out 65 mA, so the question is, how long does it need to provide the 2400 mAh required to satisfy my new NiMH batteries?

(Charging time) = 2400 mAh / 65 mA = 36.9 hours

In this way, you can work out how to fill all of your collection of various batteries without cooking or
exploding any. Any that get hot to the touch are either past fully charged, being charged too fast, or just
knackered. Better still is the use of accurately controlled pulse chargers which ask each individual battery what it would like, hundreds of times a second. Let me know if you find a cheap source!