Battery capacity

From The RadioReference Wiki

Portable/handheld scanners run off of batteries, commonly AA sized batteries, and many users want to use rechargeable batteries, to help the environment, and also to reduce their expenditure.

Rated battery capacities have been going up, and up, and up over the last 5 years or so; we've seen retail AA cell ratings go from 1650mAh to 2650mAh. But these super-high capacity cells come with two caveats:

  • They're really fragile.

The way manufactures get those capacities into what is, after all, a fixed size cell is by making the rolled-up layers thinner, and thinner... and thinner. The problem with this is that it makes the cells very fragile. Baylink has trashed at least half a dozen such cells merely by dropping them, and having them hit something hard, like a tile floor or a desktop. And given the torque generated by the strike, it doesn't really matter how they land. The problem seems most notable on cells rated 2300mAh or higher.

  • You don't get all that capacity, anyway.

And the reason is really esoteric, and best demonstrated by an anecdote.

Baylink sez:
I own a LaCrosse BC-900 smart charger, the one with separate charge controllers and displays for each of 4 slots; I got it from Thomas Distributing (doesn't everyone?) When I got a fresh new set of Duracell 2650's for my spandy new (used) Olympus E-10 DSLR, I put them in, and ran them through a refresh cycle, and sure enough, it put very close to 2650mAh into each of the 4 cells.

So I put them in the camera, and shot the heck out of them, and when the camera said "Enough!! No more" I took them out, and put them back in the charger, and charged them up.


"Huh?" says I.

But then I figured it out.

A "2650mAh" rechargeable battery will deliver 2650 milliamp-hours from its maximum stable charge voltage (usually 1.25 to 1.3VDC) down to its minimum safe discharge voltage (usually about 1VDC).

But your device may not discharge the batteries that far: it may have its own minimum safe discharge voltage, and that may be higher; on my Olymups, it was quite a bit higher, as the example shows.

And the batteries can only deliver as much power as the device will take.

The minimum safe discharge voltage will vary between devices, and I've never seen a device on which it was adjustable; the more batteries you have in a series string, the higher the manufacturer wil set it, in hopes of not reverse-discharging any of your batteries.

And it is, of course also the case that non-rechargable alkaline batteries start at the substantially higher 1.5-1.6VDC, and we don't care if we beat them to death on discharge, because they are, after all, disposable. Though this doesn't mean your device won't shut down anyway; some devices do it because they just can't run anymore, some do it to protect the battery string. And some, like camera flashes, just keep going, and getting slower and slower to recycle.

This is the reason why your alkalines seem to last almost twice as long as a charged set of NiCd or NiMH batteries, in case you hadn't already figured that out.

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