Re: Eneloop batteries

In article <jw6yi.444526$p47.286321@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
Vance Howard <itsme@xxxxxxxxxxxxxxx> wrote:

| This question is for Red Schofield (sp?).

Well, I'll take a stab at it.

| On RCGroups, a fellow keeps stating that Eneloop batteries are fine for
| transmitters, but would be bad for receiver packs because the internal
| resistance of the cells is 25 milliohms. I can't get him to explain why he
| says this. That's not very much resistance.

AA NiMH cells are known for having high internal resistances, so
people flying larger planes or planes with lots of digital servos
aren't terribly fond of them. AA cells in general are bad, and NiMH
is generally worse than NiCd. subC cells are generally very good,
which is why they're used for R/C cars and for the battery packs on
big planes with lots of servos.

http://www.stefanv.com/electronics/sanyo_eneloop.html shows this
person measuring an internal resistance 4x higher than that on Eneloop
batteries, however. But I do believe that the manufacturer's given
internal resistance figures are generally about low discharge rates
rather than high discharge rates.

I think 0.025 Ohms is pretty typical for AA NiMH cells, actually, but
I don't know if that's for high or low discharge rates. I'd guess
low, however.

In any event, a high internal resistance limits the amount of current
your servos can draw, or more practically causes a larger voltage drop
when drawing a large current. This slows things down, and under
extreme conditions the voltage can drop enough that your servos or RX
don't work properly anymore, at least until the current drops and the
voltage goes back up again.

| What is the internal resistance of a regular NiMH? What does this
| all mean anyway?

Every real-world battery has some internal resistance, and we
generally approximate it with one fixed value. This resistance goes
in series with whatever load you put on the battery, and this I^2*R (I
= current, R = internal resistance) power goes into heating your
battery rather than powering your plane.

If you want tables of internal resistance values for certain cells,
get it here --

http://www.motocalc.com/data/cell.html

That's what Motocalc uses for it's calculations, and the column you
want is `impedance', and it's given in Ohms/cell.

As for where to get them, they're $20 for a pack of 8 at Amazon --

http://www.amazon.com/exec/obidos/ASIN/B000LNI5VC/bluebirdlanefarm

I've bought a pack myself but haven't really had time to really test
them long-term yet. So far, they seem to work as well as any 2000 mAh
AA NiMH cells.

--
Doug McLaren, dougmc@xxxxxxxxxxx
Marge, this ticket doesn't just give me a seat. It also gives me the
right, no, the duty, to make a complete ass of myself. --Homer Simpson
.

Relevant Pages