Capitol wrote:


John Rumm wrote:

Ironically its a technique that would work better on a more expensive
battery pack where someone has taken the time to more closely match
the cells.


How would you match the cells? Just intrigued. Economic?

The way it is often done is with a A to D converter on a computer and a
resistor bank for a load. You charge up each cell until "full" and then
connect up to your discharge rig. This then records the discharge over
time, and allows you to plot a discharge graph. Then having done it with
a bunch of cells you can cherry pick ones with closely matching curves
for making into a battery pack.

You can buy cells that are already matched (at a price premium), or you
can match them yourself. The "fast electric" RC model guys who are
serious will typically do the matching themselves.

(I met a truly fanatical fast electric boat anorak once, who delighted
in talking me through the details of his particular boat - kevlar /
carbon fibre hull, 12 pole motor, 36 computer matched sub C cells - a
couple of grands worth all in. This boat was designed to race for 500m
round a triangular circuit - apparently it is a layout that is typically
used for record attempts. To be considered "good" you should be able to
do the three sides, a 180 turn at the end of the last, and then another
2 sides to the finish in under 20 seconds (at the time the record was
something like 14). The battery pack would typically be expected to dump
most of it charge in that time - developing a power output over 2kW!.
After each run he would then hook the whole pack up to a discharger for
a little bit before using a shunt with an a old galvo wired up to
individually discharge each cell to 1V before he would fast charge the
whole pack again.)

IME, cell life is a pot luck exercise. I find that batch A last x years
and batch B from the same manufacturer don't, in the same application
and charged in the same way. It does seem to depend on the manufacturer
to some extent, but nor reliably IME.

Hence the need for matching. Some brands do seem consistently better
than others (e.g. sanyo, panasonic) - but as you say there can be quite
dramatic differences between apparently similar cells.

--
Cheers,

John.

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