Re: Even more homebuilt charger madness

[Lee Hart <leeahart@earthlink.net>]

Roger Stockton wrote:
> the suggestion, as I understand it, is that at higher rates the
> batteries heat up less and accept charge "better" when charged at,
> say, 100A and 10% duty cycle than at 10A continuously. It may also be
> the case that the short duty cycle allows the higher charge current
> to be continued later into the charge cycle without the gassing, etc.
> that would result if you tried to maintain even 10A into the pack
> once cell voltages reach/exceed 2.40Vpc.

This is a plausible theory, and worth investigating. I don't think it is
proven yet, though.

It is *very* difficult to make statistically significant measurements
around batteries. You can run two identical charge/discharge cycles on
the same battery, and get a 10% difference in performance. So if the
first cycle used charger A, and the second charger B, was the difference
due to the charger, or normal random variations in the battery?

Measurement error is a serious problem. I see lots of errors that naive
researchers make. For instance, batteries have a huge thermal time
constant; that means it takes hours for the temperature to stabilize.
Suppose you charge one battery at 10 amps for 10 hours, and it is 10
degress hotter at the end. You charge another at 100 amps for 1 hour,
and it is 5 degrees hotter at the end. Is the high current better? No.
The temperature of the battery charged at 100 amps hasn't stabilized
yet. Its temperature will keep rising for an hour or more, as the heat
inside s-l-o-w-l-y works it way out. In the end, you'll probably find
its peak temperature is hotter than the one charged at 10 amps.

Another source of errors: When you are dealing with pulsed waveforms,
simple averaging meters give erroneous results. They underestimate the
current; the true RMS current is higher. This leads to underestimating
the current you put in. A classic example; a "bad boy" charger (nothing
but a bridge rectifier) will indicate an absurd efficiency (over 100%)
if you use average-reading meters to measure input and output current.

Batteries capacity is strongly affected by temperature. And, the
temperature is strongly affected by the current and charge/discharge
chemical reactions. A charger can appear to increase capacity just
because it causes the battery to run hotter (but shortens its life as a
trade-off).

The biggest concern I have with miracle charging schemes is that they
are disclosed in sales literature. There is an overpowering urge to rig
the tests, and throw out any results that don't sound good. Getting a
patent doesn't mean it works; it just means they convinced a patent
examiner that it *might* work (and that no one else had patented it
first).

> The real practical problem with this scheme is that it is incompatible
> with off-the-shelf "fuel-gauging".  The sequencer for this system
> would have to monitor Ah removed during series discharge and Ah
> returned to each individual module during charge, and indicate the
> pack SOC based on the least charged module in the pack at any moment;

How you measure state of charge has a big effect on the results you get.
If you charge dissimilar batteries in series, you overcharge some of
them. If you monitor and charge them individually, you can eliminate
this overcharging. But if your individual monitors don't monitor
amphours removed (to save money), then they can be as bad or worse than
a single series charger that *does* monitor amphours and controls
charging accordingly.

And as you say, knowing state of charge is considerably more difficult
when batteries aren't charged/discharged in series.

I faced many of these problems with my Battery Balancer. I chose to take
power from the pack as a whole to charge individual batteries.
Basically, it is redistributing power within the pack; no power going
out (like Rudman regulators) or in (like individual chargers). This
means a single E-meter and single shunt still correctly indicates the
number of amps, amphours, and watthours that go in or out of the pack.
--
Lee A. Hart                  Ring the bells that still can ring
814 8th Ave. N.              Forget your perfect offering
Sartell, MN 56377 USA        There is a crack in everything
leeahart_at_earthlink.net    That's how the light gets in -- Leonard
Cohen