Re: Pulse Charging

[Lee Hart <leeahart@earthlink.net>]

>> http://www.ctts.nrel.gov/BTM/pdfs/RD100_winner.pdf

Arthur Matteson wrote:
> Whodi...that's amazing. I might seriously implement that in my charger. 

(Excuse me while I put on my electrical curmugeoneer robes... :-)

One needs to understand that 'miracle' battery charging algorithms have
been 'discovered' almost continuously for the past 100 years.

Some are outright frauds, perpetrated by con artists eager to cheat
people out of their money. More often, they are 'invented' by well
meaning individuals that just don't understand normal battery behavior.
What they think are breakthroughs are just a re-discovery of the same
old techniques that have been widely used for years.

In this study, first note that their 'control' charging algorithm is not
specified, except to say it is 'constant current/voltage charging'.
Whatever it was, they only got 150 cycles of life from an Optima. Since
this is noticeably shorter than people get in real-world EVs, one can
guess that whatever they used was a 'bad' algorithm.

Second, note that they were extracting 50 amphours per cycle with the
'constant current/voltage charging', and 40 amphours with their 'current
interrupt charging'. Anyone familiar with lead-acid batteries knows that
the shallower the discharge, the longer the life. The extra life may
have simply been due to shallower depth of discharges.

Third, they used 30 amphours as their end-of-life point, which is about
50% of an Optima's 20-hour rate. Most battery test standards use 80% as
the definition of end-of-life. Using a lower end-of-life percentage
increases the cycle life regardless of the charging algorithm.

Fourth, keep in mind that a plain old stupid transformer-rectifier
charger already delivers a very high peak current at start of charge.
The current starts high because the battery voltage is low, and there is
nothing but transformer and wiring resistance to limit current. The
current rapidly tapers off as battery voltage rises. But even at full
charge, the charger is still delivering a high peak current at a low
duty cycle; for example a 1-amp average charge rate is really 10-amp
spikes for 10% of the time, at the peak of each AC line cycle (120 times
per second).

> One, why did they overcharge the batteries 5-20%?

You have to overcharge by a small percentage because batteries are
somewhat less than 100% efficient. Given the extra capacity and sudden
dropoff in life, I would guess that their 'control' algorithm
substantially overcharged the batteries, causing their early demise.

> Two, why did they still only get 40Ah instead of 50?

Because they were deliberately undercharging (to avoid overcharging),
thus less capacity. And, the shallower discharges extended life.

> Can you just double the current to make the charging time the same?

Though they don't say it here, other data from Recombination
Technologies indicated that they ran extremely high charging currents;
so high that the batteries ran very hot for the test, so total battery
life was less than 6 months. So they were already far into the region of
impractically large charging currents.

> Does anyone know if NiMH life or charge cycle can be improved by this
> type of pulse charging?

I believe fast-charging of nimh is limited by how fast the hydrogen can
be adsorbed into the plate. In most nimh cells, this rate is fairly low.
Nimh chemistry is also less efficient than lead-acid, so more waste heat
is generated during charging. So, fast-charging nimh is going to be
heat-limited, and if you apply too much charging current, the voltage
shoots up drastically, and you have severe gassing (which is fatal to
sealed cells, because they blow their seals).
--
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