Re: Even more homebuilt charger madness

[Lee Hart <leeahart@earthlink.net>]

BillDube@killacycle.com wrote:
> the scheme of sequential charging has fundamental flaws.

As Finnegan says in the movie Finnigan's Rainbow, "The situation is
hopeless; absolutely hopeless. But it's not really serious."
 
Bill is right that such a charger is fundamentally more expensive, more
complex, and less reliable than a simple series charger. But if there
are enough advantages, it might still be worthwhile. For example, laptop
computers monitor and charge every single cell individually; this makes
their chargers much more expensive. But tripling the cost of a $50
charger is not as painful as tripling the cost of a $500 charger. People
feel it's worth it for the extra battery capacity and life.

> The resistive heating of the batteries... is the square of the current

True. But is it a serious problem? Suppose you have a 50 amphour battery
with an internal resistance of 3 milliohms. If you charge it at 5 amps
for 10 hours:

        power = I^2 x R = 5^2 x 0.003 = 0.075 watts
        energy = 0.075 x 10 = 0.75 watthours

Now put in the same number of amphours, but at 50 amps for 1 hour:

        power = I^2 x R = 50^2 x 0.003 = 7.5 watts
        energy = 0.075 x 1 = 7.5 watthours

Just as Bill said, 10 times the heating. But 7.5 watthours of
heat will still cause a negligible temperature increase in a 50 lbs
battery.

> The charge efficiency (wattage) is also lowered quite a bit. You pay to
> heat the batteries instead of charging them.

This is only true when the batteries approach full charge. The higher
the state of charge, the higher the apparent internal resistance. But as
long as you don't run excessive charging current at high states of
charge, charge efficiency is almost 100% at any charging current.

> What kills it is the cost of the switches.

They are a serious expense. But the stress on them isn't quite as high
as you might think. First, they don't have to switch the full current;
only carry it. The charger can be turned off, the relays switched to the
next battery, and the charger turned back on so the relays don't switch
any power. This extends their life by at least 10 to 1.

The charger on/off switch can be a solid state device since it switches
so often.

The relays only carry the current briefly; a few minutes, for example.
Relays have a much higher current rating for intermittent use than for
continuous use.

The relays need to withstand full pack voltage when off, but they don't
need to switch it. The only time the relay might have to break full pack
voltage is if one relay was on and you accidentally turned a second one
on. In this case, it would be fuses that break the current, not the
relays.

For example, I'm using the Potter & Brumfield T92 relay in my Battery
Balancer. It is a DPST relay with contacts rated 30a continuous, 60a for
1 minute, and up to 277vac (390v peak). They sell for about $6 each. I
use a Vicor BatMod as the charger, which is turned on/off electronically
so the relays switch at zero power. Four BatMods could charge 20
individual batteries at 60 amps for one minute each. A 10-hour charge
cycle would put 30 amphours into each battery, and I could perform 3,000
such charge cycles without wearing out the relays.

> I feel bad whenever I rain on someone's parade like this. However, I
> worry that if I don't say something, someone will go through all the
> trouble and expense of building one of these widgets only to discover
> that it is not very useful. This would not be very pleasant for the
> person doing the work and spending the money, so I feel the need to
> speak up.

Thank you for voicing your concerns. It is most helpful when an expert
tells you what will (or won't) work, and WHY it is so. Then the builder
is aware of the potential problems. The problems can be dealt with by
ignoring them (if they don't matter in the intended application), by
researching them (test to see if they truly are a problem), or by doing
whatever is necessary to fix them (adding parts, etc.).
--
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