Re: Opinions [Isolated Charger]

[Lee Hart <leeahart@earthlink.net>]

Jon "Sheer" Pullen wrote:
> I'd really like a 'software' GFI that can 'self test' itself by
> turning on a deliberate small leak. But that may prove to be
> overambitious. Then again, they sell GFIs for $20, how complicated
> can they possibly be?

When you're dealing with DC (batteries), the normal current transformer
approach used for AC GFCI's doesn't work. One of the common techniques
is to deliberately connect a resistor from ground to one, then the other
side of the pack and measure the voltage drop across it.
> Only if that isolated charger is 95%+ efficient. Otherwise, you're
> trading a not-very-likely death scenereo for irresponsable use of
> resources.

Essentially no charger is 95% efficient, whether isolated or
non-isolated. 70-80% is probably typical for isolated chargers, though
90% is attainable. A non-isolated charger can be perhaps 5% better. Of
course, there is no limit to how LOW the efficiency can be; the "wall
wart" chargers supplied with many consumer products are like 10-20%
efficient.

The simple fact is that people don't care about efficiency; they only
care about what they pay. In most cases, they buy low efficiency
products because they cost less, and never add up the long-term energy
cost due to that low efficiency.

> I used to have one of those old tube tape recorders. It tickled the
> hands some, if one was grounded one had to be careful with the mic...

Most 2-wire AC products have capacitors from the AC line to ground for
noise suppression. The value of these capacitors seldom exceeds 4700pf,
which allows about 200 microamps of AC leakage at 120vac 60hz. That's
enough to feel, but not be dangerous.

> with flooded batteries, it's basically impossible to get good
> isolation.

Not impossible, but it requires more care than the average home builder
is likely to use.

My first EV was a pickup truck with golf cart batteries sitting in the
bed. I had endless leakage current problems, and when the pack was shot,
I discovered that the acid had eaten holes in the bed (what a surprise
:-)

So on my next EV, I vowed I wouldn't have such problems. I made a
battery box out of 2" styrafoam, and then fiberglassed it inside and
out. The box provided electrical and thermal insulation. Then I had zero
problem with leakage or corrosion.

The vents on flooded batteries can also be sealed so they in effect
behave like sealed batteries. Things like the Hydrocaps (which have a
catalyst to recombine H2 and O2 back into water), and automatic battery
watering systems do this. A flooded pack can look as clean and pristine
as a sealed pack if one is willing to go to the trouble.
-- 
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