Re: Opinions [Isolated Charger]

["Jon \"Sheer\" Pullen" <sheer@brassrat.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.

But why not GFI on the charger side, while it's still AC?

I assume you use a very large value resistor for this test? ;-)

It doesn't sound hard to do - can you MOSFET-switch those resistors or do
they need to be relay-switched?

> 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.

Erm, uh.

Based on the heat given off by my PFCW-20, I'd tend to put it up there at
95+%.

It warms a liter (sp?) of water about 25 deg C in operation, while moving 15
kwh from wall to pack. I unfortunately didn't measure how much my water
heated up when I did a full 20 kwh charge - I've only gotten the pack that
low once, it's kind of hard to do.

Anyway, Joe Smalley probebly has some 'hard' efficiency numbers. How about
it, Joe? This thing 95% efficient, given my pack voltage [268] and line
voltage [220] ?

> 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.

You are of course correct. Otherwise, why would people drive SUVs? But while
we as a culture do in fact currently have this broken mentality, that
doesn't mean we need to _keep_ it.

> > 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.

As Mitch points out in a excellent post, it's certainly achivable within a
home builder's selection of materials at the local hardware store, if that
home builder is aware that there is a issue. I was made aware by servicing
flooded EVs, and found the process sufficiently annoying to go non-flooded,
personally. ;-)

> 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
> :-)

[grins] I've seen this done a few times. I'm not convinced it's a good
solution.

> 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.

[grins] not to mention, your car was still operable in the winter.

> 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.

[nods] the problem is that such systems are expensive, and people [even
those who will spend $35,000 on a new car] won't put that kind of money into
their EVs.