Steve Clunn via EV wrote:
One of my problems with the setup that I have is that the Contactor is for
both driving and Battery Charging, so, the Contactor ends up being ON
almost all of the time.
One solution of course, would be to have the charging circuit separate from
the Load Circuit and I could even have the foot peddle engage and disengage
the main contactor
That's a good general case solution for problems while driving. This
will also be a big high-current contactor. It will have a relatively
high coil current; but it's only on for a relatively short time, and its
power is a tiny fraction of the load compared to driving the EV traction
motor.
but I still have to deal with charging
The contactor needed to break the DC charging circuit is far smaller,
since your charging current is likely to be much lower. That means its
coil current will be far smaller. It's easy to find a relay with 30-amp
contacts whose coil draws less than a watt. That's 12v at 83ma, or 36v
at 27ma. Maybe that's low enough not to worry about?
You can also depend on the relay being held on with only 1/4th its rated
coil voltage and current. Once pulled in, a 12v relay will stay on at
3v. So put an RC "slugger" in series with the coil. It gets full voltage
at turn-on, when the capacitor is discharged. Then the capacitor
charges, and the resistor limits the current to 14th of its normal value
to hold it in. For a 1 watt coil, that's 21ma at 12v, or 7ma at 36v. Is
that low enough?
since the chargers are not on board, I have to deal with switching
the DC input current.
That's not really so bad. The charger almost certainly won't have filter
capacitors, so the charging current isn't DC, it's pulsating AC. Relays
with AC-rated contacts are fine, because the current goes to zero 120
times a second, just like on AC.
What kind of charger are you using? Will it care if you open a contactor
to disconnect its DC output while charging? Most chargers won't care,
but a few (like the Manzanita PFC chargers) can be damaged if you open
their DC output while they are operating.
Some chargers will even shut themselves down if there is no connection
to their output. This is good with an offboard charger, since it
automatically turns off the AC without you needing a separate connection
to break AC power into the charger.
Do you always charge from a GFCI-protected outlet? If so, one way to
shut down AC to the charger without a direct connection is to have your
BMS generate a 5ma ground fault to shut it down. A *really* tiny relay
or optocoupler is sufficient to do this. The relay could be held on by
just 1ma from the batteries. When the BMS turns it off, the relay drops
out, so its normally-closed contact closes. This connects a resistor
from pack to chassis that draws 5-10ma. The GFCI sees the ground fault,
and shuts off AC to the charger. :-)
I have some transistors ( fets) that I may play with as Cor van de Water
suggested for the charger.
You could indeed build a solid state relay with series MOSFETs. But that
might be more of an engineering challenge than you'd like. There are a
lot of subtleties to get it right, so it's not killed by static or AC
line transients.
--
If you would not be forgotten
When your body's dead and rotten
Then write of great deeds worth the reading
Or do these great deeds, worth repeating.
-- Ben Franklin, from Poor Richard's Almanac
--
Lee Hart's EV projects are at http://www.sunrise-ev.com/LeesEVs.htm
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