Re: Solar Charging Port

Great idea relocating the breaker and contactor, you found another crack to
let some light in!!  There is nothing better than a simple solution to a
complex problem.  This is for educational purposes, so it leaves the school
open to expanding the technology as the students become more advanced.  I'll
be sure to post the results of the project when we are complete.



-----Original Message-----
From: ev-bounces@... [mailto:ev-bounces@...] On Behalf
Of Lee Hart
Sent: Friday, November 14, 2008 11:31 AM
To: Electric Vehicle Discussion List
Subject: Re: [EVDL] Solar Charging Port

Cor van de Water wrote:
> Why make things more complicated if you are not (yet) concerned about it
> being practical?

I agree. Get a simple solution working first; then improve on it as the
situation permits.

Since the panels generate DC and the batteries are DC, the easiest
solutions will be ones that directly connect the panels to the
batteries. Either use enough panels in series to produce pack voltage,
or rewire your EV's batteries in parallel groups for charging.

I prefer the latter solution, as it's safer (low voltage on the
connector), and tends to automatically balance the batteries) the
strings charge in parallel, so the least charged ones take the most
current).

For example, suppose your EV has a 108v pack (18 6v batteries). Wire the
emergency disconnect switch or breaker at the 36v tap, and the main
contactor at the 72v tap. When both are opened, the pack is divided into
three 36v strings.

Solar panels always have a blocking diode in series to prevent the
batteries from discharging into the panels when there is insufficient
light. In this example (charging three battery banks in parallel), use a
blocking diode for each one. I would use *two* blocking diodes per
battery bank; one in the positive and one in the negative lead. This
prevents shorts and fuse-blowing if someone forgets to turn off the main
contactor or emergency disconnect switch before plugging in the solar
charger. The diodes should be rated at about double the PV panel's
maximum current, and at least 2x the maximum pack voltage.

Also include fuses, of course. Batteries can deliver enormous fault
currents, and the fuses will prevent a fire. Put a fuse in series with
each diode. The fuse's current rating should be about 2 times the PV
panel's maximum current output. The fuse's voltage rating should be for
DC at the full pack voltage (*not* low-voltage automotive fuses)!

A bad old ASCII schematic shows the overall scheme. View it with a fixed
width font like Courier:

+36v__________|\|___/\  ________+108v to EV motor controller etc.
solar     |   |/|     \/  |
panel     |   D1    F1  __|__+
           |              ___  36v string of batteries
           |   D2    F2    |  -
        __ | __|/|___/\  __|
       |   |   |\|     \/  |
       |   |                /  main contactor
       |   |___|\|___/\  __|
       |   |   |/|     \/  |
       |   |   D3    F3  __|__+
       |   |              ___  36v string of batteries
       |   |   D4    F4    |  -
       |__ | __|/|___/\  __|
       |   |   |\|     \/  |
       |   |                /  emergency disconnect switch
       |   |___|\|___/\  __|
       |       |/|     \/  |
       |       D5    F5  __|__+
       |                  ___  36v string of batteries
       |       D6    F6    |  -
-36v__|_______|/|___/\  __|
solar         |\|     \/
panel

The scheme can be extended to any number of batteries. You just need
more switches or contactors to break the pack up into 24v to 48v size
groups, and 2 diodes and fuses per battery group.

It's unlikely that your solar panels will be large enough to overcharge
a car-sized EV pack, so a charge controller is optional. But if they
can, a charge controller will be needed. Size it for a 36v string (or
whatever voltage group you break your pack into).
--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in    --    Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net

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