Hybrid Car – More Fun with Less Gas

Question about RV solar battery charging with vehicle's alternator

register ::  Login Password  :: Lost Password?
Posted by solarbus on May 10, 2010, 7:59 pm

the Solar Bus has been going strong with our on-board PV system that
includes 8 golf cart batteries. until now I have not had a reason or
desire to charge with the alternator while I'm driving. Because our
project is educational, I want all those electrons in the batteries to
be from the solar, and there's always been enough.

lately however there are times when a little alternator charging is
desired. So I'm going to set something up.

I *don't" want to use an automatic battery isolator tied into the
vehicle's key because with those setups, it's *always* charging from
the alternator when the vehicle is driving. I want it to be manual, so
I only engage the alternator charging in the rare times when I need

I'm thinking all I need to do is run a wire between the positive
terminals of the starting battery and the aux (solar) battery bank,
with a manual switch (and a fuse wouldn't hurt). I could go from the
alternator to the solar battery but I think it will be easier to go
from the starting battery to the solar battery. Since both batteries
are grounded to the same vehicle chassis, I don't need to run a
negative wire. The wire and fuse should be sized according to the
maximum amperage of the alternator. I'll use one of those big fat
"knife" switches that can handle the current. I could use this not
only to charge the solar batteries with the alternator, but also if
the starting battery is dead for some reason I could give it a "jump
start" by just closing the switch, connecting it to the solar

I don't have that much experience with RVs and I'm wondering if anyone
has set up manual battery isolator like this and if there is anything
I should take into consideration, or if there's anything wrong with my

Gary Beckwith
a.k.a. "the Solar Bus Driver"
The Solar Bus: A solar energy educational project on wheels

Posted by M.P. Android on May 10, 2010, 9:51 pm

Check with a marine supply house; they should have a heavy-duty switch.
Boat folks often use two batteries, so if they kill one they can still get
started.  The switch is A-B-Both, so when both batteries are full you can
charge in parallel.  In your case, you can select the solar bank as you

Email on polite request in newsgroup

Posted by m II on May 11, 2010, 12:12 am
 solarbus wrote:

The alternator will only put current into the extra batteries when their
voltage is below normal. There is very little, if any, charging being
done after they are charged to their normal state.

I'd go with the battery isolator and a disconnect switch in series with
it. Leave all the solar wiring intact.

If the batteries are accessible, there are battery pole disconnects

There are many variations, but this one is inexpensive:



Posted by solarbus on May 11, 2010, 2:56 am

thanks mike, I asked also on another forum and got the same advice...
it would be better to install the battery isolator and just add a
disconnect so I can only allow it to charge when I want to.  and I
already have one of those pole disconnects you pointed to... I also
have one of the "knife" switch type, and a 200A rated on-off switch
that looks sort of like this

http://images.cabelas.com/is/image/cabelas/s7_012970_imageset_01? $main-Larg=

but it's only on-off.   any of those should work.

thanks for your advice... i'm going to go with the isolator and add a

Posted by wmbjkREMOVE on May 11, 2010, 1:40 pm
 On Mon, 10 May 2010 19:56:28 -0700 (PDT), solarbus

Charging that large a battery bank with a vehicle alternator might be
dicey. If the batteries are low the alternator will likely go to full
output for long periods, which it isn't designed for. I'd expect it to
run hot at the least. I don't have any experience with your type of
setup, but I have something a bit similar - a backup charger using an
automotive alternator outputting 2000W and belt-driven by a small
engine. The stock diodes failed twice before I gave up on them and
built a beefier external rectifier with a fan-cooled heat sink. The
other problems were overheated stator terminals (failed at the crimps)
and relatively short belt life. My advice would be to check the
alternator output at various times when charging the large bank, and
if the amps are high then be on the lookout for the weak links.


This Thread
Bookmark this thread:
  • Subject
  • Author
  • Date
please rate this thread