The vehicle's voltage regulator "sees" both batteries as a "system" during
charging.
The isolator keeps you from running down the starting battery, by not
allowing you to draw current from it by the use of big diodes. It has no
effect on what the Alternator sees as a "load". The entire battery system
is the Alternator's load once it's up and turning.
During a charge cycle (driving, RPM above idle -- note that so-called 200A
alternators often have a minimum RPM before they produce 200A), the car
battery will complete its charge first, and then until the other larger
battery bank comes up to full charge, the alternator will continue to
provide the necessary current, and the regulator will hold at around -- and
it's a big "around" -- 13.8 VDC the entire time. Current will stop flowing
into the starting battery when it's full.
A very dead battery bank will be eating up all of the charging current that
they can handle -- the majority of it, in fact -- from that 200A alternator.
Which also means, you CAN put yourself in a problem situation if you run the
bank dead, start the vehicle, drive a SHORT distance (the vast majority of
the alternator's output will go into the huge suck-hole for current that the
bank has now become and NOT charge the starting battery, since it's
stronger), stop, operate on the nearly dead rear bank, start again, operate
on the nearly dead bank...
Eventually doing that, your starting battery will NOT have enough charge to
start the vehicle.
RV owners do this when they run the load battery bank down, start just long
enough to run a few things in the back, shut down, run the back dead again,
etc. Eventually they notice the cranking to start is getting harder, if
they're smart...
If they're not, they get to a point where they can't start -- and wonder why
their "isolator" didn't work. It did, they just had a giant bank of
batteries stealing all the charging current, so they started the vehicle ten
times without giving (much of) a charge to the starting battery.
As far as the relay goes, it sounds a LOT less reliable than simple diodes
of the isolator. The diodes don't make mistakes (unless they fail), you
might.
There might be other reasons (including this quick on/off cycling) that you
might want an isolation switch besides the diodes anyway, but not as the
primary way to decide if you charge or not. The first time you forget to
cut off the relay and run the WHOLE system flat, and can't start... you'll
know why the isolator is smarter than you are. (And you feel really stupid
that a diode is smarter than you.)
A simple mechanical battery cut-off switch inline with the isolator's output
for the battery bank, would be my choice. A manual way to always be able to
completely cut off the back batteries from charging from the alternator at
all in case of problems "in the back", or a battery so dead that it IS
presenting a dead-short to the system. A "belt-and-suspenders" attitude in
case something is wrong...
Other things:
Be careful with what batteries you use in the battery bank. Normal
lead-acid batteries don't last long in deep-discharge scenarios, and even
so-called "deep cycle" batteries have a point where the manufacturer tells
you stop drawing from them when they reach a certain voltage, or they'll be
damaged. Stop at that point, always, for long-life. Don't fully discharge
lead-acids if you can avoid it. You'll get more years of service out of
them. (And they won't look like a dead-short to your charging system right
when you first start up, either... dead lead-acids will suck down anything
handed to them.)
A low-voltage cut-off in the power distribution system to the rigs, is
always a good addition for a rover. My powerpole distribution bar has this
built in, and I've hit the low-voltage cutoff before, not paying
attention... off go the rigs... buh-bye... ahh, I'd better get moving...
Also watch out for venting. Remember hydrogen gas is formed during
fast-charging, and it'll go boom if you don't vent it overboard from your
battery bank for the rigs, if they're in the cabin. Even so-called
"sealed" lead-acids have vents for this that open under high pressure.
Nate WY0X
-----Original Message-----
From: vhfcontesting-bounces@contesting.com
[mailto:vhfcontesting-bounces@contesting.com] On Behalf Of w8zn@comcast.net
Sent: Thursday, January 29, 2009 2:01 PM
To: vhfcontesting@contesting.com
Subject: [VHFcontesting] HIGH POWER
Hi all,
No, not kilowatts of RF, DC power! I have a power question about car
charging systems. I installed a 200A alternator in my little truck that is
going to be used as my rovermobile. The output side of the alternator goes
to a 200A battery isolator. One port of the isolator goes to the vehicles'
battery and the other port to my battery bank which is 5, 160AH deep cycle
marine batteries I run in parallel to charge and switch to series/parallel
to operate for my 28v amps. So all this is pretty straight forward. My
question is, the voltage regulator monitors the voltage accross the main
battery in the vehicle and as it reaches full charge, the alternator is
throttled back. What happens if the 5 batteries I've been killing for the
past hour are not fully charged? Is it as simple as the batteries needing
charge keeps the overall voltage down so the the main battery doesn't fully
charge until the remote batteries are full? I would think that the diodes in
the isolator would not have a
s much drop to the main battery as the remotes if the remotes are full.
I'm almost thinking a 200A relay in place of the isolator is a better idea.
The relay would only energize when the truck is running so when shut off and
operating, I'm not draining the truck battery.
Any thoughts??
Terry
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