Just to throw in a comment about the dual-battery setup, with questions about
high amp flows and disconnection relays. The same problem arises in solar
power systems, which I play with from time-to-time.

My understanding is that the problem occurs when your 2ndary battery is run
really flat with the ignition off. (Your primary then won't be connected to
the 2ndary 'cos the ignition is off so the transfer relay is off.)

Now when you turn on the ignition, the relay kicks in, connecting the two
batteries together. 

The problem is that you've got a full battery connected to an empty one in
parallel, and as someone else suggested, the amp flows are going to be HUGE.
Very easily much higher than the 30A or so you could generate with the
alternator/generator. 

Both a fire hazard (melted wires, explosive gas) & bad for your nice expensive
leisure battery ( starting battery will probably tolerate it more, being made
differently). 

So what to do? It's going to be too expensive to try to control the big
current flows between the two with some sort of regulator (requires big-style
power MOSFETs; say about $400?) Could try any/all of the following though:

(1) Put a low voltage disconnect unit (LVD) on your 2nd battery, set to
disconnect loads when about 50% full (10.9 v or so). This will stop it getting
really low, reducing charging currents when the batteries are connected
together. Incidentally, this will make the expensive leisure battery last
years not months; life is (roughly) inversely proportional to average depth of
discharge. Probably the best solution.

(2) Use nice big cables between the batteries. Definitely rate for 100A (too
big is impossible - well, within reason, no replies please of 'I cant get this
6in diameter armoured cable  onto the nuts'  ;-)

(3) You could try to avoid 2ndary batteries that are massive compared to the
primary; it won't put such a load on it when connected together.

[oh yes, be careful <g>, use brain, gloves & goggles when wiring batteries. .
you _really can_ weld through a crossmember (or you) if you short them out.
They tend to explode too, if I remember correctly. In fact, if an auto
manufacturer invented them now, I bet they wouldn't get past safety
regulations to be put in cars. . . . .]

anyways use info at your own risk. I think its all correct, but . . .
ben. (with the Shorter Transporter)

Topic No. 37

Date: Tue, 21 May 1996 00:00:04 -0400

>.
>
>But a problem remains when the aux battery is heavily discharged and 
>the vehicle is started.  The discharged battery can draw a very heavy 
>current load until battery voltage comes up.  Something in the order 
>of 30, 60, or even 100 amps.  This also has a disadvantage of 
>prodcuing gobs of H2 gas (acutally H2 and O2 in a exquisitely 
>explosive combination) if a conventional deep cycle battery is used.  
>Very heavy battery cables are the order of the day, in addition to a 
>sealed battery box.
>
>Isn't there an electronic device that could limit the charging 
>current in the conductor going to the aux battery?   I can't picture 
>what it would be in my mind but there must be something out there.
>
>It would need to:
>- not interfere with the terminal charging voltage
>
>- be self-adjusting to limit current to say, 30 amps but also let a 
>trickle of 1 or 2 amps thru to top off the battery when it isn't 
>fully discharged.
>
>- be electrically efficient and not create a dangerous amount of 
>heat.

With the alternator regulated to under 14 volts, you will never get much more
than 30 amps going into battery. If the battery is completely dead, it will
not conduct current well until it starts to get some charge on it. #8 wire is
more than suffecient to charge battery and feed the accesories tied to it.
 The wires from the alternator to the original battery are a pair of #12's.
 I've never seen them fry although they do get quite warm with a lot of
accesories, lights, etc being used.

Dennis.