Date:         Tue, 21 May 1996 14:14:30 -0700 (PDT)
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         Dwight Mattix<dmattix@qualcomm.com> (Dwight Mattix)
Subject:      Re: Battery current limiting device??

bounced this off a power engineer here at work re: current limiting.
so for what it's worth...

dwight

>At 1:20 PM 5/20/96, Dwight Mattix(Dwight Mattix) wrote:
>
>>>It seems the correct way to hook up an aux
>>>battery in our vans is to use a 'combiner' relay and not use an RV
>>>'isolator'.  Fine.

>Dwight, I'm not familiar with either of these.
>

>>>
>>>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.

>
>I would rather vent the H2.  H2 diffuses fast.  Best not to let it
>accumulate.  The threshold for combustion/explosion is 4% minimum H2 in
>air.  That explosion possibility continues up to about 80% maximum H2 in
>air.
>

>>>
>>>Isn't there an electronic device that could limit the charging
>>>current in the conductor going to the aux battery?
>
>This would be a switching "buck" or "stepdown" voltage regulator with two
>stage current limit.  30 amp initial current limit, backing down to a 1 amp
>"trickle" charge once the aux. battery ternminal voltage reaches 13.6 or
>13.8 or so.
>
>I am not aware of an existing product on the market like this, but I
>haven't needed one and never looked for one.  Do you want me to look for
>one?  Probably at least $100. if it exists.  The one possible problem with
>what I described is that coming off the alternator at about 14.5V doesn't
>leave much "voltage headroom" for the switching buck regulator to operate
>from.  The switching transistor duty cycle would probably go to 100%
>initially, or maybe 99% to limit the current to 30 amps.
>