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.
>