Mine is an 85 tintop.  I added the GoWesty auxillary battery setup 12 years ago with a group 41 Walmart battery.  I'll have to check my records to see if I've ever replaced that battery.  Here is the kit:  Auxiliary Battery Kit [1980-83 Air-Cooled Camper and 1980-91 Non-Campers] | GoWesty

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Auxiliary Battery Kit [1980-83 Air-Cool...
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I added a bunch of cabinetry to my van behind driver seat, making access to the Aux Battery a little hard to get to.  I also used a Greenlee sheet metal punch to provide a hole just behind parking brake lever to add a 12 VDC cigarette lighter socket; the idea was to have a convenient port operating directly off Aux Battery (I added an inline fuse).  The setup has been working just fine these last 12 years; at least I think so, it always works and I'm assuming the alternator via the relay in the kit is doing it's job; my camping electrical needs are very minimal, but I do run a laptop and some interior lighting; and also for charging phone and camera batteries.
But with Covid, my travels lately have really declined.  So....I got this bright idea that maybe I should trickle charge my Aux Battery with a cigarette plug into the nearby outlet and connected to my trickle charge?  I assume there will be minimal Hydrogen gas being emitted during the charging process but should quickly rise and ventilate out of garage.  I assume all of this would work and be safe...but wanted to pass it by the Vanagon brain trust before I do it.
Rich
San Diego