I use exactly this same system on a 911 turbo which is kept in the garage for weeks on endI'm not technical by any means. Therefore this is just for your info!
I had a VWT6 campervan prior to getting my motorhome. In the campervan the Sargent system did not charge up the campervan starter battery. I got caught out when my starter battery went flat. With the help and advice of a poster in the T6 forum, I purchased this and never had an issue again. Hopefully someone with the expertise will give you the relevant solution.
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Never let me down in 6 plus years and starts every time.
DJT