This weekend, during a camping trip, the starter battery on my 2001 VW Eurovan camper started to show signs of weakness - cranking was much slower than usual and took about 5 attempts to get the engine to start. At the time, I chalked this up to me leaving a couple of phones charging of the main battery overnight.
However this morning, starting was a no-go. Dash lights came on, and lots of solenoid clicking noises, but not enough juice to turn the engine over at all.
Then it occurred to me that this van is also equipped with an auxiliary battery for running a small fridge and some interior lights. I was in a pinch, so I hooked up my jump start cables from the aux battery to the main battery, and was able to successfully start off that. For the record, I temporarily removed the isolation relay that normally enables charging of the aux battery when the engine is running, just to be sure not to cause any damage from this unorthodox setup.
The main battery is a regular starter battery (650 CCA, 100 Reserve Capacity). I couldn't find a manufacture date, but now suspect its at the end of its life.
Conversely, the aux battery is a deep-cycle battery, rated at 60Ah. I think this one is probably original to the van, though again I couldn't find a manufacture date.
Is it safe (in a pinch) to do what I did this morning - start the van using the deep-cycle aux battery?
When I ask about "safe", I would like to know about both personal safety, and whether or not damage can be caused to the various parts of the system by using this setup.
This clearly not something I would do on a regular basis, but would like to know if it is an option next time I'm in a pinch.