Starting using a deep-cycle battery

Question

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.

Answer 1

Definitely not optimal, but as you say it'll work in a pinch. That, of course, depends upon the state of the main battery. Even when a battery is "dead", it doesn't mean that it's completely flat. It still has some power left in it (lights are dim and the starter only clicks ... but it still has something going on). The overall voltage may be down quite a bit, so it has no chance by itself to start the vehicle. Realistically, just like when you jump start from another car, it only needs a small amount of added juice to make things happen. The deep cycle battery is not designed for this type of use normally, but will make up the difference if needed.

As for if you've actually damaged anything, I'd suggest you haven't done that at all, but you've probably shortened the life of the deep cycle battery by a bit. Also, I would bet you would have noticed by now if there were any residual effects of what you do.

As long as you take the approach you did with basically taking the deep cycle battery out and using it separately, there really aren't any safety concerns beyond that of normally jumping a vehicle.

In my opinion, what you did by removing the isolation system was very wise. I would have done what you did in a heart beat if it were my only option. I would not leave myself or my family stranded if I had some other way to get my vehicle going. In the Army we used to call it a "Field Expedient Means" ... you do what you have to do.

Answer 2

This is actually a feature of most manual leisure battery control switches. They give you a bypass option to start the engine off all available batteries. As has already been mentioned, it's not something you should be doing on a regular basis but it's considered a reasonable thing to do.