Jump starting off aux battery

[blackmav]

I've been told its bad to jump start off my deep cycle battery. Its a deep cycle connected to the other main with a piranha system.
I have flattened my main battery and just put the jumper leads straight across. Done it twice.
True/false?

[-Scott-]

How long is a piece of string?

Most deep cycle batteries don't like large currents (neither discharge nor charge), but some deep cycles can handle larger currents better than others, and "large" is relative anyway.

Small battery and large motor? Not a good idea. Large battery and small motor? Probably acceptable - depending on the battery and how easily the motor starts.

The other factor is "how flat" is the main battery? If it's dead flat and doing nothing then the deep cycle is doing most of the work - not ideal. If the main battery is still turning the engine, but it won't catch, then the deep cycle is only "assisting" - so not as much current, not as bad.

Does that help?

[blackmav]

Yep. Thanks.
Car starts easy for a diesel but battery didnt have enough to turn over fast enough to start.
So no drama to the duel battery system?

[-Scott-]

I would say you're at the bottom end of the offensiveness scale, so doing it occasionally isn't something I would worry about. But I don't think you should let it become a regular thing.