David
The short answer is not long.
The long answer requires that I make some guesses. Assuming your "little space heater" puts out 1000 watts of heat. If your space heater uses 12v DC, then your battery would drain at the rate of 1000/12 = 83 amps and for a standard battery would last less than an hour. It would also probably damage your
battery. Now most electric heaters use 120v AC so you would need an inverter to convert DC to AC that is rated for over 1000 watts. The inverter would also have some losses (heat) so you'd probably be drawing closer to 100 amps from your battery. This is hardly distinguishable from a dead short. So the
answer is, don't do it.
The answer to your heat requirements is more likely met with a propane catalytic heater. These have been discussed exhaustively and can be found in the List archives.
Larry

David Devere wrote:

> Hey Everyone,
>      I'm trying to outfit my westy for the winter and was wondering how long the second battery will last if I plug a little electric space heater in for the night.  If the second battery is drained does the main battery then start to provide power?  I would rather be cold than stuck needing a jump.
> Thanks-Dave
> 87 Syncro Westy