biggest battery size??
 

For cold winter use (-40c possible), im now using a 650ta sidecar.

The battery is housed in the sidecar, so size isn't really important. (ok ok)
I will be using heavy cabling, but need to use as big a battery as possible to get the thing started, as batteries lose there current driving abilities in the cold.

The problem is, if I use a big (36Ah) battery, what is the chance of cooking the reg/rec if I drop the battery voltage down on start up? I will heatsink the reg/rec, but am I asking too much of it? so how limited am I to the size of battery?

My cub90 didn't have this problem......

Nice question ..

Provided the battery is not absolutely flat .. or short circuit .. it should not be a problem.

Under normal conditions it is the alternator that sets the maximum charge current through the rectifier/regulator .. not the battery.

----- Battery choice .. I think you may find a AGM battery may perform better in clod weather conditions .... see Car and Deep Cycle Battery FAQ, Battery Manufacturers and Brand Names List, and