Series is not an option unless 12V battery and want to run 6v lights (but wiring and charging would be tricky
)......Best to invest in as many LED lights as you can, I recently added a 6V LED and it works well and from memeory
only draws about ~2A I think the same as 5th Gen one from here;
http://www.dynamoregulatorconversions.com/led-bulbs-especially-for-motorcycles-shop.php Also having a sidecar you be able to use a higher capacity battery (or bank in parallel ) for extra storage for use at low speed, but would obviously need to be recharged with a compensatory higher speed ride after (or trickle-charge overnight)
The trouble with tail-light and pilot bulbs with bayonet bases is they are flimsy and fall apart, but this can be rectified by putting heatshrink over the join...my pilot light lost one of the SMD's so now I need to re-attach the with more micro-solder