I have a 100-Ah lead-acid starter battery linked in parallel (via simple on/off cutoff switch) to another lead-acid starter battery (110 Ah) which I use for fridge etc. I’m going to be obliged to use a piece of equipment, run most nights, which requires at least 30-35 amp-hours, so the 110 Ah starter battery, to be subjected to constant discharge-charge, isn’t going to do the job for long.

My idea is to use a third battery, a lead-acid deep-cycle of about the same capacity as the existing two, and connect it in parallel, using another on/off switch between it and the existing second battery. Thus I can continue to isolate the main (first) battery when the engine isn’t running and, if I wish, isolate the new, deep-cycle from the existing auxilary. I’ve done the maths bit and to supply extra charge and to fit with the way we expect to be using the vehicle, plan to use a 100-W solar panel (the highest capacity I have roof-space for) and link this via a regulator and meter to the new deep-cycle. Ideally, the solar panel could be switched between the existing lead-acid auxiliary and the new, third battery, the deep cycle?

Will this work?

If the panel is rated at 100W then that is probably in ideal circumstances (direct bright sunlight). In that situation you could theoretically produce 8.3 A charge current. Unfortunately that is not possible due to losses / inefficiencies so you are realistically looking at 6-7Amps in ideal conditions. Clouds / shadows etc will also reduce charge current further, if you assumed an average of 5 amps then it would about break even if you are drawing 35Ah (3 Amps). Bear in mind if you draw this load continuously (over 24 hours) you only get say 10 hours of good light per day during the summer to recharge the battery and during that time most of the solar panels power will be going to supply the load rather than recharge the battery. Worse during winter as you have a lot less sunlight. If on the other hand you are only drawing the load for 5/6 hour per night and only recharging during the day then you should be ok (during the summer at least)