Hasan -

Auto starter batteries in your app are unlikely to last more than a few months.
If that's what's available I suggest adding batteries in parallel to
increase the
A/H capacity. Starter batteries discharged below 15-20% capacity will have
short and unhappy lives.

Adding batteries in parallel is not a good solution - Meticulous,
ongoing attention
is necessary to connections - Short, heavy gauge, and clean...

Good luck.
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