Can somebody comment on sizing a solar cell for trickle charging a lead-acid battery bank without a charge controller for several months?
I found a non-authoritative source that claimed that cells with a power of 4% of the C20 capacity of the bank do not need a charge controller. - This corresponds to a charge current of about 0.23% of C20.
As far as I know, the float charge of lead-acid (if done with constant current) is typically done at 1% of C20 but requires the termination condition of dV/dt = 2.5mV/cell/hr.
So where does this 0.23% of C20 come from? Is the assumption that this balances the self-discharge of the cells perfectly?
Since the self-discharge varies from model to model, how does one size this properly? Size too small and the battery dies from deep discharge, size too large and it dies from slow overcharge?
Also: the balancing of self-discharge with a current of 0.23% of C20 does not sound quite right either. Assuming 5% discharge per month would imply a self-discharge current of the order of 0.007% of C20 - much smaller than the recommended 0.23% of C20 for "solar cell trickle charging".
Can anybody "shed light" on this for me?