The most important part of the charge cycle is the absorption charge. Since the bulk charge only recharges the battery bank to an 70-80% level, the absorption charge completes the charging cycle.

If the absorption charge is not completed fully a part of the battery plates will not cycle and are likely to become sulfated, which in turn leads to shorter period of bulk charging until the absorption level is reached. If this is done often then the battery will loose capacity and it can be quite dramatic in that after a few months it may be reduced to 50% capacity. One process to remove sulfation is to over-charge or use an equalisation voltage once the battery appears charged.

Some chargers have a timer that allows the user to adjust the duration for the required time to return the battery to full charge. In order to set the correct time, a simple calculation is required. With the help of the 20 AH capacity, you can figure out the remaining charge required for the battery bank using the following equation:

If the actual absorption time is significantly less than as calculated below then the likelihood is that the battery has reduced capacity.

If a charger measures the standing voltage, for example after 12 hours of no load, it may adjust the time of absorption noting the battery is nearly fully charged. This again can be misleading as a sulfated battery not only will have reduced capacity but a higher resistance and so the voltage will appear high when there is little charge in terms of capacity.

t = 0.42 x (C/I) and with an 80% charge

Where:

t = Absorption Charge Time (Hours)

C = 20 hr Rated Capacity (AH) [ex: 2 strings x S-550 models (428 AH) = 856 AH rated capacity]

I = Charging Current (Amps) [charger output min 5% up to max 10% of 20 hr rate]

**Example** For a single 120Ah battery charging at 12amps that equates to 0.42*(120/12) which is 4.2 hours and I doubt I reach half of that. Given that the recommended voltage is over 15v I have developed quite a problem maybe using 14.5v Oops!

Good info on https://en.wikipedia.org/wiki/Lead%E2%80%93acid_battery :: wikipedia Lead acid battery

As the batteries are 120Ah the min and max currents are 6A and 12 (5 and 10)%

The MPPT controller gives a max of 15A so it is suited to only charge 2x 120Ah at 7.5A each.

This equates to 0.42*120/7.5 = 6.72 hours