FollowupID: 757474 Submitted:
Tuesday, Apr 03, 2012 at 08:03
Battery Value Pty Ltd posted:
both negative and positive electrodes inside a cell carry some charge.
In order to increase the charge in both cells, an 'overpotential' needs to be applied - otherwise no charging current would flow.
That's why charging voltages are always higher than discharging voltages.
In a wet cell battery, the overpotential is split evenly between the two electrodes.
The moment you drive charging current through the cell (even small currents), overpotential starts to form on both electrodes.
The formation of overpotental either means the cells increase their stored charge, or the electrolyte starts breaking down towards the end of the charging process (gassing).
Now looking at VRLA batteries, there's this additional gas recombination cycle which disturbs the even distribution of overpotential between the electrodes.
The gas recombination happens along the negative electrode. Because hydrogen forms the building blocks of this electrode's overpotential, and it gets eaten away by oxygen ions drifting over from the positive electrode, the overpotential on the negative side will always be lower than on the positive electrodes.
At very low float charging currents, there aren't enough hydrogen ions left to form an overpotential at all.
And an electrode losing its overpotential during float charging has the same effect as discharging it.
One way to ensure this can't happen, is to keep the float charging current high enough to guarantee at least a small amount of overcharge on both electrodes.
In a VRLA 12V battery, sufficient float charging current can be assumed when the battery terminal voltage is between 13.6 and 13.8V.
The steady state current at this voltage is age and temperature dependent, and can be as high as 100mA in a 100Ah battery.
A 150mA rated solar
panel supplies less than 100mA for most of the time, allowing negative plate sulphation to sneak in.
And because sulphation doesn't happen in all cells at the same rate, cell imbalances will form in the 6 cell series string, which is bad news in itself.