Although there are limits to what voltage can be placed across a battery to charge it is largely governed by the current that flows through the plates. Too much and they will buckle, melt or even explode.
The first consideration in a Lead Acid battery is it's AmpHour rating commonly referencing what a battery can supply over a 20Hour period. Hence the rating is the 20h rating. For example, a 120AH @ the 20h battery can provide 6amps for 20 hours. So a bulk charge of 12 amp one tenth of the 120Ah would be fine, more may be acceptable, as much as 20 to 30 for a short period if the battery is not flat and has high resistance to a charge.
However the current is usually a by-product of the voltage which in a 12v battery is unlikely to be higher than 16v
The charge at this initial, bulk, level will be set to a voltage and any current that can be put in will be accepted. The low output from a 100w solar panel producing 19v going into a 120Ah battery that is only 50% full will have it's voltage dropped by the battery as a load and will take about 8 amps if full sun is available etc.
A maximum voltage will be set and when the battery reaches that it is assumed to be largely charged, 80% is an often quoted figure but this can be misleading in the case of batteries with a high resistance due to sulfation. Nevertheless at this point the battery charge controller will now reduce the power to the battery, that V x I and keep the voltage at (14.5) say.
The battery is now in what is called an absorption mode and will stay this way until it is 'charged' but see the post on absorption as it is not quite that simple. This was mentioned to end the concept of Bulk charging.
When using a charge controller such as a Victron MPPT 75|15 there is an yellow LED that flashes twice every second to indicate Bulk charging mode. However this does not mean the battery is charging it means the voltage from the solar panels are recognised. The following two scenarios occur:
1. The sun has risen or it's really cloudy and the output from 2 x 250 watt panels may be 0 or 1 watt. This is not enough to maintain a battery but the indicator can be misread as though there is bulk charging. So unless you monitor the charge you could be sufating the battery.
2. The power in is little more that, equal to or less then the load. Again the Victron indicates bulk charging when as above sulfating could actually be occurring.
If the above happens over a day or two in winter or more then damage may be caused that is not recognised. This has been a problem for me until I bought the Victron VE direct cable to not only monitor the charge rate but adjust the absorption voltage to compensate for low charge rates on many occasions
Created by • Last edit by on 07 Feb 2018