Parallel Connection Of Batteries
Ok, for those who have not experienced it first hand, let me reiterate the importance to avoid having lead acid batteries connected in parallel. The absolute maximum strings that one should connect MUST not cross is three in parallel. Beyond this, the batteries experience imbalance charging due to unequal current through individual strings.
Now, the question is whether this strategy applies in all the situations. The answer is NO.
To understand why, let us take a look at the reason why having strings in parallel causes unequal current flow through the strings. The batteries, as we all know, have internal series resistances as do the connecting cables that interlink the batteries in series and in parallel. The batteries in series along with the cables interconnecting them make up ONE resistance. Now imagine having all these resistances connected in parallel with small resistance between them. This is due to the fact that the parallel cables have internal resistance. Albeit small, this internal resistance is enough to form pathways for current flow with different resistances- which leads to difference in current through individual strings. Eventually, when the charging source is removed, the imbalance in string charge leads to imbalance in voltages of strings which causes internal eddy current flows, leading to small charge-discharge cycles – until all the strings reach the same voltage levels.
Charge controllers are designed to charge the batteries in 3 stages, bulk, absorption and float. During the bulk stage, the batteries experience imbalanced flow of current. While imbalanced, the flow of current is ‘into’ the battery; meaning they are all getting charged – just at different rates. Once the battery reaches absorption stage, the voltage is maintained and the charging current falls . The flow of current in the strings also reduces proportionally, but is still charging.
The above phenomena occurs as the voltage applied to the battery bank is removed. It is important to know, in this case, what happens at the controller level.
In float stage, the voltage is dropped to a lower level. This is where ‘trickle’ charge stage kicks in. This charge will solely flow into strings that are charged at a lower levels. Over time, the trickle charge would balance the string voltages out. This way, the current from from higher voltage strings is prevented from flowing to lower charged state and therefore internal eddy currents are avoided.
Having said that, it is still important to note the time horizon of such balancing. Trickle charge happens at considerably lower rates than bulk or absorption charge stages. Due to this, the battery bank will have to remain in the trickle charge stage for a long duration; more the strings connected in parallel, longer the trickle charge duration. For a bank with 5 strings in parallel, a time period of 48 hours is required to achieve noticeable balancing. Here the assumption is the cable used for paralleling is significantly big. With smaller gauged cable, this time period would be longer, much much longer.