What is the importance of equalizing charge and why is it necessary to ensure that traction batteries are not used with one or more cells being weak and/or lagging?
A traction battery is made by connecting a number of 2 Volt cells of same Ah capacity in series. Thus a 24 volts battery has 12 cells connected in series and a 36 volts battery has 18 cells Connected in series and so on.
After the cells are assembled and put on charge the manufacturer ensures that the voltages and specific gravitiès of the cells connected in series are similar and within a narrow range prior to dispatch. Such a condition with consistent voltages and specific gravities within a narrow range is possible throughout the life of the battery ONLY IF batteries are maintained properly with periodic equalizing charge. The equalizing charge is a constant charge given to a traction battery after a full recharge. The period of equalizing is 24 hrs. and the constant current to be maintained during this equalizing charge is 3.5% to 5% of the battery Ah capacity.
Over a certain period of usage – probably a few months of usage – the cells sometimes have varying voltages and specific gravities. The reasons for this can be either or a combination of the following:
1) Neglect of equalizing.
2) Irregular levels in cells during top up with water.
3) Loose connection of inter-cell cables due to the improper tightening of bolts in the case of bolt on cells.
4) Variation in cell voltages due to variation in cell specific gravities-probably due to acid addition.
5) Overfilling leading to overflow of electrolyte and consequent variation in specific gravities leading to voltage variation.
Thus we observe that the performance of the cells vary. At times the variation in cell voltages is to such an extent that the lagging cell does not accept the recharge fed in. Over a period of time, this particular cell gets discharged without getting recharged and eventually gets fully damaged & under certain circumstances, it gets reversed.
In such cases, we have one cell not working. Take the case of a 12 cell 24 Volt faulty battery. This 24 Volt battery has 11 cells good with one cell damaged. Hence it is in effect a 22 Volts battery and not a 24 Volts battery.
The power demand from a 24-volt battery if presumed to be 1152 watts it can be surmised that each cell delivers 48 Amps. When the same load of 1152 watts, if a 22-volt battery has to satisfy-the current drawn from each of the 11 cells, shall be 52.36 Amps.
Thus you observe that as and when one cell in a 24-volt battery starts lagging the load remains the same and demands higher output current from the other so-called good cells. Thus the healthy cells compensate by working harder. If such a situation continues a few other cells go down one by one thereby making the entire battery weak and damaged.
Hence it becomes necessary to ensure that no battery is continued to be used with one or a few cells weak. The lagging or weak cell/s have to be replaced immediately with a new cell and the entire battery has to be recharged, equalized and then taken for use.
However, this step is recommended only for new or batteries which are about a year old & not for batteries which are towards the end of life- say 3 to 5 years old.