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Why is it not recommended to change one or few cells of a traction battery especially when the battery is around 2 years old ?

A traction battery is made up of many 2 Volt cells connected in series. Traction battery is used for motive power.

An electric vehicle viz. platform truck or pallet truck or forklift truck which is battery operated has a D.C. motor which draws power from the traction battery.

The unit of power is Watt. It is a product of voltage and current

Watt = Volt x Amps.

For better understanding let us consider the following example of a traction battery :-

 

Rated battery voltage      :         48 V (24 cells)

Rated battery capacity    :         500 Ah

Considering that a traction battery is rated for 5 hours (C5) a 500 Ah traction battery is expected to deliver 100 Amps constant current for a duration of 5 hours to an end voltage of 1.70 Volts per cell.

The power drawn by the motor from the 48V (24 cells) battery in the normal course shall be

Watt  = Volt x Amps

= 48 x 100

= 4800 units.The motor shall draw 4800 Watts from the traction battery irrespective of the condition of the battery. In case 2 cells of the battery get damaged the situation shall be such that the required power needs to be delivered by 44V (22 cells) battery by drawing a higher current from 22 good cells thereby compensating for the 2 damaged cells.

4800 W

——————    =        109.09 Amps.

44 V (22 Cells)

In such a scenario the 22 good cells delivering a higher current than expected and designed for – get heated up with degradation of cell internal components and adverse effect on cell design life.

If the battery is around 2.0 to 2.5 years old and has been sufficiently used on a daily basis – removing one or two damaged cells and fitting brand new cells as replacement may be done only if the other 22 cells are in reasonably & uniformly good condition.

Such a possibility of 22 cells being good is rare. The battery shall be good only if the battery has been not abused & has been well maintained.

In case the so called 22 good cells are not in a reasonably & uniformly good condition & 2 new cells are put in the battery to replace the 2 faulty cells removed we have a scenario as follows :-

  1. The 2 new cells do not discharge as much as the 22 old cells & remain at a higher voltage compared to the 22 old cells during discharge operation.
  2. This means a lower current is drawn from these 2 new cells compared to a higher current drawn by the load from the 22 old cells.
  3. The 22 old cells delivering a higher current get heated up and get damaged faster.
  4. Amongst the 22 old cells if a couple of cells or more are relatively weak these cells die out. They refuse to perform creating a situation wherein only 20 out of 22 old cells and the 2 new cells working.

Thus you have a situation again wherein out of 48V (24 cells) battery you now have 2 new cells and 20 old cells working. The battery is reduced to a 44 V (22 cells) battery once again.

This saga continues forcing change of cells in a continuous & periodic manner. This does not make economic sense. Hence it is not recommended to change one or few cells of a traction battery especially when the battery is around 2 years old.

Regards 

Ramesh Natarajan

Director

Unik Techno Systems Pvt. Ltd.

J-52/4, MIDC, Bhosari, Pune – 411 026. INDIA.

(+91-20-) 27128881, (91-20) 27111798.

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