Why does a lead-acid battery self-discharge?

Have you ever wondered why your lead-acid battery gets discharged even when not in use? And why unused Lead-acid batteries are required to recharge after every few months. Not only flooded lead-acid batteries but even AGM and Gel type lead-acid batteries also get discharged. And the reason behind it same for all types of lead-acid batteries. In this blog, you will get all these answers.

Why does a lead-acid battery self-discharge?

Why does a lead-acid battery self-discharge?

All Lead-acid batteries- even when unused, discharge slowly but continuously by a phenomenon called self-discharge. This energy loss is due to local action inside the battery & depends on the level of minute impurities in battery elements & accuracy of manufacturing process control. A rise in the operating temperature is an external factor which increases the self-discharge loss.

Self-discharge loss is measured by the “Retention of charge test” which checks the amount of charge retained by a battery after freestanding for 28 days at a standard temperature.

The R.O.C test as per IS 15549 for VRLA battery and IS13369 for flooded tubular Monobloc’s accepts self-discharge loss within 10% & 5% respectively, for a new battery.

You can safely ascertain that the quality of material used & the manufacturing process control is good, simply by ensuring that the batteries meet the ROC limits as per the BIS above. A battery with poor R.O.C can have multiple deficiencies (by way of raw material input or manufacturing defects) inside and the self-discharge loss can grow by 3 or 4 times over the period of its lifetime.

How to stop battery self-discharge?

You can’t stop the self-discharge of the battery, however the same can be minimised by storing the battery in a dry room at an ambient [room] temperature of a maximum of 25 degrees celsius and by avoiding high temperatures as well as direct sunlight.

How retain the self-discharge loss of lead-acid battery?

Self-discharge loss of a fully charged lead-acid battery, as explained above can be replenished by applying a voltage of 2.25 volts/cell across the battery, for example, 2.25 Vpc X 24 = 54 volts for a 48-volt battery.

At this voltage, the battery is charged with a current which is the same as the self-discharge loss and the battery remains in fully charged condition. This is known as the trickle charge.

Float charge, on the other hand, is also a trickle charge process where DC power is fed to the electrical load connected in parallel with the battery. Here, a safe voltage for the load is applied, which can be lower or higher than the trickle charge voltage of 2.25 volts/cell.

If the applied voltage is higher than N x 2.25 where N = the number of cells- then the number of cells is to be increased to ensure that each cell does not get a voltage of more than 2.25 volts, since it will overcharge & damage the battery.

A voltage less than 2.25 Vpc is safe for the battery but does not fully compensate for the self-discharge loss hence the battery is to be given a freshening charge at some intervals.

Lead-acid battery self-discharge rate.

The self-discharge rate of flooded lead-acid batteries can be up to 8% per month, whereas for AGM and Gel Lead-acid batteries same can be up to 3-4% per month. Which is almost half as compared to flooded batteries.

Lead-acid battery self-discharge vs temperature

The self-discharge rate of a lead-acid battery is related to the ambient temperature of the battery, the higher the room temperature higher will be the self-discharge rate of the battery. However, the discharge rate of flooded lead-acid batteries is different from AGM and GEL lead-acid batteries. Here, I take two examples to explain the same.

#1 The below graph shows the self-discharge vs temperature curve for a 200Ah flooded tubular battery.

As per the above graph, you can see that at room temperature (that is 25 deg C) tubular lead-acid battery can be self-discharged up to 29% in the time period of 9 months. And at room temperature of 45 deg C, it can self-discharge up to 55% in less than 6 months.

#2 The below graph shows the self-discharge vs temperature curve for a 200Ah AGM battery.

From the above graph, you see that the self-discharge rate of the AGM lead-acid battery is much lower than the Flooded battery. At a room temperature of 20 deg Celsius AGM battery can be self-discharged up to 20%, in the time period of 9 months. which is lower as compared to a flooded battery.

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