Why you shouldn’t let your battery go completely flat

If a car battery goes completely flat, can it always be fully recharged? Livingstone

No. If a lead-acid battery is “completely” discharged, even for a relatively short time, it will be damaged.  How seriously, will depend on how long it is left in a discharged state.

If that time is relatively short (a few hours or days) the battery will probably respond to recharging and work “normally” again, but to at least some extent its life will have been shortened and its ability to store electrical potential will be reduced. 

If left completely discharged for a longer period (weeks or months) it could be “dead” – the reversible chemical reaction that enables to it generate electricity can no longer take place. Either not fully, or not at all.

Remember that a battery does not store electricity.  It stores a chemical mix that has the “potential” to generate electricity when power is demanded by switching on an electrical motor (starter, wipers, horn) or other instrument (lights, radio).  

When a battery is new and fully charged, the chemicals are primarily pure lead plates in a bath of dilute sulphuric acid. When power is demanded (by turning on an electrical gadget to compete a circuit between the positive and negative poles of the battery) a chemical reaction between lead and acid is triggered, generating electricity and progressively turning the battery’s contents from pure lead and dilute acid into lead sulphate and pure water.

This chemical process is reversible – if you put electricity “in” instead of taking it “out”, the sulphur goes the other way, restoring the plates to pure lead and turning the water back into a dilute acid.

Charging and discharging is rarely an all-the-way process because the car has a mechanical generator/alternator which goes into action as soon as the engine starts.

The starter initially takes electricity from the battery, but as soon as the engine is running the alternator starts putting puts it back.  The voltage regulator manages that process, using the battery and/or the alternator based on the balance of supply and demand.

So, in normal use, the battery is almost always nearly fully charged – any sulphating is minor and brief; the chemical reactions continue at full capacity indefinitely.

If, for whatever reason, the battery is heavily discharged when the alternator is not running (prolonged hard starting, leaving the lights on when the engine is off and such like) the lead plates become heavily sulphated. 

And if they are left in that state for a long period, the “reverse” reaction of recharging will be both slower and less complete. The battery will no longer generate full voltage, and its potential capacity is permanently reduced.

This in turn will make starting less efficient and – in the manner of a vicious circle – progressively increase the sulphation that caused the problem in the first place.

In extreme circumstances, or eventually, the battery will not recharge at all.

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