How does a car battery go 'flat'

[Sparky007]

I know the alternator recharges the battery, but if that's the case then how does a battery go flat? I don't get it?!

[Selectron]

When you switch the engine off and park a vehicle, current continues to flow from the battery to several of the vehicle's circuits. Those circuits would include things such as the alarm system and alarm-set indicator LED, the clock, radio presets memory, engine control module memory, etc. That's known as quiescent current drain and would typically be around 30 milliamps (30mA or 0.03A). I've measured my own car at a steady 23mA, pulsing briefly up to 45mA when the alarm LED flashes every few seconds, so I guess that's an average of around 30mA, although on some vehicles it might be as high as 50mA.

For a car which is driven daily, that's a negligible current drain, and the lost charge will be replaced (by the alternator) within minutes of starting the engine. Problems can arise though when a vehicle is parked for extended periods, in which case the cumulative drain over time has to be considered. Batteries have an ampere-hour (Ah) capacity rating which is specified by the manufacturer - mine is rated at 60Ah. If my car draws 30mA when switched off and parked, then over a twenty-four hour period the battery would lose 0.03 x 24 = 0.72Ah of capacity. If left parked for a week (which mine often is) then the battery would lose 0.72 x 7 = 5.04Ah, and over a two-week period it would of course be double that, so 10.08Ah of capacity would be lost, so after two weeks my 60Ah battery would have only 60 - 10.08 = 49.92Ah capacity remaining. It isn't unusual for my car to sit unused for periods of three weeks so this is something which I have to be aware of - it's a diesel engine (so a high compression ratio) and my starter circuit is not in great condition so although it will crank readily after two weeks, I know that after three weeks the cranking would be sluggish and the engine might not even start, especially in winter.

All batteries lose capacity as they age, so even a car which is driven daily can have problems if the battery is old and far below its optimum condition and no longer able to hold a reasonable charge. Other factors which can contribute to a flat battery would be, for example, if the vehicle has a fault resulting in excessive current drain, or if the alternator/voltage regulator assembly isn't providing full output voltage, or if the wiring between alternator output and battery, or the associated grounds, are in poor condition.

Another thing to be aware of is that lead-acid batteries do have a very low level of self-discharge - it's so low that it's normally negligible but it does become a factor if a battery is to be stored (out-of-circuit) over a period of many months or longer, in which case an occasional top-up charge or a continuous float charge at an appropriately low voltage level would be required if the battery was to remain in good condition.

The Car and Deep Cycle Battery FAQ website is worth reading, for anybody who has an interest in such matters.

[jdmccright]

Sorry, but batteries go dead...soda and tires go flat.

A battery can discharge slowly while the car is not driven when some accesories (radio, clock, alarm) draw current even when the key is off. As mentioned, it is very small but usually not enough for the average vehicle to regenerate during the typical drive time.

They can also go dead from age, shock/vibration, and severe thermal cycling. A short can form inside one of the battery's cells, a plate can break internally, or the cells can run dry (if not a maintenance-free type).

Hope this helps!