Most people (especially in the northern countries) have had the extremely annoying experience that in some (usually somewhat colder than average) morning the car will not start, because there is not enough charge in the battery.

The battery is a critical system component of a car. This is especially true in winter, because the colder the weather a) the more viscous are all the lubricants and thus the more difficult it is to make the engine revolving and b) the higher is the internal resistance of the battery and thus the lower is the maximum current that can be produced during starting.

Usually battery problems do not emerge suddenly but develop gradually. Therefore, by some simple measurements and monitoring of the battery state the probability of battery failure at some critical moment can be significantly reduced. What is needed is a voltmeter capable of showing reliably voltages in the range of 10 to 16 volts with 0.1 V readability and accuracy preferably not much worse than ±0.1 V. Such devices are usually available as multimeters and are able to measure also current and resistance (and many more things in the case of sophisticated models).

Dependence of the open-circuit voltage of a 12 V lead acid battery on its charge state

A most important property of the lead acid battery (which makes it very different from the lithium ion battery used to power many electronic devices) is that it should be kept fully charged all the time and never be fully decharged. Keeping battery decharged for some time will irreversibly reduce its capacity and the maximum current that it can deliver during starting the car. This happens because of a process known as sulfation. Very important for keeping the battery charged is the ability of the car’s generator to supply sufficient charging voltage when the engine runs, so that after the partial discharge of the battery during starting the engine the battery would be brought to the fully charged state as soon as possible.

Thus, to be in control of the situation, two measurements should be made from time to time:

1. Measuring the open-circuit voltage of the battery. This should be done without engine running and without any electric devices switched on. This voltage for a fully charged battery is 12.6 .. 12.8 V. If this voltage drops below 12.4 V sulfation will slowly start. The image on the right shows the dependence of the open-circuit voltage of a 12 V lead acid battery on its charge state (source: Wikipedia). If after a ride of, say, 2 hours the open-circuit voltage is not within the range indicated above then there is a strong reason to suspect that the electric generator of the car is not in order.

2. Measuring the charging voltage when the engine is running. This voltage should be in the range of 14.0 .. 14.4 V. This measurement should be done with headlights on and engine running at ca 2000 rpm. Value below 14.0 V indicates insufficient charging. Value above 14.4 V indicates overcharging. At such high voltages the water in the electrolyte is slowly electrolyzed and the electrolyte volume decreases. Charging voltage significantly higher thatn 14.4 V can damage the electric system of the car.

If significant deviations of the above voltages from the specified regions are discovered then a specialist should be consulted.



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