Flat batteries usually happen at the most inconvenient time, in the most inappropriate place and when the weather is being totally disagreeable. In fact, the battery is probably the most unreliable component in a modern vehicle. To alleviate this problem, some battery manufacturers incorporate a backup unit within the same case, to allow the vehicle to be started if the main unit fails.

A car battery can only deliver peak performance if it is properly maintained. This not only involves keeping an eye on the electrolyte level but also ensuring that the charging voltage operates within strict limits. That means a charging voltage of 13.8-14.4V for a 12V battery, or 27.6-28.8V for a 24V battery.

If the battery voltage never reaches 13.8V, then either the charging voltage is too low or the battery is on the way out. This means that the battery will be marginal when it comes to delivering the necessary current during starting, particularly in cold weather. Conversely, if the battery is being overcharged, the electrolyte will gas excessively, leaving the plates dry and reducing the battery’s amp-hour (Ah) capacity.

This can not only dramatically shorten the life of the battery but in severe cases (eg, if the voltage regulator has failed) could damage various electronic equipment in the car.

So how can you be sure that your car’s battery is being properly charged and that it is in good condition? The answer is to build and fit this Digital Voltmeter. It monitors the voltage across the battery terminals and thus provides an accurate indication of the charging voltage. It also indicates how well the electrical system and the battery cope with extra loads such as lighting, fans and audio systems.

In addition, an accurate voltmeter can quickly indicate the overall condition of the battery. For example, if the battery voltage regularly drops below its nominal value of 12V (eg, when the engine is idling or if the engine has been turned off for some time), it indicates that the battery is unable to maintain a charge (assuming that the charging system is OK).

Main Features Compact case. 3-digit LED display with automatic dimming. 12V or 24V operation. Optional remote voltage sensing. Minimum hold voltage display.