Abstract: An automotive battery charge indicator uses an algorithm expressed by fixed component values to determine if the remaining charge in the battery has fallen below a predetermined threshold. A current sensing resistor (8) (may be a Hall effect sensor or a discrete component having a resistance of the same order of magnitude as the internal resistance of the monitored battery (1)) is connected in series with the positive terminal (2) of a lead acid battery (1), and the positive side of a non-critical load (4). A percentage of the voltage across the load detected as input (6) is compared by comparator (11) with a dynamic reference voltage at input (5). The dynamic reference voltage is made up of a fixed reference voltage (7) less the voltage drop through current sensing resistor (8).

Abstract: A battery monitoring system includes a circuit for use with a battery which is one of a defined (quantified re volts supplied at known current loads at known discharge levels) type of battery such as the lead-acid batteries used in vehicles. Preferred embodiments monitor at least the battery voltage under load while measuring the drawn current and use stored numerical information in order to determine whether the battery has been discharged down to a predetermined limit; one which generally retains enough charge to perform a defined act such as starting an engine. If the battery is so depleted a non-critical load (e.g. accessories) is disconnected by a switch. Changes in the non-critical load are monitored and the non-critical load is reconnected for a predetermined period when a change is detected in order to start the engine.