CHARGING METHODS - CONTINUED
Stand-by/Back-up use (Trickle use) The application load is supplied with power from AC sources in normal state. Stand-by/back-up use is to maintain the battery system at all times so that it can supply power to the load in case the AC input is disrupted (such as a power failure). There are two methods of charging for this use.
Trickle charge (Compensating charge)
In this charge system, the battery is disconnected from the load and kept charged with a small current only for compensating self discharge while AC power is alive. In case of power failure, the battery is automatically connected to the load and battery power is supplied. This system is applied mainly as a spare power source for emergency equipment. In this use, if rapid recovery of the battery after discharge is required, it is necessary to consider the recovery charge with a comparatively large current followed by trickle charge, or alternative measures. While the type and capacity of the battery is determined by the back-up time and the load (current consumption) during power failure, some reserve power should be taken into account considering such factors as ambient temperature, capability of the charger and depth of discharge.
Trickle charge system model
(Precautions on charging)
As the battery continues to be charged over a long period, a small difference in charging voltage may result in a significant difference in the battery life. Therefore, charge voltage should be controlled within a narrow range and with little variation for a long period.
As charge characteristics of the battery are depen- dent on temperature, compensation for temperature variation is required when the battery is used over a broad temperature range, and the system should be designed so that the battery and the charger are kept at the same temperature.
Float system is the system in which the battery and the load are connected in parallel to the rectifier, which should supply a constant-voltage current.
Float charge system model
Power detection relay
In the above-illustrated model, output current of the rectifier is expressed as: lo = lc + lL where lc is charge current and lL is load current. Consideration should be given to secure adequate charging because, in fact, load current is not constant but irregular in most cases. In the float system, capacity of the constant-voltage power source should be more than sufficient against the load. Usually, the rectifier capacity is set at the sum of the normal load current plus the current needed in order to charge the battery
Sealed Lead-Acid Handbook, Page 22