Repeater Battery Backup

The intention behind any battery backup system is to keep the equipment powered for short periods of time until the AC mains is restored. No battery backup is designed or intended to power the equipment until the batteries run flat (i.e. <2V/cell). 

The SLR5500 has a built-in battery charger that allows it to connect to a 12V backup battery. When AC mains is present, it can deliver a charging current of up to 3A. This is sufficient to charge a flat 10Ah battery in around 12 hours. A 10Ah battery is sufficient to power the repeater for 3-4 hours at 10/90 duty cycle at 50W.

The charger cannot be used to charge dead batteries or batteries where the voltage has dropped below 2V/cell. A 12V car battery has 6 cells so 2V/cell would be 12,0V. It also cannot be used to charge anything other than lead-acid batteries.

The charger will provide a maintenance charge when the battery is full.

The repeater can sometimes operate when the battery supply drops to 11V but this is not good at all, as such a low voltage will damage the battery (sulphation) and will cause erratic operation. To prevent the battery from being discharged too much, a low voltage disconnect should be used (if an external charger is not used). If the battery runs too low, it will be disconnected - you will lose communications but the battery won't be ruined (and will be ready for action next time). Some of my customers have used the M148 low voltage disconnect from Kemo with good results.

If you are deploying a system that has a risk of losing AC mains regularly or for extended periods, you should either increase the battery backup system capacity to support normal operation in such conditions. In some cases, it will be necessary to install a backup generator that can power the RF site when mains is lost for longer periods.

The SLR5500 is also capable of running on 12VDC only. This is useful for installations where mains is too unreliable or impossible to install. In such a case, the RF site would run completely off 12VDC derived from solar and/or wind energy.

In this case, DC Operation Only and DC primary Source should be ticked. Ticking these options will disable the charger as these two settings imply an external charger is in place.

The Motorola R56 Standard (Manual) has some useful information on this topic. There is also a lot of information on the CADEX Batteryuniversity.com website.