Description

Alarm and Energizer Battery Backup PSU

The primary challenge with alarm systems and electric fence energisers is not so much the battery type, it’s their charging capacity.

On average, alarms systems charge a battery at 0.6A per hour and energizers at 0.4A per hour. This means e.g. a 12V 8Ah battery connected to an alarm system will require approximately 13 hours to recharge from 100% DOD (depth of discharge) and 20 hours for energizers. An 18Ah battery connected to an alarm system will take approximately 30 hours to recharge. While this is perfectly suitable as battery backup for the occasional power outages, this is clearly a massive problem when it comes to load shedding.

The only way to overcome this problem is to charge the batteries at a higher capacity however alarms and energizers by default do not support this option. The solution is to install a 6.4A backup PSU with an 18Ah Gel battery and a crossover relay to switch between the external and internal batteries when required.

The alarm or energizer will run off this external battery and only once the 18Ah Gel battery runs flat, will it switch over to the internal 7Ah or 8Ah SLA, Gel or Lithium battery. The combination of 26Ah between the internal and external batteries will provide a substantially longer backup period and also not deplete the batteries during normal load shedding schedules. If e.g. your system uses 1.25amp per hour, an 8Ah battery will last 6.4 hours (8 / 1.25 = 6.4) therefore 26Ah (8+18 two batteries combined) will provide 20.8 hours of backup.

The  6.4A backup PSU charges at 3A per hour meaning that it can charge a 7Ah battery from 100% DOD in 2 to 3 hours (vs 13 hours) and an 18Ah battery in approximately 6 hours (vs 30 hours), that’s 5 times faster than an alarm system and 7.5 times faster than an energizer. However if e.g. your battery is only drained to 50% DOD during load shedding, then only half the charging time as stipulated above is required to reach full capacity. Allowing batteries to fully recharge between power outages substantially prolongs the battery life and has more than enough capacity to keep systems running during our load shedding schedules.