Battery backup for high sites - some thoughts

The intention behind any battery backup system is to keep the equipment powered until the AC mains is restored. Lead-Acid batteries are almost always used due to their cost and ability to deliver larger amounts of current.

No battery backup is designed or intended to power the equipment until the batteries run flat – even deep-discharge 12V batteries must not be allowed to drop below 10,5V (or 50% state of charge).
To prevent risk of damage to the battery and erratic repeater operation, a low voltage disconnect must be used. This disconnects the battery from the load when the voltage drops below a specific voltage (≅ 10,5V) - service will be lost but the battery will be saved. If this condition is not acceptable, then alternatives will need to be sought.
A number of customers have reported good results when using the M148 low voltage disconnect from Kemo.

The 31-GEL from Trojan is able to deliver over 85Ah.

The SLR5500 and SLR8000 both have the ability to float charge a lead-acid battery. Neither repeater is able to charge a flat battery since this requires a significant amount of current and charge management circuitry. If this i of concern, the Motorola L1883 or an external battery charging system can be used.

If you are deploying a system that has a risk of losing AC mains regularly, or for extended periods and the impact of no radio service is too great, you should consider the following:

• Increase the battery capacity (i.e. by adding more batteries). This also usually entails upgrading the battery charging system. The Motorola L1883 (Argus) is an excellent choice as it manages all aspects of battery charging and can handle larger banks of batteries (up to 20A charge current).
• Use an AC mains generator. Most generators need anything up to 2 minutes to start so in such cases, the battery provides power until the generator starts. Here, a SCADA system would be a good idea to monitor things like fuel levels; temperatures and voltages (and would give you some control over all the equipment).
• Consider Photo-voltaic cells as an additional power source. In sunny countries, solar panels can provide power to equipment and charge batteries. In such cases, some additional equipment will be needed to manage the input and output volrages.

Estimating the capacity of the battery needed involves knowing how long the battery should provide service for (in the absence of AC mains or any other power source) and the amount of energy each connected device (load) will consume in that time. It makes sense to include some margin of error to ensure service is not lost if mains is lost for longer than expected/predicted.

For example, the SLR5500 will consume 9,5A during transmit - at full TX power - and 700mA during idle time. If the channel loading is 60/35/5 then the repeater will be transmitting for 40% of the time which means the average current consumption will be around 4,2A.
If the battery needs to provide 3 hours of service in a mains fail condition, then the battery should have a capacity of at least 25Ah (or 50Ah if a deep-discharge battery is not used) plus the chosen margin of error.

Further reading

https://batteryuniversity.com has a wealth of information on anything related to batteries.

For Motorola Channel Partners (and employees), the Motorola  R56 (6881089E50) Manual provides an excellent guide to this and many other aspects of high site installations.

You are wecome to post other information sources in the comments.