I'm working on an remote telemetry application that specifies 6 to 8 hour backup time. RTU has a small PLC and a Radio Modem as it's primary loads. Does anyone have any experience in this area? I've used battery backup before, but this is short term (maybe 30 minutes). Would I need more or larger batteries in this case?

Thanks,
Chris
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You need to size your battery for the requirements you have. Determine the TOTAL load that needs to be supplied by your battery for the full 8hr period without supply, and from it obtain the required Ah rating of the battery.

Now whether you put one big battery or a number of small batteries in your system is a matter of choice, physical constraints, costs, etc. Only you, knowing exactly the requirements can determine the optimal choice.

You need also the determine or be told what is the maximum time allowable to recharge your battery. Let me explain. If the periods without supply are not very frequent, then you can recharge your battery at its optimal rate. However if you need to recharge your battery relatively fast, as the loss of supply is frequent, then you would need a heftier battery charger. Keep in mind that your battery charger would also need to supply the load current, in addition to the charging current, if your system is a no break system.