What is a safe max. discharge rate for a 12V lead acid battery?
Ideally the manufacturer supplies the discharge rates on the battery datasheet.

A quick point: You mention you have a 12 V 2.4 A SLA (sealed lead acid) battery, but batteries are rated in amp-hours not amperes. Therefore I suspect you have a 12 V 2.4 Ah battery.

Now that we have that out of the way, a 12 V 2.5 Ah SLA battery from Power Sonic, as an example (a company that has datasheets for their batteries) shows several discharge rates that may be of interest:


Nominal Capacities:

125 mA discharge rate = 20 hours (2.5 Ah)
220 mA discharge rate = 10 hours (2.2 Ah)
400 mA discharge rate = 5 hours (2 Ah)
1.5 A discharge rate = 1 hour (1.5 Ah)
4.5 A discharge rate = 15 minutes (1.13 Ah)

Max Discharge Current (7 Min.) = 7.5 A
Max Short-Duration Discharge Current (10 Sec.) = 25.0 A


This means you should expect, at a discharge rate of 2.2 A, that the battery would have a nominal capacity (down to 9 V) between 1.13 Ah and 1.5 Ah, giving you between 15 minutes and 1 hour runtime.