I have a scenario where I want to connect an external battery charger to an alarm system's battery to keep it charged fully despite our country's frequent rolling blackouts. The battery is a larger 18Ah than the 7Ah normally used with such systems, and can be charged at a higher rate. Since the alarm panel's charger uses a lower rate, the battery does not always fully charge between rolling blackouts.
How would one connect the external charger to avoid the two chargers interfering with each other? Can I just add a diode to the output going to the alarm system to prevent current flow from the alarm's built-in charger? Would such a diode prevent the built-in charger from giving an external smart charger false state of charge readings?
Bear in mind the alarm system's built-in charger cannot be disabled or disconnected, and its charging current cannot be changed.
Edited for clarification:
The alarm system just has one battery connection (ie: same leads for load and charging).
The proposed solution is a diode from the battery to the alarm system, so that it can draw current from the battery but not charge it. The external charger would be connected directly to the battery. My main concern is whether the diode would prevent the external charger from detecting the internal charger's charging voltage, which might have weird consequences in a smart charger?
The diode voltage drop would not be an issue from a charging point of view, but it would trigger the alarm's low voltage cutout much sooner.