I've gotten several direct messages about this, so let me go into some more detail?about what happens when you use a "smart" AGM charger, an AGM battery, and a DC-powered repeater system powered off that:
Bulk Charging - Slower, or never finishes if your charger isn't sufficiently oversized
Right after install or after a power outage where the battery got drained, the charger enters "bulk charge" mode. The charger is running in constant-current, supplying its maximum charge current, and watching the DC bus voltage to see it rise to the charge setpoint.. But the current is actually being shared between the battery and the repeater gear. If your repeater isn't that busy, it is probably only drawing an amp or two to run the receiver, controller, any accessories... and so that just extends the bulk?charge time a bit. But if the repeater is transmitting a lot, you'll stay in bulk charge for a long time... and if you haven't sufficiently oversized your charger, you'll essentially never finish bulk charging.
Absorption Charging - Overcharges the battery some (if there's a timeout) or significantly
Then once the voltage rises to the bulk charge setpoint, it switches to "absorption" mode, which is a constant voltage mode (depending on the charger, there may also be a current cap) and then the charger watches for the charge current to drop to (nearly) zero before it switches to float mode. But with even just the receive load of the repeater, it'll never get out of this mode, because it can never see the charge current drop low enough. Meanwhile, if there is a current cap on the charger of less than your transmit draw, any time you're in transmit you're actually draining the battery. And if that happens, you'll cycle the battery far enough that the voltage drops low enough for the charger to switch back to bulk charge mode. Some chargers do have a time limit on absorption mode, and so they'll drop out to float mode after a timeout... but odds are that the battery will have been overcharged for at least part of that time period. If the charger doesn't have a timeout, and many don't, you'll for sure overcharge the battery as it sits in constant voltage absorption mode forever, leading to offgassing, heat damage, etc.
Float Charging - May never enter this mode, but if it does, may end up cycling the battery
Finally, if by some chance the charger does make it to "float" mode (see above for why that either won't happen or will happen too late), the voltage is reduced to the float level and the charger runs in that (lower) constant voltage mode. As long as the charger doesn't also impose a current limit on float mode, it'll just sit here, with the charger mostly supplying your gear (after having damaged your battery in absorption mode). If it does have current limiting in float mode, which some of the "smartest" ones do,, then every time you exceed that current (e.g., while transmitting), you're draining the battery, imposing yet another cycle on the battery until the voltage gets low enough to trigger it back into bulk charge mode.
Solution
The only way around this is to use an AGM charger that has full load compensation.. The ones that try to monitor and model the voltage slope to do load compensation don't do a great job with intermittent loads like transmitters cycling on and off (they're designed for the?"charging an RV battery while the lights are on" kind of use cases), so what you really need is a charger that has a separate current shunt monitoring the load side current. These exist, mostly for the off-grid solar power industry, but they're a lot more expensive and complicated to wire up than the "battery-specific charger" that you almost certainly got recommended.
Matthew Kaufman
