Planning on this setup as a 12V supply and backup:

Load connected to 12V 100Ah SLA Battery connected to a charger (Residential Gateway UPS).

Continuous load will be minimal (<0.5A), with an intermittent load that kicks on once daily at 12A for <5 minutes. This comes out to

Charger is a Residential Gateway Backup power supply. Limited data sheet info says it can output 36W max.

Based on my math there is plenty of charge current to keep the battery topped off in that average load < charger output. My concern is that when the higher load switches on, will the battery provide all the current it needs, or will the the load try to draw from the power supply, which may be inadequate?

I've found this, with a trickle charger instead of a UPS, and conflicting answers: Does lead battery + trickle charger = UPS?

My hunch is that it will mess up the charging circuit, but I have turned up nothing conclusive to back this.



1 Answer 1


Your intermittent load is 12A. This is C/8, well within the capability of a 100A battery to supply without breaking sweat. For 5 minutes, it's only 1Ah.

As long as your load can tolerate the change of voltage between 13.8v (or whatever the charger floats the battery at) and 12.something (the voltage when the battery is supplying the bulk of the load) then the load will be supplied no problem.

A dumb charger will have no problems, it will supply a safe limited current into whatever battery voltage is presented to it (as long as it's within normal limits). If it's claimed to output 36W max, then that's about 3A when the battery is loaded. When the battery reaches float voltage, it will take whatever minimal current it needs to.

It's possible (but unlikely) that your charger might be 'smart' enough to get confused by the change of battery voltage that happens intermittently. It would be a pity if it deduced that there was something wrong with it and tried to shut down. It depends who wrote the firmware for it. Check it behaves itself under your loading regime.

  • \$\begingroup\$ There's little to no info on how this particular unit behaves, so I will go ahead and monitor how it handles charging, as prescribed! \$\endgroup\$
    – dogelyfe
    Commented May 24, 2018 at 13:48

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