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.

  • \$\begingroup\$ you could put a diode from each charger to the positive side of the battery. Anode on charger and cathode to battery. ie. pointing at the battery. \$\endgroup\$ – Aaron Feb 25 at 16:59
  • \$\begingroup\$ @Aaron, good suggestion, but make that a diode with a (very) low voltage drop, such as a Schottky diode. Otherwise your battery will never charge fully. Also, I think the builtin charger cannot be disconnected to allow a diode to be inserted in its path. My guess is that the builtin charger is a simple one that only provides a “float” voltage and does not take into account the state of charge, but I might be wrong. \$\endgroup\$ – StarCat Feb 25 at 19:31
  • \$\begingroup\$ Even a low drop diode such as a Schottky can significantly change battery charging performance. With a diode you may perhaps) vever receive a proper "topping"/boost charge and battery capacity and longevity may suffer. A low Rdson FET arranged to act as a controlled diode can have negligible voltage drop in such applications. \$\endgroup\$ – Russell McMahon Feb 25 at 20:47
  • \$\begingroup\$ Since the diode would be between the battery and the alarm's built-in charger (which I'm blocking) I'm not worried about the voltage drop. The battery will still get the external charger's full voltage. Are there any other reasons why this arrangement would not work? Would the built-in charger confuse the external charger's logic despite the diode? The only other issue I can think of is the alarm's low voltage cutout would be triggered sooner due to the diode's voltage drop. \$\endgroup\$ – Thunderbelch Feb 26 at 11:55

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