Make it a schotky diode - they only have about 0.3V volt drop rather than the 0.7V of a 1N5401 silicon diode. Rip one out of an old battery-backed psu..?
Why not put a diode in series with the second battery?
That way power to the alarm can flow from the second battery but when the mains returns charge current from the panel will not be able to flow into the second one.
If the mains fails then the relay will close and the external battery will run in parallel with the internal.;
You need to add two diodes. One for each battery to enable it to supply power to the panel but prevent power form the other battery flowing into the battery. The panel's battery charging circuit needs to connect to the terminal of the battery and the diode between the battery terminal and the power intake of the panel's printed circuit board. This may require modification to the printed circuit board.Might even add the diode as well!
Electronics and systems design. My career was 19 years in communications and nine years industrial control equipment. Last 15 years self employed, trying to retire but would be bored if I did.Bernard What is it you do for a living?
Then I thought ...
If you are adding a charger for the larger external battery, why not just disable the built in charger and use a bigger battery with a separate charger ?
Early on in the thread I was thinking "diode"
You'd only need one. The external battery won't try and charge the internal one - the load will pull from both, initially more from the internal until it's charge has dropped a bit due to the volt drop in the diode from the external battery. Since you'd be leaving the second battery permanently connected,
If the mains is on, then the relay is open and the external battery is not connected. If the mains fails then the relay will close and the external battery will run in parallel with the internal
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