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This is specifically model BE500U but the same control board is used in many of their similar UPS.

I am trying to understand how these UPS devices detect their battery is present. It seems they are checking battery voltage as well as something else. Why do I say this?

Because I have been testing the circuit. If I connect an adjustable power supply to the battery terminals, I can make the UPS work with just the power supply. However if I place a diode in series with the positive lead of the power supply then the UPS will go into an error mode flashing the LED red and not run off the power supply. (Additional info: the diode was so that the UPS cannot back-feed battery charge power to the Power supply).

So I’m trying to determine why the diode causes issues as it does not prevent sensing the battery voltage. Anyone familiar with these devices and circuits? I can provide pictures if needed.

I’m sure I’ll be asked why this matters and what I’m doing, so I’ll answer. I’ve got a bunch of these UPSs with bad batteries and have a bunch of Lithium cells so am working to convert them to Lithium. I have a relay with regulator circuit and BMS to provide proper charging to the Lithium. I just need to figure out this issue and will be able to proceed.