Home made external battery pack for APC Pro 1500

[comps]

Is it possible to make your own 24v external battery pack and connect to the external connector.

If it is am I right it thinking that the uppermost terminal is positive and the next one down (the middle one) is negative and the bottom one is unused.

I am thinking of using two Yuasa NP12-12P 12Ah batteries in series.

Will the external connection charge them, can anybody see any problems?

The UPS is an APC Pro 1500

[robmir_apc]

You will find several videos explaining how to do it but in short the external connector has 3 terminals: one is positive, one is negative and the third one tells the UPS that a external battery pack is hooked and then the UPS will increase it's displayed reserve time. Basically the external Battery Pack has two 12 Volt 9 Ah hooked in series  plus another two 12 Volt 9 Ah also in series  and then both are hooked in parallel so you will have a external battery pack rated at  24 Volt 18 Ah.   The signal wire  inside the external battery pack as I recall has   50K resistor wired to the negative pole. So instead of using the 12 Ah I would  hook a couple of 18 Ah in series which will give you a lot more of reserve time and will match this UPS charging capability.  Keep in mind  you HAVE to connect the external battery pack signal wire to the negative pole because if not, the ups logic circuit will turn off the UPS  at the factory setting no matter how much charge you still have on the external batteries