Hi Pham -
We do not support the use of third party batteries with Smart-UPS and unfortunately there is no way to modify the float voltage on the charger for use with different battery types.
Do a bit of research on PROG mode. I have only just started reading about it, but it seems that it is possible to adjust some constants that are used to detect the battery voltage, so you can make the UPS think the battery voltage is higher than it really is.
In theory this will make the UPS stay on for longer, but it will likely make your batteries go really flat after a power outage (because the UPS shuts down too late) which could shorten their life. It also won't fix the problem with the float charge being too high, so this will slowly boil the water out of your battery and you'll need to top up the water much more regularly than normal.
Also bear in mind that the larger the battery the more current it will draw while it is charging. APC support advised me a number of years ago that the chargers in XL models (the ones with an external battery connector) are rated to provide more current (up to 10x the current of the internal batteries) however the non-XL models (no ability to connect external batteries) are only rated to provide enough current to handle the internal batteries. So if you use your wet lead acid batteries make sure they are equal or smaller capacity than what the charger is rated for or you might overload and burn out the battery charger inside the UPS.
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