I see this as an absolute win. Be careful, folks. Just because it’s DC doesn’t mean it won’t cause serious damage.
Edit for Clarification: When done correctly, the batteries should not arc. My problem is I did not wire the array correctly the first and a-hem second times. It only cost me one battery, which is a lot cheaper than a trip to an American ER.
UPS batteries are really dangerous because you cannot turn them off. They are also high enough voltage and current to harm you through arc flash and burns.
The other thing with small UPS batteries is that there are usually multiple batteries. There are fuses between the battery string and the circuit board but not between each battery in the string.
OP may have connected batteries wrong or he put a new battery in a string with a shorted/dead battery.
Whenever you are going to string multiple batteries together always disconnect batteries and check voltage on each separate battery to make sure that they are the same or very close in voltage. If different charge to the same level. Any difference in voltage between the cells will cause current flow that can cause a fire hazard.