That's a well-known problem with APC UPSes using non-APC batteries. The simple physical solution on one of my 1500s (that does not disrupt power to the clients)

That's a well-known problem with APC UPSes using non-APC batteries. The simple physical solution on one of my 1500s (that does not disrupt power to the clients) is to hold down the power button for about 10 seconds. It will beep, then beep again, at which point you can release the power button. If, after a clunking noise, the display still shows the same battery problem, repeat the procedure and it should go away until next time, whenever that may be. I have apcupsd running on a Raspberry Pi via USB and configured it to send a text to my mobile when the battery complaint happens so that I can perform the button hold solution. So far the periodicity is about a month between such events. I dislike having to do all this, but it is not a show stopper.