I have an APC UPS (ES 550G). If I turn it off for any reason, it will not power back up unless I unplug it, open it, disconnect the battery, and reconnect it. The software does't indicate anything wrong. I was about to order a replacement battery, but now I'm wondering if there is not a problem with the unit itself, and maybe I should replace it? Apparently power supply problems do happen with these units. Thanks.