Back-UPS CS stays off after power outage

[BillP]

UPS in question is a Back-UPS CS 500 with brand new replacement battery. During power outage, the UPS powers the connected equipment for a long time but eventually turns off as the battery is depleted. When power returns, the UPS will not turn on automatically! The UPS has been configured (with Powerchute) to mute beeps while on battery but I cant find any setting affecting the power-on after outage. Is there a defect in the unit? It is not new but is working perfect otherwise. Though totally unfit for its purpose in this state.

[BillP]

Hi Jouko,

Have you found this to happen during normal operation or through your own operational testing? 

Secret Squirrel

[BillP]

This has happened now twice during "normal operation", meaning extended real power outages. I have not had a chance to test it myself by unplugging the unit. The unit is now at a distance, with all protected devices powered off. And I am wondering if there is anything to try as I will need to drive there or if I should just bring in a replacement UPS.

[BillP · Accepted solution]

All APC UPS are designed to turn back on following a power outage once good working utility power is observed by the UPS. There are some instances however when a UPS may not return to full operation upon the return of utility power, if the utility power does not meet or exceed the hysteresis threshold for operation. For every transfer point programmed in to the MCU of the UPS, there is an alternate point in which the UPS will return from this transfer function. The two transfer points may vary by as much as 6-8volts in some situations. This variance is referred to as hysteresis.

So for example, if the low transfer point of a unit is 103vac and the utility drops below 103vac, the unit will transfer to battery power. If utility power returns to lets say 104vac, one might expect the unit to return to normal operation. This however is not the case, due to the hysteresis. The hysteresis creates a virtual window of operation in which utility power must return before the UPS will return to normal operation. The purpose of this hysteresis is to keep the unit from toggling between states of operation when observing slight fluctuations in utility power. 

Secret Squirrel