Very low runtime on low load UPS compared to similar model with high load

[ptr727_apc]

This was originally posted on APC forums on 10/11/2016

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Hi, I have two similar model, one slightly newer than the other Smart-UPS 750's, with NMC2 cards.

This morning we had a short power failure, and one UPS lasted less than 6 minutes, the other the entire outage.

Weird thing is the UPS that ran out of power has much less load..

This is from the two UPS's:

Battery Temperature
33.7°C
Runtime Remaining
5min
UPS Input
Input Voltage
114.4 VAC @ 60.0 Hz
UPS Output
Output Voltage
114.4 VAC @ 60.0 Hz
Load Current
0.7 Amps
Output VA
9.7 %
Output Watts
7.8 %
Output Efficiency
79.0 %
Output Energy Usage
2081.38 kWh
Battery Status
State of Charge
100.0 %
Battery Voltage
27.0 VDC
Next Battery Replacement Date
06/24/2021

Battery Temperature
32.8°C
Runtime Remaining
28min
UPS Input
Input Voltage
113.0 VAC @ 60.0 Hz
UPS Output
Output Voltage
113.0 VAC @ 60.0 Hz
Load Current
1.8 Amps
Output VA
27.9 %
Output Watts
37.0 %
Output Efficiency
95.0 %
Output Energy Usage
3629.90 kWh
Battery Status
State of Charge
100.0 %
Battery Voltage
27.1 VDC
Next Battery Replacement Date
04/04/2043

Notice the 28mins vs 5mins runtime, and yes I see the efficiency is 95% vs. 79%, but the load is 1.7A vs. only 0.7A.

How is it possible that the 1.7A draw will last 5 times longer than the 0.7A draw?

The UPS's are scheduled to self-test every two weeks, no errors reported.

I ran a manual self-test on the bad UPS, and the test passed, but the battery failed.

How is it possible for the test to pass but clearly the battery failed?

10/11/2016 18:07:06 Device UPS: A low battery condition no longer exists.
10/11/2016 18:07:06 Device UPS: The battery power is too low to support the load; if power fails, the UPS will be shut down immediately.
10/11/2016 18:07:05 Device UPS: Self-Test passed.
10/11/2016 18:06:59 Device UPS: A discharged battery condition no longer exists.
10/11/2016 18:06:59 Device UPS: The battery power is too low to continue to support the load; the UPS will shut down if input power does not return to normal soon.
10/11/2016 18:06:58 Device UPS: Self-Test started by management device.
10/11/2016 18:06:56 Device UPS: The battery power is too low to support the load; if power fails, the UPS will be shut down immediately.

Bad battery?

[ptr727_apc · Accepted solution]

This was originally posted on APC forums on 10/22/2016

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I replaced the battery, and runtime is back to normal.

Seems to me that the battery test is not doing its job.

APC, how about it, logs clearly shows dead battery during test, but battery test passes?