How do I know how much power is left on my external battery?

Question

Our IT consultant persuaded us to add an external battery pack to our dying UPS, rather than buy a new one.

The UPS still counts down from about 4 minutes after the AC power is removed, but continues to power-up the equipment for at least another 30 minutes from the external battery, after reaching zero.

However, the display panel remains at 0% and we have no way of knowing how long the external pack would last us.

The IT consultant told us the same as what we have been told by the manufacturer; that this is by design! (As in documented bug are features.)

Are there any methods to measure the external battery's remaining juice?

Answer 1

Open and test it. An electrical battery discharger can be used to determine battery capacities and to help identify defective batteries or cells in battery sets.

Laptop/PC Interface Lester Electrical battery dischargers can be connected to a computer via the RS-232 serial port to display in real-time, analyze, and archive discharge data.

Simple to Operate To begin a discharge cycle, you connect the two alligator clamps to the battery or battery set and press the Start button. Real-time data is available on the display. The discharge cycle will automatically terminate based on battery voltage or time, which are user programmable.

Answer 2

If you have a need to know with confidence how much power remains, you could use a digital voltmeter to read the voltage of the external battery (assuming it's lead-acid or gel cell, as most UPS batteries are). Generally lead-acid chemistry is 2.2 V per cell when fully charge, dropping to about 2.0 V per cell when "discharged" -- discharging to lower voltage can reduce a battery's recharge cycle count. This won't directly give you working time, but the voltage curve on this type of cell is almost linear, so you can read the range as a percentage of remaining capacity -- i.e., 2.1 V per cell is about 50% charge.

As others have said, you shouldn't depend on the UPS to continue working, but use the time it gets you to perform an orderly shutdown, so you don't wind up with damaged filesystems, lost data, or possibly even hardware damaged from an abnormal shutdown. The time it takes to restart after a shutdown is nothing compared to the time to recover after a filesystem gets partially randomized by open files or an interrupted write.

Answer 3

As stated in the Wikipedia entry on uninterruptible power supplies:

The on-battery runtime of most uninterruptible power sources is relatively short (only a few minutes) but sufficient to start a standby power source or properly shut down the protected equipment.

You state:

However, the display panel remains at 0% and we have no way of knowing how long the external pack would last us.

The IT consultant told us the same as what we have been told by the manufacturer; that this is by design! (As in documented bug are features.)

A UPS is generally meant to provide just enough power to shut down the attached equipment in a controlled manner, so the “…is by design” is a fully correct answer, to be interpreted as “You get the specified amount of time to shutdown your equipment, with guarantee.”

The fact that the UPS has capabilities beyond that, is your guarantee (the safety margin, crumple zone, …), and nothing that you should try turn to advantage in other ways.