Just thought I'd point out a very good PowerPoint (.ppt) file titled, "Care and Feeding of Rechargeable Batteries"

As a short overview, the author has done well.

http://qcwa70.org/careandfeeding.ppt

The article includes technical info, and also information which can help us get the most out of our UPS systems, laptop batteries, etc.

It will also help you determine if a manufacturer or vendor is making overly "optimistic" claims about how long a UPS will keep your computer up and running in the event of a power failure.

In particular, it includes a graph showing the "Characteristic Discharge Curves" (CDC) of Sealed Lead-Acid batteries (a.k.a. Gel-cells) used in many Un-interruptible Power Supplies. This helps us separate the advertising hype from reality.

For example, using the discharge curve I can determine that a UPS that contains a fully-charged 12V 10Ah gel-cell will be able to power my UPS for about 12 minutes...NOT the manufacturer's claim of 30 minutes. When powering one specific computer, this UPS draws 20amps from the gel-cell. That is a discharge rate of 20A/10Ah or 2C.

Following the curve, I can see that the battery will deplete to 10.5 volts in about 12 minutes. (note that the graph times are non-linear)

I chose the 10.5 volt threshold because the "safe" Vmin for a gel-cell/SLA battery is 1.75Volts per cell x 6 cells, equals 10.5Volts. Any lower, and I definitely risk damaging my backup gel-cell battery. If you are conservative and want a decent safety margin, add 0.6volts (0.1V per cell) for a total of 11.1volts. Using that threshold value, the UPS will keep my computer running for just a little over 9 minutes...a significant decrease.

BTW, this particular UPS automatically shuts itself off at about the 10.7V level, which could end up permanently damaging at least one cell in the US$25 battery.

I have compared the CDC chart with several 12V gel-cells at varying capacities and with several xC discharge rates, and this particular chart is in very good agreement with my measured results.

It also accurately predicted the real support time of my UPS.


The article also includes a table that shows the minimum voltage a cell can be taken before damage to that cell occurs, Vmin. For example, you generally don't want to discharge any individual gel-cell cell below 1.75 volts. Remember that in a 12V gel-cell battery, there are 6 cells.

You can use Microsoft's PowerPoint Viewer (included with XP) or freeware OpenOffice to view the entire presentation. I strongly recommend viewing the entire presentation.

For those without access to a PowerPoint viewer, there is also a copy of the "Characteristic Discharge Curves" at
http://www.gamewell-fci.com/datasheets/CS-2500.pdf
In the .pdf document look a couple pages down for the figure labeled
"Characteristic Discharge Curves 1109grf1.tif"
(however, note that in this copy of the graph, the two curves to the far right are mis-labeled as 1.0C and 0.5C when in fact they should read 0.1C and 0.05C respectively. They are properly labeled in the .ppt file)

The 0.05C curve is the "typical" C/20 discharge curve used by manufacturers to label the "Ah" rating of their gel-cell battery.