Battery Capacity Testing Questions

[steve1515]

I have two of the following batteries that I would like to test for capacity. ( http://www.klb.com.tw/dbf/WP7.2-12.pdf )
I have an electronic load that let's me do this and looking at the data sheet, I figure that I would set it to draw 1.224A until the voltage drops to 10.20V. This should take 5 hours if the capacity is at 100%.

My question is... Wouldn't this ruin the battery? I always thought that on a 12V SLA you shouldn't go below something like 12.25V. 

How would I test these batteries and not ruin them? Maybe bringing them down to 10.20V just a couple of times would be ok? Can someone please clarify the correct way to do this?

Thanks!

[Psi]

1.75 per cell is the safe no-load cut off point.  So that 10.5V for a 6cell SLA.

However, due to the internal resistance, if you measure the voltage while discharging you're not reading the actual cell voltage.

So, either you disconnect the load every so often and check for 1.75V or you use the general rule of.. 1.5V cell cutoff for 1C discharging (so 9V)
(google for SLA discharge cutoff graphs to find the voltages for other discharge rates).

As long as you recharge it to full as soon as the testing is complete and stay above 9V (while loaded above 1C) you shouldn't damage the battery.

[steve1515]

Thanks for that info!

Now, looking at the spec sheet 1C is 7.2A. Should I test at that or one of the other discharge rates?

[Psi]

To confirm that capacity matches the rated capacity you should discharge at 0.2C as this is usually how the capacity was rated at the factory.
Edit: looks like your battery's capacity is rated for 20H discharge at 360mA

However for a more real-world capacity rating you should discharge it at 1C or whatever current the battery is usually discharged at based on application.

[digsys]

Now, looking at the spec sheet 1C is 7.2A. Should I test at that or one of the other discharge rates? 

That's up to you and the intended use. The cutoff capacity will decrease as your test current increases.
IOW, IF you intend to drain it at say 1C, then you may only get 80% capacity out of it, even though you can pick up
the other 20% by dropping to C/10 for example. A rest period, at say 1C, will also give you a bees dick more.

[steve1515]

Ok. That sounds good. I was just a little afraid to drop the voltage down so low. I wasn't sure if my testing would permanently kill the batteries. Looks like I know what to try out now. And at 1C I can test faster! 

[Psi]

For comparison, it looks to be about 50-60% of rated capacity at 1C discharge on a brand new battery. (reading the datasheet)

In my experience with SLA batterys, if you get to the point of wondering about the capacity then it's probably stuffed.

[steve1515]

These batteries are pulled from an APC UPS battery pack. My UPS's use two of these types of batteries in series to get 24V. I believe that when the pack goes bad, often times it's just one of the batteries that needs replacing and the other is still good. I figured that I could test them out and see how good or bad they really are.

[MarkL]

In a series string, it's generally not a good idea to mix batteries of differing ages.  The higher internal resistance and other possible cell failures in the old battery can lead to reduced capacity and premature failure of the new battery.  And usually, when one battery is at the end of its service life, the other one is not far behind.  It's always better to replace them as a set.

If after testing you still have significant capacity left in one of the batteries, I would use it up in a different 12V application.

[digsys]

+1

[steve1515]

...
It's always better to replace them as a set.

If after testing you still have significant capacity left in one of the batteries, I would use it up in a different 12V application.

Will do. That sounds like a good idea. 

[SeanB]

Getting close to UPS battery change time again. Means I will have 144 7Ah batteries to play with.

[calexanian]

Getting close to UPS battery change time again. Means I will have 144 7Ah batteries to play with.

Ring Launcher!!!!!!!

[SeanB]

They ripple down to other UPS's and to things like alarms and such. A lot are dead though so most go to the scrappie.