What we have learned about Batteries

[galewind]

Now, this is newsworthy in and of itself, though I'm not sure how much of it is true.

*Apparently* FIRST is moving away from the Exide ES18-12 battery, and moving to a battery by MK (the ES17-12). This came from the Batteries Plus vendor when I tried to confirm some information about ordering new batteries (as we were going to order more for this coming year).

I don't know anything other than that -- I tried to follow up with the FIRST "KoP Engineer", but they did not return my call.

Again, this is ONLY from the battery vendor "Batteries Plus", and I HAVE NOT been able to confirm any battery changes and/or official vendors yet.

[Al Skierkiewicz]

Matt,
Interesting data and a great visual on how the battery charger works. As you will note in all the batteries, the charging voltage is a minimum when the charge current is at maximum. This is a great display of how the battery changes impedance with the charge. I am guessing you were monitoring the charge current with an ammeter inserted into the lead between battery and charger. Although the current appears to be pure DC it is in fact pulsing. This accounts for the slight differences between batteries for specific currents at specific voltages. It also accounts for the apparant high terminal voltage near the end of charge as the battery impedance gets higher, the voltage drop inside the charger become less, so the peak voltage rises to near 14 volts. (13.8 volt theoretical for a lead acid battery) The phenomena you noticed when the LED changes state is the portion in the charge curve when the charger begins a "delta" check on the battery. This alternating cycle is the charger taking a look at the difference in battery voltage under a slight load. The charger samples the battery voltage as it looks for a reduction in change over time. As a lead acid cell nears full charge it will continue to draw current but the terminal voltage rises at a different rate. As you can see in the graphs, the sudden upturn in charging voltage signals that the battery is nearing full charge, yet it continues to draw current. At this point, most of the charging current is being turned into heat as little or no additional chemical reaction can occur. Of particular interest is the fact that the battery is listed as a 12 volt battery yet produces 14 volts at full charge. This is phenomena is known as "surface charge" and it will disappear after a short period of time or under load. Although this testing shows that all your batteries are taking a charge at roughly the same current, it does not give an indication of how the battery acts in discharge on a robot. Load testing vs. time is a good indicator of the condition of your batteries in practice. Thanks for all the info. Can I use this in a presentation some time?

[Viper37]

Use away! I would be glad to know someone is using it for good!

One thing I forgot, we drained the batteries not by using a load tester, but by affixing 5 lamps to the batteries, and draining them down over the course of a day. THe actual testing was done the same way, but with a single lamp i believe.

[eugenebrooks]

More reason to wait until we see the game rules and KOP before buying equipment and supplies that may be affected by the rules!

Specification sheets available on the net put this battery at the same size, weight and capacity* as the EXIDE ES18-12, so this possible change would be transparent, except for the possibility of ruling out use of prior year's batteries for competition matches. Lets hope not. Some teams have acquired quite a collection of batteries, over the years, and carefully maintain them.

* Some web sites list it as 18 amp hours, and some list it as 17 amp hours.
* Some web sites list it with a maximum discharge current of 90 amps, for 5 seconds, instead of 230.

Any change is possible, we won't know until we see the KOP...

Eugene

Originally Posted by galewind

Now, this is newsworthy in and of itself, though I'm not sure how much of it is true.

*Apparently* FIRST is moving away from the Exide ES18-12 battery, and moving to a battery by MK (the ES17-12). This came from the Batteries Plus vendor when I tried to confirm some information about ordering new batteries (as we were going to order more for this coming year).

I don't know anything other than that -- I tried to follow up with the FIRST "KoP Engineer", but they did not return my call.

Again, this is ONLY from the battery vendor "Batteries Plus", and I HAVE NOT been able to confirm any battery changes and/or official vendors yet.

[Al Skierkiewicz]

The original batteries were speced at 17.2 Amp Hours several years ago. 18 AH was likely the marketing department trying to boost sales. Be careful, although BA is telling you it is the battery, it might just be their cross reference to the actual battery. Rule of Thumb, don't buy anything until kickoff. Murphy is in full force!