Chief Delphi - Battery Best Practices

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Re: Battery Best Practices

Hey Al,
Thanks for your informative post about the West Mountain Radio unit. We bought one awhile back and have yet to really understand how best to use the thing. We can't seem to test a load with more than 7.2 amps and even then the results are hard to interpret (for us).

Would you mind laying out a few steps and settings you use to test it?

Thanks very much in advance.

Quote:

Originally Posted by Al Skierkiewicz (Post 1120862)

Brandon,
These batteries can and do regularly drop down below 7 volts without damage. Testing with simply a voltmeter is not enough to determine if a battery is good enough for competition. Only a load tester of some type will give you an indication that the battery can actually supply power. A voltmeter reading of 13 to 14.5 volts is normal for a battery that has just been removed from the charger. This battery, however, is nominally 12 volts under load and that is the reading that will show on your dashboard when communicating with the robot. That reading comes from the battery connection on the analog module of the Crio through the jumper to port one. That is actual battery volts as measured at the PD. Without a true load tester, using the robot and dashboard while running the drivetrain may give you a pretty good idea of battery health. You can expect to see a voltage drop when starting but it should return to near 12 volts after one second. I highly recommend the West Mountain Radio CBAII or III USB battery tester. These devices will fully test batteries for AMP HR ratings taking an hour or two to perform the test. The results are available in graph form and can be saved in order to perform annual tests on batteries and compare the results. Please remember that our batteries are specified for about 400 charge/discharge cycles maximum. Our use causes that number to drop as a match will draw far more current than the battery was designed to deliver. I would expect life to be in the 300-350 cycle range.

Re: Battery Best Practices

Aaron,
The 7.2 amps is sufficient. set your cutoff voltage to 7-8 volts, which is the cutoff for most of the battery spec sheets. It will take more than a hour to run the test. After a few minutes the battery voltage should fall a slight amount. This is due to any remaining surface charge and the all of the components reaching a set temperature. After that the voltage should remain relatively constant until the end of the test when the curves start to drop off to the 8 volt limit. The program runs constant amp hour calculations and should match the battery curve very nicely on a good battery. What you are looking for is a drop of 2 volts during any part of the test. This is one of the cells giving up. I have a had a few batteries where one gives up and then a second gives up a little later. The cell loses capacity if the electrolyte is not full in that cell or if one or more plates have broken within the cell. On a rare occasion a battery will start off at 10 volts indicating a cell is shorted. Once the test is complete, save it with a unique ID that you also mark on the battery. That way you can track the same battery from year to year and watch for end of life when the battery no longer can ~18 amp hours. The program also allows you to overlay several curves so you can look at the same battery annual curves or you can compare batteries you want to use for competition. The 7 amp discharge is nowhere near what the robots will pull during a match but remember you are testing to manufacturer spec in this test and that is really the only valid test to determine if the battery meets spec. I also recommend that if you are testing a suspect battery, try moving the cables around and tap on the battery to see if you turn up any intermittent voltage dips. We added an Anderson connector to our CBA II to make it easy and all of our chargers are also so equipped with the APP connectors. We do not use alligator clips and neither should you.
If you search, I put up some curves a long time ago to demonstrate all the possibilities.

Re: Battery Best Practices

Quote:

Originally Posted by Al Skierkiewicz (Post 1145519)

If you search, I put up some curves a long time ago to demonstrate all the possibilities.

Is this the one?
http://www.chiefdelphi.com/forums/showpost.php?p=766746

Here's one from Hugh Meyer:
http://www.chiefdelphi.com/forums/showpost.php?p=916447

Re: Battery Best Practices

Yep, but I didn't want to make it that easy.

Re: Battery Best Practices

I am also a proponent of the West Mountain Radio CBA III. I agree at first, you will not know how to interpret the results, especially since the graphs auto-scale to fill the screen.

The real value of the results shows up when you overlay the test results. Then you see the differences between tests. I test each battery at the beginning of the season and do an overlay of all the batteries. It becomes easy to see which batteries are the strongest and which are getting weak.

The other use is when a battery is suspected of getting weak. Test it again and overlay the results with the test you did at the beginning of the season.

Bad cells and broken internal connections are quickly spotted with the CBA III.

If a team was short on money and can't afford the Battery Beak and a CBA III, I'd take the CBA III. It's only shortcoming is it can't be used quickly in the pits. You must be disciplined enough to test all your batteries ahead of time.

Another recommendation I make is (and I think some of the posters here were alluding to it) is to number your competition batteries and use them in sequence. Don't fall into the trap of saying "Battery 3 was strong that last match, let's use it again." Pretty soon, you have overused battery 3 and it's your weakest one.

Batteries need to be given time to charge, as well as time to cool after charging. Having more batteries than you need and rotating them in sequence allows them to be used equally and be given the best operating conditions.

A two minute match along with a few minutes of power on before a match should not come anywhere near depleting these batteries. If you run a match, then charge them, they should be able to be charged back to full capacity fairly quickly. You don't need one charger per battery. A 1-2 or 1-3 ratio should be enough.

I find that the most damaging thing to the batteries is practice and demos. We get playing with the robot and run it for 10-15 minutes until someone notices the battery won't drive the robot anymore. By then you've severely depleted the battery. They only can take so much of that.