Battery Best Practices

[fox46]

Both the teams I mentor use these: http://www.tecmate.com/u_accumate_pro5.php

They are an affordable, compact and brain-dead solution to battery charging. We have them wired up to battery connectors so all you do is plug your batteries in and when the light turns green you're good to go. If you have five batteries which are always plugged into one of these and take them home at the end of the competition day to charge you should never be left without a battery. If for some reason you need to charge a battery at a higher rate, you can connect multiple terminals to one battery to boost the charge rate.

An important note about the batteries we use. The ONLY way to reliably test if a battery is indeed good is to use a load tester. This is a fancy voltmeter with a large resistor (heating element) inside it which loads the battery heavily to check that it is still capable of supplying sufficient voltage under heavy load. Last year Cybergnomes went to 6 motor drive and we found that the performance was extremely different between the new batteries that we had obtained that year vs the old ones. After load-testing them it was found that nearly half of the older batteries were incapable of supplying the same power as new when under load. The "failures" were attributed to storing the batteries at the end of the year and leaving them in a depleated state. With a lead acid battery it is very important to exercise it during the offseason. If you simply store it and forget it, the charge level can deplete. If these batteries are left in a depleted state they are prone to sulfation which is a formation of sulphides internally. These crystals can severely degrade the battery's performance. The best solution to avoid this problem is to charge your batteries or even better, cycle them (discharge, charge) regularily during the offseason. The phrase "use it or lose it" applies here. This type of failure can be very difficult to diagnose because the battery will test full voltage and without a loaded tester will show charged however, as soon as you apply a load the voltage nose-dives and you realize the battery can only store a fraction of what it should.

[Al Skierkiewicz]

Originally Posted by Superllama12

Some major advice: if the battery drops below 10.5 volts, it will get damaged! I would advise to check the batteries pretty frequently, and make sure they don't go below 11v. Also, when they charge, their max is 14.4v, but it will drop to 13.8v (its max rest charge) afterwards, so don't freak out if they seem to charge and then drop again!

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.

[pwnageNick]

FRC Team 2949 PWNAGE just finished building a new battery cart. It consists of 12 batteries, 9 charger banks, a BatteryBeak, Spill Kit, 2 DeWalt chargers, extra AndyMark Battery Clips, and a removable lid. Some pictures below show the setup.






The DeWalt chargers and spill kit still needed to be added since these pictures were taken. The DeWalt chargers are mounted on the plate on the top middle of the cart (in-between the chargers). The spill kit goes under the 2 3-Bank chargers. Eventually with some more fundraising done we will switch out the 3 1-Bank chargers for 2 more 3-Bank chargers.

I would be happy to answer any questions about it. I had just uploaded a couple of pictures of it to CD a couple of days ago, but apparently hey still haven't been approved... anyway let me know what you think.

More pictures can be found at our fb page (Like us!):
http://www.facebook.com/pages/pwnage...09248172427910

-Nick

[adengler]

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.

Originally Posted by Al Skierkiewicz

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.

[Al Skierkiewicz]

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.

[Ether]

Originally Posted by Al Skierkiewicz

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

[Al Skierkiewicz]

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

[Doc Wu]

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.

[cheesepuffgd]

1) My team takes all of our batteries from the last 3 years to competitions
2) My team takes a "battery cart" that has 4 chargers connected to a single wall outlet
3) My team puts each battery through a cycle of discharges over decreasing loads using the incandescent lightbulbs also on the battery cart. this conditions the battery and can restore some lost capacity. The battery cart is fully automated with an Arduino microcontroller and can condition 2 batteries at once. for electrical schematics and datasheets and eventually source code for the Arduino, search "team 955 battery cart" on google and select the first option.
4) besides charge them?
5) don't store batteries partially discharged for long periods of time.

[Ether]

Originally Posted by cheesepuffgd

My team puts each battery through a cycle of discharges over decreasing loads ... this conditions the battery and can restore some lost capacity

Can you provide a link to an authoritative source for a technical discussion of this phenomenon?

[Al Skierkiewicz]

Originally Posted by cheesepuffgd

1)
3) My team puts each battery through a cycle of discharges over decreasing loads using the incandescent lightbulbs also on the battery cart. this conditions the battery and can restore some lost capacity.

5) don't store batteries partially discharged for long periods of time.

I suspect the attempt here is to remove some sulfation on batteries that have sat around for a long (many months) time. Batteries self discharge over time so it is impossible to store partially stored batteries. (see battery sheet on self discharge specifications) However, it is best to charge batteries before storage and keep them in a temperature controlled environment. i.e. Don't allow them to winter in an unheated garage or to sit in an closed shed during the heat of summer.

[tsaksa]

Originally Posted by Al Skierkiewicz

I suspect the attempt here is to remove some sulfation on batteries that have sat around for a long (many months) time. Batteries self discharge over time so it is impossible to store partially stored batteries. (see battery sheet on self discharge specifications) However, it is best to charge batteries before storage and keep them in a temperature controlled environment. i.e. Don't allow them to winter in an unheated garage or to sit in an closed shed during the heat of summer.

I have also known of some people doing this with brand new batteries since SLA batteries reach top performance after a few proper charge discharge cycles. A friend of mine actually holds a world record for an electric vehicle race, and does things like this. The random discharge cycles a new battery might experience are not likely to be ideal in conditioning a new battery if left to chance. I have never tried this though because from what I understand the benefit is modest at best, and doing this to incorrectly can actually increase sulfation and other battery problems. I might try it someday, but right now it has not made it very high up on the priority list.

What we do right now is to never deeply discharge any battery. We try to always get a battery onto the charger immediately after use to avoid sulfation. We also store the batteries in a cool but not cold location and top up the charge regularly (at least monthly, preferably weekly) on any unused batteries. And our newest competition batteries do not get used for driver testing or software development where they can easily be over stressed before anyone notices. Someday we will try to increase our knowledge of and attention to proper battery care. But for now just a few basics seem to have made a significant difference for us.