Battery Best Practices

I have a few questions about batteries at competitions.

  1. How many batteries do you take?
  2. How many chargers?
  3. How do you test, charge or otherwise prepare your batteries for competition?
  4. Do you do anything special between matches to keep your batteries ready?
  5. Any other advice?

Our team usually takes 4 batteries, and that gets us through usually pretty well. As far as chargers, I would suggest at least one per battery, but you could get away with just 2 or 3 chargers if you must. For testing the batteries at competition, we have a device (I think it’s an old automotive battery tester) that puts the battery under load and reads it as good or weak. To charge the batteries, we charge them over night before and during competitions, and during the day on Fridays and Saturdays, we change it to the full 6AMP charging rate and try to always keep one full charged battery on hand. We also separate full charged batteries from batteries that need to be plugged in. If there is ever an open charger, we plug in a battery unless the battery is >95%.
If it comes down to elims, you can always ask other teams if you can borrow batteries/chargers. They’ll be happy to loan you for the afternoon.
Hope it helps!

For testing, we have a voltmeter with the tips of the cables cut off and soldered to a battery connector. We normally bring one charger per battery, and about seven to the competition (though this is probably a bit overkill). Some major advice: if the battery drops below 10.5 volts, it will get damaged! :ahh: 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!

The Andymark Battery Plugs provided in the KOP are an excellent way to indicate that a battery is charged.

Both the teams I mentor use these:

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.

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.

  1. We generally take as many batteries to a competition that we can. In previous years this number has ranged from 4-6 ‘competition’ batteries and 2-3 ‘Practice/Testing’ batteries.

  2. Recently, we’ve taken 3 Chargers to most competitions, this is usually enough at most regional events until later in eliminations when battery changes happen more frequently.

  3. In previous years we never had a scientific method of testing batteries, we’ve usually just relied on pre-season robot testing to find out what batteries are the best.

  4. After a match ends and the robot is brought back to the pits that battery is immediately put on the charger and one of the charged batteries is staged to go into the robot for the next match.

  5. Sometimes you’ll come across an older battery or two that won’t quite make it through an entire match but will hold enough of a charge to run the robot for some period of time. We usually set these batteries aside for pre-match testing and practice field use. In years that we’ve used the compressor, we’ll often use these batteries to initially charge the pneumatic system. If you don’t have enough batteries to do this, then you can always leave the battery from your previous match in your bot up until it’s time to go into que for your next match and use that battery for testing and pneumatic charging. No reason to drain a fresh battery before a match just to see if everything’s working, IMO.

Team 612 takes 9 batteries and usually 4 to 5 chargers. Our logic is that with 9 batteries, that is normally the maximum amount of matches a team would play in the playoffs. 3 QF matches, 3 SF matches, 3 Finals matches. Its also nice to have extra batteries just in case an alliance partner needs one.

1640 brings around 9 batteries and 7 chargers to competition. This lasts us through finals if none break, and usually gives us one to loan to an alliance partner or opponent.

My number one recommendation is to clearly number all your batteries and chargers. We also test all the batteries beforehand with a CBA III and test any questionable chargers as well.

At competition, we use a comprehensive match chart which includes the match number, battery and charger numbers, and battery performance (namely actual starting and final voltage from the dashboard if they’re abnormal). We also keep them all carefully sorted in the pit, using a standard sequence as well as checking the charge lights on the SC-600A (highest amperage). We’ve also purchased a Battery Beak, which we’ve been using in the shop and plan to use in the pits. We looking to build a battery cart in the off-season, as the current setup is sort of table-space intensive.

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You want to keep up with your batteries. Buy this

I’m using one of these this year. We only have four batteries at present, so knowing the capabilities of each is critical. I have recorded measurements of all batteries at well-defined points. E.g. after each use in a testing session, after each charge session, after each idle period. I expect to use the information as evidence that we need more batteries for competition.

My only objection to the device so far is that it comes with a lanyard (around the neck) that doesn’t split. Safety hazard in a mechanical environment. ::safety::

I second this. We ordered one through AndyMark and have found it very useful to give a quick charge rate and reasonable idea of battery health.

We bought a battery beak this year and think it’s great. If we monitor the internal resistance of our batteries over time using the beak, is that a fair substitute for a CBA III for a team on a budget?

We are a team on a bit of a tight budget looking into a load tester. Does anyone have any experience with the attached relatively cheap tester from Harbor Freight?

I would recommend that no team use the linked battery checker. The minimum load is 100 amps. While FRC robots can draw that much, the battery will likely show up deficient on this tester. In the hands of an inexperienced user this tester could cause some internal damage to the battery. The CBA can run a duplicate test to the manufacturer printed test to calculate amp hour rating. The CBA tests can be stored on the host computer and compared over time with the same battery or with all batteries. The Battery Beak is a quick tester that determines battery condition at 1 amp and at 15 amp and makes calculations to determine internal impedance and battery condition. The Beak is a good quick tester while the CBA tests over a two hour period. The Beak fits in your pocket and requires no special cable, the CBA does not fit in your pocket and requires a USB connection to a computer.

    • 7 or 8 competition batteries, 1 “pit test” battery for testing code, charging pneumatics in the pits, etc.
    • One triple charger and two single chargers (5 batteries max charging at once).
    • Our EE mentor spends about a week discharging and recharging batteries before competitions start. He reconditions the batteries, and then load tests them. Those which do not pass the competition load test become “pit test” batteries, and they are replaced with brand new batteries for competition use. All passing batteries are then labeled “Competition OK 2012”
    • We label our batteries by number, and keep a running list of how often they’re used during a competition, and what their individual voltages are when they’re finished charging. We put 1 charged battery in the robot, take 1 extra charged battery on our cart to the field just in case, and have at least 2 charged batteries ready to replace them in the pits.
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We tested our batteries this year as follows:

  1. Acquire battery
  2. Plug battery into dc to ac converter
  3. Attach volt meter to leads
  4. Plug Halogen bulb into converter
  5. Turn on converter
  6. Turn on bulb
  7. Record Battery start voltage
  8. Wait for battery to drop below 10 volts
  9. Record time taken
  10. Label battery accordingly

It took awhile, but worked fairly well for reheating pizza.


What are the approximate dimensions of this battery charger? We’re considering building a new battery cart (wooden box containing chargers, slots for batteries, and AndyMark battery plugs. On wheels.) and this seems almost perfect.

The Genesis NP18-12R battery (2012 FRC KoP) datasheet1 indicates that it is rated up to 150 amps, so in theory you could measure it with a 100amp load tester, but:

  1. The datasheet does not list any manufacturer specs at that high discharge rate, so any assessment based on testing done at that rate could only be relative to other batteries tested the same way.
    1. In the wrong hands, testing at 100amps could damage the battery or be a real safety hazard.

Use something else.


FWIW you can buy 300 watt 12 volt bulbs at a pool supply store. Not the safest thing to do either.

We just used a shop light that we had lying around.