It depends on just how inefficient/efficient your robot is, but as a general rule of thumb, swap the battery every one or two matches if you can.
That’s impressive. If we put in a battery that isn’t at full charge at the beginning of the match, it dies during the endgame or slightly before. I don’t know how long they last beyond a match length, because we’ve never tried it. We know it doesn’t work for us.
We have a cycle of 6 batteries. After experiencing several inconsistancy issues with older batteries last year, we purchased 6 new ones for this season. Even then, we discovered at our second competition that one of the batteries was a dud and we could not last half a match before we saw some failure in robot performance.
Please keep in mind that perhaps as much as 25% of teams are running a battery into the ground by match end. These are due to a variety of factors but mostly due to inefficient drive trains or multiple turns with sticky wheels. With some experience it is easy to pick out robots that would die if matches were extended.
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Kara this sounds very interesting and I would go on a limb and say it is not the batteries themselves but how your team may be treating them. If you have gone through seven batteries in two years, I would look mainly at your chargers and how students are handling them. While it may not seem evident, handling the batteries by the cables or lugs directly can cause major damage, along with the obvious of dropping a battery will damage it. The plates that create the chemical reaction in them can shift or even pull out partially through mishandling.
Also what type of chargers are you using, and what connectors are on them? If you are not using a “smart” charger at the right current rating, you can be over or undercharging the batteries, thus again causing damage. Finally I would look at the connectors on the battery chargers, if they are alligator style clips, change the connector. The gator clips scratch the anderson connectors contacts and create an inefficient connection later on when in the robot. I would cut off the gator clips if present and swap them out for anderson half connectors, the same as used on the battery to make a solid connection.
As for failures with these batteries, if you see the battery dropping in 2 volt increments that is showing a failed single cell (per 2 volts) and means it is time to replace that battery.
Kara,
Even a new battery with a fresh charge will give you low voltage if the load is high enough.
I have uploaded a battery test I performed for team 177 back in 2006. You will find it here.
This was the last time I did an “exhaustive” test. I usually only test suspect batteries…
We did the tests because the team thought that we had “bad” batteries. Several batteries had been marked as “bad” by the pit crew during a regional competition. That year, the game was “Aim High” and the requirements of the shooter mechanism, drive system, pneumatics pump, et cetera, was so great that the battery voltage would crash during a match.
The drive team needed to learn that, if they were pushing against another robot and shooting at the same time, their shooting distance would be compromised.
You will also note that the batteries did not significantly degrade over time. In fact, ignoring batteries dropped on a concrete floor or having the terminals ripped off because they were handled by the wires, I don’t think we have had a single battery failure in less than 7 or 8 years of use and abuse…
We have always used the KOP chargers. We always store the batteries in a charged condition for the summer. Storage temperature is always between 55F and 95F (we have unheated/unconditioned storage). We always charge a battery as soon as possible after use. We try not to leave batteries on a charger more than 48 or 72 hours.
I would suggest that your team invest in a clamp-on DC amp-meter and a good battery analyzer and do some engineering in the off season to understand what is going on.
And before I hear from the peanut gallery: Yes, the current KOP batteries are a different part number but, IMHO, their performance should be very similar…
Regards,
Mike
Edit: At our next competition, I tested two batteries as they came off the field after a match to measure the charge left. This was my criteria for recommending “A battery will definitely last two matches and you can get three matches from a battery if you really need to.”.
My husband and I procured a battery load tester for the team. We load test the batteries at the first of the season to see if there is a need to buy new ones. Anything that is 50 to 70 percent is relegated to use for shop use, not competition. They can also be used in the stands for the laptops, if necessary. There was a battery cart built this year to hold six chargers and six batteries. The team cycles through them to make sure we have not power issues. My husband has also been teaching the students to solder the connections and shrink-sleeve to insure no loose connections.
Linda,
I hope that means soldering the wire terminal after wire insertion instead of soldering to the battery terminal. I have frequently suggested teams use an external tooth lock washer between the wire terminal and the battery terminal to prevent slippage. Using locking washers on the hardware is simply not enough to prevent loosening on this critical termination. All of the above with heatshrink added is an ideal solution.
We take all the batteries to a local auto parts store for load-testing at the beginning of each season. They will test them for free.
The batteries need to last for a single match w/o impairing performance. Normal engineering practice is to design for a 2X margin so our robot generally will run 2 matches on a single charge but we NEVER do that (always replace the battery for a new match).
HTH
Also, many teams have not taken care of their batteries properly, so they are starting off with batteries that are incapable of even being close in capacity to a new battery.
I’m sure that a lot of the issues WERE caused by improper treatment of the batteries. As much as we stress how important it is to not carry a battery by the cables, it inevitably happens occasionally when someone is in a hurry between matches. Some of the batteries that we retired and replaced this year were as old as 2007, and had suffered through four full seasons of use and abuse. I know that a few of them had indeed been incorrectly charged.
We do use the Anderson half connectors instead of the gator clips. I don’t know offhand what kind of chargers we use. We replaced our age-old ones this year because we had the money, and the old chargers were going bad in one way or another. We also acquired a load tester during the fall, and have used it.
I believe that, though I don’t think that it was the case for us this year. We had one battery that consistently died during matches. As none of the other ones gave us problems when fully charged, I’m inclined to think that the failures were not coincidences related to gameplay.
Thank you both for your advice. Next time I have a chance to talk to my mentors with the batteries next to us, I’ll pass on the information.
Kari,
Batteries have a finite life. The batteries we use are speced at 400 charge/discharge cycles under normal use. Our use is pretty heavy with long periods of no use in between. I estimate a team can get realistically no more than 300 if their robots use a lot of current or 350 if they are more efficient. If you are still practicing with 2007 batteries you are getting some decent life.
Our team uses 5-6 batteries changed out after EVERY match. We want our robot to have top performance during every match. We usually have a lot of motors and an air compressor that runs a lot of the match, so our battery is pretty low by the end of the match.
Also we buy all new batteries every year. Batteries from the one year previous are used for practice in the shop and programming and testing the bot in the pit.
Now to the question that you originally asked, I think that you could extend the matches a bit, but no more than 1 or 2 minutes.
Honestly from a field coach position, the matches are very fast, and done before you know it. Also I am not sure that I could take the stress for another minute or two. 
When I drive the practice bot we usually go for about 10-15 minutes before I see a noticeable drop in the performance of the robot (as in, it stops driving top speed). Sometimes a little bit longer.
Past robots haven’t been quite as efficient, sometimes they have only lasted like 5 minutes or so.
No matter what though we always change the battery after every match, just to be safe.
I’d say that if you are just barely making it through a 2:15 second match with a full battery, there is a serious problem. Really anyone should be able to go at least double that without absolutely having to change the battery.
Here is a 50A load battery characterization on our 2006-1 SLA conducted Jan 19, 2006 (note: typo on graph)
DMM is directly connected to battery terminals (so excludes all connector & wiring resistance voltage drops)
Vterm recorded & plotted every 10 seconds until Vbatt drops to 7.0v
started at -3 minutes with .5A fan preload to illustrate ‘soft’ voltage area between ~13.3 - 12.6v becoming 'hard at ~12.6v at high discharge rates
‘hard’ useful robot current occurs at .018 ohm avg internal battery resistance
then plummets quickly
note: Battery Terminal voltage drops 1v for each 50A of load
4 CIM drives with friction belt contact to floor (or pushing opponent)
start up currents can hit 400-500A for few hundred milliseconds resulting in
an 8 to 10 v drop from the ‘hard’ Vterm!! (excludes connector wiring Vdrops
(dynamic current: use Oscope across .001ohm shunt to verify!! scale 1A/mV
or use 120A CB as shunt in pinch for this, its R closely controlled ~.00096ohm
also useful for rough robot DC current: DMM 200mV scale across 120A CB)
400A: 12.6v-8v = 4.6v to keep CRIO, Radio etc alive else it resets!!
500A: 12.6v-10v = 2.6v CRIO, Radio etc reset!!
Advice:
Charging: do it ASAP after every use, float charge ALL other times
abide by mfr spec max fast charge, (guaranteed to do no harm)
Matches: new battery every time
… used batteries =Increased Rint reduces max bot fps + pushing power
Fast charge (~5A) gets you to only ~85% upon completion
…remaining 15% float mode takes >=3 hours!! use your Auto dual rate charger and leave it on. have one or two batteries on charge overnight for critical matches for speed power edge needed.
Load curve reveals much about State of Health of a Battery
…expected life, speed and motor torque loss as time passes due to Vdrop resulting from higher internal battery resistance as it gives up its energy
Note upswing of voltage after heavy load depression is Normal
… internal I^R generates heat: chemical reaction result: increasing voltage for a short period! isn’t science wonderful?
if robot consumption expected closer to 100A average run Batt test at 100A!
main challenge: find Load Resistor(s) to take heat for about 10minute for 50A .25ohm test, or 5 min for .125ohm 100A test
(swapmeets often sell 200W AL heat sink 1% WW resistors inexpensively
series parallel as required P=I^2R size for 12.6v initial voltage-1v / 50A)
Dale… 2 attachments data, graph + imbedded calcs as Rint changes
note: my email has changed from that on the attachments
now DaleScience@aol.com
Dale,
Nice charts. I would like to caution teams on a few things. Charging a battery while warm or hot will disrupt the charge cycle. Our batteries are better added to the charger after several minutes of cool down. While our batteries will regularly undergo several hundred amp loads, it is not wise to continuously draw high currents. As you point out, there is sufficient current drawn through the internal resistance. At 100 amps, the internal resistance is dissipating 110 watts at manufacturer’s spec. This continuous high temperature can cause permanent damage. I believe that many of the early deaths we see in the AGM types are due to this mishandling. I have recommended the West Mountain Radio CBA II or CBA III battery analyzers for many years. While drawing currents in the 7-10 amp range, these analyzers will produce curves similar to yours, calculate AH ratings and allow curve overlays of the battery as tested over it’s lifetime. The one volt drop in your curves is indicative of the surface charge being lost. This is one of the most confusing phenomena for teams who only measure their batteries, right off the charger, with a VOM. For those teams that wish to duplicate your method, a drop in 2 volts indicates a cell failure. Those batteries should be recycled or relegated to a ‘practice only’ status.
clarification…
Al’s comment on 2v drop refers to a rested battery after charge under NO load…
Graph of data taken show TWO quick declines:
… the first is surface charge and exists from 13.3 down to 12.7 or so under low load dissipates in 3 minutes… under no load can exist for hours
… the second upon 50A load declines immediately and is due to Batt internal resistance… Vdrop = .02*50A =1v
… then holds relatively unchanged until the battery is exhausted and again drops off quickly (decline#3) as the internal resistance rises to & >.1 ohm
… de K3MNN originally from Pittsburgh PA --… …–
Dale,
The 2 volt drop is not the 1 volts surface drop in your graph. Frequently, a damaged battery or one nearing it end life will have one cell drop out causing a 2 volt drop in the load curve. You can see this in my posts with the CBA II graphs. Please forgive any confusion.
This thread has strayed from the OP’s intent, but we buy 10 new batteries every year. We put so many hours on them throughout the season that they inevitably end up not being as good as new ones. We practice during the current year with the previous year’s batteries. We never use a battery in a match more than once. We won’t even use a battery that’s been on the bot in the pit for system checks/testing.
You can never have too many batteries or be too careful with them. It is the most critical part on your robot.
On topic, matches will not get any longer unless more than 6 teams are on the field at once. Just won’t happen from a scheduling standpoint.
We tell Gail at the beginning of the season “We need enough batteries for the season.” and so she orders 110 so we never have to use a battery twice…
Just kidding…
Like Cory, we order a new set of 12 every season and use them only at competitions, never back to back (usually only once per day or twice at the most for each competition).
We use the previous year’s batteries for practice and still swap after each practice match…
The 2 year old and older batteries are for the scrapbots that we use for defense practice…
As for how long…depends on a lot of things… Our 2006 robot used 10 motors I think including several that ran almost all the time (two big CIMs, two FP’s), 4 little CIMs in the drive, and two bane bots plus the pump and we would almost kill the battery in 2 minutes…
I was actually thinking about 4v4 with one robot off the field in a “pit area”, and something like :30 + 4:00 . From this topic, it sounds like the batteries could likely last for that long…