I was arranging a group purchase for new robot batteries when one of the other teams indicated that they would prefer two specific batteries due to their lower internal resistance (Yuasa NP18-12B or Panasonic LC-RD1217P). I have never considered a battery a source of competitive advantage beyond bringing enough new-ish batteries to an event. I’ve gone down the CDbatteryrabbithole but we haven’t had this conversation since the motor count went to “in any quantity”. I’m interested to know how other teams are managing batteries in practice.
Team
6364
Competitiveness
Regional Eliminations
Competition batteries
5 batteries 0-2 years old + 1 code battery
Battery Model
Mixed
Battery replacement
Replace competition batteries after 2 years. No testing
There’s other factors at play (chargers, practice use, robot complexity, etc…) that could be added if important.
We take batteries out of competition service after they have an internal resistance much over 0.021Ω according to a battery beak. Over 0.030Ω and they’re no longer good for even practice and we recycle them.
We test our battery every year with a tester that checks resistance and voltage over a constant load. Each battery test takes about 8 hours to test and we can display it over a graph to see what batteries are good and bad. We normally recycle a couple from the year before and we buy new batteries every other year to keep our quantity of batteries to what we need.
We also have batteries for practice and comp bot labeled
We use the battery beak to test our batteries often and we record the data in a spreadsheet. This allows us to see degradation over time as well as how the battery reacts to different use cases. We have not noticed a significant difference between brands.
One of the biggest factors in battery life is in how you charge and treat them. Never pull them below 6v under load, don’t leave them sitting discharged for long periods of time, and invest in a nice battery maintainer. We have one of these for in the shop that has a max charge current of 2A. We keep our batteries on this 24/7.
We also have two of these that we only use at events as charging at 6A can be quite harsh and has been known to significantly degrade batteries.
We have batteries that are still in good condition from 2014 that we use as practice and backup batteries.
They plug into the USB on a computer and do everything for you. I highly recommend using one. We finished our testing last week and only had 1 bad battery from last year. That’s pretty good because normally our demos and summer activities kill 3 or 4.
I agree with James. We use a smart battery charger that charges at 6 amps initially and tapers off. We read the battery documentation before purchasing it, and have been using the same charger for a decade or more without issues.
The biggest help with batteries we had, was to stop using the quick 6 amp ,3 bank changer other then at competitions. The rest of the time use a nice little slow charger. With a 17 amp hour battery, a 6 amp charger pushes the battery over the 1/3 amp charging (rule/guideline/strong suggestion) creating more heat in the battery and shortening is life. We don’t have the really cool 10 bank slow charger like @vanjan14 pointed out (thank you by the way, that’s going on the team wish list) just a few smaller 2 amp chargers.
6A “smart” chargers are great when you need to charge quickly such as at an event, but put needless stress on the battery when you don’t need a quick charge.
I would never recommend the 2A charger we use for anything but shop and battery storage use.
This is a timely issue since I talked to a West Mountain Apps person yesterday at the St. Charles, IL hamfest. I use the CBA-IV tester at 7.5 amp discharge to run a test that will take about 2 hours. My results for that discharge rate is 12.5-13 amp hours, exactly as predicted by manufacturer specifications for these batteries.
In general all battery makers specify 5.5-6 amps as max charge current. With smart chargers, the cycle will check for battery charge state at regular intervals and will back down charge current to fit the profile for our batteries. In checking several chargers I have found start currents at near 6 amps and within a few minutes they will back down closer to 5 amps. The charge current will back down throughout the charge cycle.
Do not believe the charging is killing our batteries, it is the discharge during a match. These batteries have a predicted charge/discharge life of 1200 cycles. With the rapid high current discharge during a match, we reduce that life to about 400 cycles, typical. Remember, each CIM motor is 131 amps in stall, or each time you robot starts moving.
Virtually all batteries are made by just a few manufactures, how many is a trade secret. That means that internally they are very similar. Any differences in the specifications (amphour, discharge current, life span) should be examined against other manufacturers. At least one Chinese manufacture lists their battery as 20 amps hours because they test it at different load currents.
If you choose to purchase a West Mountain analyzer (or other products like wire or APP connectors), please let them know you are on a First robot team. I have talked to them in the past about robotics and I want them to know the impact they are having. In speaking with them yesterday I found that there is an extended software package that is optional. It allows testing to be configured for periodic high current and low current testing. This is a normal load for ham radio where when transmitting the battery is required to give high high current while when receiving the current is vastly reduced. Ham operators like to predict how much battery they have when they are operating portable (in the field, parks or camping).
Last year 2910 started buying batteries from MK battery directly. Prices are very reasonable, ≈$35 per battery including shipping. About 8 new batteries each year has become normal for us.
We are pretty picky about which batteries we will run at competition. Using the battery beak, If a battery is showing >0.017 Ω we wont use it. Our robots the past couple years have not been traction limited, pretty much full weight and geared for around 16 fps. We have been preventing brown out with SRX current limiting. The battery beak will indicate around 110% to 130% charge at the start of the match and it is normal for us to end a match with a battery under 60% charge. Using a battery showing <0.018 Ω prevents us from hitting brown out protection at the end of matches.
To extract the maximum short term performance out of our batteries we have found that running them hard helps. At a competition we will tend to only alternate between 2 batteries as much as we can. Charging them with the AndyMark 3 bank fast charger. By the time we are getting deep in eliminations those 2 batteries are getting noticeably warm. The internal resistance is down around 0.012 to 0.013 Ω and when we are waiting for the match to start the voltage on the driver’s station is showing close to 13 V (sometimes over 13 V). I’m sure this way of abusing batteries is not good for the life of the battery but for getting the best performance for only a few matches it really seems to work.
So our batteries are showing 0.00 and 0.001 according to our Battery Beak. Is this normal? Should they be fully charged to do the internal resistance test?
Please remember that the internal resistance shown includes the series resistance of the wires and terminals. Your high readings may be in fact a loose connection, dirty terminals on the battery or a bad crimp.
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