Do teams get rid of the their old batteries after some time? By that, I mean, even if they are running ok?
We have been getting rid of ours after 2-3 years, but wondering if we are thinking about it wrong. Perhaps we should keep them until they are proven to be malfunctioning. Thoughts on this?
Thanks.
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2-3 years sounds about right. We buy 8-10 new batteries every year and recycle ones older than 3 years. The ones we keep that arenāt new get used for practice and other random stuff.
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This is semi-off topic, but are you supposed to keep the batteries always plugged in? Our team has kept our batteries mostly unplugged except for meetings because I think someone thought that it would be a fire hazard to have them always plugged in.
After 200 - 400 discharge cycles FRC batteries will lose 20% (or maybe a lot more) of their original capacity. The deeper the discharge, the worse the capacity loss. If you are practicing and competing on a small number of batteries, itās pretty easy to approach that life span in a year or two.
We tend to buy 6 new ones every year, and reserve them for competition (recharging them periodically until then to combat self-discharge). We relegate previous year batteries to practice duty, and recycle any older ones.
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We keep them plugged in. I would advise all teams to do so. In general, most of the chargers that teams are using are āsmart chargersā (and if they arenāt then they should be but thatās another story). These chargers provide a float voltage and do a whole bunch of things to keep the batteries at peak performance.
These batteries are happiest when kept full of potential energy and donāt really enjoy being drained. FRC applications tend to be very unkind to these batteries and so itās best to obtain new batteries to ensure all of the chemical ingredients are kept fresh.
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These charts are for the MK which are (arguably?) the best battery in use for FRC robots. All others are lesser.
Itās a little tricky to know the discharge depth. You can get an idea of it by looking at the DriverStation log details such as this example on DS at the moment:
--------
Duration : 01:43:52.38 sec
Errors : 0
Warnings : 533
Messages : 643
Brownouts : 7PDP #: kJoule
0: 0.00 1: 0.00 2: 0.00 3: 0.03 4: 0.00 5: 0.00 6: 0.00 7: 0.00 8: 0.00 9: 0.00 10: 0.00 11: 0.00 12: 0.00 13: 0.00 14: 0.00 15: 0.00 16: 0.00 17: 0.00 18: 0.00 19: 0.00 20: 0.00 21: 0.00 22: 0.00 23: 0.00
Total: 0.03
My guess is the discharge depth isnāt very much as the robot motors wonāt go below about 11 v. You can see that storage time kills batteries.
We bought the basic computerized battery tester and are happy with the information it provides (see CD threads on batteries). Each test - actually each and every discharge - shortens the battery life so donāt over-test (somewhat short tests and not too often).
Itās actually much lower than this. Not only do we approach 100% depth of discharge with every match, the discharge rate approaches 10-20C, with 40C peaks. Discharge rate is correlated with battery sulfation, which reduces cycle life further. Unfortunately itās very hard to find data in what happens when you discharge an SLA battery at 20C, because most studies consider even 5C to be extreme.
I need to pull more studies together, but Iād estimate we get less than 100 cycles out of a battery before the ESR increases to an unacceptable level (say, 25mohm).
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To pick off the lingering question:
Sure! We put a band of blue tape around these older batteries to identify them, and theyāre our preferred batteries for control system and programming work, demos, powering an inverter in the stands for our laptops, and other similar things where we wouldnāt want to put undue loads on a competition-grade battery.
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Please post any links you have. This is useful info.
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OK to leave batteries plugged into a smart charger. Not a conventional dumb charger. Overcharging will cause gassing which is bad for AGM batteries. Plugging them into a charger once a month or so to top them off is as good as leaving them on all the time.
Consider this as solution.
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Even worse than that!
Below is a plot from a 18Ah battery datasheet (this can be generalized to all FRC batteries Ā± a little bit). The scan was awfulā¦ apologies for thatā¦ what we have is:
X-axis: number of cycles
Y-axis: % of rated capacity
Curves: 100%, 50%, and 30% depth of discharge
For optimal performance we stick to that ~100 cycles where performance is at or above 100% of the rated capacity. I assume 100% DoD for all matches/most practice time. I marked the plot in red for this info.
We also buy ~8 new batteries per year and only use those for matches (assuming they test out okay).
Well, yes, but actually no, but actually kinda.
Soft sulfation occurs normally during discharging, but is totally reversible. Hard sulfation (and loss of capacity) occurs if the battery is left not-fully-charged for a prolonged period of time.
An annual, or semi-annual, desulfation charge cycle will keep oneās batteries in better shape for longer than using āregularā chargers.
No, storing at a high a high state of charge is fine for a few months, it is not necessary to store on a float charger. Recharging every 6-12 is recommended for AGM batteries though (our batteries are AGM lead-acid).
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Given the very high number of discharge cycles we are āsuppposed to getā I kept my much lower guess to myself. Iāll follow you with āwe, too.ā Iām thinking 0-50 cycles for a good battery through some practice and the competitions. Then 50-100 cycles for general use in the shop - practice, prototypes, programmers, driving at outside events. Then the batteries are pretty well shot.
Iāll add with this edit that Iām not saying itās the cycles that killed the battery. All those cycles were at a high discharge rate and batteries are stored for many months. All factors combine to shorten the life but we donāt throw away a battery because itās old; dispose of them when they donāt work right for what you want to use them forā¦
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This is absolutely true, however, from experience, most teams are not actively tracking the battery state and ensuring batteries are charged before letting them sit uncharged for long lengths of time so my recommendation has been (and will keep being) to store FRC batteries plugged in and charging. Iām not going to die on that hill but itās what Iāve come to recommend.
One day, weāll get smarter batteries but it aināt this year and it likely isnāt anytime soon.
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I was reading a great paper from OSU yesterday, Iāll do a writeup soon.
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I think the batteries are going to be what FIRST uses to throttle down and somewhat level the playing field between high resource teams and mid-low resource teams. Putting more high powered motors on your bot is just going to drain your battery faster. That will change some of the thought process to what is a more efficient mechanism that would still be powerful enough to do the task. I could be wrong though
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I will look forward to the write-up. My understanding is that it depends a lot on the float voltage of the charger. If the charger has a ācycleā float voltage (14.4 to 15V for MK) then it will cause gassing over time. Standby voltage use is lower I think for this exact reason (13.5 V to 13.8V for MK).
There are some other things that complicate the issues such as desulfation cycles. The specifics of lead acid chargers are much more obscure than I would have hoped for in such an established technology.
What is frustrating is that the RealPro chargers that andymark sells advertise the ability to have the float state and a 5-stage charging cycle but nowhere do they specify what the float voltage is or what the 5-stage charging cycle means.
Product page with a section on DeltaVolt
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A good doc from FIRST itself on conservative battery best practices (new comp batteries at least every second year; never keep a battery longer than 4 years; test batteries by doing X and get rid of bad ones) would be helpful. Our school admin only seems to fully trust the info from FIRST & they make up rules as they go along otherwiseā¦ Like weāre only allowed to charge batteries during meetings now because a 5Ā±year old pre-COVID battery leaked overnight while charging once last year (maybe it was dropped at some point & finally failed). The fact isā¦ That battery should have been recycled long ago. All our pre-COVID batteries should have. They were still around because our school advisor liked keeping batteries around that could hold any charge & be potentially useful, and they looked ok on the battery beak, so we kept them. We need to learn better & do better.
I did get our new coach to agree to buy 6 new batteries a year & keep no batteries longer than 3 years, so that should help. But I need to find a way to reverse that charging ruleā¦ From what Iām reading here it is the opposite of best practice.
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Thereās a bit of a catch to this. High-resource teams pay attention to efficiency and power curves for their motors and calculate their gearing ratios accordingly. Iād reckon the vast majority of FRC teams donāt really do this. They just āeyeball itā and if it works, it works.
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If I were in this situation, Iād try to buy enough battery chargers to charge all the batteries I care about (9? 12?), put them all on a power surge protector, and turn the thing on every time the team is at the shop, and turn them off when we leave. That should be enough, I would think.
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Tell me youāre in a toxic relationship without saying youāre in a toxic relationship.
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