Average Energy per match
 

I am doing battery capacity fade studies at my work and wondered how their results could be applied to these robotics competitions, but ran into the question of how much energy is actually used per match.

Has anyone ever collected any data on the average energy used by their robot in a given set of matches? This is going to different per robot, per match but I imagine throwing out some extremely outliers I'd think you can find a pretty accurate average value.

My initial thought is that the battery we use currently is relatively poor for the amount of money we spend and the shelf life they retain. To lose 20% capacity in 6 months by only sitting on a shelf makes me wonder about the impedance growth rate associated with these batteries.

Re: Average Energy per match
 

MORE Robotics (1714) has an amazing battery charging system that tracks each battery's usage and life. I'm sure they could shed some light on your question.

Re: Average Energy per match
 

We test our batteries, and have not seen the drop-off that you suggest (20%). In fact, some of our best batteries are not the new ones, but ones that are one or two years old.

If the manufacturer says 20%, they may be hedging their bets in case the battery is used in a critical application, or (more likely) are basing it on statistical sampling and covering the worst-case: for instance 99.999% of the batteries may lose less than 20%.

Re: Average Energy per match
 

In preparation for launching the PD, our interns created a battery monitor. It logged the battery voltage and current a couple times per second to an SD card. I have the information gathered at Mayhem In Merrimack. It is either 2meg zip of .pngs or a 2gig folder of raw data. If you PM me your email address I'll forward it along.

I was hoping that CrossTheRoad or AndyMark or other would sell the devices, but there wasn't much interest. All it is is a pic, a hall effect sensor and a microSD connector.

Re: Average Energy per match
 

It makes me wonder why we don't take more effort to make sure we keep ours charged up and in use all year :)

Energy use per match depends on a lot of things, and the game has a lot to do with it...we used up most of the capacity of the battery during matches in 2007...this year the batteries lasted quite a while.

You should be able to figure that on average, roughly one quarter to one half the capacity of the battery in each robot will be used in a match. So figure two batteries worth of charge per match (6 robots) as a starting guess.

Re: Average Energy per match
 

Any chance plans and code for this device could be published?

Re: Average Energy per match
 

Sure. It is just a lab tool - it isn't cleaned up or productized, and finding where the source went might take a few hours. I could email them or publish them somewhere. How do you want them?

Re: Average Energy per match
 

It is not my dropoff, that is from MK's datasheet.

How did you make that assertion that MK is incorrect? I doubt they are "hedging their bets".

What is the maximum SOC you saw when you first charged them? what C rate did you charge them with (C/10, C/20)? Now two years later when using the same C rate what is the SOC? Were you at the same temperature both times?

Or did you use some type of impedance spectrometer to correlate that to capacity?

Just because your robot still runs doesn't mean the battery hasn't lost capacity, back to my original question. Maybe your robot will function normally even if you lost 40%? What is the average energy it expended per match? Did you test it now two years later under the same conditions?

@squirrel, I wouldn't recommend charging them fully for long term storage from what I can tell 40-60% seems to be the sweet spot for %SOC with regards to reducing impedance growth. Also, keeping that ambient temperature in check is huge.

@Eric, check your PM box in a few minutes

Re: Average Energy per match
 

I can try and get usage data from the battery charging station for the last few events and demos the robot's been to - We always swap batteries between matches just to be safe, so the station should log the "before" and "after" voltage for a match with a 2009 robot. I'm personally not too familiar with the inner workings of the charging station (it has big buttons and you hit them to start charging), but if it's trivial I'm sure I can get data from a mentor.

Re: Average Energy per match
 

The only thing I see that is close to 20% over 6 months on this brochure is the charge retention, which doesn't sound like what you are describing. Can you post your information?

Re: Average Energy per match
 

I remember someone coming up to our team at IRI in 2008 and putting some sensor on our battery cable to check power usage. I believe it was used for making sure everything went right with the new control system this year. I'm pretty sure it was a FIRST guy.

Re: Average Energy per match
 

Yep, we used that data as well, I just don't have it handy at the moment. Thanks!

Re: Average Energy per match
 

www.mkbattery.com/images/ES17-12.pdf

that link is very interesting though.

Re: Average Energy per match
 

Please do not confuse charge retention with capacity retention.

Charge loss is reversible; we generally re-charge batteries after each match.
Capacity loss is irreversible, it is the permanent loss of (some of) the ability to re-charge (or more accurately to deliver that energy back to the load)

A charge loss of 20% over 6 months is typical for lead-acid chemistry, no surprises there. For capacity loss, focus instead on "life expectancy" on that same data sheet.

IF you want to prevent significant capacity loss, keep the battery charged. Industry standard is 3 months on the shelf before it needs a recharge. We try to use that as a maximum.

The greatest factor of permanent capacity loss is allowing the battery to sit in a discharged condition. This allows the formation of 'hard' lead sulphate crystals which cannot be reversed by charging. As the sulphur comes out of the electrolyte, it loses effectiveness and the crystals 'clog' the lead oxide reducing the ability of the chemical reaction to occur, both reducing capacity.


Anyhow, to the original question: the charger can deliver 6 amps, it takes about 1.5 hours to recharge a battery, so we've used 9 Amp-hours as a rough estimate, about 50% of the battery capacity.

Re: Average Energy per match
 

ah, correct, I misunderstood their terminology. So their 20% loss is due to their internal ESR discharging it on the shelf and not referring to their internal impedance growth (your 'clogging') where the electrons remain lithiated in the anode/cathode material.

My main interest in knowing the average energy was to see if a) this battery is overkill and b) if something like an ultracapacitor stack could replace it since there is no significant capacity fade associated with a UC stack (among other benefits).