BatteryBeak: Utility of making internal resistance and state of charge measurements

Hi all,

I’m attempting to learn useful ways to track battery health/status using the Battery Beak. I’m looking for verification on the things I’ve learned and any suggestions the intelligent, experienced people like you can provide.

I plan on using the Battery Beak on the ES17-12 battery model from MKPowered. I’ve met some resistance (no pun intended) from members on my team that think the Battery Beak is too simple and underplay the utility tracking the internal resistance of a battery can offer. I want to convince them otherwise.

From my understanding, tracking the internal resistance is useful for the following reasons:

  1. The first measurement done with a new battery allows you to see if the battery matches the manufacturing specification or if it deviates (and the size of that deviation)
  2. The health of the battery (the physical condition of the individual cells and of the electrodes and electrolyte of those cells) can be determined from measuring the internal resistance.
  • No change or a small change can mean a battery is eligible for competition.

  • A medium change from when the battery was new can mean a battery is eligible only for practice or workshop testing.

  • A large difference from when the battery was new can mean a battery is eligible only for laptops in the stands but not robots

  1. The treatment that a battery is given and/or manufacturing defects. By noting the speed at which the internal resistance changes (by calculating the slope of the function describing the internal resistance measurements), one can determine if a battery is receiving proper or improper treatment over its life. If the measurements show a sudden jump, this could indicate a situation where a battery cell has failed.

Simply put, by the end of build season, batteries with low internal resistance and slow growth in their internal resistances are prime candidates for use in competition.

Are there any issues with my understanding or useful reasons I’ve missed? Do any other teams track internal resistance through time?

At the beginning of the season, we measure the CCA of the batteries. We use the best ones last so they don’t get too much use. Then, before we pack for competition we remeasure and reorder them from 1 to 8. We use the highest rated batteries first.

How do you do that with a battery beak?

A battery beak can be used to diagnose two battery problems:

First, It provides an internal resistance (derived from the voltage at 1 and 18 amps). The lower the better and internal resistance generally charts the health of the battery. The “Good”, “Fair” & “Bad” ratings are determined by internal resistance only. This, therefore, provides a ranking method. Beware, though, because there tends to be some noise in this signal.

Second, the voltage at 0 amp (and % charged) will let you know if your battery loses a cell. A fully charged battery should read >100% charged on the battery beak. If you’re down around 90% or lower and your charger indicated a full charge, you are down a cell.

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The internal resistance of the battery will vary with temperature, state of charge, how long it has been off the charger, age and how many times it has gone through a charge/discharge cycle. While the Beak is a good check for battery condition, you likely will get better data with a different test method. Nothing beats a Beak in your pocket to know you are ready to play with a known good battery. If you are trying to check against manufacturer’s spec, you must test under their conditions. The stated test data varies from manufacturer to manufacturer which is why the MK battery and Enersys are rated for different amp hour capacities when they are identical batteries.
I recommend the CBAIV from West Mountain Radio which happens to be located close to you. While the test takes a couple of hours for each battery, it allows you to overlay tests and print results. It will display real time amp hour capacity throughout the test and then store the results. You can vary the load up to 150 watts which allows you to set the discharge rate to match the specifications or to increase the discharge to simulate robot conditions. One of the best parts of this tester is the ability to see if individual cells have reduced capacity and if there is internal arcing due to damaged plate connections or inter cell wiring.

I thought that the battery beak checked the health of the battery by looking at the cca. Most of the battery tools that I’ve seen look at both the internal resistance and the CCA, but I guess the battery beak doesn’t. Anyway, the same strategy in my earlier post could probably be used with the internal resistance.

As a quick test, the Beak is great. We use one all the time.

IF you can maintain about the same conditions (see Al’s post) for each test, then monitoring the battery’s internal resistance over time can give some good clues as to what’s happening inside. But consider these as relative measurements, not absolutes.

The West Mountain Radio Computerized Battery Analyzer (CBA IV) actually discharges the battery at a rate you can set - stock is up to 150 W which means over 10 Amps, and you can buy an ‘amplifier’ load for IIRC 450 Watts.
However, even knowing the actual number of Amp-hours the battery can deliver at 10 A doesn’t tell you what it will deliver in competition. So again, this should be considered a relative reading, valuable to monitoring battery health, but not for checking against manufacturer specs.

If we focus on FRC, what we “really” need to know is covered by the Beak: A relative indication of health, and present State of Charge.

All that being said, WinDnDusT, #1 and #3 aren’t reasonably easy, but #2 is really what you ‘need’, and is very do-able with a Beak. (Just one caution: If the battery is “bad”, it is - don’t use it for the stands, as it may be more hassle than it is worth. Recycle it instead.)

You can’t with a battery beak you need a load tester.

A load tester is the best item to see how a battery can respond to the load that it will see in a match.

The typical automotive load tester will put a 100 or 125 amp load on the battery for as long as you hold the “test” button.

Or for 15 seconds, whichever comes first1 :slight_smile: They get very hot after that.

1most of the inexpensive ones I have seen warn you not to hold the button more than 10 or 15 seconds

So as to not confuse the masses, there are several terms here that need explanation and a little tie to FRC robots…
CCA is Cold Cranking Amps and is measured at 0 degrees F for 30 sec and is rated by how many amps during this test will bring the battery down to 7.2 volts. Wholly meaningless measurement for our use at normal indoor temps. Small batteries do not specify CCA for most manufacturers.
CA is cranking amps (@32 F degrees) in car battery specs but is charge/discharge rating in our battery specs. Not the same measurement! 1 CA~1 Amp Hour. For our 18 amp hr battery this is 18 amps but measured at 25C.
Amp Hour is a rating that specifies how long the battery will deliver current until the terminal voltage reaches a certain level (@25 degrees C and that level varies from manufacturer to manufacturer.) It does not specify the max current nor does it indicate a time at any current other than that shown. It is typically measured at 1/20 of the amp hr rating. That is an 18 amp hour battery delivering 1/20*18 amps= 0.9, amps will take 18 hours to reach ~8 volts. At higher currents, the time decreases. An Enersys is speced at only 6.5 minutes delivering 54 amps continuous to fall to 8 volts terminal voltage.
Internal Resistance is an equivalent resistance that causes the terminal voltage to drop in proportion to load current. This is due to the voltage drop (Ohm’s Law) across the internal resistance. This resistance varies with charge, temperature and age. A typical new, fully charged, robot battery will have 0.011 ohms of internal resistance or about the same resistance as 11 feet of #10 wire.
Battery Testers of this type use high current loads, 60 and 125 amps or simply 125 amps and produces a meter deflection to show good or bad. A car battery (.003 ohm) has an internal resistance typically 1/4 of our battery (.011 ohm) so this tester will not give you a good idea of the state of the smaller battery. I cringe at teams that use this battery tester as it will continue to load the battery as long as the switch is held “on” which under certain conditions can damage the battery.
The Battery Beak as stated above makes two quick measurements, one at 1 amp and one at 18 amps and uses those measurements to calculate the internal resistance and from there an indication of battery state. If a cell is dead or shorted, it will show up during this test. If the cell is reduced capacity, it cannot show it as it takes some period of time for the cell to discharge. Only a long term test can reveal reduced capacity. Fortunately, this type of battery failure is rare and usually is a result of rough handling (dropping). I have seen defective cells capable of producing current for half the time other cells produce. These batteries will reduce by 2 volts for each bad cell, their terminal voltage under load. A no load test (open circuit voltage) on these batteries will show good.
Long term testers such as the CBAIV will show all of these issues and you can vary the load and conditions to use it to test other batteries as well. The down side wilth any long term tester is the amount of time it takes, and that it must discharge the battery to perform the test. (See above post)

What method does your team currently use for determining the condition of batteries?

*Attached is a graph of the discharge curves at 77F for the Yuasa NP18-12B battery, taken from the Yuasa datasheet for the battery.

That battery is rated at 17.2 AH. So the 0.2 CA discharge curve would correspond to roughly 3.4 amps discharge rate. According to that curve, the battery voltage should remain above 11.5 volts at that discharge rate for 2 hours. That corresponds to roughly 6.8 AH discharged.

Now take a look at this CBAIII curve for a battery with a weak cell1. Note the sudden drop-off in voltage at about 3.2 AH into the test, caused by a weak cell.

125 amps for 15 seconds is only 0.5 AH, so it’s not obvious that such a test would identify the bad cell. In fact, I have a lawn tractor battery with a bad cell that passes the 125A test when freshly charged.

If you don’t have the budget for a CBA tester, there is a home-brew solution. Go to the auto junkyard and find a nice sealed beam2 36 watt 12V headlight. Hook it up to the battery and monitor the voltage under load for 2 hours3. That’s roughly 6 AH. If the loaded voltage drops below ~11.5 volts, stop the test. You may want to mark that battery not for competition use.

BTW, the lawn tractor battery mentioned above (which passes the 125A test) fails the 2 hour test.

1 reproduced from this post, for those who don’t have PDF reader installed

2sealed beam for safety (do not use a replaceable bulb). no cracks or chips or signs of damage. competent adult supervision required. always use safety glasses.

3digital voltmeter with sufficient accuracy can be had for $5 (or less, on sale) at Harbor Freight

Not exactly, Al. While the principle is the same, the Ah rating comes from the maximum constant discharge possible over a specified time period (20 or 10 hours, usually) falling to a specified terminal voltage. So an 18 Ah battery can deliver 0.9A for 20 hours, ending at about 8 volts. (But, in agreement with Al’s point, that does NOT mean 1.8 A for 10 hours…)

20 hours is usually used for larger batteries, 10 hours for smaller ones. You need to read the specs o see what the manufacturer used.

Also the exact ending voltage is a standard, but I don’t have that info handy at the moment. It is around 8 volts though.

There doesn’t seem to be a standard which is why the curves for the two legal batteries are similar but the ratings are different. If you overlay them the differences are minimal but the values are different. In looking at the curve linked in Ether’s post, you can see all the curves nose diving to 8 volts but many of the curves are truncated so as to not confuse the customer. The MK VLRA Technical Manual specifies amp hour based on 1.75 volts per cell.
“Ampere Hour Capacity at 20, 6, 3 and 1 Hour Rates:
Ampere hour capacity is a unit of measure that is calculated by
multiplying the current in amperes by the time in hours of discharge
to 1.75 volts per cell. These are nominal or average ratings.”

Thanks for all your responses! (Don, did you know that your team was sitting behind mine at Championships during Einstein matches? We were thrilled because our team numbers are sequential)

Al and Don,
I think I see the errors in my understanding. I was assuming it would be easy to track the contribution made by age (sulfation) to the internal resistance. I now understand that due to the multiple contributing factors (temperature, state of charge, amount of surface charge) to internal resistance, this is difficult to do. So I probably won’t use #3 for making decisions, but I am still curious to try it as an experiment (using a controlled process to measure internal resistance). I also see that #1 isn’t really doable because I don’t know the conditions during the manufacturer’s test.

So I think I was a little optimistic about tracking internal resistance measurements over time with the Battery Beak.

For #2, I gather that the most important thing is to make sure that you don’t use a battery that the Battery Beak measures as ‘Bad’. I think I see from the comments here that an internal resistance measurement of ‘Good’ or ‘Fair’ from the Battery Beak isn’t the same as a clean bill of health because this type of test cannot reveal reduced cell capacity.

Does this imply that a battery can have reduced capacity without having an increased/abnormal internal resistance?

(Btw, Al and Ether, thanks for your explanations of discovering reduced capacity via long term test)

My team’s method isn’t really controlled. It’s more like ‘This battery looks new, it’s probably good’ or ‘Oops, that battery died during the match, let’s not use it again’.

We don’t have a person who has the domain knowledge to create battery management processes and policies for us. And we really felt the consequences of that this past year. We started off the 2013 season with 6 new batteries and by the end of the season, not one inspires confidence for use in matches. We took all of our batteries to a neighbor team’s workshop and borrowed a machine that put a load on the batteries. We put a 200 A load on the batteries and not one of our batteries made it 5 seconds before dropping to unusable voltage. This did not contrast nicely with the neighbor team’s batteries which lasted much longer (around 35-45 seconds I think).

So I’m trying to gain the requisite knowledge to be able to step into this role.

The cells with reduced capacity that are shown in Ether’s post and my discussion are cells that for one reason or another have some of the plates within the cell damaged or detached. The change in internal resistance is very small but the cell has less available power density and so it dies out sooner than the other cells. I recommend that teams do not try to pull 200 amps from a battery for extended lengths of time. Yes, robots do pull more than that but it is for much shorter periods of time. (except in extended pushing with another robot) The effects of this high current, extended test, is to warp the internal structures of the cells and raise the internal temperature to the point where cell relief valves vent the higher pressure (and water vapor) to atmosphere. I also believe that the separator (absorbed glass mat) may become damaged as well producing another type of internal resistance that discharges the cell constantly. In these batteries, the terminal voltage will fall in multiples of 2 volts after sometime (several hours to a few days) from being removed from the charger. All of the test methods including a voltmeter will show this defect when the cell has become completely depleted. In worse case events, the discharge is extreme enough to cause heat, leakage, case swelling and steam.
Above all, remember that our batteries have a stated life of only 400 charge/discharge cycles under optimal conditions. Our severe use cause this life to be reduced to perhaps 300 cycles or less.

Al, thanks again!

I now have a much better understanding of what the Battery Beak is capable of telling me in terms of internal resistance.

Thankfully, pulling the 200A from the batteries was the first and only time we’ve done that (although I realize the risk for damage isn’t mitigated because of this). I’ll advise my team against any tests of that sort in the future.

I am also looking into the CBA IV from West Mountain Radio that you mentioned. Out of curiosity, would the CBA IV allow you to find the Peukert constant of a battery? I know the (approximate) value of the constant when the battery is new, but I also know that it changes as the battery ages.

Also, I’m interested in keeping to the 50% percent rule for our batteries. Assuming that rule applies to our robot batteries, will the Battery Beak assist me here? If I take a state of charge measurement before using a freshly charged battery, do some work with the battery, and then take another state of charge measurement from that battery, will I know the true amount of capacity removed from the battery (In units of % rather than Amp hours)?

I am guessing you are trying to make a judgement based on usage to predict battery life. There are some methods to do that but none are very accurate since the load varies with each match. The CBA will allow you to make a calculation to get Pleukert’s constant but it will take several tests at different loads to do that. Unfortunately, the life of the battery and the temperature are constants you cannot readily measure with any accuracy. I am not sure what your 50% rule is, can you elaborate? I do have some rules of thumb I consider in battery use. These are merely observations reduced to writing…

  1. If you deplete the battery in one two minute match, you have an inefficient drive system. If your robot suddenly starts to do this, you have a broken drive train.
  2. Don’t use 6 motor drives simply because you have they are the best. I can show you more teams that eat a battery than get a benefit from the multi-motor drive.
  3. If you are only using 4 motors and succumb to rule #1 above, you have chosen the wrong final gear ratio for the game. More important if you are only using two motors.
  4. A battery should last long enough to drive in two or three matches minimum before running down.
  5. Never trust any of the above rules, always use a freshly charged battery.
  6. Even batteries that come right off the charger can be bad and fail in a match.
  7. The robot is talking to you, listen to what it is telling you.

In troubleshooting a robot electrical issue, I watch the battery voltage monitor on the DS dashboard. If you are driving and the voltage is falling to 6 or 7 volts you are going to have a problem soon. At sustained voltages of less than 6 volts, the regulators in the digital sidecar start to drop out, killing drive to your robot. At less that 4.5 volts, the power supplies on the PD cease causing the Crio and/or the radio to reboot. There are a number of reasons that could cause this. Too much load current, dead battery, too long of a #6 wire run, improper crimp or loose connections on the battery, the main breaker or the PD, will all contribute to low voltage.

I think he means do not exceed 50% depth of discharge of the battery.