Battery Testing

[FIRST JerseyKid]

Does anyone have a good way to test a battery? I know there's a battery load tester, which I think tests for the internal resistance of the battery.

I think, that usually a battery is bad when it has a lot of internal resistance and when it can't hold a 'full' charge anymore, correct me if I'm wrong or missing anything.

Can someone explain to me how the Load Tester tests the battery, I have experience in ac and dc circuits and I think I have an idea of how they do it.

[KenWittlief]

funny you should post this, I just bought a used Marine VHF handheld radio on ebay, and Im sitting here testing NiCad batteries while I type.

A battery tester will only give you some indication of the present charge left in the battery. As the battery charge is used up the internal resistance does increase (as you mentioned). The result is when you hook the battery into a significant load, the voltage across the terminals is lower than it would be if the battery was fully charged.

For non-rechargeable batteries this is about all you can do. Momentarily connect the battery to a significant load, and measure the voltage. The resistance (load) that is significant for a battery depends on its size (capacity).

For rechargeable batteries, the best way to test them is to cycle them (this is what Im doing right now). You charge the battery fully, then you connect a constant load. For example, I am testing a 7.2V 600mAhr NiCad pack. I have it connected to a 10W 10 ohm resistor. I connect it to a DMM, write down the time. When the battery voltage falls to 6V (about 80%) I stop the test and note the elasped time.

The capacity of the battery is the averege current * the time duration. In my case the 600mAhr battery is putting out about 420mAhrs (lasting about 35 minutes). Not bad for an older battery, but not new either.

If you dont have time to sit and check the DMM every few minutes, there are data logging volt meters (like my Keithley 175) that you can setup to record the voltage once a second, or once a minute, or once every 10 minutes.... so you can let the test run and come back later and see how long it took for the battery to drop.

Usually you run the test with the battery loaded at a rate that will discharge it in one hour. For something like the Exide batteries in the FIRST KOP, that would be an 18 Amp load, and the battery should take about an hour to drop 80% of its voltage. (sometimes finding a good load is a challenge. 18A * 12V = 216W. That would be a pretty big resistor (expensive). For large 12V batteries, you can get 12V 50W light bulbs that fit in a regular light socket. They use them in campers and RVs - most RV stores carry them, and some places like Home Depot. Car headlights work well too)

Also, if a battery has not been used much, you might need to cycle it a few times (charge, drain, charge, drain...) to, well sorta wake it up.

[FIRST JerseyKid]

Okay. So my next thing is...

I want to design a board that I can plug battery's in and do a load test on them, take readings using an ADC, feed the ADC to a PIC chip that records the values and does math and outputs to an LCD.

Does it sound like a good idea and are there any special things I should know?

My thought is its mainly a voltage divider with my circuit having a large resistance and the battery having its resistance. The PIC should be able to calculate the resistance of the battery and the capacity...

your thoughts?

[KenWittlief]

so you mean like testing an alkaline battery to see how much energy is left?

Your description is correct if that is what you want to use it for. DuraCell has good spec sheets for their alkaline batteries on their website. You can use the graphs to estimate the energy left in the battery by measureing the voltage with a specific load on a battery.

So depending on whether you are testing a AAA, AA, D cell.... the load or the voltage curve will be different.

Duracell Alkaline website page: http://www.duracell.com/oem/primary/...lkcapacity.asp

[FIRST JerseyKid]

I want to try to test a battery's capacity and it's internal resistance.

[KenWittlief]

you mean, you want to run it until its dead, so you can say "EverReady AA's have 400mAhr of capacity, and Duracell AAs have 440mAhrs...."

that kind of testing?

also, do you mean rechargeable batteries, or primary cells, like alkalines?

[FIRST JerseyKid]

I want to read its fully charged energy capacity and it's internal resistance.

[KenWittlief]

someone else correct me if Im mistaken, but I dont think you cant take an instantanious reading on a rechargeable battery that will tell you its full-charge capacity.

The only way I know to do that is to run it through a full charge, discharge cycle, like I described in my first post.

For your idea, if you fully charged the battery, and then measured its internal resistance, that might an indication of how worn out it is. I think you would have to do extensive testing to determine the relationship between the internal resistance of a fully charged battery to its remaining capacity. Also the temperature of the battery would probably be a significant factor for your calculations.

And I think you would have to find that relationship for each size and type of battery you want to check: NiCads, NiMH, LiIon... AA, C, AAA...

it sure would be an interesting project!

[Richard Wallace]

West Mountain Radio makes a nice battery analyzer that works the way Ken describes. It came up in an earlier thread. Several teams have used this analyzer with good results.

[Al Skierkiewicz]

As Richard pointed out the West Mountain tester(CBA-II) will give a graphical display of the discharge characteristic of a battery. It can only discharge the robot batteries at about 7.5 amps but that is enough to show you the state of charge and it gives you the amp-hour rating of the battery. The beauty of these little devices is that for about $100 you get a nice little USB device that will save the data and print battery labels in a 3" cube. West Mountain has a new product (I think at the urging of the FIRST community) that will discharge at much higher currents but runs about $500. The tester will show if individual cells have reduced capacity and has given many teams an indication of a defective battery out of the box. If you are looking for internal resistance measurements there are fairly expensive devices that will do that. I know of a SnapOn product that tests state of charge and internal impedance in a few seconds of testing. It is about the size of a small brief case and the cost is pretty high.

If you are trying to model the discharge of the robot battery in use by somehow comparing or calculating from the amp-hour rating, that will be nearly impossible. There are so many factors that are involved in battery chemistry, discharge rate, peak current, etc. during a match that calculations are fruitless. Wildstang built a current test fixture that we used on several robots during the 2002-2003 seasons. It was able to measure current from several probes on the robot and main battery terminal voltage. The data was ported out through the dashboard port along with time marks provided from the RC so we could correlate current demand against time while watching replays of a the match in which the data was being generated. The data was surprising and we learned a lot about robot current draw but nothing that could lead us into a predictable battery life curve. We could accurately predict that certain robots would fail at some times during a match due to low battery voltage and we could also predict that certain robots would drain the battery by match end. Armed with that info, teams still took no action to correct fatal errors in their design. Thankfully, IFI saw the need to have a backup battery on the RC which didn't correct all problems but reduced the time it took for a robot to recover from a low voltage condition.
One thing to keep in mind at all times, the internal resistance of the battery is 11 milliohms, draw max current (400 amps) from the fully charged battery and the terminal voltage will drop to 7.5 volts. A robot that will draw high current throughout the match will cause that voltage to drop to perhaps as low as 4 volts later in the match when the battery is discharged. 4 volts=robot dead! Take that same battery out of the robot and measure with a DVM and it will read 12 volts, no load, but won't run another match.
Ken, you ought to look into the CBA-II it is a pretty cool device.

[Ben Piecuch]

I'll post this here, and in CD-swap as well... Does any team in the Connecticut area have one of these Battery Analyzers? We have, unfortunately, had some dead battery problems this year, and would love to see how our batteries perform. We're willing to drive anywhere in CT (and maybe even NY and MA) to have our batteries analyzed. Or, if anyone is coming to the Bash at the Beach in October, we could possibly do some testing there.

Please email or PM me here, thank!

BEN

Originally Posted by Richard

West Mountain Radio makes a nice battery analyzer that works the way Ken describes. It came up in an earlier thread. Several teams have used this analyzer with good results.

[Richard Wallace]

Originally Posted by Al Skierkiewicz

One thing to keep in mind at all times, the internal resistance of the battery is 11 milliohms, draw max current (400 amps) from the fully charged battery and the terminal voltage will drop to 7.5 volts. A robot that will draw high current throughout the match will cause that voltage to drop to perhaps as low as 4 volts later in the match when the battery is discharged. 4 volts=robot dead! Take that same battery out of the robot and measure with a DVM and it will read 12 volts, no load, but won't run another match.

Emphasis added.

Thanks for jumping in here, Al. I think it bears repeating that simple voltmeter measurements do not indicate very much about a battery's readiness. Too many teams neglect battery management and suffer the consequences.

[KenWittlief]

I think Eric has the right idea to make a small device that can measure the energy (charge) remaining in a battery, esp if it was designed to only work with one specific battery (the KOP Exide for example). Measuring the voltage with a load applied does give you a good indication of where you are on the discharge curve.

But now he has me wondering. I dont think you could come up with an instantanious check that will tell you how much capacity is left in the battery (ie, is this a new battery, or one that will only last 30 seconds after its been fully charged)

but I wonder if you could do a short charge, discharge sorta thing to a battery, and get some indication of how well it will hold a charge? Design a tester that would put 2A into the battery for, lets say, 30 seconds, then pull 2A back out for 30 seconds, and see how much the voltage changes?

That might be feasible. Eric might be on to something here.

[Al Skierkiewicz]

Ken,
Again there are too many variables for that. The age of the battery (how many charge discharge cycles) affects the surface composition of the plates. A battery capacity has a lot to do with how much of the plate area is actually exposed to the electrolyte. As the lead oxide builds up, less surface is in contact with the chemical. There are some testers available from automotive suppliers that put an instantaneous load on the battery and measure the current. Unfortunately, many of these devices are attempting to discharge over 100 amps and the meter isn't calibrated for a lower amp hour battery such as ours.
Ben, the CBA at 7.5 amps will take a few hours to discharge the battery to the specified terminal voltage of 8 volts. We will typically put a battery on test when we start the class time and watch it periodically throughout the night. Recent batteries have shown a failure in one or more cells over the discharge. Graphically, the battery will run at normal discharge until one of the cells stops producing current, at that point the output voltage falls by 2 volts. We have had at least one battery that had two cells with reduced capacity. Without doing a CSI on the battery, I suspect defective attachments to the individual plates in those cells. Reduced plate surface area=reduced capacity=reduced charge current/chemical action.
I don't have a graph here at work, but I do believe I have some at home. I will try to post this afternoon.

[Alan Anderson]

Originally Posted by KenWittlief

...Measuring the voltage with a load applied does give you a good indication of where you are on the discharge curve...

That presupposes that you know what the discharge curve looks like for that specific battery (with that specific load, at that specific temperature, etc.).

You can come close to measuring how much energy is left in a battery by looking at the voltage under load, but only if you have already well characterized that battery's behavior in advance. Devices such as the CBA-II can do that characterization unattended.