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

[Ether]

Originally Posted by evanperryg

We figured it was a fluke and threw it on the charger. about 5 minutes later, I go and check this battery to see if it is ok. The thing is hot(as in burn-your-hands hot) and still at 8v.

What model charger were you using?

[Al Skierkiewicz]

[quote=evanperryg;1287356...We figured it was a fluke and threw it on the charger. about 5 minutes later, I go and check this battery to see if it is ok. The thing is hot(as in burn-your-hands hot) and still at 8v. Long story short, nothing is a fluke with a battery.[/quote]
This is not unusual, the battery shorted two cells internally and the heat produced was coming from the energy conversion in those two cells. The battery was likely producing steam and vapor as well. Often a close examination of the battery case would reveal evidence a drop or severe shock.

[Ether]

Quick pic of poor man's battery discharge tester.

Battery voltage is on the left. Battery amps on the right^*. Meters can be had for less than $5 each at Harbor Freight.

Sealed beam headlight is from my 68 Camaro, I think. Found it in a pile of old auto parts. Not quite 36 watts.

Battery is from my lawn tractor.

For actual use, the wiring should be cleaned up, and the headlight secured safely on a fireproof surface.

sealed 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.



^*The ammeter is just to confirm the lamp's wattage. You don't need the ammeter for the test if you know the wattage.

[WinDnDusT]

Originally Posted by Ether

It's an interesting question. I haven't seen this specific question answered here on CD.

Are there any teams out there who regularly measured and recorded internal resistance and state-of-charge using battery beak, before and after each match? Would you be willing to post your data (and procedure^*) here?


^*for example, how long did you wait after charging (to let surface charge dissipate) to measure the before-match values, and how long did you wait after the match (to let the battery stabilize) to measure the values

No exact answer yet. This might mean an opportunity to conduct a science experiment with the Battery Beak.

[WinDnDusT]

Al,

Thanks for all the explanation and rule of thumb list.
I wasn't originally thinking about using a tool to measure the current draw of the robot but now I'm curious about it. Although I understand the difficulties you mention in finding the right tool for the job.

How would a team figure out the robot is drawing too much current from the battery (before repeated abuse kills the battery)? It sounds like there isn't an easy way to figure this out so most teams don't think about it and get lucky/unlucky.



Hugh,
Thanks for all the info. It's very useful to see actual data measurements.
Now I know what to expect when working with our own batteries. I might use these templates too.

[Ether]

Originally Posted by WinDnDusT

How would a team figure out the robot is drawing too much current from the battery

Use an analog input to measure voltage drop across the main breaker, or across the wire between the breaker and battery (-). In software, run the samples thru a simple low-pass IIR filter and monitor that, and/or integrate the samples to get a rough metric of depth of discharge.

[Al Skierkiewicz]

Win,
The more than one match rule is hard to beat. If you go through several of your batteries and each dies towards the end of 2.5 minutes in normal driving, you are too inefficient. If the battery dies before the end of the match (exhibited by robot drive "stutter", Crio reboot and/or loss of comms) you are severely shortening the life of your batteries. I do not have any hard data on this but my gut (observation and anecdotal reports) tells me it is likley reducing by a third to a half the lifespan of a typical battery.

[AllenGregoryIV]

What do people consider the typical lifespan of a battery? I have teams ask me this and I never have a good answer since it depends on so many factors.

[Al Skierkiewicz]

According to the battery spec sheet, it is 400 charge/discharge cycles under normal conditions.

[Ether]

Originally Posted by AllenGregoryIV

What do people consider the typical lifespan of a battery? I have teams ask me this and I never have a good answer since it depends on so many factors.

From the MK spec for the ES17-12:

200 cycles...100% depth of discharge
225 cycles....80% depth of discharge
500 cycles....50% depth of discharge

[evanperryg]

Originally Posted by AllenGregoryIV

What do people consider the typical lifespan of a battery? I have teams ask me this and I never have a good answer since it depends on so many factors.

We don't really do it very scientifically. Basically, if the battery is over 3 years old(or underperforms moderately), we designate it as a 'practice battery,' one that still can hold a charge up to around 11.5v but not one that we would use in a match. They are very useful, as a power source for our scouting system, in the shop when we don't need the nice batteries, or in applications that don't need a good battery(robot cart underglow, for example). If a battery is severely underperforming, it is designated as a 'doorstop,' and it is left to sit until the next hazardous materials pickup.

[MrRiedemanJACC]

Ether,

Thanks for the pic of the homebrew test information. Our team is looking at purchasing the battery beak, but they are out of stock at Andymark (could get it direct from Cross the Road, but we still have credit left at Andymark). So I'm thinking about trying your test as an alternative to purchasing the West Mountain CBA (will try to get one of those next year).

Also, there are three connections on the back of the headlight I have, which is also from an old Camaro - but a '79. The light is labeled GEH6024. Do you happen to know what each one might be for? I'm assuming high beam, low beam, and ground? But I'm not having much luck in finding a diagram to know which is which.

Any other suggestions on the data we should take? I was thinking that we should record Volts and Amps every 10 minutes for the 2 hours). But from your previous post it sounds as though the 11.5 volts at the end of 2 hours is our determining factor to tell if the battery should be used or not.

Thanks for your and others help on this topic.

[Ether]

I believe you are correct that the 3 terminals are hi, lo, and ground. Connect the battery thru your ammeter¹ to two of the terminals and measure amps. Whichever gives you closest to 3 amps is what you want.

You can measure the volts periodically if you wish. Use the manufacturer's published discharge curves as a reference for what the voltage should be² as time progresses. If the battery voltage drops well below where it should be, you can stop the test - the battery is weak. If you're getting good readings and you stop the test too soon, you may get a false result - you may think the battery is OK when in fact it is not.

WEAR SAFETY GLASSES and FOLLOW ALL SAFETY PROCEDURES.


¹ make sure the ammeter is set to 10 amp (or more) scale or you risk burning it up.

² ask if you need help deciphering the curves