Ally,
The chances that you have a bunch of bad battery chargers is very unlikely.
I agree with the suggestions of turning on one thing at a time to identify a fault.
Another possibility: I have seen are loose connections at the battery causing high resistance and large voltage drops. Last year, I had a team at WPI with bad connections on all of their batteries…
Regards,
Mike
Ally,
There are a few answers here. The batteries that are from previous years, 2007 or earlier have likely reached their maximum life. If they cannot hold a charge, it is time to recycle them. Our batteries have a useful life span of a maximum of 400 charge/discharge cycles. If they are run hard (drained in one match) you can expect that to be much less. If you have used them for a significant number of demos and off-season play then it is likely that even this year’s batteries could be at the end of their life. A battery load tester can tell you for sure. A voltmeter cannot tell you anything about the battery without a load. The one I recommend is the CBA-II from West Mountain Radio. It is expensive but worth every penny. it even allows you to test the battery and overlay it with last year’s test of the same battery to track age differences. We have been using mine since 2004.
For those that might be wondering, yes batteries can fail right out of the box. Dropping, holding by the wires and poor storage can kill batteries. Over charging, higher than 6 amps, will also kill a battery no matter how much you think you are monitoring it. With AGM batteries, the higher voltage encountered with high current chargers, can punch through the glass mats and cause hot spots inside the battery.
Alright, I’ll check these things too, but I won’t be able to get into our workshop until January so it’ll take some time to find out what it is. I really hope we can find the problem before it accidently gets recycled and put on our new 2011 robot!
We have tried just having it plugged into one thing at a time. We tried all this during the elimination rounds at Ramp Riot this year. We currently have one or two batteries that actually work. The 2010 battery we got is one of the worst ones. I don’t remember exactly which battery works the best, but it was the only one consistently working for us.
A sharpie is a good thing to have on you at all times. Mark the suspect batteries as “practice only!” Mark your new ones with the date you opened the box and put them on the charger. Mark the side or the top, never the bottom. You will wear off important info if you mark the bottom.
Sammy,
Remove all the snap action breakers and then see what happens. The only thing running should be the Crio and the wireless access and the camera if you have one. If you have a camera, pull the plug on the bottom. If the battery doesn’t discharge, then add two breakers that feed your drive train and see what happens when you drive. If the battery stays up, add two more breakers and repeat. Typical electrical problems are frayed connections at the Crio power input connector or the power output connectors on the PD. The same is true for power feeding the sidecars and Crio interface cards. Check all the wiring on the PD as this is another area where a frayed wire will contact and adjacent lead.
Another common problem is through the camera which has a power supply common to the camera case. Plug the camera in last if you have one. When you are telling us the battery is falling to 8-9 volts is that reading on the dashboard/Classmate or with an actual voltmeter across the battery terminals? It is not uncommon for there to be a failed 50 amp battery connector on the robot side that could be dropping the voltage. A bent contact, a poor crimp, a damaged terminal on the PD all could give the effect of low voltage to the robot.
I believe we have tried using a batter load tester. I’m not totally sure, but I think so. As Ally said, we are unable to get into the workshop until January to test the batteries any further.
Thanks everyone for all these ideas! We’ve been kinda lost about what is wrong and where to go from there, but these should really help us out
Each battery has a number (as well as the year we got them) and we are planning on keeping a record of the voltages of each battery/how they ran when we tested them.
In 2008, we did 2regionals and a scrimmage before atlanta, and had the battery mounted on it’s flat side in the robot. We then lost two matches at Atlanta because two of the six batteries we had been using would die during a match. They would read 13volts on a meter before a match, but under load it dropped to 6 volts or so. We eventually figured out that one or more of the glass plates within the battery must have cracked or have been compromised. We now use a 50 dollar load tester from napa auto parts to make sure the batteries are still ok when at competition.
New students see those batteries as industructible heavy objects and don’t realize how careful they need to be with them. I also recommend mounting them as vertical as possible to reduce the stress on the plates inside.
You have any of several issues:
You’ve read several suggestions to eliminate #3.
#1 is indeed unlikely, but possible. After a full charge (as indicated by the charger) measure the battery voltage with a multimeter. A normal reading for a battery just off the charger would be over 13.5 volts. Let the battery sit for a day, to eliminate the ‘surface charge’ and a normal reading should 12.6 volts.
#2 is easily determined with a load test. Al’s recommendation of a West Mountain Radio CBA is excellent, but they are not free.
Instead, take about 10 feet of #10 wire (or 8 feet of #12), which should be about 1 Ohm. Measure the length of wire with an Ohmmeter (be sure to comensate for the resistance of the meter’s test leads) to verify about 1.0 Ohms. (A high-wattage 1 Ohm resistor (25 Watts capacity) will do as well)
While measuring the voltage across the battery terminals, connect the 1 Ohm load (the wire or resistor) also across the battery’s terminals. Count exactly 15 seconds. Write down the battery voltage at the 15 second mark.
A Very Good battery will read over 10 volts. An OK battery will read over 8.5 volts. Bad battery will drop below 8.5 volts. (NOTE: These numbers are debatable, but that’s what I use).
If in 15 seconds with a 1 Ohm load your batteries all drop below 8.5 volts, and start above 12.5, they’re bad. Buy new ones.
What causes a Lead-Acid battery to go “Bad”? Other than usage, sitting in a discharged state (terminal voltage less than 12.4 volts) for any period of time. Like over the summer. This kind of battery loses 50% of its charge in 90 days. On our team, we make sure they don’t go more than 60 days without getting charged. Even then, we lose 3-4 batteries each year.
OK Sammy,
We will be waiting for a report in January. There is a possibility that you have one or more failed chargers and that in the normal course of events, all batteries have been touched by the bad charger. That is unfortunate but should not be ruled out. I am going to guess that the batteries have been used often and are just dying of old age.
Doug, the glass is a woven mat between the plates so there is really nothing to break. The lead plates can become dislodged with abuse and break off the common terminal inside a cell. In this case, the battery will test normal even with a momentary load tester. The CBA will show this damaged cell by a drop of 2 volts after a period of time. Black in the attached plot is a normal (2 year old) battery showing 16 amp hour capacity, the blue shows a two cell failure, green shows a single cell failure and the red shows the signs of intermittent plate connections within a cell followed by several cell failures.
Don, any advice on safe (ie sparks and heat) and effective (ie minimize extra resistance due to poor connection) ways to do this ?
Also, where did the “10 volts after 15 seconds at 12 amps” rule come from?
According to the datasheet, a new battery should still read 12 volts after 16 minutes at 18 amps.
**
Let me give a short thanks to Al and another plug for the CBA (computerized battery tester).
http://www.westmountainradio.com/product_info.php?products_id=cba3
We’ve used this at his suggestion now for 3 years, and even gone so far as to rate our batteries 1-10 based on their tested capacity. It takes a while, but is definitely worth it to weed out the batteries that are simply worn out.
He said to measure during discharge. The datasheet values are after discharge has stopped.
Joe,
As the datasheet does not give all the measurement specs, I would have interpreted this as load curves with calibrated load resistors. Generally, with the load removed the terminal voltage would rise. With a high impedance voltmeter, there is no voltage dropped across the internal impedance of the battery.
I interpreted Don’s #10 wire test as a go/no go test for his team, not a definitive battery test.
“He said to measure during discharge.” He did indeed. It would still be informative to know where the “10 volts after 15 seconds at 12 amps” rule came from. Is this a home-grown FRC metric based on experience? Or is it published somewhere?
“The datasheet values are after discharge has stopped.” The datasheet didn’t mention that fact anywhere. I guess this must be a common understanding among those who work with battery specs? It would be interesting to know how long the testing is paused at each datapoint to allow the voltage to rise before recording it. Perhaps that’s part of some battery-industry spec. Also, question for Al: in the CBA graph you posted, are the voltages open-circuit voltages or are they volts under load?
**
The graph you posted in this thread was for 1 amp load. Do you have a 12 amp graph (of the FRC battery) you could post?
Also, as I understand it the CBA is computer-controlled. Is it possible that it is momentarily opening the circuit before taking each voltage reading?
**
The curves are generated at 7 amps or about 0.5 C (see curve menu at right side of graphs). From the CBA webpage…
“Unlike a resistor load it has an electronic constant current load that is controlled both with software and electronics.”
I don’t think taking a reading after removing the load will give any valid information. I do not recommend that anyone use a voltmeter alone to evaluate the state of charge on an SLA battery. The input impedance of modern voltmeters is sufficiently high to prevent an accurate reading across the internal impedance of the battery. As this “resistance” changes with battery age, temperature and state of charge, the load test will tell exactly what the battery is capable of delivering in a curve that can be compared with both the published battery data and with past tests of the same battery. The one amp shown was generated when I overlayed the four previous tested battery curves onto a single page within the program.
Don suggested applying a 12 amp load for 15 seconds and measuring the voltage under load. Above 10 volts is Very Good. 29
Ether cited the battery discharge graph in the datasheet for the FRC battery which shows 12 volts after 16 minutes at 18 amps. 31
Joe stated that the voltages in the datasheet graph are taken open circuit. 33
Al stated that he believes the datasheet graph is showing load curves 34, and that to his knowledge the CBA graphs are load curves too. 36
Al’s 7 amp graphs show a healthy battery still reading over 12 volts (under load) after 43 minutes at 7 amps. Even a very sick battery (blue line) still reads over 12 volts (under load) after 17 minutes at 7 amps. 30
Can somebody help sort this out ?
**
I think you’re off by a factor of 100, Don. Isn’t ten feet of #10 wire actually more like 10 milliohms?
(A single ohm would only draw about 12 amps, which isn’t nearly enough to give useful information about the health of a battery after just fifteen seconds.)
