Re: First: 2008 Batteries are D-Y-I-N-G.
 

One of our games in the quals in Israel were affected by that dead cell battery.

The electroincs team-member swore to me 1000 times that he saw it full.

and it did say so later, but on the field it didn't show that way.

Re: First: 2008 Batteries are D-Y-I-N-G.
 

We had one bad battery this year, but that was due to the top of the battery coming loose and leaking somewhere. We disposed of it properly, but it took a few minutes to figure out why it wasn't working. I noticed that the top was lose, peeled it back, found a rubber cap missing and an empty cell.

Speaking of battery problems. Anyone know why the battery on my laptop will last for 30 minutes at 70+% then instantly (and I really many instantly) go to 7%. It's about a year old.

Re: First: 2008 Batteries are D-Y-I-N-G.
 

Please add another team to the list of documented issues. We also competed at Kettering this weekend and had batteries go from 12V to 5 or 6 during the match. They were extremely hot when we took them off. We only have 07 and 08 batteries as we are a newer team. We will segregate and monitor. Is there anyway to absolutely check for a "dead" cell?
Thanks,
Jean-Ann Webster
Mentor HAZMATs - 2145

Re: First: 2008 Batteries are D-Y-I-N-G.
 

Jean,
These batteries are made up of 6, 2 volt cells in series. A bad cell will be indicated by a two volt drop in terminal voltage. If the cell is truly dead, the battery will read 10 volts or slightly higher when removed from the charger but not higher than 12 volts. Each cell is made up of several plates, interleaved positive and negative, with a glass mat separating the plates. Should the electrical connection to any of the plates be broken then the cell will have reduced capacity. If any of the plates should be touching then the cell will self discharge causing high internal heat. There are two tests which will determine which defect is causing the problem.
Charge the battery and remove from the charger. Attach a voltmeter and monitor the terminal voltage. A good battery will show slightly higher than 12 volts for several minutes and then fall to 12 volts. If it begins to fall below 12 volts in the first 30-60 minutes and then remains at 10 volts (or a multiple of 2 volts), you have a battery with damaged plates touching in an individual cell. One area of the case should be warmer than the rest of the case. If the battery remains at 12 volts, then you must load test it. We use the Mountain Radio CBA-II. With this computer based tester, you can draw a steady 7 amps and chart the terminal voltage and it will calculate the amp-hour rating. A cell with a few broken plates will be indicated by an drop in terminal voltage of 2 volts before the battery becomes discharged. The following picture compares three bad batteries with a good battery using the CBA. You will see in this graph the normal discharge curve in black, two highly reduced capacity cells in blue, a single reduced capacity cell in green and a internittant cell in red. The "green" battery was reported to be suspect during practice at home where the robot was run for 15 minutes at a time. The "red" and "blue" batteries were also discoveered during practice but the intermittant nature of the red battery confused everyone until this test. Without the constant discharge, all of the batteries showed as 12 volts at the OI, when the robot was not moving except the "red" which showed a varying voltage.
It is likely, all of you have batteries that have one or more cells that are diminished capacity without knowing it. This is particularly true of batteries that are from 2006 or earlier due to the number of charge/discharge cycles they have been put through.

Re: First: 2008 Batteries are D-Y-I-N-G.
 

33 has had issues with a couple batteries, but we determined it was due to really hard impacts. This years game lended itself to some rather spectacular crashes. 1 battery had a visible deformation, and the other had a noticeable hot spot right after the match. As Al said we suspect internal damage on both of those batteries (the bulged one was rather obvious.

As pointed out by others, make sure you read up on your batteries, Lead acid, NiCD, NiMH, and Li-Ion all have unique peculiarities to them. Some like to be deep cycled, others will get destroyed doing this. Some have memory, some don't. Some give off a superficial full-charge when really they aren't full. Assuming whats good for one type is good for another can be very dangerous and at a minimum, costly.

Re: First: 2008 Batteries are D-Y-I-N-G.
 

1075 was also at Kettering this weekend (Thanks 910, 67, and 2619), and we too had battery issues. In the week leading up, we had a battery that showed full charge and dropped immediately under any load (even the RC was enough to instantly kill it). We've had a couple of them go bad with swelling on chargers, including 1 this weekend that wasnt even ours, we had borrowed it from another local team and brought it with us since we knew ours suck. I gotta say, i'm not a big fan of the MK batteries. The Exides took a much harsher beating and were fine. We still have at least one of the old Exides in active service for practice and off-season events. Its the best battery of all the ones in active service and based on its case is no less than 4-5 years old (I think it was a 2004 kit battery).

That being said, we've swelled a few of the old exides too, but I think we only have 1 good MK now, and we've had 6 of them.

Re: First: 2008 Batteries are D-Y-I-N-G.
 

Sounds like you have too powerful of a charger.

Re: First: 2008 Batteries are D-Y-I-N-G.
 

Using a lower current charger will extend the life of your batteries. However, I think Al has shown that this is a separate, more painful problem.

Re: First: 2008 Batteries are D-Y-I-N-G.
 

If you check the MK battery site you will find that the team battery specifies charging at 5.4 amps. That is why the robot rules have stipulated a smart charger of 6 amps or less as the only charger allowed for batteries on an FRC robot.

Re: First: 2008 Batteries are D-Y-I-N-G.
 

If it has a Li based battery, I have heard that if they are deeply discharged, they will show an artificially high charge level right after being pulled off of a charge cycle. I know this is tru of my cell phone. If it dies, it takes a really long time to get a full charge in, however it says full after only a short while, it will completely discharge in a short while.

Re: First: 2008 Batteries are D-Y-I-N-G.
 

At competition (where the borrowed robot swelled) we ONLY use the kit chargers (this year and previous years). At home we have occasionally used our bigass charger designed for car batteries (Motomaster 2A/35A/200A), but we only use it when time is of the essence and we dont have any charged batteries because we know its bad for the batteries.

In any case, I think the Exides were built to a much more robust standard, which is why we're able to put them through so much more abuse. The MKs just don't have the durability required to stand up to a full lifespan in an FRC bot.

Obviously, we don't leave batteries on a charger like that for more than a couple minutes at a time, that would just be stupid.

Re: First: 2008 Batteries are D-Y-I-N-G.
 

I can't even begin to describe how bad an idea this is: This practice is a liability.

Re: First: 2008 Batteries are D-Y-I-N-G.
 

When charging any battery, some of the charge current is given up as heat within the battery. When you try to ram a lot of current, the internal heat does not have time to migrate to the outside of the battery case. So you may think by checking the case that the battery is doing fine at the higher current. However the internal (spot) heat may have exceeded the boiling point for the electrolyte causing excess gas pressure and spot heating of the plates. If the electrolyte actually boils enough, the plate becomes coated with residue which reduces the available plate area and therefore reduces the amount of current density in the battery. At some point, the pressure relief valve may also open releasing some moisture. The max current specification on the battery is dependent on the amount of electrolyte in contact with the plates. If you reduce the plate area, the amount of available electrolyte or raise the temperature of the battery, the output current is affected.

Re: First: 2008 Batteries are D-Y-I-N-G.
 

I've been doing FRC for 8 years, and without a doubt, the Exides we used 01-06, we're so much more reliable. Last year, by the time we got to Atlanta, our batteries (unknown to me at the time) were so unreliable. I know I'm not a battery expert, but I thought I knew enough about keeping them charged, marked, and tested to know that we can use them on our robot. Unfortunately we lost two matches due to a low battery problem. We all blamed our battery crew - they swore it was a well charged battery - and later we determined that two of the batteries will reading 13+ Volts and then drop to 6-8 Volts within seconds of the start of a match. Very frustrating. The problem definitely got worse as time went on - most likely to violent robot action/crashing (or a student dropping/rough handling).

I'm glad to know I'm not the only one unhappy with the MK batteries from last year.

Re: First: 2008 Batteries are D-Y-I-N-G.
 

That can also be an artifact of the way battery charge level is calculated by the computer.
Think of pink fiberglass insulation, but squished a bit to make them somewhat stiff (but not rigid). Some call AGM (Absorbed Glass Mat) batteries by the name "Fleece" batteries, the fiberglass sheets look like wool fleece.
That indicates a broken electrical connection inside the battery, either between cells or from cell to terminal. Usually caused by mishandling. The connection can handle the microamps of the voltmeter, but not the milliamps (or more0 of a real load.

Don

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