Re: Which Battery to Buy?
 

This is the reason I can't recommend a Werker brand battery in this application. Ours were hand picked from separate shipments and both showed the same problems. Not a good sign for Werker in FRC. Their large batteries for NECA racing were great however. I will have to check on the tester as it is expensive and they don't let me play with it much. It lives at another mentor's house who owns it.

Edit: This is one of the ones that we use to evaluate our batteries. It is a micro-processor controlled tester that measures the internal resistance of the battery at no load, while measuring the voltage.

Re: Which Battery to Buy?
 

Andrew,
The linked tester looks interesting but the documentation doesn't really document how each test is performed. I have contacted the manufacturer to see if I can get any additional info. My suspicion is that the load test is performed for a very short period of time that is not controlled by the human tester. That makes the test fairly benign. The PBT-50 looks like a more appropriate tester for our size batteries but both should work.

Re: Which Battery to Buy?
 

This stuff is all fairly far outside of my wheelhouse. I know how batteries work and I can take care of them well enough, but my desire to get into the numbers and specifics of testing and data logging is about as strong as my daughter's desire to clean her room.

With that said, our electronics mentor told me that it doesn't technically do a load test. It finds the internal resistance and measures the voltage, then works as a calculator to work through a little ohm's law and gets it's answer. The only downside of the lower models of the Midtronics is that the batteries we use don't generate enough amperage to fully make use of the test. For example, they will most always return a reading of their being a bad or failed cell in the battery simply due to the size of the battery. The PBT-300 allows you to look down to 100 CCA which allows our batteries to be tested to our needs. We will have to test the PBT-50 sometime though and see how it works.

He did mention that the West Mountain Radio tester is something he would be interested in as he has used them before, but $200 is a lot of money for us and we need to make sure we can fund registration before we look at anything like that.

Re: Which Battery to Buy?
 

Andrew,
I received a very informative reply from Midtronics Tech Support on these testers. It is my understanding from the reply that the CCA test does not use a load and does not require a fully charged battery. The test makes a calculation based on a "conductance" test. The test generates an AC waveform that is used to determine the internal conductance of the battery. This is an indication of the available plate area within the battery. That value is then used to generate a CCA display in the unit. This value when used with other tests that the device performs can determine the true health of the battery without a lengthy test. The manufacturer does recommend that any odd readings be performed a second time after the battery is fully charged as well. The full set of readings, if interpreted correctly, can point to a battery with a failing cell or a battery that is nearing end of life and therefore good for practice but not competition worthy.
Tech support also provided me with a document titled "Transportation Battery Testing Q&A" that describes the test in more detail. I also did a search of their website and found this page...
http://www.midtronics.com/transporta...t/technologies
It has several informative articles on batteries and testing. My contact suggested that the larger tester may not give you the most accurate readings on FRC batteries as they are designed with "wet" cell batteries in mind. Since our batteries are not specified in CCA, users will have to come up with a "good" CCA value when using this test.
I would like to thank Midtronics tech support for their helpful and hasty reply. I was fairly impressed with their website and product line. It is easy to see why they are the official battery tester for NASCAR. I would like to see one of these testers in use if anyone has a unit at an event I attend.
Tech support suggested that a MDX-640 might be a better device if a team is looking at Midtronics testers.

Re: Which Battery to Buy?
 

Al,

Thanks for the update, it is a nice little unit and seems to do a great job for us so far. Something seemed wrong to me when I said internal resistance. Conductance sounds better.

The only funny reading we get is the meter tells us we have a failed cell no matter what. Our established CCA value for a competition ready battery is anything over 180 CCA. If they drop below that they are used for practice or for emergencies. A good new battery will typically be in the 230-250 CCA range, with some higher and some lower.

Re: Which Battery to Buy?
 

Andrew,
I can't give you any ideas on your problem at the moment. The failed cell indicator may be coming from the test measurement as it compares to a wet cell auto battery. The tester uses the conductance test to make these determinations. The doc I saw specifically relates the conductance to the available plate area. As a battery ages, sulfation and other things affect how much plate area is exposed to the electrolyte. This is proportional to the available current. The less area, the less available current.

Re: Which Battery to Buy?
 

Tangentially related, I can't wait for the price of LiFePO4 batteries to drop a tad more and then we can skip this lead nonsense completely :cool:

For comparison, the batteries we use are really only 3-6 AH in the FRC context (their apparent capacity is reduced as a function of current draw. This is demonstrated on most manufacturers spec sheets).

The only advantage of lead currently is it's peak current capacity is pretty high compared to it's actual capacity (it's C rating) per dollar.

We'd need to use a LiFePO4 pack larger than 6 AH just to get the peak current we'd need. That's not a huge issue though, it'd let teams run the same battery for multiple matches. Teams like ours would drop from 12 batteries to 3-4, which at current prices makes them cost essentially the same.

We'd also be replacing them less often annually. I think with that they'd actually SAVE money at this point already.

Re: Which Battery to Buy?
 

Learned my lesson with assumptions :rolleyes:

But I agree with this 1000%. Cannot wait to get to lithium, hell even NiMH would be better.....

I may have rigged a LiPo Pack (do not do this...) for my team's FRC bot in the past for in lab testing uses only and man that was heavenly.....

Re: Which Battery to Buy?
 

The price of LiFePO4 (Lithhium Polymer) and LiCoO2 (Lithium Ion) batteries should come down quite a bit in no time, as Tesla's working on creating a large factory in Nevada.

With the massive supply it will create, we should see prices drop quite a bit.

I can't wait till all of this is complete!

Re: Which Battery to Buy?
 

Except for one thing.

Tesla's focus is going to be on making batteries for Tesla vehicles, or I miss my guess. Not a general "let's make lithium batteries" plant.

Now, I'm not going to rule out them getting some of the technology down and releasing it under patent or what-have-you and someone else building a "let's make lithium batteries" plant, but that'll probably be about 5-10 years after it opens at the shortest, 17-20 years more likely.

That said, I know nothing about what Tesla's internal workings are and even less about making a lithium battery. So I could be totally off the mark.

Re: Which Battery to Buy?
 

They actually are partnering with Panasonic heavily, and intend to sell (or Panasonic sells, I forget the exact details) batteries to all markets.

Re: Which Battery to Buy?
 

I agree. Tesla wants to make batteries primarily for it's own vehicles. However, I am sure that they will devote a small portion of their factory to produce consumer-grade batteries with it's own technology.

Re: Which Battery to Buy?
 

Realize Tesla wouldn't exist on car sales alone. A large chunk of its viability is from tax incentives & selling electric car credits to conventional car companies. One hopes it is a seed to bigger & better things. For the battery factory to be successful, at least in the near term they are going to have to make batteries for other than Tesla cars.

We run a LiFe battery on our T Shirt cannon. It great. Lasts for a Football game that normally takes 2-3 FRC batteries. A bit of a pain since it takes a separate charger.

Re: Which Battery to Buy?
 

That is true that it takes a separate charger. My project mentor for Engineering works for SRP and he told me that Lithium batteries are quite safe when used PROPERLY. The only difficulty is charging because as soon as it is full and you continue to dump charge into the battery, it heats up quite rapidly and goes through thermal runaway. This can result in catastrophic events such as rupturing and the spewing of poisonous gasses. However, one nice benefit of Lithium batteries is that they can output a lot of current safely (as long as the wires can handle it). With these SLA batteries, to pull large amounts of currents, we need large batteries, which escalate in price quite quickly. You can get Lithium batteries off HobbyKing, which can provide a much higher current output, and that too for a reasonable price.
Also, one of the nice things of Lithium batteries is that if they are well-maintained, their discharge voltage is quite stable until depleted, where they drop down significantly.

Also, I don't think Lithium batteries require as much maintenance as SLA batteries, which need to be fully cycled every few weeks to maintain it's quality. As long as you bring the voltage down to between 3.7-3.85v per cell, it can be stored with almost no worries.

Also, the self-discharge characteristics of Lithium-based batteries are very good, with a very slow self-discharge rate. Just stow your battery at 3.85v and it should be in perfect conditions after a long summer break, that is if you make sure they are still in the 3.7-3.85v range when you get back.

There are some other emerging battery technologies coming together. My mentor had spoken about Sodium (metal) Chloride batteries, where it uses the energy output from the reaction between sodium (Na+) and chloride (Cl-) to power the systems, and probably some sort of electrolysis to break up the molecules again. These batteries have a magnificent charge capacity, but come with the drawback that they need to be heated to around 300 degrees celcius to melt the sodium.

Please feel free to let me know if I stated any wrong information. A lot of this is from when I was doing battery research for the MicroCar that I want to design.

Re: Which Battery to Buy?
 

Yash,
Those first two things are some of the reasons we don't want them on FRC robots. Our batteries get dropped a lot during competition.

I am not sure what you mean by SLA battery maintenance. These are sealed batteries and there is nothing you can do for maintenance except to charge. While they likely will not tolerate no use for a year or more, a few months is not a problem.
I recommend you check out the Battery University site for some really good info on batteries.