2013 EnerSys Genesis NP18-12B KOP Battery/FIRST Choice Weak?

[Al Skierkiewicz]

Guys,
We chose the 7.5 amp/0.4C discharge rate as it gave an fairly accurate calculation of the design AH rating for the battery while using a test that lasts a little over two hours. We experimented with adding a shunt load to increase the discharge current and as expected the AH rating went down nearly identical to published curves from the manufacturer for the higher discharge rate. Remember that the 18 AH rating is generally accepted for discharge rates in 0.1 C range. Discharge rates above 0.4C will give quicker results but will not display anywhere near the rated AH.

[Hugh Meyer]

Chris,

Our new KOP battery tested at 15.3 AmpHours. The Battery Beak Rint was 0.018 ohms and the charge % was 105. This is on the low side of normal. Most of our batteries test in the 14 - 17 range. This was the first test. Sometimes we see a subsequent test of a new battery measure higher.

Attached is the data from the CBA tester. You should get one. They are a great tool!

-Hugh

[Chris_Elston]

Thanks Hugh...so it was LOWER than normal huh? Ok thanks for confirming that.

[Ether]

Does anyone know: What causes the sharp bend in the curve at 11 volts? What is going on inside the battery at that point?

[AhAhPatel]

Originally Posted by RyanN

Over the years, Team Fusion has received a few defective EnerSys Genesis batteries. Enough defective ones that we don't even consider using them now.

By defective, I mean the internal resistance is too high to run a motor. They read a good voltage powering the cRIO and other small loads, but as soon as a heavy load is applied, our robot would reset.

Our number's aren't that high though. Maybe 3 or so defective ones? We also had problems with them splitting open on year, while the MK batteries seemed to not split open at all. (2010, side mounted cradle... it was a bad design).

Are we required to use the batteries in the KOP or are there other options?

[FrankJ]

Originally Posted by AhAhPatel

Are we required to use the batteries in the KOP or are there other options?

You are required to use the battery specified in the rules. So you do not have to use the battery specifically supplied in the KOP, but do have to use one of these two:
A. MK Battery (P/N: ES17-12) or

B. EnerSys (P/N: NP 18-12)

[protoserge]

Be wary of the cheaper "replacement" batteries. While these are pretty much identical to the legal batteries in terms of performance, they are not legal.

I'd still recommend picking a few of the replacement batteries up for driver training and non-competition practice since you won't be abusing your competition batteries and extend their life. They're also about half cost.

[JDL]

I load tested ours. Poor results, dropped from float voltage to 10.9 volts instantly, never even triggered my scopemeter that was set at to trigger at 50 amps. It's back on the charger and will be tested again this weekend.

[Al Skierkiewicz]

JDL,
Instantaneous drops of 2 volts signal a defective cell. Check the archives for one of my previous posts on CBA curves showing several of these types of failures. Although some of the batteries were pretty old at the time, most drops on new batteries are due to mishandling. Often dropped from a distance onto one corner. My intuition tells me that several plates break off the connector plate within the cell resulting in a short across some or all of the cell.

[JDL]

That's what I figured too. The corners looked ok, but the fact they just hand you a battery sans box or anything at kick off doesn't inspire confidence in me that they are well cared for.

[Gregor]

Originally Posted by JDL

That's what I figured too. The corners looked ok, but the fact they just hand you a battery sans box or anything at kick off doesn't inspire confidence in me that they are well cared fore.

It should have been in your black KoP tote.

[JDL]

Originally Posted by Gregor

It should have been in your black KoP tote.

Oh is that how they did it this year? Some years we've just been handed a loose battery. Still just tossing the thing in with all the other junk ain't great.

[sanddrag]

Before buying new batteries, is there a definitive word on which is better, the genesis or the MK?

[Al Skierkiewicz]

Both batteries are identical in that they are the same size, weight and construction, i.e. AGM. The slight differences in performance specs are due to the different test methods used by the two companies and the specification chosen for amp-hour and discharge. Actually, all batteries in this size will have nearly identical performance and will carry specs in the 18 amp hour range.

[simpsonboy77]

Originally Posted by Ether

Does anyone know: What causes the sharp bend in the curve at 11 volts? What is going on inside the battery at that point?

I pulled off my chem book from the shelf so I don't give out false information. I am also going to assume a semi-ideal battery, basically I'm going to assume the concentration gradient of everything in the electrolyte will be constant. This is the reason you can overcharge a cell and also over discharge a cell.

If the reaction is moving from left to right side then it is discharging

Positive electrode: PbO2 + H2SO4 + 2H+ + 2e- <> PbSO4  2H2O

Negative electrode: Pb + H2SO4 <> PbSO4 + 2H+ + 2e-   

Overall cell reaction: PbO2 + Pb + 2H2SO4 <> 2PbSO4 + 2H2O

H+ is notation for the hydronium ion, its just hard to type it in here. H30+(aq) e- is for an electron or the hydroxide ion OH-(aq)

If the cell is fully charged then the plates are lead and the electrolyte is sulfuric acid. As the cell becomes more discharged the concentration of H2SO4 will decrease, and the concentration of lead sulfate will increase.

We can apply the Nernst equation.
V(cell) = V(standard cell potential) - RT/(2F)*ln(Q)
V(cell) is the current cell voltage
V(standard cell potential) is how much voltage the cell gives nominally, ic 2 for SLA, 1.6 for Li-ion etc.
R is the gas constant
T is temperature
2 is the number of electrons transferred between anode and cathode per reaction.
F is faraday's constant
Q is very important

Q is the reaction quotient, basically its the concentration of sulfuric acid divided by the concentration of lead sulfate. This is simplified from the actual definition because we can ignore other terms as they are not aqueous.

ln(Q) = ln(concentration of sulfuric acid) - ln(concentration of lead sulfate)

What makes batteries really nice is that the sum of the number of lead sulfate and sulfuric acid molecules must be constant. This is assuming the battery isn't venting a whole lot, leaking, or lead sulfate drops to the bottom or significant sulfation on the plates. For the short term these are fine assumptions, and honestly I don't know if I could take all of these into account properly.

Lets say the battery starts with N moles molecules of sulfuric acid and lead sulfate.
N//volume of electrolyte is concentration of sulfuric acid + lead sulfate

ln(Q) = ln(concentration of sulfuric acid) - ln(N/volume of electrolyte - concentration of sulfuric acid)

ln(Q) = ln(concentration of sulfuric acid/(N/volume of electrolyte - concentration of sulfuric acid))
This is going to relate DIRECTLY to how much charge the battery moves.
charge = current times Time. By discharging it reduces the concentration of sulfuric acid in a linear rate, yet the cell voltage is going to be reduced in a logarithmic fashion.

V(cell) = V(standard cell potential) - RT/(2F)*ln(Q)