No Title -> General discussion / questions -> Allowing overweight Nissan Leaf modules as a step towards greater use of Lithium Ion in electrathon vehicles.
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TOPIC: Allowing overweight Nissan Leaf modules as a step towards greater use of Lithium Ion in electrathon vehicles.
Over the last couple of months, I have been researching commonly used technologies and best practices in electrathon vehicles. One findings is that the biggest challenge facing electrathon is related to the battery. Advanced battery technologies are becoming ubiquitous in modern life while electrathon continues to focus on Lead Acid batteries.
There appear to be six factors limiting electrathon teams from migrating to advanced batteries:
1. Financial investment in Lead Acid.
2. Knowledge investment in Lead Acid.
3. Cost of other technologies.
4. Complexity of other technologies.
5. Safety concerns.
6. Rules.
I have experimented with various advanced battery packs. I have found, that it is possible to build Lithium Ion battery packs of more capacity than the standard lead-acid packs given the 15 pound weight limit. But, to be honest, I would be hesitant to ask a high school student to design, build, and maintain one of these battery packs.
My main concern is tendency to put all of ones allowable weight in battery cells while shortchanging safety features. There is a well founded reason that electrathon required unmodified lead acid cells.
This has lead me down a slightly different line of research. Which manufactured batteries would be most suitable for use in electrathon vehicles? After looking at several alternatives, it appears that Nissan Leaf modules would make a reasonable accepted standard for Lithium-Ion Batteries.
My initial tests indicate that a set of three modules come in slightly over 1000 Watt-Hours for a one hour discharge.
-Nominal capacity per module is between 450 and 480 Watt-hours per module.
-Tested capacity per module for a 1 hour discharge is closer to 350 Watt-hours.
-Price per module is between $120 and $150 per cell.
-Enclosed aluminum housing.
-Sturdy terminals with beefy cell interconnects.
Nominal voltage for the three module pack is 22.5 Volts which enables teams to continue using their existing motors and controllers. The problem is that because of the aluminum casing on the modules and the slightly lower energy density of the individual cells compared to other Lithium-Ion chemistries, the weight of a three module pack is about 25 pounds.
It seems likely that one could build a Electrathon Lithium Starter Kit around these modules and a simple BMS for between $500 and $600.
Would the rules committee consider approving the Nissan leaf cells even though they are overweight? If so, Ill continue this line of research. If not, I continue looking for other solutions.
28.1 Any type of the following sealed production batteries may be used as long as their weight does not exceed:
I think there is a good argument that the sealed production battery is the cells that Nissan bought to make their modules. They weight 1.7624 lbs each so 8 would be under the 15 lbs limit. As they are packaged, this would 2 in series , 2 in parallel, this would only be 15.2 volts nominal. It would require cutting open the can to set them up as 30.4 volts.
The 3 module approach offers a turn key solution and, based on your testing, puts the batteries right below the 1,000 Watt limit but the 12 cells would be over the 15 lbs rule.
The big advantage is that they should last many seasons of racing without being that much more expensive than a pair of Optimas. Since lithium tend to be damaged by over discharge, they would require a BMS that shut them off before fully discharged.
Electrathon America is set up so that the rules can be rewritten if there is enough interest. My understanding is that Optimas were too heavy for the original rules but enough members wanted to run them that the rules were changed to permit them.
The Leaf batteries seem like they might be a good fit. i think there was talk of revising the rules at the end of this season, so the more information you can gather, the easier for people to vote intelligently on the issue.
I read about some of your work a few years ago making packages out of foil pouches. I gave it a shot and came to the conclusions that if you a) buy top end cells b) go light on safety features and c) have the skills to build your own battery pack you can make a winning battery pack. However, I would hate to be the inspector at the track trying pass judgement on one of these builds in the couple of seconds they have before they need to move on to the next vehicle.
My shift away from DIY battery packs came when an EC5 connector I used to connect the battery pack to the load started to melt and burn a hole in my work bench :( An EC5 connector is assembled by inserting a wire into a hollow tube and then filling the gap with solder to make a secure physical and electrical connection. The problem is that it is impossible to see from the outside if you have a good solder joint or if you have a cold joint.
Yes, the durability of these modules is an important benefit. They are big beefy cells which run nice and cool. I think, in the long term, they will end up being cheaper than lead acid as you can get many more cycles out of them before they start to fade.
Two key safety guidelines are starting with a balanced charge and adhering to the cut off voltage. By using a decent quality charger (BMS) in the $100 - $200 you can get a balanced charge. Most, if not, all motor controllers have a programmable cut off voltage which will shut the controller off if the battery voltage drop too low. I have not seen many teams use this feature even for lead acid. If a team follow these two guidelines, I don't think they need an onboard BMS to remain safe.
The downside is that for the last couple of years, Nissan has been increasing the rated capacity of their pack by about 2 amps per year. I am using 2012 and 2013 cells. The 2015 cells might have already climbed out of the electrathon power envelop.
As an aside, what do you recommend for charging your 1KW packs Lithium packs? There is a pretty big price jump between the RC chargers and the larger commercial chargers. Most of the RC type chargers seem to be designed with the expectation that they will be used once or twice a month for a couple of years. They are not standing up to daily testing on 1Kw packs. Since I am using them at between 33 and 50% of their rated capacity, the manufactures have been good about their warranties so far.
"Two key safety guidelines are starting with a balanced charge and adhering to the cut off voltage. By using a decent quality charger (BMS) in the $100 - $200 you can get a balanced charge. Most, if not, all motor controllers have a programmable cut off voltage which will shut the controller off if the battery voltage drop too low. I have not seen many teams use this feature even for lead acid. If a team follow these two guidelines, I don't think they need an onboard BMS to remain safe."
A good top balance (all cells charged to the same finish voltage) does not guarantee that the cells will get to empty evenly. Especial as the pack gets older, the difference in capacity will mean that one cell might be empty while the rest still have charge.
The other problem is that most of us do not use the controller low voltage cutoff because voltage sag under load will trip the cut off prematurely. Some controllers can limit current to keep the voltage above the cut off but it is still hard to find a setting that balances performance against battery protection.
I am running a Manzanita Micro BMS which has a digital line that rests at 0 volts until any single cell goes below my minimum set point. When the cell goes below the set point, the line goes to +5 volts and I get a warning light. If I am at full throttle, I know it is sag and I do not ned to worry much. If it is staying on when I am off the throttle, it is time to pull off the track.
As to your question about what charger, we have a Manzantita Micro PFC 50 that is our workhorse charger. It is a big powerful versatile charger but more expensive than is necessary for Electrathon. The smaller PFC models might be a good option. You can set the maximum voltage and then use the amp dial to set the charge current rate. We recharge in less than an hour. We could recharge quicker but usaully we are sharing a generator. The charger has an open source connecter that can connect to a BMS signal to shut the charger down. Connected to the Manzantita Micro BMS, it cycles the charger on and off as the top balance resistors get hot.
We only run one battery pack. We recharge between sessions. This was an advantage with lead acid because it warmed up our pack. So far, we have had no issues with the Kokam's getting too hot.
I agree that more research needs to be done on the necessity of an onboard BMS in electrathon vehicles. So far I have been having successful by organizing cells into three packs: practice, good, and best.
After a discharge, the entire pack goes on the charger and I record total voltage and individual cell voltages. If a cell discharges more quickly than others it gets moved to a lower pack and the best cell from that pack replaces it. This way the difference cell voltages after a discharge are within a few hundredths of a volt. It does require the extra manual step of recording cell voltages. Are teams going to remember to do that in the excitement of preparing for a race or when they are in a hurry to get home after a pratic session?
Yes, voltage sag has presented many opportunities for me to shoot myself in the foot :( I have been playing with a cruise control device which automatically sets the vehicle's throttle based on battery voltage and time remaining in the event. To measure the unloaded terminal voltage, the systems turns off the throttle until the voltage climbs back to a steady state. Then the unloaded voltage is recorded and the throttle resets. Lead acid batteries can take several seconds to restabilize.
Randomly shutting off the throttle for several seconds can't be the best way to handle this (especially in traffic). But, I just haven't figured out anything better yet.
Yikes $3,500 for a charger. I looks like a beauty, but out of my price range. David
-- Edited by dfarning on Friday 11th of September 2015 07:21:40 AM
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No Title -> General discussion / questions -> Allowing overweight Nissan Leaf modules as a step towards greater use of Lithium Ion in electrathon vehicles.