i want to preface this by sayng im not the most knowledgeable in terms of what i know on the electrical side, im more of a mechanical guy.
i was having a discussion with some of the other members on the forum and part of that discussion was the rule about lithium batteries cannot exceed 1kw, there is no power limit on lead acid just weight, why is there a restriction on lithium? i get that there lithium is more dense and has more power, but if there is a weight limit that should be the only limitation right?
the only thing i could think of was evening out some teams that have more money than others, but im sure there are lead acid batteries that are better than others that will meet the 73lbs limitation that would probably be more expensive than others.
I think you'll find that 73lbs of lead-acid batteries is LESS than 1kwh. I have tested several different lead-acid battery packs and the best I have ever gotten is 940wh. Lead-acid just doesn't have the energy density of lithium. Also, section 28 of the handbook says "the weight limits are meant to keep the available power under one kw/hr, or about the same as the current standard class."
So those in the advanced battery class are likely getting at least 60 more watt/hours than those of us in the standard class and hauling around 58 less pounds while doing it.
It's almost enough to make me switch to lithium...
hmm, in Tampa we actually separate lithium and lead acid class, maybe we need to stop trying to do a direct comparison and classify them as separate divisions, in Pensacola and Alabama they just combine them
-- Edited by 4TRIDENT4 on Thursday 1st of March 2018 06:19:07 AM
Yeah the distinction between advanced battery and open classes hasn't always been observed especially in Pensacola.
I think this will have to change in the future. Probably the first time they have a high school team running lithium.
Seems like some organizers see it as "high school" class and "everyone else".
yep i hear you on that, i was also wanting to open up lithium to a little less restriction, that way they can truly be "experimental class" and be able to try different things especially with the battery.
-- Edited by 4TRIDENT4 on Thursday 1st of March 2018 08:12:41 PM
One of the big challenges for writing rules that promote good competition in electric racing is getting the battery rules right. First, if one team has a much better battery, then it will not be a competition. Electrathon handles that by requiring all teams to use a commercially available battery which means that teams can only run something that all the other teams can buy as well.
The next potential problem is what if there is a super battery that goes bad after a couple of uses? We want rechargeable batteries but it would be hard to define, must less test and enforce, a specific cycle life. So a team with a big enough check book could run these super batteries and throw them away after each race. Again, not good competition, just who has the biggest budget.
So Electrathon rules limit capacity to 1,000 Watt hours even if you can stay under the specified weight limit. This means that finding a better battery can give a minor weight advantage, such as a 10 lbs battery when everyone else is running 15 lbs. It does not give a team the dominant advantage that a larger capacity pack would and makes it unlikely that anyone will spend huge amounts to save a few pounds.
The Electrathon rules are cleverly written. We do not have the ability to easily test pack capacity, so we use the capacity claimed by the battery manufacturer. Even if one team was able to convince a battery manufacturer to underrate their batteries in their specifications, it would not benefit the team because everyone else would be able to purchase the same packs.
There is still tons to be learned about getting the most out of these batteries without going over the 1,000 Watt hour rule.
I agree with your assessment.
Whoever came up with the rules for Electrathon is clever indeed.
They managed to find the middle ground that allows for ingenuity and innovation while still keeping a relatively level playing field.
The battery weight restrictions tailored to individual chemistries is a prime example.
One of the things I have come to appreciate most about Electrathon America is how most of the rules are primarily safety related and how they don't try to dictate every facet of your car design leaving you free to experiment.
The rule book is pretty great with removing any ambiguity. Archer's comments are right on what my thoughts are.
Also add that the wording hints that batteries should be off-the-shelf with a charge rating, rather than a prototype capacitor ("according to the manufacturer's data").
For example, my battery is 16% under the maximum weight but just 4% under maximum rated storage; for the benefit of having a production battery and BMS that has hence been built and tested before, I'm happy to accept the capacity limit as it keeps the energy potential close for all cars.
And, it's an easy performance measure when checking if you can use it (Oh that battery is 24V and 45AH? That's 1080Wh, too much, can't use it...)
Yeah it seems like the more batteries you use the lower your WHs become.
Probably due to the increasing weight of non-active components. I.e. plastic casings.
I've been around lipo and liion batteries now for the past four years now and know the rules for the 1000Wh cap are a little misleading.
Whoever wrote it didn't know that most lithium based batteries according to the manufacturer data sheet, either on the battery or a spec sheet for liion batteries, you only run them down to 80% of capacity.
Meaning if you have a lipo or liion pack rated for 1250Wh cap, you can only effectively pull 1000Wh out of it.
1250 * 0.8 = 1000
The rules should be written more clearly in the case of lithium based batteries.
In 2019, I ended up purchasing four Tattu 6s 14000mAh packs that worked out to 310Wh each according to the Wh rating on the pack. When doing the math, that worked out to 1242Wh * 0.8 = 994Wh. In reality, I probably don't have my power meter calibrated 100% but I was able to pull 1045Wh from the pack. That tells me my equipment is pretty spot on and close enough to work with.
The only catch now for 2021 is I noticed lithium polymer (Lipo) is not in the rules anymore. Only lithium ion (Liion) and lithium ion phosphate (LiFe). I'm already looking to purchase around 120 liion 18650 cells to make four smaller packs that I'd just then plug in together to make one big pack.
I can concur with that. We have a battery made up from 16x LiFe Powerizer cells labeled at 64 Whrs (discharge rate unknown). So, the labeled capacity for the whole pack is 1024 Whrs, but our constant current discharge tests at a 1 hr rate result in around 800 Whrs. Even a very slow discharge rate over 5 hours doesn't bring it up much - around 830 Whrs. So the 0.8 factor sounds reasonable for estimating actual functional battery capacity.
However, I am not sure there is 20% capacity left in the battery. At the end of our tests, the battery voltage is clearly dropping fast. There was not much left to squeeze out. I put the difference down to real world conditions vs optimum lab conditions... and a bit of marketing exaggeration.
Whoever wrote it didn't know that most lithium based batteries according to the manufacturer data sheet, either on the battery or a spec sheet for liion batteries, you only run them down to 80% of capacity.
Hi Zaine,
I understand your point that some manufactures recommend that you do not discharge your lithium batteries below 20%. The difficulty from the rule enforcement point of view is the difference between 'should not' and 'cannot'. Lithium batteries will deliver that last 20% if the driver continues to drive. It will shorten the cycle life of the battery but otherwise, they will work fine. Do you have a suggestion how to write the rules to make sure teams are only using the legal limit on energy?
<...I noticed lithium polymer (Lipo) is not in the rules anymore.>
What people general refer to as lithium polymer cells are, in fact, lithium ion polymer and thus a type of lithium Ion and are still legal for Electrathon ( https://en.wikipedia.org/wiki/Lithium-ion_polymer_battery )
Are you using a BMS. Some types of BMS seem to use up energy. I wrote up some of my experiences here: https://electrathonamerica.activeboard.com/t66140971/proev-tests-an-18650-lithium-pack/?page=1#comment-66333252
Of course, marketing exaggeration is also a good possibility.
We have balancing boards on the pack, but nothing else, so not a full BMS. The pack was fully charged before each test.
I have noticed the balancing boards consuming energy, and annoyingly they don't have any low-voltage cutout system, so if you leave the pack for long enough without charging, the balancing boards will kill the pack. Not good.
Electrathon handles that by requiring all teams to use a commercially available battery which means that teams can only run something that all the other teams can buy as well.