More on Balanced Systems: Thoughts While Riding

Poise

I’ve been riding around a lot lately, and thinking about balanced systems, and how complete systems are designed, as well as thinking a lot about the design challenges that teams like the Buckeye Racing guys faced putting a bike together for the Isle of Man race.

Here’s what got me going.  Running RC lipo with a microscopic pack, I had a bike that weighed about 210lbs, a range of 10 miles, and astounding acceleration.  Except it wasn’t as astounding as it could be because the pack was so small.  Even if you can pull the amps, and the batteries can deliver the amps, when the whole pack is tiny you’re tapping it for all it’s worth, and at some point the voltage starts to sag.  Once the voltage drops, you don’t go as fast.  The answer would be to build a bigger RC lipo pack, or a bigger pack that can do what RC lipo does.  A123 is a good example, or a nice array of god-awful expensive “lipo”.  Kokam would be good and if anyone wants to buy me a Kokam pack, well, go right ahead, I won’t stop you.  Hell, you can even have my last good kidney.

So then I put in a new pack with CALB LiFeP04 40ah cells.  I get a good, 30 or so mile range, the bike weighs about 275 now and the packs are easy to maintain.  Since the nominal voltage is lower the pack is now running at 80V at full charge.  As a result, the overall acceleration is slower, but also, those cells sag a lot more than a lot of the current cells, so the voltage, on hard acceleration, will drop down to as low as 60V.  No worries, it pops back up, but that’s now 60V driving the system.  Remember, volts = RPM, so now at 60V I have much lower potential RPM.  Since that means lower top speed, I geared the bike up a bit.  Now at low end, it’s working harder, so guess what?  The voltage drops…  giving me another hit to my acceleration.

I changed my balance point.

Overall, my top speed is good, my acceleration is fine but I’m not pulling wheelies anymore, and the bike is still a blast to ride with a decent range and easy to maintain batteries.  With the taller gearing it’s a pretty well balanced system.  But it has one bottleneck – the current supplying ability 0f those batteries.  Now, if I were to bust out that bottleneck by putting in some high amp batteries I’d be running at a whole higher level of performance.  Higher top speed, faster acceleration, to a serious degree, not just a little tweak.

Then, it would be time to find the balance to that system.  Suddenly I’m dumping huge amps, so my guess would be my motor is going to be getting hot and my controller is going to start to complain…  but that’s only a guess.

It’s kind of like back when you took the air cleaner off your Dad’s car, and suddenly it ran well.  There aren’t a lot of things you can do to most well-evolved systems to get that kind of quantum-leap in performance, but that’s what it’s all about.  Finding the bottlenecks, eliminating them.

Lemmee see now.  Balance.  Bottlenecks.  Kind of a yin-yang thing, huh?  Zen and the Art of Electric Motorcycles?  …hmmmm

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5 responses to “More on Balanced Systems: Thoughts While Riding

  1. Sagging is of course highly dependant on how many C you pull from the cells. If you go for 400A which is 10C on a 40Ah cell it’ll be much worse than on say twice as big cells, which would only see 5C. Also the new grey CA-series CALB cells have been found to sag a lot less than the old blue cells. Haven’t had the pleasure of getting up close and personal with those yet though.

    • “…new grey CA-series CALB cells have been found to sag a lot less …” This, I’m afraid, is heresay unless you have a source that’s done some good testing. I’ve found no conclusive tests.

      And please don’t quote Jack Rickard lol.

      But, to bring us back to balancing the system, if you ran larger cells you’ve got more weight… would that offset the sag? Enquiring minds want to know. (But can’t afford the purchase-for-testing.) Not, unfortunately, a “quantum leap” step, (as Kokam, for example, would be) but just a fine-tuning of your balance.

      • Yes, it’s hearsay, and I probably never will be able to verify it myself directly, since if I some grey cells later I’ll most likely get bigger than the 40 Ah I have now so the comparison won’t be reliable. Unfortunately this is probably true for most people – very few have the means to purchase different cells of the same size just to compare them.

        I don’t see how weight would have anything to do with sagging if the amp draw stays the same. You will however lose acceleration if you increase weight and thus need to continue the amp draw longer to reach the same speed. So, more stuff to think about…

        Where can you buy Kokam?

      • Maybe I wasn’t clear… sorry! The bigger cells give you less sag, right? But now you have more weight. So more to pull. So the question is, will the advantage of the higher voltage get sucked away by pulling the increased weight?

  2. “will the advantage of the higher voltage get sucked away by pulling the increased weight?”

    That is a good question and something I didn’t come to think of right away. In my case I’d add 15% more weight if I doubled the number of 40 Ah cells (including me as the driver) and I think 15% might actually be pretty close to the sag, but no battery has zero sag. One thing is for sure – this gets complicated fast and weight isn’t obviously the only thing affecting acceleration. I don’t think this can be typed to smart either, one would need to test…

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