For me, it was primarily about quality & consistency, followed by longevity & aesthetics.
My first few eboard builds were made using Cheap Lipo From Hobbyking… mostly 6S 8000mah Zippy Flight Max Bricks.
For the first few builds, they seemed pretty good… but i soon realised there was a problem! they were not all the same, on my fourth or fifth one the quality seemed to drop off, the physical size changed & there were a lot more puffy/bad cells starting to appear when they hadn’t before, i assume hobbyking frequently change suppliers and just whack the same label on.
However, this was a major problem for me as i was just starting my business making custom eboards for people, i invested in 20 aluminium cases that were a perfect fit for this particular zippy flight max pack i was using… well as it turned out the tolerances for pack dimensions were crazy big… some varied by 3-4mm in width, height & length… so eventually the batteries that i was receiving wouldn’t fit in the cases anymore…
I then started searching for suppliers who could custom make the LIPO pack to my specifications, this was cool, i actually got some really high-quality packs rated to 60C! I was able to make really slim builds now with much more AH, not having any problems with puffing or bad cells either, but i was still only at 6s.
WHEN THE VESC ARRIVED ON THE SCENE…
This changed everything…
Because the vesc could handle up to 60V it seemed logical to make higher voltage packs, Mostly because I was having lots of heat problems when my heavier mates were trying to climb hills… so i was sort of forced to go higher voltage in an attempt to reduce current throughput.
The other contributing factor was that most of my mates had no fucking idea what a LIPO balance charger was… They keep fucking the battery charge sequence up and one even caught fire and nearly burnt down a house…
I wanted higher voltage & also wanted onboard charging via a built-in BMS… this way my big non-tech-savvy mates can ride up hills without over-heating and charge without any prior knowledge about proper battery care.
This is where it got interesting…
So I started asking a few lipo manufacturers about higher voltage packs with built in BMS, most of them had no idea what i was talking about… Which was surprising… It turned out that most factories that made LIPO didn’t use or understand the purpose of a BMS
After months & months of researching the ebike scene it appeared there was a common pattern, most battery packs that have built in BMS were made using 18650 format… it didn’t really dawn on me at the time, i just assumed it was because it was a better technology, which it is for our use case… but there is more to it…
Of course, it is possible to build a LIPO with a BMS! But the reason it is rarely done in the industry is because of these two points.
LIPO is a high output, low cost, lightweight battery technology which has a much shorter lifespan (charge cycles) to other chemistry like li-ion. Adding the cost, complexity & weight of a BMS sort-of defeats the key advantages of a lipo pack. To justify the cons (cost, complexity, weight, size) of using a BMS a battery must have a longer life span so that the cost of adding the BMS gets spread out over a longer period of time & often can make the cost per charge cycle equal to or in most cases better than a LIPO - so, in the long run, you sort of get the BMS for free & you also get the benefits of a BMS in exchange for a slightly larger back that cost more upfront. So the point is, the upfront cost is not that critical when considering a battery you should calculate the cost per charge cycle, longevity & how well it matches your project requirements.
Weight & Power Density, adding unnecessary electronics not only adds to upfront cost & can limit power output (most common BMS can’t pump out 200A), but it also adds to volume & weight. You might not realise it but one of the biggest industry for LIPO is UAV & of course RC hobby industry… All these unmanned aircraft have payload restrictions/limitations, every gram counts!.. so LIPO is the obvious choice, You need a small, cheap & powerful pack that is designed to be removed & replaced frequently. There is no need for each pack to have a charging circuit because it can be charged outside the craft whilst the craft is being used. There is no real benefit of longevity (charge cycles) because the pack is so easily swappable & fairly inexpensive to replace - it’s a consumable item!. In an electric vehicle designed for transporting humans, you don’t really want to be limited to short trips & have to replace batteries frequently, especially if you could simply install a larger one in the first place! Human grade vehicles tend to be more robust in design with sleek aesthetics & integrated batteries to help improve the form, not so much the function. Also, The total weight isn’t going to change if you have the battery in a carry bag or already integrated into the vehicle, so just install it! bigger & heavier is normally ok for EV… OK… So the primary difference between EV & UAV is that the pilot goes with the EV & Doesnt with UAV, so for EV having small separate packs that need to be swapped out regularly is counter intuitive. With UAV though it makes a lot of sense to have smaller & swappable packs…the pilot is stationary and outside of the craft, they can easily have a bag full of batteries charge cycling ready to install once the UAV has landed. The weight & quantity of the spare batteries doesn’t impact on the performance of the UAV because they are not onboard. In fact, smaller high-output packs improve UAV performance because of the power to weight ratios.
So to summarise, for EV you tend to want to go with longer life & bigger packs with built-in charging circuitry so 18650 Li-ion fits well, for small hobby grade toys you pilot remotely you want the lightest cheapest chemistry with no onboard circuits.
Furthermore… the aesthetics of the 18650 cell are also important, they never change size! even if you change brands! they are consistently the same size! which is really important when you are designing & investing in expensive molds for your EV & battery enclosures.
Finally, the Quality! 18650 type cells are made by very large well known mega companies that spend billions to get 1-2% gains & have strict quality controls… Samsung, Panasonic, LG… so you know you are getting some quality chemistry with proven performance… LIPO’s are mostly brandless silver pouches with colourful stickers attached, who knows where the fuck they come from… If you do spend time researching & testing you can find good LIPO, I certainly did.
I will happily say that I will never use lipo again on an EV