Electric cars are fantastic compared to ICE vehicles, but E-Bikes are even better. Much lower environmental impact and far more suited to medium range travel such as commuting. Here in NL they sell in huge numbers, far faster than electric cars. But they also have their limitations: a regular e-bike tops out at 25 Kph, and will do a very limited distance on a single charge. What powers your average e-bike are Lithium-Ion cells, usually of the 18650 variety, capacities vary but the very best cells you can get at the moment that are still affordable top out at about 3400 mAh per cell. A typical e-bike has about 40 to 50 of these, in a 10S4P or 10S5P arrangement.
My first e-bike, a pretty crappy one but enough to get my appetite whetted had a 500 Wh battery, enough for a 55 km trip one-way, and it would be dead on arrival, range anxiety to the max. After shopping around for a bit I bought a secondhand second battery to give me either more range or a way to get back home if the first one ran out. This works, after a fashion. But 25 kph isn’t a lot, on my (very) elderly 10 speed I would be way faster than that. The good bit about that e-bike for me wasn’t the speed but the fact that I didn’t need to exert as much force when starting up, which as the result of a previous bike accident is still hard for me (lots of steel and screws in one of my legs).
Doing this for six months was an exercise in frustration, I’d always be tempted to use my ‘normal’ bike because cycling an e-bike ‘over the top’ of the motor is something that gets old very fast. But my car usage dropped by more than half and that by itself was enough of a reason to further pursue this project.
Some more searching and I hit on the concept of a S-Pedelec. It’s a pretty weird cross-over between a bicycle and a moped, top speed 45 kph, but the range is even worse. Other issues that that technically it is a moped (it needs insurance and a license plate) and in many places you are forced to ride in traffic, which is anything but safe. Even so, I got one, a Riese & Mueller ‘charger’. Fantastic build quality, super good brakes, really stable to ride. But the range is even worse than with the normal e-bike on account of going faster. A full 500 Wh battery will take you 45 Km on a good day. Driving around with three spare batteries in bags and swapping them out is not nice and the weight distribution on the bike also wasn’t ideal, especially not because there is some slop in how the bags are mounted and with that kind of weight in them you get a real kick every now and then.
So I decided to increase the range of the bike by building a larger battery. This is where I pretty much dropped into the rabbit hole of battery pack manufacturing and the Bosch e-bike system in particular. I made a giant file of notes on how these batteries work (I’ll post this separately one of these days), how the BMS in them works and talks to the main controller (which is located in the motor) and how the charging process works, as well as how to repair them after they - inevitably - break. After documenting all that I realized there are a couple of major sticking points: for one, the Bosch BMS is part of a DRM setup that pretty much prohibits using 3rd party batteries, for another, adding many cells to the existing BMS is risking it bricking itself due to its inability to balance such a large pack, the Bosch BMS is a bit nervous about changes to it’s world as perceived through the sense wires that it does not understand and the easy way out is to shut down completely. I have a couple of these BMS’s now that refuse to release the battery to the high voltage bus even when connected to perfectly good battery packs.
I found part of the solution on Ali Express, a device known as an external balancer. As a neat little extra it has a bluetooth comms module built in that allows me to monitor pack voltage and the cell groups just in case it breaks or I messed up something during the build. I built a small (10 cell) test pack, tested it with the BMS and the bike recognized it, even while the external balancer was running.
Then I ordered 190 Samsung E35 cells (from nkon.nl, which are a fairly standard thing in e-bike battery packs. This cost a pretty penny, close to 600 euros. But given that a 500 Wh battery pack costs roughly the same its still a bargain. A BMS was sourced from a defective pack (busted cells due to water ingestion, a common problem with the Bosch rear carrier mounted battery packs).
I watched endless youtube videos on pack manufacture, spot welding techniques, troubleshooting and most interestingly, what tends to go wrong with battery packs. After a number of videos on this theme I realize that this isn’t exactly safe. You are connecting 10’s to 100’s of batteries in series and parallel configurations that allow the liberation of all of that energy in a very short time. Working on a pack that size is like working on a live bomb. I have great respect for Lead-Acid batteries, Lithium-Ion is at another level still.
Bit by bit the geometry was worked out, with some false starts and then it all came together, a 10S17P configuration was possible, but the pack geometry came out really weird. The reason why is that the pack lock and pack connector stayed in place on the bike, I did not want to butcher it and there is a limited amount of space in the frame. Initially I wanted to mount the pack on the rear carrier but some conversations on Hacker News and a test ride with some bricks on the back convinced me that this was a bad idea. Center of Gravity too high, and too far towards the back changed the riding characterists in a way that made the bike unpredictable and dangerous in wet or slippery conditions. So in the frame it all went, and my 190 battery plan changed into a 170 one. More Ali Express safari sessions sourced a suitable welder, battery supports and sturdy adhesive paper for insulation purposes.
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