The Lithium upgrade
Some of you who follow my blog might have been wondering where I’ve gotten to this last few weeks. I’ve not been idle on the upgrade front, that’s for sure. In fact I’ve been spending almost all the spare time I can get on my biggest project to date, – that is the much covetted Lithium cell upgrade. After many weeks and about a hundred hours of work, I’ve finally succeeded!
As you can see, I have a fair amount of catching up to do on my blog and guide, as I’ve only recently began to start work documenting this. A lot was involved: In addition to designing and building the Lithium (LiFePO4) pack itself, I also had to come up with a way of charging it.
I could have gone for an off-the-shelf Battery Management System (BMS), but I’d heard to many horror stories about people ordering these things only to find themselves little support or documentation to assist them with actually getting it up and working. Instead, I elected to build one from scratch from a much used, well-known and proven design conceived originally by Gary Goodrum and Richard Fechter. The Goodrum-Fechter BMS is not available pre-assembled, but comes as a bare circuit board whose parts must be acquired and assembled separately. This meant a lot of work, but I opted for this route because of the strong support that this design has from the e-bike community, the good documentation that comes with it to help users assemble and test it, and the solid forum support offered by the board’s designers.
As detailed in my previous blog entry, the pack I eventually settled on was a 24s2p, 76.8V nominal, 86.4V charge voltage, built out of 48 Headway LiFePO4 38140S 12Ah screw-tap cells. It weighs just over 19Kg, versus the 48Kg of my SLA bank, so I’ve shed nearly 30Kg!
My 76.8V LiFePO4 Lithium pack
I elected to mount mine vertically. It wouldn’t fit flat as it was too wide, so I sacrificed some seat space as before. The advantage of this arrangement, though, is I can more easily remove it if needs be. There is also just enough space down the side to slide the BMS down alongside it so it is flush with the battery case. Not only is this a convenient arrangement that keeps everything nice and tidy, but the side of the battery box can act as an extra heat sink to protect the BMS from getting too hot.
Here’s the final arrangement. I had to take a chunk out of the seat, but I’m quite happy with the result. My Lyen’s controller is en route back to me, so I’ve only had chance to use it with my cheapo controller, but I’m delighted by the improvement to its all round performance.
The final assembly
The reduced weight and peppier voltage levels means it accelerates and handles better, and doesn’t have to struggle so much with hill starts. I’ve yet to put it to a proper range test, but so far it shows every indication of having better range than it did before. Though the Lithium bank is a slightly lower capacity (24Ah) than its previous 28Ah SLA bank, much of that power was previously wasted hauling the much heavier ‘lead sled’ into motion from stops, and fighting gravity up every incline.
All that remains to be done here is a bit of weatherproofing, to protect the bank and battery compartment from the wet when poorer weather arrives. I might also add a couple of brackets to more firmly secure the pack in place, and prevent it from shaking about on rough roads.
The Charging System
Unlike an SLA battery bank, which can just be series charged with few balancing problems, a lithium bank must have its charge levels very carefully managed. Any power from a charger unit therefore needs to go through a suitable BMS which manages the distribution of the power to the individual cells of the bank.
Most of the work I did involved constructing the aforementioned Goodrum Fechter ‘Zephyr’ unit. The architects of this board simplified the process as much as they could by giving electronic parts lists (BOM) that could be drag-and-dropped into the online shopping cart of the U.S. components store Mouser.
The completed unit is shown below. It has a perspex lid so that the cell-level LEDs are visible and can be monitored if needs be. In everyday use this isn’t really necessary – a single charge-status LED on the end tells you when it’s finished, but it can help identify bad or weak cells if problems develop with the pack. It also has an end of charge (EOC) adjuster that let’s you fine tune the point at which the charger cuts off and finishes the cycle.
The BMS has three cables. The grey Anderson connector and the 24-pin ATX connector go to the battery pack. The first serves the main terminals, and the other manages the individual cells. The black and red Anderson connector is connected to the charger itself. I’ve used good, thick 1mm2 tap wires to keep resistance down and allow for higher charge rates.
The charger I’m using is a BMSBattery EMC-900, which supplies the 86.4V charge voltage at a rate of 9A. However, I’ve wired the BMS up with nice, thick wiring so that I can take advantage of the 20A charge rate that the GF Zephyr is capable of handling, if needs be, and can always just get a bigger charger.
I’m delighted with the improved charge rate, which is almost four times what I got with my 2.5A SLA charger. In my latest test-run, I ran the bike for ten miles and then charged it up again. It was fully charged again in just under an hour, as compared to the 4 hours or so it would normally take under the old SLA system!
Here’s the final arrangement for charging. The BMS fits snuggly down the side of the lithium pack and is connected in between the charger and the battery pack.
The Lithium pack on charge via the BMS
The BMS can also be used as a cell-level LVC cutout system via throttle pass-through connectors which power down the throttle signal if any cell voltage becomes dangerously low. I haven’t got that up and running yet, and am simply monitoring voltages manually with a multimeter for now. I may go with an alternative, and simply use the BMS for charging, rather than having the BMS permanently attached.
Even at 9A, the whole assembly usually runs cool as a cucumber. The only time it gets even slightly warm is at the end of the charge cycle. This is when the ‘shunting’ occurs that protects individual cells from overcharging and bleeds off overflow from already charged cells through big resistors on the back of the board.
Over coming days and weeks I’ll continue to catch up with documenting the details of the upgrade, including the BMS build, testing, and troubleshooting. I’ll also detail setting up the charger to get the exact voltage I needed to make it work just right with the BMS. In the mean-time I’ll be putting the pack through it paces witha more extensive range test, and also seeing how it performs once I’ve got my sporty Lyen’s controller back in place!