The motor itself is capable of 6kW+peak power with a bit of statorade, but my battery and controller aren't up to snuff
I'm personally pushing 60kph :) But it's pretty scary at those speeds even with the best tires I could find and pretty good disc brakes, so I'm generally at 20mph or under.
I'm planning a 4kWh battery eventually... but not yet.
It's a super budget build for now - a BTwin Rockrider 520 with a battery from salvaged modem packs and an overvolted KunTeng controller and a hub motor. It's super fun though and I've done over a 1000km on it which is very nice.
How much does the whole thing weigh? Mine is at 29 Kg, and that's with only a 500Wh pack, I figure about 6 Kg/KWh so by the time I'm done building the new pack it will be around 40 Kg in total or so.
I haven't weighed it yet, but:
The bike itself is 15kg. The battery weights 48g per cell, so at 140 cells (14s10p) that would be 6.7kg, but I'll round up to 7.5kg for the wiring, mounting, etc...
The motor and controller and screen are around 11kg or so. The hub motor is very very heavy!
Summing it all up it's around 35kg. Sounds about right from hauling it around.
It's not technically street legal, no. But I'm far from the worst offender - a lot of people ride what's basically a 125cc with a hub motor on the end, and they're sold everywhere, so I'm never going to get in trouble for it.
35 Kg is doable, as long as that battery doesn't run dry!
Mine really isn't pleasant if you end up without assist, it's doable but I really hate it, especially uphill. Can't wait until I have that pack ready, but I'm not going to hurry it, I'd hate for it to fail halfway or so, and that's a lot of tinkering to be done still (and a lot of welds...). But the batteries are already here, the balancer as well, I have a BMS from a scrapped pack and a good idea of what the housing should look like, if my test pack works I'm off to the races, then it is mostly straight up assembly and otherwise I'll need to re-think the way the BMS is hooked up.
Let me tell you, the added weight is nothing... On a hub motor when you don't have a connected battery, say because the BMS tripped as you ran dry, the motor converts a signficant amount of your kinetic energy into electrical energy and then to heat. You can really feel it cogging. I could swear the two weeks I ran it without a batteris saw my thighs get significantly bigger!
Thankfully in my city there are plentiful subway stations with elevators, so when I did run dry once, I was able to pedal my bike into the subway and take it home.
It is serving me quite nicely without those kinds of mods. I'm not looking to set any speed records. Just needed something to help me get up the damn hills when I have a load of stuff. It does that better than hoped, so I'm happy.
Do you have a link for your bike? Mine is a Riese & Mueller 'Charger', a 2016 model.
https://www.pedelecforum.de/forum/index.php?attachments/uk_2...
The battery pack is quite anemic (500 Wh), so I'm building a much larger one, I do 130 Km round trips with some regularity (twice per week) and it takes two of those fully charged to fully discharged; and then recharged at the destination which is effectively murdering them, hence the need for a larger battery. A single pack has about 35 Km range. I want to be able to do the trip one-way on 80% full to 20%, which I've worked out to about 1000 Wh plus some reserve. The pack will likely be 160 cells, which is pretty heavy for an e-bike, but the frame has enough room for this.
Currently figuring out how to hook it all up and how to fool the controller in the motor to believe that my oversized battery is a genuine Bosch product without letting out the magic smoke from the toy level balancing unit in the BMS.
[0]https://www.radpowerbikes.com/products/radwagon-electric-car...
This is the bike (image ripped from an ad, not that bike but the exact same model):
http://imgs.advertentiex.nl/upload/Te-koop-prachtige-christi...
So you can see that in the back there are no options other than to either completely change the frame or to go for a single speed and neither is feasible. I could maybe expand the space between the forks in the rear so a wider hub with a motor + cassette would fit but that risks cracking the aluminum frame.
The bike has a rear hub that is geared, so there is no space there and this being a cargo bike means that an after market mid-motor can't be placed (it would run right through the middle of the main spar. That leaves the front wheels for the electric drivetrain. I'm not afraid of slipping or sticking, electric motors tend to be pretty well behaved in situations like this, the difference in speed between inner and outer wheel in a turn will be readily absorbed by the motors and discarded as heat, and they won't be overpowered to the point that they will start turning independently, each motor will run at a leisurely 125 Watts or so. The one situation where that might happen is if one wheel comes off the ground but in normal use that should never happen.
This is the kind of kit that I'm eyeing to do this conversion with:
https://www.fietsunie.nl/Babboe-Big-bakfiets-elektrisch-make...
This is a pretty common thing here in NL and rather than build the whole drive train from parts I'd go for a kit.
This is the balancer (not a BMS, just an active balancer):
https://www.aliexpress.com/item/32945565819.html
What's neat about this one is that it uses two supercaps as a chargepump so it can rebalance quite effectively without producing a lot of waste heat or wasting power (of course you waste some but a small fraction of a resistance based balancer, which just burns off the power in the cells that are highest voltage).
I'm building a small test pack (20 cells) right now, and if that works properly I'll scale it up eight fold for full range. It will still plug into the standard charger and I'll have a switch that will allow the pack to be charged to 100% capacity in case it is needed for an even longer trip.
It's a good idea to take care to balance the cells. That said, 1A of balance current is very overkill.
I actually ran tests as I was curious. I cycled my battery 10 times at 1C. The parallels only went 0.03v out of balance, over 10 hours of discharge and 30 hours of charge. That's at the top end, so really I had what, 0.5Ah of imbalance, over 40 hours of non-stop use in the very worst scenrio. So the 40mA of balance current from my BMS is actually enough even in the worst situation.
In reality you'll be doing at most one battery cycle a day, maximum two, so you'll actually be needing only 4 mA of balance current!
Because of that I decided that the 40mA was all that I needed and I didn't add any really heavy duty balancing circuit. The one that came with my BMS was more than enough, even though like you I was quite skeptical at first.
So I think you should keep it simple. If I were you I'd solder the balance wires of both BMS's as parallel and go on with my day knowing that whatever I'll throw at it, it will stay balanced. Plus there is less that can go wrong :). But then again, I'm not the one not to enjoy some overengineering
The thing is that when you have as many cells in parallel as we do, for you probably 15?, the imperfections between each cell really do balance out and even with the pretty knackered cells I went with it will stay in balance.
Yes, 1A is overkill, 100 mA would be plenty (and likely still too much), but the Bosch BMS has maybe 1 mA balancing capability and that's too little for a pack that size so I decided to throw in something beefier and this was the smallest one that had bluetooth out so I can monitor it, a pack that gets unbalanced too much is a good indication that something is wrong, for instance a weld that's not done well, or a cell that is on the blink. Those things are best caught early with homebrew packs of this size, you really don't want to have a battery fire.
A good, new pack stays within a few mV over a whole charge/discharge cycle. The reason is that if you start off with a balanced pack the laggards during discharge will be the first ones to be topped off so it all balances out in the end modulo the difference in internal resistance, and this leads to the various cells dissipating a bit more or less heat (as well as the cells on the inside of the pack being better isolated). That's cumulative and over a longer number of cycles you'll see the pack drift more and more out of balance.
So it's not so much a matter of me being skeptical as it is knowing the exact limits of the Bosch BMS and the fact that if the pack ever goes out of balance more than that that it will brick the BMS, and that's a nuisance, coupled with the fact that this particular balancer had some more useful features.
16 cells in parallel, and you're right that the cells tend to self balance to a large extent. But it's also a really good way to monitor the pack so it's dual duty for me, wear monitoring (which has safety implications) and balancing all in one for less than $100, which on the total cost of this project is quite doable. The cells I'm using are $6 a piece (Samsung E35s, 3450 mAh) and 160 of them is already quite the outlay.
AS for the battery fire, that is why I suggest using a second BMS and running the phase wires from there - a good BMS such as the one I use will cut off the power when there is an even slight imbalance, which is my cue to investigate it and avoid a battery fire :) - while that second BMS would handle the balancing.
But paying for 6$ cells, my oh my, that does make it worth it :) I paid all in all 70 cents a cell and even then I was quite skittish :P
70 cts / cell?? Wow. Consider me somewhat jealous, that's an absolute bargain. But better keep a good eye on them then, a FLIR isn't a luxury, it is a good idea to keep an eye on the cell temperature using a FLIR while charging, bad cells will stand out immediately (as well as later on, when they are possibly warmer than their brothers and sisters around them which should be at ambient fairly soon after you stop charging, that's a good indication of a partial short in a cell). I've already spotted some bad apples like that which when checked out on a reference charger seemed to be perfectly ok. It also helps to spot bad welds, simply load up the pack and look at the interconnects, any bad welds will be higher temp because of their increased resistance.
Thankfully I didn't have to spot-weld. I used cells from modem batteries, so they were already spot welded - I was able to very rapidly solder to the spot welded tabs which already had some spots to run balance wire through, and the batteries were connected to each other 3 by 3. Then I was able to solder the "balance" wires which were quite thick at each cell to put them in series and it worked fine.
The pack as it is now does not heat up at all. That's to be expected I guess because I'm charging it overnight quite slowly and I'm not really drawing more than 1C.
I had multiple cells that I threw out - I all ran them through a balance charger and fast-charged then fast-discharged them. Any cell that got warm, out of balance, or didn't hold the expected capacity was thrown out. I ended up disposing of 25 or so cells, but I had order an extra 46 anyways so it didn't matter.
It also helps that I let the cells aside after a full charge for a week or so simply because of the balancing process. Any cell that self-discharged I knew wasn't good. No FLIR yet but I'd love if :)
If you think 70 cents per cell is good, batteryhookup has 35 cent cells at the same time... They were in worse condition at a pretty low capacity though so I passed on that.
So that's why you really need that Bosch BMS. Then the bad news: the Bosch BMS is on the edge of what's safe, the FETs are underpowered (just a hair under rated maximum current and voltage), not cooled, there is a 40V Schottky diode there which regularly operates at or even slightly above it's ratings, there is a single chip DC-DC convertor which also isn't cooled properly and which tends to blow. So I don't particularly trust the Bosch BMS from a safety perspective, I've seen a whole bunch of these now (about 15) and a good number of them had blown up. And that's with the standard battery. The one that I'll be hooking up to it is four times more powerful. Any short circuit protection in the old BMS is going to be terribly undersized so I need something in series with the Bosch BMS to ensure that if there ever is a short (say in the motor) that it's the battery that gets disconnected well before the wiring goes and I simply do not trust the Bosch BMS not to fail 'closed' in which case the whole thing will turn into a giant fuse. That's the kind of scenario that I can really do without.
On more than one pack that I've opened I've found evidence of small fires, typically BMS balancing wires that shorted out (there are no fuses there, obviously) and that could have ended quite badly. The wires from the pack to the motor are substantially thicker so then the question is what cuts out first: the BMS or the wires. I simply want to play that as safe as possible and aim for the disconnect to happen fast enough that the wiring doesn't light up.
Another option is a very fast fuse in line with the battery terminals, that's definitely a possibility as well (and easier to mount and less chance of it in turn breaking than yet another BMS in series). Anyway, work in progress and to be continued, nice to find someone on HN who has a lot of knowledge about this!
Waiting for a while to see if there is a self discharge is a very good method to spot flawed cells before you assemble a pack but such defects can also develop at a later date, so please do keep a good eye on your pack, especially if - as I suspect - you are using reclaimed batteries, you can't be 100% sure that they have never been deep discharged.
Safety is one of the main reasons why I decided to go with brand new high capacity brand name cells, yes, it's a lot of money, but even so, the whole thing will cost about as much as a 600 Wh commercially available pack for this bike and have more than 3 times the capacity (and a better form factor to boot, and will last a lot longer).
Yes dendrites could develop later on. But the process of dendritic growth is quite slow and gradual.
Because of that, before the cells start getting too hot and threaten thermal runaway, what will happen is that the pack will run out of balance faster than the 40mA my BMS can balance. And shortly after my BMS will trip protection and I will be safe. This is actually why BMSs have a lower balance voltage - 1mA is of course not enough but they tend not to exceed 50mA so they will catch damaged cells very early removing all the risk.
As for deep discharge, you might enjoy this paper : https://www.nature.com/articles/srep30248 which also includes a short literature review.
The TL;DR is that damage is relatively limited during deep discharge even to 0v. There is plausibly possible capacity degradation but it not very different from accelerated wear and tear, though the authors did not detect any.
What is really dangerous though is discharging a battery pack to 0v - when that happens it's easy for some cells to gain a negative state of charge, which is when the nasty stuff that actually causes fires and damage happen.
Copper dissolution according to this paper in more modern batteries does not even really happen until -12% state of charge. Indeed in modern higher capacity batteries they found no detectable damage at all until -12% soc.
Because of that, I am very confident from my setup - the risk of internal short circuit is incredibly low given that I know that the cells were always in balance, and any slow-forming internal short circuit will trip the protection much before there is any fire risk.
It's very interestinf though from reading the paper that the deep discharge risks essentially disappeared at 0v from pure coincidence as capacities increased though, I'd have expected somewhat the reverse :)
Agreed that your setup is pretty safe though, especially if you have already cycled the pack many times and see no deviation between the cells.
I've figured out the layout, it - unfortunately - won't be possible to put the pack in the frame triangle, there are too many details that will become messy, space is constrained, mounting options are limited and wiring the pack would be a nightmare with a lot of odd groupings of cells which would have a lot of potential for mistakes, so I'm going with the 'boring' option of making a rear carrier based box that will house the battery. A port at the bottom will connect a cable with a custom made plug from an old pack to the port on the bike, which can then still do double duty to charge the pack.
From a center-of-gravity perspective it's not ideal, but it does allow boosting the pack to a full 200 cells in 10S20P configuration, which should be more than enough to do my runs and have room to spare. Can't wait to get it tested :)
>I'm not quite sure what 'modern' means in this context but all of the batteries that I've seen so far (Samsung, LG, Sony) caution in their MSDS against deep discharge both from a functional risk as well as from a safety perspective and there are a number of pretty knowledgeable people online who are on the record with their reputations at stake cautioning against this
Basically by modern they mean higher capacity than the ones in the 2006 study that found dendrites at 0V - those were early 2000s era tech, shoddy 760mAh cells. They found that the higher the specific capacity, the higher the resistance to Cu becoming dissolved.
As far as constructor experience I do agree with you, it's preferable to avoid it. Though one factor is that people with extensive commercial experience will probably have had to work for very cheap, low capacity 1600mAh cells that are definitely more prone to this, and quite old arrays. I definitely recommend against it too, it just seems to me that it won't result in a fire anymore. I wouldn't take the risk though with a cell I know was discharged to 0 or beyond.
I should do a write-up on battery pack failures, I've collected a small mountain of data on this, it might be useful to others.
Many thanks for the rear carrier information, this is something I will have to really deal with somehow, the frame gap on this bike is just too bloody small and I don't want to permanently alter the bike in case I want to sell it. I've done quite a bit of cycling with kids on the back so I'm used to having a lot of weight there (and one on the front too, total bike weight in excess of 130 Kg), but it's something that will have more effect the faster you go.
What's definitely out is an off-side pack or to have it split across the carrier bags, that's asking for trouble, both from a mechanical point of view and an electrical one (you'd be running quite a few wires between the two sides). At least a carrier one is going to be centered on the bike.
Cells are arriving tomorrow, for now I've decided on a 10x20 grid (so a few more cells ;) ), 21 x 41 cm plus some room in front to house the BMS. the box will be fairly low, about 90 mm altogether so that should help keep the COG low, the carrier is rather low on this bike so on the whole it might not be that bad. I'll be sure to be extra careful with wet roads and I categorically refuse to cycle when it's icy (I have a bunch of steel in one leg from a previous accident so falling again is really not an option).
At least my adapter works, so I won't need to butcher the bike wiring harness.
If you want to take this out-of-band: [email protected]
And I'm working on an extended range pack (2000 Wh, the bike is a speed pedelec doing 30 mph).