Will very cold weather allow me to slightly push C limit on BBSHD controller?

[Ph03nx]

Hi everyone, a little intro here but if you want to skip to the heart of the problem see below.

Here in Montreal, we go through all possible seasons, including long and often humid winters (November to April). People's biggest expense here is related to car fueling and maintenance. Since COVID19 started, we've been trying to cut down on expenses and my solution has been to place all bets on ebike technology. Some people think I'm crazy to ride in -20 temps but to them I say, there are no bad temperatures if you have the right gear. With a North Face jacket, a BBSHD equipped ebike and quality studded winter tires, believe me everything is possible! I even built a very simple super wide rear rack extension with thick plywood and ratchet straps allowing me to mount a huge storage box to carry quite a lot of groceries. The experience has been great and winter riding is super fun!

***I can make a post with pictures on how to build this easy, cheap and super efficient upgrade if you guys are interested.***

I now do 100% of all my transportation by ebike, thus saving us on avg 4500$ of car expenses this year over : Insurance, gas, general maintenance, anti-rust treatment, mandatory winter tire swapping, car part breaking down due to our very OH SO shitty asphalt roads and so on... Incredible! 

Pictures of my bike when in normal and "cargo mode"

https://ibb.co/qB9GY9j
https://ibb.co/sj1VQFd
https://ibb.co/cQKd9VY
https://ibb.co/wY3rFfr
https://ibb.co/TKK7Cgt
https://ibb.co/bdJWpqR

Heart of the problem

My next challenge is to find the right configuration for inter-city winter commuting in very cold temperatures (-20 ish, 25 to 50 km ride).

My strategy is to hookup 2x 52v17.5ah batteries (Home made with LG MJ1 3500mah cells) in parallel and use as little throttle as possible for better range. After testing this setup, I realised that plugging 2 batteries in parrallel helps reducing heating which is normally a good thing. The drawback is that in -20 temperatures the cells get even colder which translates into even more rangeloss.

My idea is to program the BBSHD in a way that the battery heats up just enough to bring the cells back to a suitable internal temperature. I use settings similar to Karl's special sauce and other than the standard 30a limit, I make sure the controller and motor never have to perform more than a 1500w workload. I know that the BBSHD can easily do 2000w without any issues so considering my riding style, I'm not worried about smoking the motor. The concerns is the stock controller... of course.

With the added wind factor, the mosfet's heatsink gets super cold during -20 commuting. With this in mind, do you think I can push the drive's amp limit to maybe 32 or 35amp without overheating the FETs ? The cells wouldn't even need to be very hot to the touch... just a bit of heat at the core of the cell would already be an improvement I think.

I'd love to have your thoughts on that!

 

[amberwolf]

(25 to 50 km ride).

My strategy is to hookup 2x 52v17.5ah batteries (Home made with LG MJ1 3500mah cells) in parallel and use as little throttle as possible for better range. After testing this setup, I realised that plugging 2 batteries in parrallel helps reducing heating which is normally a good thing. The drawback is that in -20 temperatures the cells get even colder which translates in even more rangeloss.

How about simply insulating the battery compartment thoroughly (styrofoam padding, etc), and if necessary include heating pads (like those for reptile terrariums) between the insulation and the batteries? Well before any ride that needs the extra heat, take the battery compartment (or whole bike) inside for charging, and also plug in the heating pads to the wall.

Include some temperature monitoring (any kind of remote or wired thermometers that you can place the sensors inside the packs), so you can ensure the temperature only reaches what you want it to (very warm but not hot, staying well within the cells' safe limitations, perhaps 90-100F). It's also possible to use thermostat control of the heaters (they even make powerstrips specifically for this in reptile habitats), but it's easy enough to do manually.

Once the pack is at the temperature you need, unplug the heaters, install the battery compartment on the bike, and go ride. If the temperature monitoring can be done in a way that you can access it during a ride, you can watch if the cells get hotter or colder and how much so.

It's quite possible that simply having enough insulation around the batteries would be enough to keep them warm enough for long enough, without preheating the pack (but still keepign it indoors until you ride), but you'd have to experiment with that.

If preheating isn't sufficient, you could also wire up the heaters to run off the batteries themselves during the ride. Just install a powerstrip inside the box that you've cut the wall plug off of, and wire that to a plug that can go between the output of the batteries and the inptu of the controller, and then plug the heaters into the powerstrip when not having them plugged into a wall. The voltage will be less than half of the wall voltage, so there will be a lot less heat produced, but they'll still work and still keep heating the packs.

Alternately, just always have them wired up to the pack outputs (with a switch so they don't drain the packs dead left unattended), and leave the packs on the charger (so the packs don't drain whiel preheating them) until th3ey've reached the temeprature you want them at.

With this in mind, do you think I can push the drive's amp limit to maybe 32 or 35amp without overheating the FETs ? How about simply increasing the bike's weight, would that alone do the trick ? 

I don't really understand; if you make the bike heavier, (enough heavier to make a significant power draw to create heating), you're decreasing your acceleration, making your ride harsher (bad roads, right?) and harder on all parts of the bike and everything mounted on it (including you).

Also, making the bike have to work harder simply to keep the batteries warm will reduce range and unnecessarily stress all of the electrical system (batteries, controller, motor).

And you'll have to ride harder than usual to make the system draw that extra power; it wont' draw the power unless you are forcing it to do so.

 

[markz]

https://milwaukeetool.com/Products/Work-Gear/Heated-Gear/Heated-Jackets

https://www.homedepot.com/b/Tools-Workwear-Heated-Clothing-Gear-Heated-Gloves/N-5yc1vZ2fkomi8

https://www.marks.com/en/innovations/thermalectric.html

 

[Ph03nx]

Thanks amberwolf. I thought of heating the compartment but I was worried that the heat wouldn't distribute equally amongs all the cells (inner vs outter cells) or create hot contact spots. Your idea of insulating solves that because I would end up with a heat bubble, I like that! But before I get into building a heating system, I still want to see if I can use the cold to my advantage and up the current limit a little to make the cells generate their own heat. I know its not an issue for the motor or the battery because a lot of riders run the BBSHD on 52v batteries at 40 amps amperage using more powerful controllers with no issues. The motor is built to handle 2000w with ease. And for the batttery, the cells barely need to be hot to the touch... just a bit of heat at the core of the cell would already be an improvement.

I mean, if its doable, then my problem is solved without having to mod anything and that would be great! I agree that I will propably loose range but not nearly as much as if the batteries are dead cold. It would be like choosing a lesser evil, unless of course my controller fries the second I go past 30amps. Thats what I want to know. I never tried it so I was wondering if someone ever tried to incrementally increase the c limit during winter time and what to expect with that approach. You're right the extra weight probably wont help at all lol.

 

[amberwolf]

And for the batttery, the cells barely need to be hot to the touch... just a bit of heat at the core of the cell would already be an improvement.

The catch with heating up the batteries by pushing them harder (rather than external sources) is that this is damaging to them, and will decrease their lifespan (vs decreasing the range on the particular ride you're doing this on).

So, sure, you can probably do what you want...but you'll need new batteries that much sooner by doing it. How much sooner depends on exactly how hard you push them, how good they actually are (quality of cells, how well matched, how well-suited to the application, etc).

unless of course my controller fries the second I go past 30amps.

It's unlikely to do it like that, but remember there is a delay between the heat being generated within the FETs, and it passing thru first the insulation between them and the next thermal stage (whatever mounting bar it has for them), then thru that thermal stage, then from that stage thru whatever insulation there is between it and the case, and then thru the case itself to the outside air. Even with the best heatsink pastes between things, and the best heatsink materials, its' still not an instant thermal path, and if there is sufficient heat generated in a FET so that it cant' dissipate fast enough to at least the first stage of the heatsink, that's when things fail. Colder temperatures work well to start with, and as long as this higher FET temperature is in bursts, but it works less well as things inside begin to soak up the heat and be less able to pass it along.

Personally, I don't think that you're going to see much change in the *average* current thru the battery, and thus not much change in the *average* temperature--your usage, if it's typical, will see only slightly higher peaks of current for a second or two as you accelerate from a stop. As soon as you pass the point where it takes more power than you had before to do what you're doing, you're back to exactly the kind of loading you had before, etc.

To "take advantage" of the higher current limit to cause continous cell heating greater than you had without it, you would have to force the system to operate at that higher current limit continuously. That might be by always riding in the wrong gear for the speed you want so the motor has to work a lot harder (which stresses gearing, chain, motor, etc), or riding enough faster all the time to take that much more power (whcih stresses tires, wheels, brakes, gearing, chain, motor, etc), or as you suggested before of adding enough extra loading (weight, etc) to the bike to force it to use more power all the time, etc.


Something else to consider is that if you had to run it at 40A constantly to get the heating effect you're after, and previously it was at 30A constantly, you're using 40/30 = 1.34 times as much current, so you get around 3/4 (inverse of that) of the range you would otherwise.

(it wouldnt' be constant in either case, so the range hit isn't likely to be that bad, but it's just an example to point out the potential issue with wasting power to cause heating intended to fix range loss from cold--you'd have to actually test it under both conditions to see what happens.)

 

[amberwolf]

I even built a very simple super wide rear rack extension with thick plywood and ratchet straps allowing me to mount a huge storage box to carry quite a lot of groceries.

Oh, I also wanted to note that you'll have better ride quality / handling if instead of putting stuff up on a rack, you put it in panniers centered around the axle instead. That's how I built the best of my cargo bikes over the years (both pedal and electric-motor-powered), some of which can carry up to a couple hundred pounds depending on the density of the load and the conditions, etc., without causing handling problems. Anytime stuff has to go up over the wheel(s), handling suffers, usually greatly.

(the SB Cruiser trike I built for even bigger loads of a few hundred pounds also has a low-down cargo area, but the dynamics are a lot different than on a bike)

If there is any lateral wiggle in the rack, it also makes handling worse (even with panniers) becuase you get "tail wag", which is worse when pedalling as you're already shifting weight back and forth, but still happens even with legs still and motor doing all the work. THe taller the wheel (and higher the rack off the ground), the worse this problem tends to be.

Since I can't see your externally-linked images, I don't know how your rack is setup, so the above may not apply to you. If you attach the images directly to your post, then anyone that can see your post can also see your images.

 

[Ph03nx]

Wow great stuff amberwolf! Thank you for taking the time to provide detailed feedback!! What you say makes a lot of sense. The heating system seems like a much better idea now. Sorry for the images, I don't know why I'm having trouble uploading the files. If you have simple rear rack designs that I could inspire myself from, I would love to see them!