Thursday, January 14, 2021

Total rewire and 1KW-hour LiFePO4 battery conversion

While the blog might have been idle, life hasn't stood still. My dear old Bakfiets is still going, even if it got a break for a year when we were living in the middle of the Outback, where you just needed to look at the ground to get a puncture.  

Its done well over 25,000km now, and after the third battery pack dying, I got sick of this happening, and decided that I wouldn't buy another one of the standard packs. Also, they are no longer made, as Bakfiets switched from the Azor electric kits to the Shimano electric systems. I looked at replacing the dead cells in the packs, but that's also a pain, as it requires un-spot-welding and re-spot-welding an awful lot, and I don't own a spot-welding machine.

But with COVID19 messing with importing bikes, the Australian dollar being low again and all that, I figured I didn't want to fork out a pile of cash on a new e-Bakfiets, either.  So I decided that I would completely replace the battery pack with something custom, and fix several annoying problems that always annoyed me:

1. The standard battery pack was only ~400W hours, which means <2 hours of full boost.  Church is close to an hours ride away, so a return trip would typically result in a flat battery, and that was before the batteries start losing their capacity, which they would do quite quickly when used every day, because...

2. ... the official battery packs were LiPo-based, which while lovely and small and light, are not the most long-lived chemistry. Also, I don't really like having a battery system that can go critical easily if I have an accident.  I much prefer LiFePO4 batteries for their much longer useful life and intrinsic safety.  The weight and size are not really an issue when you are talking about an e-Bakfiets, since I have this 100L box on the front, and the kids are now too big to ride in it, so I rarely need the full volume.

3. The original battery packs have to be removed to be plugged into charged.  This is just plain annoying, especially as the sled can be a bit sticky at times, and is just a bit fiddly to move when you get home in the cold rain in winter etc.  So I wanted an approach that would be as simple as possible to plug into charge.

4. The original charger is also as slow as a wet week: 6 hours of charging for <2 hours of riding.

5. After adding an airhorn, front flood-light and rear super bright red LED light, the wiring was a mess, with everything having to connect back to the battery tray on the rear carrier.

So I looked around at what batteries I could order directly from China, and found a nice 40A 7S (~27V) LiFePO4 battery pack, i.e., 1,080W hours, for less than the list price of the original ~400Wh LiPo packs.  In fact, the LiFePO4 battery itself was about 1/3 the price, but then air freight to Australia for a single battery doubled that. This more or less matches the 8S LiPo for voltage, so can be used without an adapter. For another US$90 they included a 20 Amp charger: That's right, I can recharge this 1KW hour monster in 1/3 the time of the original.  At 250W maximum power I should get around 4 hours of full-assist riding, for only 2 hours of charging.  Much more civilised.

It took a while with COVID and postal hilarities caused by ordering while we were living in the middle of the Outback, but the battery and charger finally arrived last week, and today I finally had the time to start stripping everything down, and rebuilding with the new wiring.  I decided that I wanted to put as much as I could in the cargo box at the front, and have only the bare minimum wiring visible externally. I ended up being able to reuse almost exclusively the existing holes in the cargo box, including a couple of the drain holes that don't get much use, now that the kids don't ride in it.

The whole episode only took about half a day, including remembering how I had the various things connected together, including those add-ons I mentioned earlier, which are controlled using a cheap motor-bike handle-bar control which has a momentary horn button and click-on-click-off headlight button.  In the process I also discovered that the old airhorn had given up the ghost, so I replaced that as well (I had a spare on hand, as I had suspected that the old one was dying). 

The only thing that isn't working perfectly is the battery level indication on the Azor handle-bar display.  I assume that this gets the voltage from 3-pin connector that came from the original battery, and which I haven't connected.  Alternatively, it might determine it from the primary battery voltage.  Either way, it is showing empty. But then, it has been doing that with the original battery packs for years, as soon as they drop a single cell, thus lowering the maximum voltage. So that's not really a big inconvenience. That said, I have ask Dutch Cargo Bike if they can get me some more information on that 3-pin connector, to see if I can find a way to get the battery level showing on the handle bars. The big LiFePO4 pack does have a display, but you have to press a button on the battery to activate that, and it goes off again after a few seconds.  Also, with my setup, that's inside a battery box under the flat tarp cover of the Bakfiets, so not accessible while riding.  Anyway, if I charge it up every night, I shouldn't ever have a problem with it going flat on me.  And if it does, I can now recharge about 8x faster than the original charger.  

So, in short, I'm pretty happy with the result.  Here is the battery pack and wiring after all the work:


The battery is in the big black metal box, to make sure nothing falls across the big fat terminals of the battery, since fully charged it does contain enough energy to boil about 30L of water... The black box to the left of that is the charger, which fits conveniently in the gap. The relay screwed into one of the hold seat-belt harness holes is for the airhorn, which draws 20A at 24V, i.e., almost half a kW -- it is really loud --  because the handle bar switch can't carry that much current.  The handle bar switch comes in through the other hole.  At the moment there is one cable that just comes in over the top edge, which I would like to tidy up further, which is the cable to the absurdly bright rear light (its the same type you put on portable antennae to ward of low-flying aircraft).  But all up, I'm pretty happy, and it is miles better than it was before.

And now the pictorial journal of the process I went through...


First, remove seat and original battery, so that I can get to the little electronics enclosure: 

Here is that insanely bright rear LED light.  It draws 10W on average, with a 25% duty cycle or so, so has a peak power consumption of around 40W of LED.  Cars now leave a nice gap behind me when stopping at the lights at night.

Then under the battery compartment, we have the actual motor controller:

This doesn't need to go there, since the motor is at the front of the bike. So if I move that into the box, we can avoid having a lot of wiring going to the rear carrier and back again.

Next step, sit the battery in the bike and think about where everything will go:

Here is the nice little integrated display on the battery. If I could make it stay on, it would be really useful.

Now, originally, I was going to use these big fat leads to hook everything up, since we have to deal with peak currents of up to 50A (full boost plus that 20A air-horn). But in the end I didn't need to.

So here is the 20A charger with its nice big fat alligator clamp fittings.  I removed those later and put eyelets on that I found in the shed, and hard-wired it onto the battery, as you will see later:

Again, a quick fit test and further think with the cover for the battery box in place. The battery box is there to avoid problems if a bit of water gets in under the cover.

Now, speaking of the old ugly wiring, here is where that relay has been sitting for the past three years:

Completely function, but completely ugly. You can also get a hint of the general mess of the wiring prior to today's work.

Now, back to pulling apart the wiring loom for the original battery system. Most of it is hidden in that little compartment behind the seat-post, and connected to that little black controller box we saw earlier:

Its a bit of a pain to get out through the hole in the bottom, but its just a case of fiddling with it for a while. I also had to start removing the 5 connectors to the big battery interface terminal. This blessedly used spade-connectors, which I was able to just slide off, and make mating connectors for, later:
After all that. I had the controller box out and alone:
And the rear carrier was now looking like this: The metal plate and wire running on the top are for the rear light:
One of the enduring mysteries is what this brown-yellow-blue 3-pin part of the battery connection interface do.  It has a 2A fuse inline on the blue, and connects to the Azor luxury display on the handle bars.  Is it backup power? Is it battery level indication? Both? Or something completely different? I'll let you know when I find out:
Next I removed the key lock that cuts the power from the battery to the controller.  I like the idea of having a key-interlock for the power, especially since I have a 139dB airhorn switch on the handle bars that kids love to press if they figure out it is live. So I decided I would keep that, and hook it up with the new battery. Unfortunately it can't stay on the carrier, because the battery is not there, so I will need to have two separate keyrings with me in future. A bit of an inconvenience, but nothing major:
Then we have a bunch of wiring modifications I made when I hooked up the airhorn and bright lights.  I used vampiric crimp connectors, so that the result would still fit in that tiny electronics compartment:

However, with the new setup, that wasn't a problem, and they were both ugly and a pain to work with, so after I removed them to be able to disassemble the wiring loom, I re-wired without them.  The Deutsch connectors at the top of the picture above I added when adding the lights and airhorn, to make the loom easier to disconnect, which I was rather thankful of today.

And here are more pictures I made so that I could make sure I ended up with functionally equivalent wiring at the end:

So now I was more or less at the point of having a bunch of suspiciously similar looking cable ends, that I had to start matching back up, to reassemble:

So more gazing in the box to think about how I would hook things up, and removing the bench seat and seat belts to make more space:
This was when I had the brain-wave to use the holes from the strap mounts to hold the relay in place, and to feed one of the cables through:
So now that ugly patch of wiring with the relay etc, is no longer visible from the rear:

Around by the front-wheel, there was also a mass of wiring, which I was able to shorten and simplify, feeding it through one of the front drain holes. The other front drain-hole is taken by the mounting bolt for the airhorn. The airhorn itself can be seen in the lower-right of this image:

From the other side at the front we can see the flood-light, and how I did still have to feed one of the cables over the top of the front of the box. This goes over a slightly damaged part of the box, that was roughed up by carrying the kids bikes on top of the cargo box every day for three years. I might file a small notch in this area in the future to let the cable sit out of harms way.

So now the wiring was starting to get joined up, but was still unsettled in the cargo box, and I had to come up with a plan for how and where to mount the controller box:

The controller is the heart of the rats-nest of wiring, and eventually should probably get a nice tidy box to conceal the rats nest:
But for now, I used one of the bench seat supports, and just put the rats nest of wiring on the floor below it. It can't really go anywhere else, unless I want to cut and lengthen the speedometer and pedal crank sensor cables, which were really made assuming the controller would be right next to the crank and rear wheel. We can also see the eyelets being used to connect the wiring to the battery. I salvaged one of the 30A fuse holders from one of my dead original battery packs to protect against short-circuit:
So now its all really starting to come together. The lid can fit on the battery while it is connected, and I have decided on using the spare space to fit the charger:
The charger is, as I mentioned, hard-wired to the battery as well, via another pair of eyelets. These also act as spacers to enable the screw terminals to hold solid.  Writing this now, I just realised I should put another fuse block on the charger leads for complete safety...
And finally, with everything hooked up, and a retainer strap over the battery, using the bench seat supports as anchor points.
So apart from a couple of odd wires, from the outside, when the cover is on the cargo box, there is not really anything to give away that the wiring and battery system has been completely re-built using a custom battery and charger solution.

Let's see how it goes over the coming months. But for now, I'm just glad to have lights and a horn again for safely traversing city traffic, and a battery that works to let me use the motor when the need arises.

1 comment:

  1. Hi! I rewired my battery, too, but using a different (much cheaper) pack in the same enclosure. I was initially trying to just go with the BMS that came with the new pack but this didn't seem to work with the three "extra connectors" of the battery unconnected. No display switched on, no power... I had thus to connect the old BMS to the new pack which turned out to be a major soldering and "no space in the box"-PITA. Did you ever find out what the purpose of those connectors is? It seems you got things to work with the same controller without them, anything special you did? Thank you in advance.