it's not off-topic as far as I'm concerned. How's your MX-5 beast coming on Steve?
Dual-motor Austin Mini
I've been talking about this for long enough and I've changed my mind about how to do it countless times, but I've finally made a start on my Mini conversion.
I've got to come clean first and say it's not a completely authentic Mini, as it was rebodied some years ago with a glass fibre shell. Otherwise it's a Mini through and through, complete with leaks, ageing 1275 A-series engine and gokart handling.
She's currently buried outside under 12 inches of snow, but I've bought a spare front subframe so that I can get the motors mounted and refurbish the front running gear over winter. I found a couple of Prestolite 7" motors a while back, via an ebay contact. They're rated at 130A continuous at 36V. I've advanced them by eight degrees and plan to run them at around 120V each.
The motors have actually been shortened by around 40mm since I took the photo below. This was done by removing the internal fan, and machining back the alloy "legs" on the drive end. It was the only way I could get the two motors to fit across the subframe and also gave me a decent length of plain shaft to mount the drive sprocket on. I'm not going to worry about whether it's unbalanced the motors yet...
Each of the motors will drive a chain sprocket mounted on the inboard ends of the half shafts, so no gearbox, no diff. I'm busy making up two pairs of bearing carriers that will each support a short shaft with a sprocket on one end and a CV joint on the other. I'll post some photos in a few days when I've got something worth showing.
Now to the first of many daft questions: I want to keep this conversion as simple (and hopefully cheap) as possible. I've been looking at options for providing vacuum and 12V power and the simplest way I can see to do that would be to use the tail shaft on one motor to drive a mechanical vacuum pump, and use the tail shaft on the other to drive a lightweight alternator. The question is, if I have the motors wired in series to a single controller, is the small difference in load on each motor likely to cause any problems?
I've got to come clean first and say it's not a completely authentic Mini, as it was rebodied some years ago with a glass fibre shell. Otherwise it's a Mini through and through, complete with leaks, ageing 1275 A-series engine and gokart handling.
She's currently buried outside under 12 inches of snow, but I've bought a spare front subframe so that I can get the motors mounted and refurbish the front running gear over winter. I found a couple of Prestolite 7" motors a while back, via an ebay contact. They're rated at 130A continuous at 36V. I've advanced them by eight degrees and plan to run them at around 120V each.
The motors have actually been shortened by around 40mm since I took the photo below. This was done by removing the internal fan, and machining back the alloy "legs" on the drive end. It was the only way I could get the two motors to fit across the subframe and also gave me a decent length of plain shaft to mount the drive sprocket on. I'm not going to worry about whether it's unbalanced the motors yet...
Each of the motors will drive a chain sprocket mounted on the inboard ends of the half shafts, so no gearbox, no diff. I'm busy making up two pairs of bearing carriers that will each support a short shaft with a sprocket on one end and a CV joint on the other. I'll post some photos in a few days when I've got something worth showing.
Now to the first of many daft questions: I want to keep this conversion as simple (and hopefully cheap) as possible. I've been looking at options for providing vacuum and 12V power and the simplest way I can see to do that would be to use the tail shaft on one motor to drive a mechanical vacuum pump, and use the tail shaft on the other to drive a lightweight alternator. The question is, if I have the motors wired in series to a single controller, is the small difference in load on each motor likely to cause any problems?
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If it performs this well on half voltage and with the current limited the MX-5 going to be a real rubber burner. Look forward to seeing more photos and hearing performance details.
Does your insurance company know that it goes a little faster than your average milk float?
Does your insurance company know that it goes a little faster than your average milk float?
Hehe, I have a traders policy, but even if I didn't I would be showing them the figures for Continuous power not 10s peak
OK, winter’s gone and I’ve still not finished the battery pack, but at least I’ve made a start. The problem has been trying to work out how to stuff almost 200 cylindrical cells into a Mini boot. The 10Ah LifeBatt cells I’m using are fairly old tech by now and they take up a lot of space. After lots of measuring and scratching my head I finally made another compromise, and decided to install the pack in place of the rear seats. I just wasn’t happy with the idea of having expensive and fairly volatile lithium cells installed in what little crumple zone the Mini has. Besides, this is just a proof of concept, so details like this can be fixed in the next version.
I also gave up on the idea of mounting the individual cell chargers on the busbars. It seemed a neat idea at the time, but I’ve now got a better appreciation of how much waste heat these little DC converters put out – way too much. The next idea was to mount the converters on a fan-cooled aluminium plate above the cells. The plan is to build five of these 5p 8s modules, so just over 24V and 50 Ah per module. The DC converters are fed from a 40V supply, and are current limiting. The real issue with these that they have an efficiency of just over 80%, and when they’re hooked up to a parallel string of five empty cells they put out about 25A and get very hot very quickly. I keep them cool with a 12V automotive AC blower, but this just doesn’t seem the ideal solution for mounting inside a plastic car…
So the latest plan – liable to change tomorrow – is to go the conventional route and buy a decent charger, probably an Elcon. The Vicor DC converters won’t go to waste though, as I’ll use them for occasional cell balancing. The pack will be protected by Cell-logs and relays on each module that will short the throttle and switch off the charger when HVC or LVC are reached.
By the way, if anyone has any spare unused or little used LifeBatt or PSI 10Ah cells lying around in Europe, I’m interested in buying them. I need another couple of dozen to finish off my pack.
I also gave up on the idea of mounting the individual cell chargers on the busbars. It seemed a neat idea at the time, but I’ve now got a better appreciation of how much waste heat these little DC converters put out – way too much. The next idea was to mount the converters on a fan-cooled aluminium plate above the cells. The plan is to build five of these 5p 8s modules, so just over 24V and 50 Ah per module. The DC converters are fed from a 40V supply, and are current limiting. The real issue with these that they have an efficiency of just over 80%, and when they’re hooked up to a parallel string of five empty cells they put out about 25A and get very hot very quickly. I keep them cool with a 12V automotive AC blower, but this just doesn’t seem the ideal solution for mounting inside a plastic car…
So the latest plan – liable to change tomorrow – is to go the conventional route and buy a decent charger, probably an Elcon. The Vicor DC converters won’t go to waste though, as I’ll use them for occasional cell balancing. The pack will be protected by Cell-logs and relays on each module that will short the throttle and switch off the charger when HVC or LVC are reached.
By the way, if anyone has any spare unused or little used LifeBatt or PSI 10Ah cells lying around in Europe, I’m interested in buying them. I need another couple of dozen to finish off my pack.
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An update as promised:
My first attempt at making the stub drive shafts wasn't good enough, so I bought a small lathe and made up some new ones. Also made a chainguard.
My first attempt at making the stub drive shafts wasn't good enough, so I bought a small lathe and made up some new ones. Also made a chainguard.
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Stripped away all the DC converter gubbins from the battery modules. 5p8s LifeBatt 10 Ah cells. Just need to add the Cell-log and relay unit. I plan to use just three of these to start with, to stay comfortably within the 90V limit of the Alltrax. I have enough cells to make five modules, and I'm still looking for more PSI cells if anyone has any spare.
Fitted a proximity sensor to the motors – the same one Tesseract recommends for the Soliton. And I found a nice rpm switch from Intellitronix in the US. The rpm limit can be set using dip switches, and it works perfectly with the proximity sensor. The switch is connected to a relay that shorts the throttle input.
Fitted a proximity sensor to the motors – the same one Tesseract recommends for the Soliton. And I found a nice rpm switch from Intellitronix in the US. The rpm limit can be set using dip switches, and it works perfectly with the proximity sensor. The switch is connected to a relay that shorts the throttle input.
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Made another chainguard for the top of the subframe. The welding isn't great, but I always seem to be in a hurry. Fitted a pair of Albright 182B reversing contactors, one for each motor of course. I still need to make polycarbonate covers for these.
I found a nice forward/reverse switch at an auto jumble. Originally an auto selector, I just had to reposition the switches and add the F/R indicators.
And I used some of the little Vicor DC converters and four LifeBatt cells to make a 1p4s auxiliary battery. With my limited range, 10 Ah should be plenty for now. The DC converters take an input of 36–72V volt, so two of them will be connected to one half of the pack, and the remaining two to the other half. That should ensure balanced drain on the pack and will let me go up to 144V in the future.
I found a nice forward/reverse switch at an auto jumble. Originally an auto selector, I just had to reposition the switches and add the F/R indicators.
And I used some of the little Vicor DC converters and four LifeBatt cells to make a 1p4s auxiliary battery. With my limited range, 10 Ah should be plenty for now. The DC converters take an input of 36–72V volt, so two of them will be connected to one half of the pack, and the remaining two to the other half. That should ensure balanced drain on the pack and will let me go up to 144V in the future.
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Made a simple frame to support the Alltrax and blower to cool the motors. Sorry about the dull picture, but sunshine is in short supply in the UK right now.
The thing just to the right of the blower is a throttle body from an MG, which is nice and light. I fitted a 0-5k throttle position sensor from RS components, as the original TPS only had a range of about 1–4k, and the Alltrax won't work with that.
I plan to upgrade to a proper controller eventually, but for now I just want to get this on the road. With the motors in parallel the Alltrax will supply each motor with 72V and 225A, for a while...
The thing just to the right of the blower is a throttle body from an MG, which is nice and light. I fitted a 0-5k throttle position sensor from RS components, as the original TPS only had a range of about 1–4k, and the Alltrax won't work with that.
I plan to upgrade to a proper controller eventually, but for now I just want to get this on the road. With the motors in parallel the Alltrax will supply each motor with 72V and 225A, for a while...
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By the way, your battery pack design looks a lot like what I did for my booster pack.
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Thanks Adam. I get loads of inspiration from everyone else's projects – but the mistakes are all my own.
DIYguy: Those A123 packs look great. I've not seen the large cylindrical A123s in a DIY project before. How did you get hold them?
Thanks for the info about the Vicor modules. The micro converters I'm using do current limit at 20–25A, though I wouldn't want to use them this way as they get very hot and fizz quietly. I've charged a 5p block of almost empty LifeBatt cells with a single DC converter, but it takes a lot of cooling as they're only 80% efficient. I can dig out the datasheet if you like. One useful feature is that you can parallel up to three of the converters and designate one of them as the master by adding a single resistor. This gives you a 60A single cell charger.
Studebaker:
Same here, and I wish I'd bought one years ago. Even a little lathe like mine opens up so many new possibilities. My dad was a shipyard fitter and turner, so I felt it was in the blood
Motors are in, finally! The mini has been on blocks on my drive for too long.
It's bit crowded in the engine bay, but everything fits, just. I need to cut a couple of small holes in the inner wings to make sure the motor shafts don't rub.
It's bit crowded in the engine bay, but everything fits, just. I need to cut a couple of small holes in the inner wings to make sure the motor shafts don't rub.
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I've just finished top-balancing the pack. Each 24V/50Ah module has its own cell-log and relay board to provide low-voltage cutoff to the throttle input and high-voltage cutoff to the charger. The relay outputs are wired in parallel.
The fourth module is only partly populated at the moment as I'll just be running 26s to stay within the 90V max for the Alltrax controller. The battery pack will sit in a rack in the back seat of the mini, and will have a polycarbonate cover and insulators between each module.
It's taken several days of tweaking to get the cells closely balanced, but they look pretty good now. I'm using a pair of 30A 36V Curtis chargers in series. The chargers are each adjusted to finish charging at around 45.5V. All cells finish charging at 3.51±0.02V at the moment.
Just got to install the pack and finish wiring now...
The fourth module is only partly populated at the moment as I'll just be running 26s to stay within the 90V max for the Alltrax controller. The battery pack will sit in a rack in the back seat of the mini, and will have a polycarbonate cover and insulators between each module.
It's taken several days of tweaking to get the cells closely balanced, but they look pretty good now. I'm using a pair of 30A 36V Curtis chargers in series. The chargers are each adjusted to finish charging at around 45.5V. All cells finish charging at 3.51±0.02V at the moment.
Just got to install the pack and finish wiring now...
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