And so to lithium…

In the beginning…
It’s been in the back of my mind for a while now to upgrade my scooter to lithium, and I’ve been watching the prices of lithium cells gradually fall for a year or two now. But having noticed a slight deterioration in the performance of my lead-acid batteries over the winter I realised that after 2 years and about 2000 miles of riding, it would not be long before I would be looking for some form of battery replacement.

Replacing like-for-like is the cheapest solution in the short term, but is a tad short-sighted as it’s generally accepted that lithium batteries can last up to 6 times as long as their lead-acid counterparts, but cost nowhere near 6 times as much to switch to (probably in the region of 2-3 times the cost of lead-acid). There are many advantages to lithium over lead-acid, not least their tendency to retain a large portion of their voltage until they are nearly exhausted, for the voltage not to “sag” when drawing large amounts of current, and their lighter weight. The drawback is that lithium cells are very sensitive to being overcharged or overdrained, meaning that a Battery Management System (BMS) has to be installed to ensure that any cells that reach their maximum voltage are not charged any further whilst the other cells continue to charge (balancing), and that if any cell reaches a preset minimum you can be warned, and it can even cut power to the controller to ensure that no further discharge occurs. This obviously adds to the cost but can still bring the whole system in at about 3 times the cost of replacement lead-acids, depending on which BMS you choose.

With this in mind I gradually put together a plan to perform the upgrade; I can fit approximately 70 or so volts’ worth of cells in the original battery box (thus regaining the underseat space I lost with the 72V upgrade), but I wanted to maybe gain a little extra performance at the same time. I realised from a previous attempt to remove the original 4 batteries from the battery box that this was not going to be possible without some serious “modification” of the box as it appears when it was welded to the frame it distorted, trapping the batteries in there for good. With this in mind I decided that a slightly larger box could be accommodated within the frame, and that this could be dimensioned to suit 50 lithium cells, giving me 80 volts! Add to this the weight saving (about 20Kg of lithium vs. 47Kg of lead-acid) and you can see that I may gain more than a little performance🙂

The BMS
As for a BMS, I looked at some of the “off-the-shelf” options and was not really taken with any of them. Then one evening I decided to register on the Battery Vehicle Society forum, and discovered that a few of the members on there had been developing a “home-grown” solution based on microcontrollers. This idea immediately appealed to me as I am a Software Engineer by trade and always enjoyed doing what we call “embedded work”. So I set about ordering some bare PCBs from the guy that originated the project, and as it happened he had a 25-cell “slave” board which is perfect for my needs (I’m going to arrange the 50 lithium cells as 25 “supercells” of 2 cells in parallel, which will give me a capacity of 24AH at 80V (not a lot less than the 28AH of lead-acid I currently run). Needless to say this is going to increase the cost of the project as it not only performs the function of a BMS, but also monitors and displays many other parameters (current, temperature etc.). You can even connect the pulse output from the controller to show an accurate speed reading, and attach a radio-controlled remote display so that you can monitor the charging process indoors!

Sourcing the parts to populate the boards proved to be a bit of a headache as this project has been a “work-in-progress” for about 3 years now and has undergone a few revisions, and no-one has produced a definitive up-to-date parts list. However with a bit of head-scratching and perseverence all the correct bits were identified and ordered, and within a few evenings of receiving them I had the “master” board (the one that does all the clever stuff) and 2 of the 25 circuits on the slave board populated and running. One thing I did save some money on is the display. The system is designed to drive it in a TV-like way (composite video), and most people use the LCD monitors designed for reversing cameras in cars. One of these would be way too large for me to fit anywhere on the scooter, so I came up with the idea of adapting an old Casio pocket TV that has been no use since they switched off the analogue TV transmissions. It already has a composite video input, and I reckon the display will fit nicely into the dash of the scooter where the (soon to be redundant) battery meter currently sits. For more information on the BMS, see here.

One of the problems with this “home-grown” approach is that in order to customise it to your own needs you need to be able to edit the software and reprogram the chip. It’s written in PIC Basic Pro, but having looked at the price of this compiler (and not being a great fan of the Basic programming language anyway), I decided that I would attempt to rewrite the Master code in C.

Fast-forward a few weeks and the C rewrite is almost complete, the slave board is completely populated and working, the lithium cells have arrived and been assembled into a pack, and work has started in earnest on the actual conversion.

Getting my hands dirty – the build
The first thing to do was to strip the bodywork off the scooter and attempt to get the old batteries out. This was not so easy because of the aforementioned “Chinese welding” job which had distorted the top of the battery box inwards, trapping the batteries in the box. This was soon remedied with judicious use of the hacksaw, halfway across the bracket. This meant that I could double the bracket up and drill and bolt it back together, with the side of the box in its intended place.

Batteries removed, I then realised that the box had a “front” which would need to be bent down to accommodate the longer battery pack that I was installing, which would also mean the pack could slide in from the front as the seat-support crossmember restricts access to the battery box from the top. A couple more hacksaw cuts of the spot-welds at the top of the front panel, and a bit of brute-force bending later, and I had a compartment that looked something like this:

The modified battery compartment

Next thing was to see if the pack was actually going to fit in there – it was a tight squeeze under that crossmember, but it went in!

The pack in situ

I’d realised for a while that in order to accommodate the extra length of the pack I’d need to cut some of the plastic and have it poking through, and make up a “power bulge” to cover it up. What I didn’t know was how far it would protrude, so I was pleasantly surprised with this:

Of course now it had to come back out again, as I still needed to connect all the wires from the BMS slave board, a job I was not looking forward to.

Delaying the inevitable, I moved on to charger wiring; I had purchased a couple of the highly-recommended Meanwell 48V power supplies which have adjustable voltage, and can be modified for “constant current”, a requirement for a lithium charger. I had done the modifications a couple of weeks ago and attempted to adjust the “series” voltage down to 92V which I reckoned would be about right for the number of cells involved. Unfortunately I could only go down to 93V but I’m hoping the extra 0.04 volts per cell won’t make that much difference! I’d already tested the current limit modification by attaching electric fires, power tools etc. to produce a load, and then turned the current down until it had just started limiting – I’ll possibly adjust this up a bit when I actually have the supplies connected to the battery.

I need a 12V supply to power the master board during charging as I obviously won’t have the ignition on which will normally power it, so I’ve made up a little board which allows me to have 2 separate 12V supplies plus a line that will be shorted to ground when the charger is plugged in so that the processor knows it’s in “charge” mode. This board also has a 6V regulator on it to provide a supply for my pocket TV for when it’s installed in the dash🙂 The charger 12V supply will come from my scooter’s original 12V DC converter which is connected across one of the 48V Meanwells.

One of my dilemmas was how to connect the current sensor – this needs to be “in line” in the main 80V wiring. Luckily Greg Fordyce has just produced a few PCBs just for this purpose, one of which he kindly sold me. So the sensor is now firmly soldered to the PCB, along with 2 stonking great 170A cables🙂

I’ve decided that the slave board will actually be taking up some of my underseat space after all, just in case I need to disconnect it in a hurry. This will only occupy about an inch in the bottom of the space so I’m going to make up a “false bottom” that will sit over the top of the board so that I can still stow my essential wet-weather gear and towel to wipe off the seat🙂

I’m going to mount the master board and the little 12V/6V daughter board mentioned above in the compartment under the footplate which was presumably originally for the battery on a petrol scooter. I think there’s just enough headrooom to mount it on the 2 little pillars that are already there and drilled to accept self-tapping screws. If not there may be some more plastic-cutting going on!

I finally plucked up the courage to connect up the BMS wires to the battery pack and all was going extremely smoothly until I was 3 wires from finished when I just touched the connector with my meter probe, and there was a blinding flash and a puff of smoke, and this was the result:

Oops!

I’m still not sure how it happened – I did have the negative battery terminal connected to the shell of the connector (but not any more – it’s out on a separate connection now!), so whether I accidentally shorted one of the pins (possibly about 74V) to the shell with my meter probe (which you can see is now a little worse for wear). Luckily, that connector was only on there for testing purposes so it was expendable, and miraculously a peek inside the important connector housing showed no apparent trauma at all (also since borne out by actually plugging the slave board into that connector, which worked perfectly!🙂 ).

The next job is to install the battery pack back in the scooter and find a suitable mounting point for the current sensor, and then I’ll be pretty much ready to switch it on and test if it runs!

That’s pretty much the state of play at the moment and although the job is nowhere near finished yet, I’m going to post this now as I’ve been writing it over a period of a couple of months and it could end up like War & Peace!

Finally, an accurate speedo check

I’ve finally had a chance to do a proper speedo check since the new tyres went on, and it looks like my speedo accuracy has improved from (a not very scientifically-measured) 20%, to 15% optimistic. I borrowed a friend’s iPhone and used the app that shows the route you take and you can get statistics on the journey. I carefully noted the maximum speeds I achieved according to the speedo – these were 40mph on the outward journey (which came out at 34.61mph actual), and 35mph coming back (30.45mph actual).

It also seems to confirm that my top speed hasn’t increased significantly (if at all) since the change of tyres, as under the old regime I used to see about 42mph which translates to 35 when corrected by 20%. I think I have seen 42mph under certain conditions since the tyre change, which comes out at 36.5 – hardly earth-shattering!

That’s two things!

(apologies to those who might not have got the breakfast cereal reference🙂 )

As my first experience in riding in snowy conditions earlier this week, there are 2 things I have done to my scooter that I’m really glad I did:

1. New tyres – the K62s are *so* much better than the rubbish that was on there originally.
2. Controller speed limits – when I built my latest controller I set 2 speed limits that are throttle-related, which I set to 30% and 70% respectively. This means that for a certain throttle opening it transmits less power to the motor, enabling a much more controlled pull-away in slippery conditions. When you get onto an un-slippery road you can just flick the switch up to the next setting while riding, giving a fun gear-change boost effect too!

Referring to some of the other comments I made in my previous post, I still haven’t been able to verify the actual speed I’m getting (although I may have some more news on that soon), but the speedo still seems to show approximately the same speeds as it did before so I assume I am getting a higher top speed! And since I’ve had the new tyres on I don’t seem to have even come close to grounding the centre stand when leaning into corners! Oh – that’s another two things🙂

As I hinted at earlier, watch this space for a speed check…

New boots!

I really didn’t fancy another dodgy winter on the original plastic Chinese tyres, so a few weeks ago I bought a pair of Continental K62s, which I finally had fitted on Friday. Following a recommendation from Mike on the Electric Motoring forum I went for the 3.50 profile instead of the original 3.00s, as these have 4 potential advantages:

1. They give an effective wheel diameter increase, which results in a less optimistic speedo reading.
2. Said wheel diameter increase also results in a slightly higher theoretical top speed – theoretical because that also depends on other factors such as available current, wind resistance etc.
3. The bike now has a bigger ground clearance, so it can be leant further into corners without the risk of grounding the centre stand, and…
4. The chunkier tyres now make the scooter look much more “butch”!🙂

A quick road test today confirms that it now feels more sure-footed, but of course I need to do some proper tests to work out how much more accurate the speedo is and what my true top speed is now. I do feel that the acceleration has suffered very slightly, but a quick phase wire check put my mind at rest that any extra current it may be pulling is not causing any extra heating effect.

The only disadvantage that I know of so far is that the increased wheel diameter has drastically reduced the clearance between the wheels and the ground when using the centre stand, which is only ever likely to be a problem when testing the motor – given that the balance of the scooter front-to-back is marginal anyway, it would be all too easy for it to tip backwards while the back wheel is spinning, resulting in it shooting forward off the stand! I notice there are some convenient-looking holes in the bottom of the centre stand, so there may be some capacity for screwing some form of spacers onto the bottom of the stand legs – more when I have experimented.

So let’s see what difference it’s made to the daily commute!

Quick update

The resistor mod I mentioned in my last post has now been done, along with replacement of the phase wires from the controller as the originals, although thicker than those going to the motor were still a little on the thin side.

As a result, regen now feels a lot more natural and doesn’t seem to cut in so suddenly, enabling me to ride a little smoother, although in my short test it did still seem a little on the powerful side. However, this is with the programmable regen level set to its highest again, so if I feel I need to back it off a bit I can just plug my laptop in and set it to the middle or lowest setting.

The upgrade of the wires to the motor has had to be postponed again, as I removed the back wheel and took all the bolts out, but nothing seemed to be moveable without using excessive force, so for fear of damaging something I just put it back together until I have the time to try again. At least the controller mods are done now so I will only need to disconnect the wires and remove the wheel again, leaving the controller where it is. I have now had a few tips on what needs to be done to get the wheel apart, and apparently it does require application of quite a bit of force in the right places.

So when I’ve got the time and the courage to have another go, that’s what I’ll be doing!

Upgraded – at last

Well, the upgrade I mentioned has finally been done, in spite of many jitters along the way. I usually enjoy electrical/electronic work but for some reason this job seemed a lot more daunting than previous projects I’ve undertaken. Maybe it was the need to reinforce the tracks due to the high currents involved, and the potential for a spectacular disaster if I got it wrong, or the rather intimidating 80W soldering iron I had bought for the job (which is very painful to touch, as the scar on my finger now bears witness), but whatever the reason I found myself putting it off more often than not.

But gradually it came together, and eventually gathered momentum the closer I got to the finish. The final thrust was completed last weekend, and I was particularly pleased the initial test showed all my existing options (reverse, cruise, regen etc.) all worked exactly as before with no “debugging” necessary.

That was the centre-stand test – now it came to actually road-test it. I set the current limits really low to begin with, just in case current limiting wasn’t working properly and the thing decided to launch itself up the road! I also tested it on a straight bit of road, and not on my gravel drive for fear of pebble-dashing my garage!

All seemed well so it was time to turn the current up a bit. This has been made much easier because I’ve brought the programming interface out to a header into which I can plug my laptop so I can reprogram on the bike. I decided not to go as high as I might have done because I am still on my original 1400W Ego motor with its rather thin phase wires and I didn’t want to risk burning anything out.

The road test showed a marked improvement in acceleration and hill-climbing ability, and all available from a standstill! I checked the motor temperature after my short round-the-block run and it was stone cold – I also wanted to check the phase wires but as it was getting dark I found it difficult to locate them. However the first morning after my run into work I checked both – motor still cold, but phase wires a little warm. On the run back in after lunch I used a bit more throttle most of the way and the motor was slightly warm and the wires a bit warmer, so I’m not going to risk pushing the current up any higher for the moment.

I now have some thicker cable that I’m hoping to replace the phase wires with, which will hopefully not get warm. I can then increase the current gradually and decide totally on motor temperature when to stop increasing.

The other thing I need to do is reduce the regen a bit – I find it much fiercer than before, and although it’s not too much of an issue in the dry, because it feels like the back brake has been applied quite sharply when it cuts in I’m a bit concerned it might lock up the back wheel in the wet. I have changed the programmable “level” down to its minimum, and although it’s much more controllable it still cuts in a bit harshly which makes it difficult to ride smoothly. So when I upgrade the phase wires I also intend to change a resistor inside the controller, which should reduce the intensity of the regen.

All in all I’m really pleased with this latest upgrade – for the first time since I got the scooter I’m not riding everywhere at full throttle, and it’s great to know that I’ve got a bit of acceleration in hand!

Throttle problem solved

As I was going away for a week’s holiday last week but not leaving till the Monday, I decided it was high time I did some of the maintenance I’ve been putting off for far too long. This involved the 175A cable upgrade I’ve had the bits for since last November, and maybe a bit of investigation into what I’ve often referred to as my “throttle problem”, which in some cases in the bad weather has manifested itself as a complete loss of power for a number of seconds. It always recovers, but I’m sure you can imagine how frustrating and unnerving it can be not knowing if you are going to suddenly lose power, especially if you have someone following very closely.

So over the weekend before going away, I started taking things apart, safe in the knowledge that if I didn’t get it finished I still had a day to get it all back together after my holiday before the inevitable return to work. I disconnected all the battery cables and removed the controller, and started making up the link cables with the new 175A cable.

Taking a break for a cup of tea, I decided to open up the controller to see if I could see anything out of the ordinary. A first scan showed all the cables apparently in place and soldered correctly, but looking over it a bit closer I suddenly noticed a dry joint! For those not familiar with this term, it is basically where a component has little or no solder on one of its legs, resulting in intermittent contact and thus unreliable performance. It turns out that the particular component would cause exactly the problem I have previously described, which is sudden loss of power without warning, and in a very extreme case could have blown up the controller completely!

So a quick application of solder to the offending contact, reassemble the controller and it was back to the cabling job. As predicted I ran out of time to complete this, so I had to leave it until after the holiday.

On my return I finished putting the new cables together, and reassembled everything, only to find that the ignition switch had no effect. In my reluctance to disturb too many of the existing cables I had bypassed them where they go through the loom, not realising that the takeoff for the ignition switch is somewhere down there! So I insulated the bottom of the old cables at the bottom of the loom, and reconnected them at the top (although they were now a *very* tight fit in the connector at the top along with the new, very thick, cable).

All back together again, and I took it for a quick test round the block. What a difference! I can now roll on the throttle gently and get consistent drive, and the motor sounds generally quieter and smoother at low speeds. Riding to work the next day showed speeds comparable to or greater than those I have had to date, and not a hint of hesitation or cutting out.

I wasn’t too confident about having the cables jammed into the old connector, so this weekend I fitted what’s known as an “Anderson connector” in place of the old ceramic block, which has a much larger hole for the cables to fit into, so I now feel confident that none of the cables are going to work loose.

The cable upgrade now means that I am in a better position for my next intended upgrade, which I alluded to in my last post. This involves an uprated controller board and FETs, and now includes a higher wattage motor as although my existing motor would probably handle the higher currents involved, the weedy cables feeding it almost certainly wouldn’t, so there should be a motor with my name on it winging its way from China soon, I hope.

This upgrade will not affect the top speed of the scooter, as I will still be running at 72V and as I think I have said in the past, speed is a function of voltage, and acceleration a function of current. So the upgrade should result in greatly enhanced acceleration, which I have found to be a source of frustration on occasions, as my existing controller has what they call “soft start” which deliberately limits the current when pulling away from a standstill. It’s frustrating to the point that I find myself pushing the scooter forward with my feet just to get it going sometimes!

So my next post will probably be after I have completed this upgrade – more when I have it!


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