Hi,

I was at Middlezoy this weekend.

I would like to attend on the Saturday. I will be bringing and driving Terry Sander's Stutz.

Regards,
James

Hi, I will be coming (in person). 100%, final answer, lock it in.

Work on my cyclekart has been very slow. I have been working on getting my controller, battery, motor and necessary ancillaries connected up. The minimum battery/motor/controller connections were a bit of a practical challenge, rather than a logical challenge. Making the heavy cabling for 100+ Amps is very different to what I've done before.

But the biggest problem has been understanding the other peripherals, what the connections are and how they are used, and importantly, which are unnecessary. The circuit diagrams and manuals are literal conversions of the Chinese words which don't make much sense, e.g. the electric door lock, the turn handle, and many more. Also the way that the sentence are structured are completely different too, so the meaning is often completely different from what you might guess.

After months of toiling over the circuit and the set-up method, I finally connected it up into what should be a minimal working system and tried it out on the bench. The motor turned a few times and then stopped. The controller suggested that my freshly charged battery was too low voltage and refused to work with it. I popped it back on the charger which charged it for about 10 seconds before declaring it full. But when I tried it with the motor again it continued to declare it too low. After all of the struggle I had had, that was the last straw - so I bought another controller that seems to be fairly popular in the electric moped/motorbike community.

My new controller is a FarDriver ND72300. FarDriver make a large range of controllers from 1kW to 40kW and they can even be used in heavy quadricycles and light cars. It is much more complicated than my 'old' controller and the documentation is better, but still not enough on its own. Luckily there are helpful expert user videos and moped forums that have filled in a lot of the gaps. It has cruise-control, regenerative braking, short-term boost (using field weakening for higher speeds), a CANBUS interface, bluetooth, a phone app for set up and diagnostics, including graphs and recordings. The app allows you to define a hundred or so parameters to customise the profile of the functions at different speeds. It really is the business!

I bought a matching display and compatible handlebar throttle unit with built-in switches to make my initial experiments easier. This helped but didn't make it trouble free - some pins in the connectors for the throttle and switch were not clipped into the connectors and had fallen out, and one connector had the pins in but in the wrong positions. These took a few hours to sort because the cable colours were also different to the ones from the controller!

Eventually I got it all together and it worked a little bit but then it too complained about the battery voltage. But with the phone app connected I was able to see that the controller was measuring the battery at 37V, not the 53V that I had measured after charging. A quick check with a multimeter showed that the controller was right and that one cell was showing 0.3V rather than 3.3V like the rest. I disassembled the battery, replaced that cell with a new one and suddenly I had a completely working system. What a relief! What a joy! Now all I need is the rest of the cyclekart, but that doesn't represent the same level of technical risk - it is a well trodden path.

I do wonder if I could get the 'old' controller to work now, but I've moved on. I am looking forward to tweeking the regen to give the feel of engine braking and having a Boost button for overtaking, like having KERS. But that'll have to wait until I've got a complete cyclekart.

After a busy time with new solar cells and subsequent roof problems, I finally got a chance to get back to CK things today, and gave myself a bit of a scare with a short circuit on my battery.

I had assembled all of the cells previously and connected all of the sense wires for the BMS but not connected up the BMS. So I finally connected the BMS and struggled with the Bluetooth BMS app to configure the parameters for the battery. I couldn't get it to balance the cells. The English in the instructions is pretty poor, so too is the English in the app, for example dialogue boxes have two buttons labelled "Disagree" and "Determine" which I assume mean "Cancel" and "Okay".  I can't make any sense of some of the error messages at all.

So I decided to use the touch screen that I recently bought. Unfortunately, the wiring for the display, which should only use the CAN/RS485 connection, actually uses two pins of the UART connection that the bluetooth transmitter uses, so I can only use one interface or the other, not both. And they both seem to have different functionality. Grr!

Some of the instructions seemed to suggest that the BMS doesn't perform all of its operations until the battery is fully charged for the first time, so I decided to do that with my new 15A charger. The charger worked a treat and only took half an hour to complete the charging. However, the BMS still would not balance the cells. The charging curve of the cells is very flat until the cells are 90% to 95% charged but then it rises quite sharply. This means that the voltage difference between the cells increases as the most charged ones approach 100%. So, with the recalcitrant BMS not helping me, I used the cell voltage display as a guide and used a small power supply to charge each cell individually. After spending and hour or so doing that, I had reduced the difference between the cells to about 50mV but I wasn't happy with that and it kept diverging as it settled after charging. So I decided to disassemble the battery and to wire all of the cells in parallel and charge them all together to 3.6V.

The disassembly process is a little bit tricky because you can blow up the voltage sensors in the BMS if you disconnect the sense wires wrongly. I managed that and was starting to disassemble the cells, removing the +ve connection of the battery pack, when I dropped a washer and inadvertently let go of one of the loose joining busbars which naturally decided that it wanted to swing down and touch the -ve terminal of the battery. There was a flash and I suddenly had droplets of molten metal bouncing on my dining table and onto the floor. Luckily the busbar was loose and didn't make a good connection but there were a couple of further sparks before I managed to get the offending busbar away by using the allen key that I had been using moments before.

As far as I can tell, the only lasting damage is the the burn marks on the dining table and some material deposited on one cell's terminal. That cell was warm to the touch but all of the cells seem happy - they are rated at 150A, so easily sufficient to blow holes in thin metal like the busbar without ill effects. So just a nasty scare.

I shall be re-configuring the battery structure to ensure that there is always an insulation barrier between cell terminals so that I can't repeat the experience. Meanwhile, the battery is fully disassembled ready to be connected in parallel tomorrow.

I have not been able to get past 2 major drawbacks.
1. The cost of the drive system will be at least £1000, more than my entire kart.
2. How will I recharge the battery fast enough to make a day’s racing achievable.

1) The cost...
I have spent £370 on a 3kW motor and a matching controller through Alibaba from a company called Wuxi Helanda Mechanical & Electrical. That was the total cost including delivery, import duty and various other charges, but I was very pleased with this price. It is intended for a large 3 wheeled transport that is common in China but it remains to be seen if this is suitable for a cyclekart.

I have currently spent about £450 on the batteries, which was £300 for the cells, £130 for a BMS. I got the cells 'new second hand' for a very good price because they were surplus from another project and had cost over £1,000 new in 2020. I noticed that similar cells are available through AliExpress with 60% off making them only a little higher than the price I paid.

I have also bought a 15A charger specifically for my battery configuration from AliExpress for £110.

So I am standing at £930. I am actually going to split my cells to make 2 batteries, so I'll need another BMS which will bring the total to just over £1,000. But I recognise that the implementation is completely unproven and I might well end up reworking it (i.e. spending more money).

2) By my estimates, a typical 10 minute racing session will use about 12Ah at 48V. My cells are 15Ah (Headway 40152's), so I can get away with using a single string of 16 cells. Note: the cells can happily delivery 150A, so I don't need to parallel them.

As I have 56 cells in total, I am intending to make a second battery that uses 32 cells in a 16s2p arrangement, so that I can take part in longer events (~20 mins). I intend to make these batteries interchangeable (although I doubt it would ever be as fast as a F1 pitstop). So with my 15A charger running full time charging one of the batteries, I can drive for 10 mins per hour all day. I'd expect to arrive with both batteries charged and to always swap the batteries, to keep the charger running. However, I might move up to a higher power charger (I've seen a 30A one) because even the single string can be charged at more than 15A.

That all assumes that power is available and that I am allowed to use it. If not, and for places that are off grid, then I think I'll need a 3KVA petrol generator - so about another £300 into the mix. I might claim that the generator is to cover home power cuts, as we have had 2 in the last year, so I can put that expense in a different budget!

A higher power charger and a generator are going to push the cost of going electric close to £2,000. But I'm not going to invest any more for now until I know 1) that the cyclekart works, and 2) that a rule change won't make my hardware obsolete.

I had a question about batteries on my build thread. I think it would fit better here.

Could you use the 48v battery from an ebike?

That depends on 3 main things: The current limit of the pack, the total energy capacity of the pack, and safety.
Note: I assume the question is about road-legal ebikes, off-road bikes are different as there are no rules and no power restrictions and may well be user designed.

Current Limit
========
UK CycleKarts are limited to 3kW continuous/ 5kW peak (which typically means for 10 seconds before the motor overheats).  Road legal ebikes are limited to 250W continuous. So we are using 12x the power. This means that road legal ebike battery packs are not intended for the sort of currents that a CK will draw. 5kW peak is the CK motor mechanical output but its electrical input will depend on the motor efficiency, if this is 75% then the battery needs to provide 6.7kW, which is about 140A at 48V. Whether that will quickly destroy the cells in an ebike battery depends on the particular cells used and the number of cells in parallel in the pack.

Battery Capacity
==========
Also a CK battery will typically need to last for a minimum of 10 mins of racing (but some events may be longer, e.g. 24 minutes so a battery swap might be needed). During a race, drivers are usually on full power for about 60-75% of the time, so 10 mins of racing is about 7.5 mins on full power. 3kW output means 4kW of electricity which translates to 83A at 48V. This means the pack needs to be able to supply 500Wh before its voltage dips significantly. What fraction of discharge represents a significant drop in voltage will depend on the battery chemistry but it is likely to be between 60 to 80% discharged. So you probably need a nominal capacity of 700-800Wh but more would be better. I don't think that typical ebikes batteries are in this range.

Safety
====
There are many different Lithium chemistries and most of them need special care and some are particularly sensitive to over AND under charging. Under failure conditions they can result in serious fires and explosions. There is also the danger of crash damage causing short circuits, battery rupture etc leading to fires. On a ebike, the rider is likely to be thrown clear in a crash, not so in a cyclekart. Other racing clubs (I'm thinking of the Hacky Racers) have virtually restricted the approved chemistries to only LiFePO4 by imposing restraints on other types. LiFePO4 is much safer because its electrolyte requires much higher temperatures to ignite and generally needs an existing fire to get it going. LiFePO4 also has other safety benefits when handling the cells and when abused. It also has a long life, long shelf life and it maintains its voltage longer under discharge. But LiFePO4 has significantly lower capacity for a given volume or a given weight, which means that it is probably not used in ebikes - but I've not checked.

Note: this is my view, others might disagree...

An alternative is to parallel a group of cells and charge to 3.5v, this way they are all guaranteed to be the same voltage, with a series arrangement the overall voltage is not necessarily divided equally across the cells. 

The cells I have fall into two groups - those that were partially charged and were resting at 3.33V and some which were resting at 3.1V. What I was doing was charging 8 of the cells that were low until they were closer to the partially charged ones BEFORE pairing them to make a 16s2p pack. Whilst they were charging I was manually checking their voltages to see that there were no outliers. What I didn't want to do was to pair a 3.3V with a 3.1V because, with a the cell resistance of about 8 mOhm,  I'd get a current of 25A between them. I weld 16 gauge steel at those currents so I wanted to reduce the difference as a first step.

It is rather a hit and miss affair trying to charge them to get a particular resting voltage as the voltage whilst charging is quite different to the resting voltage. I studied the published charging and discharging curves for these cells so that I could estimate their different SOC and then timed the charging of these cells based on that. That came out pretty close and I got no sparks when pairing cells. This is all experimental for now and I expect to fiddle around with them again before the final battery build.

The BMS that I've bought does balancing and I expect to use that capability but it may not be the best way to do it. However, my BMS only works with 16 cells in series, which I can't charge with my small 30V power supply (new charger is on order, hopefully arriving tomorrow). In use, I intend to avoid letting the cells get below the 80% discharged state so I would not expect to get into serious cell difference issues in the short term. (Do advise me if you disagree)

My one worry in this is with paired cells: If I make a single series chain of 16 cells then the BMS can completely monitor each cell, but when each cell is actually a pair of cells in parallel then one weak one is more difficult to detect. So I think it may be prudent to regularly (but hopefully not too frequently) disassemble the pack and check all of the cells individually.

For now, I just want to get a pack together to test out the motor and controller.