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Re: Another EV bike :)

Posted: Wed Feb 19, 2014 10:07 pm
by ChrisBarron
17 days later....and the four packs are complete ! :D

Only one is fully charged and balanced, but it will be enough to start playing about for now.

I'm working out a scheme for a balancer system based on the 'flying capacitor' method, where the cell with the highest voltage charges a capacitor, which is then discharged into the cell with the lowest voltage. The problem with this method is the speed is very slow, not least of all because you don't fully discharge the capacitor, it only discharges down to the voltage of the lowest cell....

But what if the ca pacitor is replaced with a small pack of lithium ion cells ? :o

More specifically, two small lithium packs to provide full isolation. One can be discharging into the cell with the lowest voltage while the other is being charged by the cell with the highest voltage. There's some relay switching involved, lots of relay switching to be precise but with only a couple of dc-dc converters things could work quite well.

Or I could shell out for an isolated dc-dc supply and connect it in between the highest and lowest cells using relays.

I have plenty of ideas to play with as you can see.....

I should probably spend a bit of time sweeping the garage floor to clear it of plastic offcuts and bits of random offcuts.

Re: Another EV bike :)

Posted: Thu Feb 20, 2014 2:45 pm
by ChrisBarron
I've had a contact from a battery company in the far east, if anyone is interested. They can weld tabs to each cell for a further $0.35 per per cell

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We can confirm all the batteries are high quality and we will be responsible for these batteries.
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Re: Another EV bike :)

Posted: Tue Feb 25, 2014 9:42 am
by mattcarr
I think it would be alot of effort to try and shuffle the power around the pack to balance it. Would it not just be easier to monitor the voltage of each section of the pack - would cell balancing not happen within each part of the pack naturally as all the cells are connected in parallel?

Re: Another EV bike :)

Posted: Tue Feb 25, 2014 6:43 pm
by ChrisBarron
Yes there is a natural balancing within each submodule of 21 cells.
I hadn't considered the idea of paralleling each submodule with the equivalent module in another finished battery pack.

I suppose that could work quite well actually ! Four 21-cell modules in parallel equates to 84 parallel cells.

I don't expect to have to balance the packs very often, but after assembly they're all over the place and need to be brought in line with one another. After that they could potentially stay well balanced if they're connected in parallel across batteries too.

I'm building in a balance lead connector to each of the four battery packs which could easily deal with 10Amps. It is made up of 11 wires, one connected to each of the nine inter-submodule cell contacts, one at the top of the pack and one at the bottom. The top and bottom contacts will be paralleled anyway via the plus and minus terminals, so I just need to parallel each submodule across the finished batteries (with a decent sized fuse between as a precaution !), using 9 wires.

Okay, that's cheered me up a bit !

I've started work on the voltage monitor, which I suppose now only needs to monitor the voltage between 10 paralleled sub modules.

I was looking at using a microcontroller with a 12-bit A/D, and adding potential dividers or multiturn potentiometers to trim the voltage from each submodule. I've also considered using the very interesting LTC6802-2, a nice purpose built part, which could be cost effective. But I already have my 'one microcontroller per submodule' design, which works out to cost just about £1 per submodule. A voltage monitor with LCD display, as well as over and undervoltage alarms and control signals could be built for as little £20. I can also use almost the same code to add temperature monitoring via the addition of one microcontroller, with all micros connected to a common data bus, each one optoisolated from the others, just in case.

Re: Another EV bike :)

Posted: Sun Mar 02, 2014 7:40 pm
by ChrisBarron
Just waiting for a few more microcontrollers to arrive and I'll have the full pack voltage monitoring system built and ready to spit out data to a suitable host interface with a display.

I've made up the harnesses that will connect all pack submodules together toa remote junction box. This will have a standard interface into which I can plug other devices, such as an active balancing device (only expecting to need it once when all packs are joined together) or a resistor discharging unit.

I connected all 4 packs in parallel and charged them slowly up to 41.2V. I didn't go higher because one submodule was already at 4.22V then.

I picked up the motor and held it under my arm and decided to drape the 2 supply leads on the battery terminals. When I do this with the lead acid pack the motor kicks quite hard and there's a good arc at the terminals. When I do this with the lithium pack the effect is much more dramatic and so I feel that the output power could be very interesting once it's all installed !

Re: Another EV bike :)

Posted: Tue Mar 04, 2014 9:25 am
by ChrisBarron
I just caught up with Jehu Garcia's episode 25, where he got a real world test of his 18650 battery. (detail starts at about 10 minutes)

Very encouraging.

He has 150 cells in parallel, I have 84 in parallel. My system voltage is much lower than his but my bike is much lighter than his van. !

It also looks like in episode 27 ... verview-vl he is acknowledging the problem of how to go about testing individual cells.

I thought there would be other solutions before I built my mass discharger, but it seems not to be the case.... I only built my own because I had everything I needed to hand.

I think i'll make a video covering the testing of cells and the assembly into battery packs.

Re: Another EV bike :)

Posted: Tue Mar 04, 2014 12:04 pm
by Kevin Sharpe
Jack Rickard at EVTV is about to start playing with 18650's as well... should be fun :D

Re: Another EV bike :)

Posted: Wed Mar 05, 2014 12:50 am
by ChrisBarron
Really ? :lol:

He hated my battery monitor design, when I requested ideas for an acronym for it he suggested 'B.O.M.B', because as far as he was concerned all battery fires had begun with a BMS. But he wasn't prepared to distinguish between a voltage monitor and a balancer at that time. I'm going to be using that same design on these packs of mine :)

Anyhow, I did manage to put a small video together which hopefully covers the salient points of my construction methods. Here it is (apologies for the poor quality, it's really just for information)

If anyone has buying power, he has. I expect he'll buy up all sorts of cells and find which are the real duds and which are solid reliable. There are quite a few RC guys who have already done a lot of that work for us. I think I saw claims of an 18650 with 5200mAh capacity, which turned out to be about 1800mAh

I also saw this, a warning about fake 2400mAh 18650's on ebay the casing was opened up and it was clear that the cell inside was smaller than AAA size and a lot of packing material was holding it in place ! It turned out to have a capacity of 66mAh !

Most branded ex-laptop cells which I rejected can do much better than that !

The more people who buy 18650's the lower I hope we can see the prices go :)

Re: Another EV bike :)

Posted: Wed Mar 05, 2014 1:35 pm
by mattcarr
fakes always appear. I remember reading of an imported EV from china that reportedly had several sets of batteries fail - so the purchaser decided to chop open a battery and found that half of the lead plates were missing and there was sand in the bottom to make up the weight !!!!

Re: Another EV bike :)

Posted: Wed Mar 05, 2014 9:36 pm
by ChrisBarron
mattcarr wrote:fakes always appear. I remember reading of an imported EV from china that reportedly had several sets of batteries fail - so the purchaser decided to chop open a battery and found that half of the lead plates were missing and there was sand in the bottom to make up the weight !!!!

They probably considered selling them as a new type of battery 'lead sililcate'. It wouldn't be a lie !

With that in mind I've emailed a few PC repair shops around the area and a couple have replied to say they have a small mountain of ex-laptop batteries and would consider my offer of between 10p-15p per cell.

Another pack or two might come in handy, and would eliminate any of the risks of buying new but fake cells !