E-max Conversion to Thundersky LiFePO4 Cells

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PJD
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E-max Conversion to Thundersky LiFePO4 Cells

I've been procrastinating on writing this, but my "economy" conversion of a e-max sport to LiFePO4 power, using Thundersky LFP40AHA cells is now complete.

In the US/Canada, the cells are available from Electric Motorsport (EM) of Oakland, California (http://www.electricmotorsport.com/store/index.php and in limited quantities from Patrick Rentsch residing in the same area. Please PM me for info on purchasing from Patrick. EM sells them for $100 each and presumably has a warranty. Patrick sells them for a flat $87 each, including shipping, but all sales are final. He does, however, charge and discharge the cells before shipping.

The main issues for the conversion are physically fitting the cells in the scooter, and finding a suitable battery management system. You will want to upgrade the battery monitoring instrumentation a bit to protect your investment - as over-discharge is the most effective way to damage a LiFePO4 cell.

1. PHYSICAL INSTALLATION

As far as fitting sixteen cells in the scooter. I'll let this picture do most of the explaining:

//i109.photobucket.com/albums/n77/PJD123/CellInstall.jpg)

The battery box did need some minor widening, (using a small jack and wood blocks) to accommodate this cell arrangement. Also, you may notice that the DC-DC converter was relocated to the frontmost battery box clamped on to the front edge using a small aluminum plate. This was needed because at it's original location, there were two protruding mounting nuts which interfered with this battery arrangement.

Here's the installation with all the connections and battery management leads installed:

//i109.photobucket.com/albums/n77/PJD123/emaxthundersky.jpg)

The positive terminal of the pack is the cell at the left end of the front row of five; the negative terminal is the front-most cell.

The Thundersky cells are taller than the stock batteries, but I was pleasantly surprised to find that no raising of the seat well was required - but I did have to cut the protrusions off where the front mounting studs/nuts were used. Also, holes had to be cut where the seat and rider weight could potentially press down on the right and left front-most battery terminal bolts.

The battery hold-down bar is not used. Attaching the seat with the two rear bolts plus the front screw near the seat hinge works fine.

It is important to secure the batteries - they must not bounce or rattle in the battery box in use. The fit in the box is already pretty snug for all cells, but for additional assurance, I injected expanding builders-type expanding foam filler between the battery box and the foam ribber padding on the sides of the box. This foam secures the cells while still (hopefully) allowing extraction and replacement of individual cells. Note that Thundersky recommends clamping the cells together to resist swelling in use. This largely applies to the LCP (LiCoO2) cells, not the LFP cells as LiCoO2 cathodes expand and contract with charging/discharge, but LiFePO4 cathodes exhibit much less volume change.

The Thundersky cells are sealed and non-spillable, but use a liquid electrolyte with some air-filled head space. For this reason, I believe they should be installed as close to vertical as possible.

The older members of this board will recall that I'm running my e-max;s in switchable 60 volt mode using an extra 20AH SLA under the seat as a "booster" battery when a top speed of 45 mph/75kph. So, the real battery box that holds the single SLA in the stock setup now holds one of the now-two separately charges booster batteries. They are still SLA's for now, as this is an "economy" setup, and no 60 volt (20 cell) battery management systems are available. But one may be available soon.

If a 20 cell/60 volt system is desired, there is room for possibly two cells laying on their side in the rear-single battery box, and one more cell in the rearmost battery box. But, I'd recommend instead sacrificing some storage space and putting all four additional cells in the seat well. They can be secured with large-diameter type hose clamps.

2. BATTERY MANAGEMENT AND CHARGING

One of biggest problems was finding a suitable battery management system (BMS) that would be affordable, and compatible with my stock e-max chargers, which I was wanted to continue to use as an economy measure. For the e-max, we only need two thing from a BMS - a means of preventing any single cell being discharged below 2.5 volts, and a means of balancing the cells at about 2.65 to 2.75volts per cell. Many battery management system have too many features for our purposes. Particularly, we don't need a current limit cutoff that the BMS's have on these packs being sold for e-bicycles - which have a limit too low anyway.

I had originally hinged my whole project upgrade on the Bob Mcree designed charge-management and low-voltage cutoff board - only to see the whole project fall through. (go to the endless sphere forum - "new 16-cell BMS" thread) So, I ended up spending more money than I'd like on the the low-voltage cutoff 9LVC) board and less than optimal dual TP210 aeromodeler pack balancers sold by Gary Goodrum here:

http://www.tppacks.com/products.asp?cat=26.

The LVC board was mounted to a hardboard cover over the hole in the bottom of the seat well, and the balancers plugged into the connector, as shown here:

//i109.photobucket.com/albums/n77/PJD123/BMSsmall.jpg)

The LVC is primarily designed for e-bikes, not larger scooters, so the LVC's signal wires are connected a bit differently. They are connected to the throttle leads with a pull-up resistor added to the throttle signal wire so if the LVC detects a low cell, the throttle signal get grounded out, shutting the motor down. In practice, this will take the form of a studdering or stumble - just like the LVC built into the controller.

The stock e-max charger has a switch-to-CV charging voltage of 59.5 volts. The cutoff current for end-of CV charging is adjustable via a 10-turn pot on the charger control board. I turned it down from about 500 mv to about 250 mv. At 59.5 volts, this charges each cell to 3.72 volts if perfectly balanced. The dual TP210's don't do a perfect job - but generally keep them between 3.70 and 3.76 volts - good enough. The TP210's can be left kept connected full time, but since that are a dissipative type of balancer - bringing the high cell down to the low cell. I prefer to use them only during charging. This means that charging is no longer quite the plug-and-forget process that it was, since you need to unplug everything fairly soon after charging is finished.

The LiFePO4 cells hold a higher end-of-charge voltage than lead acid, so the float stage of the charger never gets actuated.

Thundersky recommends 4.25 volts per cell, but the consensus is that for maximum life, a LiFePO4 cell should be charged to no more than 3.65 to 3.75 volts. During charging, once the voltage reaches 3.65 volts, it shoots up exponentially, so the amount of extra charge capacity between 3.65 and 4,25 volts is only a few percent.

I am using the 2.1 volt version of Gary's low-voltage cutoff board - the only other readily available version being a 2.7 volt version. The recommended minimum voltage for Thundersky cells us 2.5 volts, but this goes down to 1.5 volts at -35C. So interpolating, that's 2.2 volts at 10C.

While a pack monitor, like a Pak-Trakr or Cycle Analyist could be used, I opted to just use a self-powered digital voltmeter for economy. If the voltage is sagging to 42 volts under load, I better be close to home.

3. PERFORMANCE

The performance gain from the cells is impressive. Even with my two SLA booster cells, the scooter weighs about 40 pounds/20kg less than the old setup. Range is increased from about 20 mi/32 km with new SLA's to at least 33 mi/53 km. The pack provided much more consistent voltage during discharge, only sagging below 48 volts when 85% discharged. All the riding so far has been done in summer weather, however.

There is no requirement that LiFePO4's be kept fully charged, so the pack can be treated almost like a fuel tank. The general thinking is that liFePO4's will have the longest calender life if kept partially discharged.

PJD
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Re: E-max Conversion to Thundersky LiFePO4 Cells

Here's an addendum on the cost of the project, then maybe this board will allow me to fix typos in the original article.

The cells cost about $1360 including shipping. To charge and individual cells for testing, and get the pack balanced for first use, and to adjust the originally proposed BMS board which never materialized, I bought a cheap Mastech bench top power supply - about $80.00 - but that's a necessity for any kind of messing around with electronics. Then, an upgrade to the power supply- replace it's cheap adjusting pots - $60.00. The LVC was $85, and the dual TP210's were about $195. Panel meter $30.00.

Cost of the upgrade itself, about $1670.

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Re: E-max Conversion to Thundersky LiFePO4 Cells

Congrats Paul - I know you've been contemplating this for quite some time. I assume these are 40Ah LFPs, what were the amp hours on the SLA's replaced? (I'm trying to get a handle on the increase from 20 to 33 miles).

It sounds like it was "worth it".

(BTW, no pictures are showing up when I read your post).

John H. Founder of Current Motor Company - opinions on this site belong to me; not to my employer
Remember: " 'lectric for local. diesel for distance" - JTH, Amp Bros || "No Gas.

antiscab
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Re: E-max Conversion to Thundersky LiFePO4 Cells

The increase in rage from 20 to 33 miles is due to the increased usable capacity.
The usable capacity at 1C of the original silicone batteries to 100%DOD was 15AH, rather than the 20AH quoted.
So the total was 30AH@48v.

The thundersky's give 36AH at the 1C rate.
His pack, like mine give 40AH@60v (because hes using a booster pack.)
thats 1.44kwh vs 2.4kwh, hence the greatly improved range.

btw, PJD, welcome to the lithium club :D

Matt

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah
Vectrix 60Ah Lithium Tyres Fuel Registration Insurance cycle analyst 2 x TC Charger & MC
conversion

PJD
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Re: E-max Conversion to Thundersky LiFePO4 Cells

John,

Matts explanation is about right.

Both the old pack and the new Thundersky LiFePO4 pack are rated at 40AH, but because of the Peukerts effect for lead acid, only about 24 AH are usable in my hilly area with frequent discharging at 2.25C. But nearly all the 40AH are usable from the LiFePO4.

In my case the range comparison may seem to be complicated by my use of the extra lead acid booster batteries. But, these are used on an intermittent basis via a "boost" switch when I need to go faster or climb steeper hills, in these situations the current draw from the main LiFeP4 pack isn't reduced, so their effect on increasing range is small.

When the bank account recovers and I can find a reasonably priced 60 volt charger or power supply, I might buy more cells and convert the scooters to full time 60 volt use.

By the way, Gary Goodrum is coming out with a new design for a 16 cell BMU. It can be chained with additional boards for larger packs. Hopefully it will be for real this time.

antiscab
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Re: E-max Conversion to Thundersky LiFePO4 Cells

PJD,

heres a 60v charger:
(ebay item) Battery Charger Electric Scooter Bike 60V 2.5A

youll still need a BMS to go with it, but the price is reasonable

Matt

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah
Vectrix 60Ah Lithium Tyres Fuel Registration Insurance cycle analyst 2 x TC Charger & MC
conversion

PJD
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Re: E-max Conversion to Thundersky LiFePO4 Cells

Matt,

At 2.5 amps, it would take a long time to charge the pack.

I think the the Gary Goodrum BMS can use an ordinary switchmode power supply. I have seen ones up to 20 amps on e-bay. Someone on the the Endless sphere forum provided information on changing the voltage on them. Remember that a "60 volt" charger needs to actually be about 75 volts.

I forgot to ask - how is your pack doing performing? How many km on it?? What kind of management system do you have?

I have a correction - after carefully monitoring a few charges, it appears the TP210 balancers are not "dissipative", rather, they seem to move charge from the high cells to the low cells- like powercheqs do, by their capacity is lower.

They work surprisingly well considering their low capacity - and sure are needed. The cell voltages get very, very sensitive near full charge. The one or two cells that are only slightly ahead of the others in state of charge would probably go to 5 volts or more - with the low cells never getting past 3.55 volts if I didn't have the balancers.

antiscab
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Re: E-max Conversion to Thundersky LiFePO4 Cells

PJD,

yeh it will take a long time to charge, depends on how far you go each day i suppose. The reason i mention them is because ive just bought a bunch of the 48v chargers for my car conversion (2.5A to charge a 90AH as an oppurtunity charger). their 3 stage chargers, not sure on the terminating voltage on the 60v models, but its 59v on the 48v model.

My thundersky pack is doing well, ive done about 5000km (3000miles) on them so far. Ive noticed a 15-20% reduction in range due to the colder weather (due to voltage sag as opposed to AH loss). so atm my range is only 50km, but on warm days its back up to 60km. im running 60v full time now (i cant go back to 48v...sooo slow).

My BMS is the one from www.evpower.com.au . It does dissipative balancing at end of charge (this is only very minor in reality, the pack stays in balance by itself mostly, the max dissipation current is 0.6A), and disconnects the pack from everything in case of under/over voltage on any cell.

btw, where did you get the TP210 balancers from?

I tested the pack at high discharge with a carbon pile tester. did 320A at 1.7vpc, fairly impressive, the performance appears to be scaleable (ah 200AH pack i know of does 1600A at 1.7vpc).

Matt

Daily Ride:
2007 Vectrix, modified with 42 x Thundersky 60Ah
Vectrix 60Ah Lithium Tyres Fuel Registration Insurance cycle analyst 2 x TC Charger & MC
conversion

PJD
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Re: E-max Conversion to Thundersky LiFePO4 Cells

btw, where did you get the TP210 balancers from?

Gary Goodrun at TPPacks. The URL is on my original post. Since that are used by aeromodelers, I'd imagine there would be an Australian dealer somewhere.

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Re: E-max Conversion to Thundersky LiFePO4 Cells

By the way, Gary Goodrum is coming out with a new design for a 16 cell BMU. It can be chained with additional boards for larger packs. Hopefully it will be for real this time.

Is that the one that Bob designed and there's that really long post about over on Endless Sphere? If so, I thought it looked great because, as you mention later, it can be used with any power supply capable of putting out enough volts for the pack.

Do you know anything about timing or pricing of this?

BTW, the XM-3500Li does have some form of BMS for the charge cycle. As soon as I get my bike I'll see if I can find out more about it.

John H. Founder of Current Motor Company - opinions on this site belong to me; not to my employer
Remember: " 'lectric for local. diesel for distance" - JTH, Amp Bros || "No Gas.

PJD
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Re: E-max Conversion to Thundersky LiFePO4 Cells

John,

Gary's new BMU is a different design.

Haven't heard anything from Bob in couple months. The new design will not have the 8-amp charger limitation that the old one did. It's best feature is that no bulky DIY-built heat sinks are required. But, don't expect the roll out of the new BMS to go too smoothly.

The new BMS will only be in kit form for now. Haven't heard the price, But I think it will be well under $100.00.

vinnie
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Re: E-max Conversion to Thundersky LiFePO4 Cells

While reading this I was suddenly reminded that Ken Hall, maker of Paktrakr, located just north of me here in Colorado is also working on a BMS for Li. I'll try to remember to email him for a status report...

Vinnie
Broomfield, CO

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Re: E-max Conversion to Thundersky LiFePO4 Cells

PJD and Vinnie -

Keep us posted with what you find out, the world is waiting for a good and affordable LiFe BMS solution (OK, maybe not the whole world - but there are a few of us waiting, right?) ;-)

John H. Founder of Current Motor Company - opinions on this site belong to me; not to my employer
Remember: " 'lectric for local. diesel for distance" - JTH, Amp Bros || "No Gas.

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Re: E-max Conversion to Thundersky LiFePO4 Cells

RIGHT!

Grandpa Chas S.

PJD
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Re: E-max Conversion to Thundersky LiFePO4 Cells

I never posted some pictures of the final BMS setup I settled with. They are an early version of the Gary Goodrum 4 to 24 cell BMS's Chas has been discussing in another forum.

Here's the completed 16 cell BMS - It clamps the charging voltage at each cell at 3.68 volts and provides a low voltage cutoff at 2.1 volts. The LED's indicate when the cells clamping voltage has been reached, giving a visual verification of when the cells are balanced:
//i109.photobucket.com/albums/n77/PJD123/a321cc79.jpg)

The board was mounted on a piece of hardboard which covers the opening in the bottom of the seat well:
//i109.photobucket.com/albums/n77/PJD123/18c4a152.jpg)

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