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Old 11-05-2014, 08:25 PM   #1 (permalink)
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Building a battery pack (cheaply)

Well, since my long(er) term goal is to drive an electric car, and however much I want one, I really can't afford a Tesla Roadster, I'll have to DIY one.

In this topic I want to document the process of me building my own battery pack (on the cheap), Everything from gathering materials, to assembling + me figuring out what BMS I want, charging it, etc..

First some (personal) history: Ever since I got my drivers license, and through random internetting stumbled upon the ForkenSwift conversion, I really want a DIY Electric car. However, a few things were holding me back.
1: in feb/2010 the regulations for a DIY conversion were made much stricter, requiring EMI/EMC certified parts (which were hugely expensive), which made the plan financially impossible. (they're now freely available from crashed EV's and hybrids)
2: battery writeoff, when you write off 20 cents a km on the cells alone, its not a very financially wise plan. Battery cost has come down a lot since 2009 though.


While random-youtubing, I stumbled upon a video of a guy building battery packs out of old laptop batteries. This is really quite briliant because:
- Its the same type of batteries in the Tesla cars. (18650 cells)
- Once a single cell in a laptop battery fails, the whole pack becomes useless to a laptop, but most of the cells inside will still be very much useable, even if they're partly worn.
- OEM laptop batteries use brand name cells, they're probably better than cheap no-name chinese cells, even if they're worn a bit, they might still have lower resistance, more capacity, more cycle life and cost less at the same time.
- Dead laptop batteries are of no use to anyone else, so should be cheap. The trick is finding them before they go to the battery recycling plant.

And thus, my quest began.

Chapter 1: materials.

I've been looking on eBay at laptop batteries. 1 guy selling them for $9-10 a piece, thats not really cheap enough, but its a source, so I'll keep it in mind.
Then I spotted an auction, starting bid $1, Buy it Now $30, for 95 batteries.
By the description of the seller, I concluded that they should be all 6 cell batteries, so 570 cells (give or take a few.) Sadly, the downside of this deal was that I had to hire a courier to bring them to me, they were in the UK, and I'm not. None of the normal mail companies want to move such a large quantity of li-ion, so I got a man-with-a-van to do it for $100. Still a good deal, $1.36 per laptop battery, or $0.23 per cell, thats a lot cheaper than the next cheapest eBay seller. I'm keeping my eyes open to find more batteries in bulk like this, this gives me the EV smile.

I might contact the national battery recycling company and see if they're willing to send some laptop batteries my way at a small fee. Probably not, but it doesn't harm to try, right? Re-using is the best form of recycling, in my opinion.

I bought a few rolls of nickel-plated strip, I plan to spot-weld one side of the cells, and solder the other with fuse wire. So should 1 cell short-circuit internally, it doesn't draw hundreds of amps from its parallel brothers untill it catches fire, it'll only draw a few amps and the fuse will blow. Apparently, thats how Tesla does it? I also bought a roll of 30 gauge solid copper wire for the fuse wire.


I looked at mounting the cells, theres those cheap plastic clip-together endplates.. well "cheap", they're pretty much the same price I just paid for the cells, so eh, I might have to think up something else. On the other hand, they're pretty solid looking, a good way to securely mount the cells. I'm not quite sure yet how to proceed on this part.


Chapter 2: equipment

Next up, I'll be fixing up some testing equipment to sort the good from the bad.
The idea is to strip the batteries to bare cells, charge each individual cell in a cheap 1-cell charger, toss them in a box for 2 weeks to weed out the ones with stupid self-discharge, then put them in individual dischargers (automate somthing with an arduino) to measure their capacities and sort them by that.
Dead ones go to recycling, weak ones might power something like an e-bike or an alternator-delete or some solar led lams or something?, and the good ones will be mix&matched to get balanced capacity parallel groups.

I'll also need a spot welder, I already have a microwave transformer, so all I need is 2 turns of fat wire, and a wooden handle with 2 nails or something to prod the cell tabs with, and a foot-operated switch to turn it on. Not too difficult.

Chapter 3: BMS.

Well, theres plenty of options to pick from here. I'll get back to that later, I guess. Building comes first.

Anyway, thats it for now, more will be added as soon as it happens.


Last edited by AlexanderB; 11-05-2014 at 08:51 PM..
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Old 11-06-2014, 12:38 AM   #2 (permalink)
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I will watch with interest.

My own ambition is to mount a motor/generator and find a suitable controller/charger/BMS. The money will run out somewhere in that process. I don't know now where the money for the battery will come from, so likely it will be something like this (and borrowing against future gas savings). Maybe with Prius packs.
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Old 11-08-2014, 02:16 AM   #3 (permalink)
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I don't think any one is cheaper than I am and I have build 2 LiFePO4 packs.

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Old 11-08-2014, 02:21 PM   #4 (permalink)
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Im not sure you can combine those 2? Not all batteries are created equal and the "cheap" ones usually have a manufacturing defect. You really need to buy 10% more than needed and test them for capacity, then rack and stack em.

Once you got the voltage and amps you need, then depending on the chemistry you need a bms and charger.

I see a lot of lifepo deals on ebay, but I think I rather stick with a prius pack as they dont need a bms, just a grid charge to balance them out.
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Old 11-08-2014, 02:39 PM   #5 (permalink)
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Quote:
Originally Posted by freebeard View Post
.... Maybe with Prius packs.
Please note that prius Nimh packs are heavy compared with lithium one and will have a short life time if used with high DOD (depth of discharge). They are made for hybrid: plenty of small charge/discharge.
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Old 11-08-2014, 02:45 PM   #6 (permalink)
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Quote:
Originally Posted by AlexanderB View Post
Chapter 3: BMS.

Well, theres plenty of options to pick from here. I'll get back to that later, I guess. Building comes first.

Anyway, thats it for now, more will be added as soon as it happens.
I use peter perkins bms
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Old 11-08-2014, 04:31 PM   #7 (permalink)
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Please note that prius Nimh packs are heavy compared with lithium one and will have a short life time if used with high DOD (depth of discharge). They are made for hybrid: plenty of small charge/discharge.
How is the plug-in-Prius different?
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Old 11-08-2014, 05:17 PM   #8 (permalink)
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Quote:
Originally Posted by AlexanderB View Post
I'll also need a spot welder, I already have a microwave transformer, so all I need is 2 turns of fat wire, and a wooden handle with 2 nails or something to prod the cell tabs with, and a foot-operated switch to turn it on. Not too difficult.
I do have a small welding kit, unused as of yet, and I would be happy to let you use it for this project.
PM me if you feel like dropping by in Nieuwegein
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Old 11-08-2014, 07:27 PM   #9 (permalink)
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Quote:
Originally Posted by oil pan 4 View Post
I don't think any one is cheaper than I am and I have build 2 LiFePO4 packs.
Ah, yeah.

I wonder though, is spending all that money really worth it? because I think $350 worth of cells and whatever worth of extra materials just to replace a $100 starter battery is kinda.. pricy.
And thats just for a little battery, not a full EV pack.

Quote:
Originally Posted by Cobb View Post
Im not sure you can combine those 2? Not all batteries are created equal and the "cheap" ones usually have a manufacturing defect.
So thats why I don't buy those. I buy OEM packs, usually those have sanyo or sony or some other brand name cells.

Quote:
Originally Posted by Cobb View Post
You really need to buy 10% more than needed and test them for capacity, then rack and stack em.
Since I'm buying used, I'm under the assumption of a worst-case scenario, 50% of the cells are useable at about 75% of their original ratings, 25% is useable for low-power stuff and has poor capacity, and 25% is just broken or has only a few % of its original capacity left.
We'll see how close I was to this estimate.

Quote:
Originally Posted by Cobb View Post
Once you got the voltage and amps you need, then depending on the chemistry you need a bms and charger.
Indeed.

Quote:
Originally Posted by Cobb View Post
I see a lot of lifepo deals on ebay, but I think I rather stick with a prius pack as they dont need a bms, just a grid charge to balance them out.
Eh, I don't like the NiMH cells that much. LiFEPO4 would be nice though, but is still pretty pricy compared to how low I need the price to be to make it work on my 'beer'-budget. (as I don't drink, I have an excuse to spend some money each month on other things.)

In fact, talking about cheap, electric driving is really not cheap.
Electricity here is €0.20 ($0.25) per kWh at home, €0.25 to 0.30 ($0.31 to $0.37) at a public charging station. (theres no free stations)
Using 200 wh/mile as a worst-case scenario, that would work out to $0.05/mile to $0.06/mile.
I pay $0.14/mile in diesel, and another $0.12 or so per mile in vehicle tax. (with my current amount of driving a month).

So battery writeoff will have to be covered by the difference in tax and 'fuel' cost, not a whole lot of room to play with.

Assuming 15000 mile/year, divided by 365 is 41 miles a day. Lets round up by a bunch because I don't spend every day behind the wheel doing the same exact trip, and say I want at least a 100 miles of range. At 200wh/mile, that would mean a 20 kwh pack. This pack would do 150 full cycles a year, or perhaps a ton of part-cycles adding up to 150 full cycles.

Assuming a cheap lithium pack can do an optimistic 1000 cycles, that would write off in 6,6 year. At 15k miles a year, that would be 100k miles. (yay, back to round numbers.)

Lets assume you manage to find some very cheap lithium packs for $500/kwh, so a 20kwh pack would be $10k. $10k divided by 100k miles is $0.10 a mile. Not bad at all, considering our fuel prices in Europe, that would save me actual money in the long run.

As long as the pack doesn't blow up after 250 cycles.
Or I have to loan the 10k with so much interest it doubles the total price.
Or it turns out you can't actually buy any new lithium packs for that slightly optimistic price.
Etc..

Interestingly enough, a 24 kWh Nissan Leaf pack would only be about $6.5k from the dealer? Also not a bad deal, if it holds up to 1k cycles or more.

So thats why I bought "5kwh" of li-ion cells for $120, even if a large portion of the cells are bust or have less than original capacity, thats a very very good deal. Now lets hope I can find more near the same price, that would mean I might be able to trade in my diesel for an electric.

Quote:
Originally Posted by planetaire View Post
Thanks. A whole forum about EV's and a subforum full of BMS, thats very cool.

Quote:
Originally Posted by Xist View Post
How is the plug-in-Prius different?
Its got li-ion cells.

Quote:
Originally Posted by RedDevil View Post
I do have a small welding kit, unused as of yet, and I would be happy to let you use it for this project.
PM me if you feel like dropping by in Nieuwegein
That would be very nice I'll let you know when the time comes.

Last edited by AlexanderB; 11-08-2014 at 07:54 PM..
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Old 11-08-2014, 09:18 PM   #10 (permalink)
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Id give you an award since you figured it out. I dont know of a single American who has successfully used the PP BMS. On the Insight forum there was a group buy to make a lifepo4 insight if not a PHEV mode, but only 1 guy has come close to making it work, not counting Peter.

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