# 3.7V 2400mAh Lithium Batteries (2-Pack Blue) - $6.40



## Gavin1977 (Sep 2, 2008)

Well, personally I probably wouldnt go with these because...

For a 10kw pack, your looking at ~1100 batteries, at a cost of $3600. Price looks good, but you have 1100 cells to wire up. Guess you would have to try and solder them with a large tip, and ensure that you are quick enough not to cause damage to the cells.

Also what is the C rating? if these are basically laptop cells, then you are probably looking at 5C as an absolute max, which gives you 50kW peak.

Anyway, if the C ratings are good, and you have the means (and the patience) to wire them up, then they are probably a very good deal.


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## Technologic (Jul 20, 2008)

Gavin1977 said:


> Well, personally I probably wouldnt go with these because...
> 
> For a 10kw pack, your looking at ~1100 batteries, at a cost of $3600. Price looks good, but you have 1100 cells to wire up. Guess you would have to try and solder them with a large tip, and ensure that you are quick enough not to cause damage to the cells.
> 
> ...


My guess is batteries this size can be had for half that from china directly?

So... if you have the patience to do 1100 cells... might as well get them for manufacturer's cost.


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## david85 (Nov 12, 2007)

I'll have to double check my notes, but I think they can be had for $2 each. Actually that would make them compatative with LiFePO4 even factoring in the 5 year life span.

The Lion 18650 is probably the worlds most mass produced lithium battery, so volume production has driven cost down considerably. It some one could wield them and make a good BMS for them, they would be viable. Just be warned though, these are the kind that can explode, so proper cases, wiring and BMS are critical for safe and reliable operation.

Peak C rating will be way higher than just 5C though.


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## Technologic (Jul 20, 2008)

david85 said:


> I'll have to double check my notes, but I think they can be had for $2 each. Actually that would make them compatative with LiFePO4 even factoring in the 5 year life span.
> 
> The Lion 18650 is probably the worlds most mass produced lithium battery, so volume production has driven cost down considerably. It some one could wield them and make a good BMS for them, they would be viable. Just be warned though, these are the kind that can explode, so proper cases, wiring and BMS are critical for safe and reliable operation.
> 
> Peak C rating will be way higher than just 5C though.


Assuming 3.7v is a nominal continuous voltage, these cells at $2 each would cost about $0.22/wH (errored on the low side)

Seems to back up my assumption that $0.25/wH is the lowest end for prismatic cell packs we can probably see with massive volume 

C rating SHOULD be about 10C ... maybe as high as 15C peak.

Granted that's a hella a lot of cells... I'd almost look for a cell maybe twice that big to fit the bill if they can be had for around $0.25/wH... just to save wiring time and the possiblity of dead batteries.

BMS modules could be done for a group of 20-30 stacks that can shut down the entire stack if something goes wrong... seems possible. You'd need to parallel the stacks to each other, not series. Paralleling then series should help stabalize the charging some.


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## david85 (Nov 12, 2007)

You would need a series-parallel configuration for best results. Perhaps connect enough single cells together for a 100-200ah 3.7V module, install the BMS voltage clamper, and wire as many such modules together in series as needed to get the desired voltage. That way a single dead cell cannot cut power to the whole vehicle.

Nothing new about the idea. You can get the specialized resistance wielder that can spot wield them to a conductor plate for under $4k. This is after all, the basic configuration that AC propulsion has been using for years. I'm not positive if they use the exact same cells but they are close.

Would need to make a proper jig to install all of those cells though, and even then would be rather labor intensive to assemble an average sized pack. Once set up though it would be quite cost effective assuming you had a reasonable option for BMS.


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## Technologic (Jul 20, 2008)

david85 said:


> Nothing new about the idea. You can get the specialized resistance wielder that can spot wield them to a conductor plate for under $4k. This is after all, the basic configuration that AC propulsion has been using for years. I'm not positive if they use the exact same cells but they are close.


lol you don't need a contactor welder 

it should be possible to press fit the units or solder them with a decent setup/brazer.
What would be good though would be seperate BMS modules for every grouping of parallels... not too labor intensive I'd imagine... and the option of contact welding isn't worth the time IMO for such cheap replacements.

The 1100 pieces is the only thing that'd stop me.

Fabrication could be simple relatively. If you figured out the parallel number you could cut copper sheet and simple hot press it (with another thicker sheet over solder balls)... either that or simply use a weldable solder... which would take forver.

You could for instance take a sheet of 1/16th" copper.. cut it to fit over the positive and negative terminals for a group of say 30 cells... then take two larger pieces of steel/aluminum about 1/2" thick then bolt the two pieces of steel together sandwiching the entire group of cells and bending the copper on the positive lead around the "knob" end. 
SHOULD work effectively. if you did this in one long 1 battery thick piece... you could then manually touch up the connections for each battery while you press the copper to it.

That kind of soldering might not even be necessary... you could just press the entire thing together and pure tension of bolts could hold it decently well.


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## david85 (Nov 12, 2007)

I guess that would work for a one off or some other low volume, but I would be weary of using that method for some one other than myself. Just not really sure how reliable that many press fit connections could be over the life of the battery. Road bumps and vibrations could take their toll.

But I'm just guessing at this point.

Although those 1100 cells would in theory only cost you about $2200. It is tempting considering the performance and cost advantage.


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## Guest (Jan 14, 2009)

I have been thinking of using sprinkler tubing (the thick stuff and putting an adjustable bolt in each end to squeeze the batteries together and in between the batteries would be a thick piece of silver or copper washer to give a soft but metallic point for the batteries to connect. I'd then have a screw terminal on each end for attaching wires. It may work for a small pack like a 72 volt pack but 100ah or 200ah pack. I could afford $2500 for a bunch of batteries. May only need half as much for my 72 volt system. I was actually going to use NiMH AA batteries to make a small pack for a bike and to test my theory. Li batteries would be better as each battery is 3.2 volts or so unlike the low voltage of the NiMH batteries. 

Pete : )





david85 said:


> I guess that would work for a one off or some other low volume, but I would be weary of using that method for some one other than myself. Just not really sure how reliable that many press fit connections could be over the life of the battery. Road bumps and vibrations could take their toll.
> 
> But I'm just guessing at this point.
> 
> Although those 1100 cells would in theory only cost you about $2200. It is tempting considering the performance and cost advantage.


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## Harold in CR (Sep 8, 2008)

What about that tiny spot of corrosion, where one battery contacts the other, as in remote controls. You have to pop the cover and spin the batteries with your thumb, to break that spot of bad contact, so the batteries make a good connection. 

I used to build ni-cad "C" cells, using 7 cells, to make a metal detector pack. Used to solder the contact with plain solder. I know this isn't capable of high amp draw, BUT, This idea looks interesting. 

Down here, it's a use what you can find environment, and, I might get a decent price, shipped in from Cheena, because we are heavily dependent on supplies from Cheena ??? Might get a hitchhike in someone's container ??


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## Technologic (Jul 20, 2008)

david85 said:


> I guess that would work for a one off or some other low volume, but I would be weary of using that method for some one other than myself. Just not really sure how reliable that many press fit connections could be over the life of the battery. Road bumps and vibrations could take their toll.
> 
> But I'm just guessing at this point.
> 
> Although those 1100 cells would in theory only cost you about $2200. It is tempting considering the performance and cost advantage.


I'd be rather worried a contactor welder's heat would ruin the battery. Battery terminal oxidation could be counteracted by placing the tension cells into a larger airtight system than just vaccuuming all of the air out.

obviously there's easier methods for a series/parallel, than parallel-series


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## david85 (Nov 12, 2007)

Technologic said:


> I'd be rather worried a contactor welder's heat would ruin the battery.


Actually thats why a resistance wield is the safest way to do it. The heat does get high, but its very localized and it happens so quickly so there isn't much heat that can escape and heat up the rest of the cell. The machines are designed to do this and have to be properly calibrated, but if done right it is the safest and most reliable way. Much faster too.

Soldering involves waiting for heat transfer to heat up the parent material enough to melt the lead solder. The max temperature is lower, but the net amount of heat going into the cell is in fact higher.

These guys carry a line of spot wielders for this: http://www.powerstream.com/


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## Technologic (Jul 20, 2008)

david85 said:


> Actually thats why a resistance wield is the safest way to do it. The heat does get high, but its very localized and it happens so quickly so there isn't much heat that can escape and heat up the rest of the cell. The machines are designed to do this and have to be properly calibrated, but if done right it is the safest and most reliable way. Much faster too.
> 
> Soldering involves waiting for heat transfer to heat up the parent material enough to melt the lead solder. The max temperature is lower, but the net amount of heat going into the cell is in fact higher.
> 
> These guys carry a line of spot wielders for this: http://www.powerstream.com/


I still like the idea of a tube and doing a series parallel >,> with a very tight pressure on the ends of the tubes... you could do sets of 5 like that... then wire them in parallel again, then series.

Say 15 batteries in series, then 15 sets of those in parallel then 5 sets of those in series...

You're talking about a hell of a lot of batteries here... and 10 batteries in series going out isn't a big deal... just pull that one out and start over (it's also a manageable amount of units to check... only about 75 vs. checking a faulty battery in a parallel configuration... you could narrow down the modules fast that way... well faster than doing 1100 battery checks that's for sure.

If one of these batteries in a cell does go out... you're really gonna neeed to replace it promptly otherwise it will imbalance all of them and probably end up breaking/ruining some of them.


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## Guest (Jan 15, 2009)

After thinking about it for awhile it would actually be best to use an anti corrosion grease between batteries and that will help prevent loss of contact. Having the batteries in a tube but not welded will make replacement cells easier but keeping the connection may prove difficult over time. Maybe!

Pete




Technologic said:


> I still like the idea of a tube and doing a series parallel >,> with a very tight pressure on the ends of the tubes... you could do sets of 5 like that... then wire them in parallel again, then series.
> 
> Say 15 batteries in series, then 15 sets of those in parallel then 5 sets of those in series...
> 
> ...


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## david85 (Nov 12, 2007)

The cells in the tubes would be connected in series? How would you balance the individual cells?


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## Guest (Jan 15, 2009)

I hear chatter about that same thing but all batteries are made up of cells. How does any battery maintain balance in all it's cells. A 6 volt Golf Cart battery has three cells but all somehow remain balanced. Same with a 12 volt battery with 6 cells. Same with AGM and any other battery chemistry. So all batteries are built of cells then those cells are in series. Why would you think it would not work? Just because someone says so or because of what real issues? I am proposing building a Battery then hook them in series. No different than what I currently have now is how I see it. It is just that it's not a normal lead acid flooded battery. If it won't work then that's OK too. 

Pete : )





david85 said:


> The cells in the tubes would be connected in series? How would you balance the individual cells?


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## Technologic (Jul 20, 2008)

david85 said:


> The cells in the tubes would be connected in series? How would you balance the individual cells?


I'm not aware of any electrical reason that series wiring is more unstable than parallel...


Besides the natural imbalance in batteries causing overdischarging... is there a problem doing a series first wiring set?


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## david85 (Nov 12, 2007)

Parallel allows current to flow to the path of least resistance. As cells get more charge, they resist the current more and more. Higher charged cells in a parallel wired pack don't "get in the way" of current that is trying to get to cells that are not charged as much.

In series, each cell gets current in a specific order, and if one cell in the string happens to have a little more charge than another, well, the electircity will still "force its way" through the pack as the charger tries to bring the net battery voltage up.

Lead acid batteries are much more tolerant of overcharging than lithium are. The internal resistance of the cells themselves are enough to regulate each cells safely enough. Usually all that happens is the extra unusable current in higher charged cells are converted into heat and gas. Its not ideal, but lead acid batteries will self regulate to a limited degree.

Lithium (cobalt oxide) cells have lower internal resistance than lead, so they would accept an overcharge much more readily. That can damage the separator barrier and lead to thermal runaway.

I hope that makes sense...


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## Technologic (Jul 20, 2008)

david85 said:


> Parallel allows current to flow to the path of least resistance. As cells get more charge, they resist the current more and more. Higher charged cells in a parallel wired pack don't "get in the way" of current that is trying to get to cells that are not charged as much.
> 
> In series, each cell gets current in a specific order, and if one cell in the string happens to have a little more charge than another, well, the electircity will still "force its way" through the pack as the charger tries to bring the net battery voltage up.
> 
> ...


Yeah I realized that after I started googling it. Series wiring sets would allow for isolation and ease of replacement. I wasn't aware that the Lithium batteries might stray voltages that much. I mean discrepencies of 0.05v or less are not relevant and won't lead to overcharging in such small 10-15 battery "packs". You'd need 0.1-0.3v differences in cells to make that difference. Proper matching should negate a lot of that long term?


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## david85 (Nov 12, 2007)

You could probably get away with it if the cells are good quality and you made sure they all had the same charge when they are first assembled, but the question is....would there be enough safety built into the system should something go wrong and a single cell gets out of sync with the rest of the pack? The thing is they don't last forever.


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## Qer (May 7, 2008)

Before you consider bolting together the cells so they can't vent the normal way in case of an electrical malfunction, read this report:

http://wikileaks.org/wiki/Toyota_Prius_A123_car_fire_investigation_report_2008

I'd say this is a VERY stupid idea. Don't go there.


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## cruisin (Jun 3, 2009)

I have built many batteries using 18650 cells that I buy with tabs. Currently running a 70vw with 130v and 220ah for a total of 28.6k. It is real easy, but time consuming, to wire them up. The key is to use the proper size wire to cary the ah per battery. I can be reached at [email protected] if you need info.


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## onesojourner (May 6, 2008)

cruisin said:


> I have built many batteries using 18650 cells that I buy with tabs. Currently running a 70vw with 130v and 220ah for a total of 28.6k. It is real easy, but time consuming, to wire them up. The key is to use the proper size wire to cary the ah per battery. I can be reached at [email protected] if you need info.


 very interesting. could you post some pictures/info on what you are doing exactly?


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