# Switiching to lithiu LiFePo4



## Biostudent (Jul 13, 2010)

Hello,

I am looking into switching to lithium batteries sometime in the future. I wanted to inquire what I should look for and what the prices are like. I have a 72 volt system. I saw this battery http://www.wgeso.com/goods-84-72v+100ah+LiFePO4+Battery+Pack.html. Its 72 volts with one battery. Is it possible to use that battery? With lithium batteries will I get the same range if the ah is the same or I should get more because of reduced weight?

Any other costs involved in switching to lithium?

Thanks


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## Powered By DC (Jan 3, 2009)

The battery pack you linked to is priced a little high. It is for 24 cells and better batteries can be had at a cheaper price. 

I sell several types of lithium batteries here http://currentevtech.com

I am in the process of bringing in a container of Thundersky cells. I just started the process so it will be about 8-10 weeks before I have them in stock. I can get certain sizes from other vendors I know in the US that may have them in stock. You can email me for specific lead times.

All of the prismatic cells I sell come with all of the hardware included

You may also want to concider a BMS (battery management system) if you are switching to lithium. I personally think they are a must, but their are lots of opinions on this subject. 

I have a few BMS options available on my website


There are lots of different Lithium batteries out there and lots of vendors do your homework before you make your purchase.
I personally think the Thundersky batteries are the best bang for the buck 

Best Regards

Dave Kois
Current EV Tech, LLC
http://www.currentevtech.com
253-988-5020


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## CFreeman54 (Jan 14, 2009)

That pack you were asking about consists of 24 cells @ 3.2V per cell (typical voltage for LiFePO4). Reasonable price is $1.20-$1.30 per amp hour (i.e. a 100 Ah cell costs $120-$130). Go with a known manufacturer....Thundersky and Sky Energy seem to be the most used by people on this forum. Range will be a little better than PbA (for same Ah capacity pack) both because of reduced weight and less Peukert effect...but remember regarding Ah capacity PbA's are usually rated at the 20 hr rate and Li Ion at the 1 hr rate. A 100 Ah (@20 hr rate) PbA battery may only be 50Ah at EV current draw rates.

You will need a BMS for the li Ions, with functions that monitor overvoltage and undervoltage and shunt current through a resistor when the cell is full (when charging). I have Belkronix BMS boards on mine. I think Manzanita Micro makes a BMS too, but I don't know much about theirs. The BMS adds about $1000-$2000 to the cost depending on your pack size, and how many BMS boards you need. Your pack is small so it shouldn't be too bad. Remember shipping charges...EV Components used to add shipping from China charges AND shipping from their warehouse in Washington to where ever you were....and those charges add up.


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## dtbaker (Jan 5, 2008)

EvolveElectrics.com seems to be a good place to start... they stayed completely out of the evcomponents.com fiasco and have a good record for timely delivery.

Also, you don't HAVE to have a Battery Management System (BMS). Many people do use them with Li because they want to make extra sure all the cells stay in balance. If you over or undercharge one cell it can be damaged. The chargers 'see' the total voltage, and cannot adjust charge to an individual cell without a BMS. Personally, I don't think Li get that far out of balance that often to justify the cost of a BMS, so I am planning on periodic manual checking and balancing.


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## JRP3 (Mar 7, 2008)

CFreeman54 said:


> You will need a BMS for the li Ions, with functions that monitor overvoltage and undervoltage and shunt current through a resistor when the cell is full (when charging).


Or not  I run with no BMS. Though it hasn't happened, I can kill and replace a lot of cells before it adds up to $1000-$2000. I do see the value in having overvoltage signalling to shut off a charger. I see no benefit to shunting at all, you'll never get any more range than your smallest capacity cell can provide.


> The BMS adds about $1000-$2000 to the cost depending on your pack size, and how many BMS boards you need.


Or buy the Clean Power Auto mini BMS advertised on the right for about $13 per cell plus $30 for the head board.


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## ewdysar (Jun 15, 2010)

Biostudent said:


> Hello,
> 
> I am looking into switching to lithium batteries sometime in the future.... With lithium batteries will I get the same range if the ah is the same or I should get more because of reduced weight?
> 
> Thanks


For a battery pack with the same voltage and Ah rating, you will see small performance/range improvements due to decreased weight. However, if you are switching from flooded cells, you should see a dramatic increase in range due to Puekerts Effect. 

Here's a quote from one of my posts on another board: "There are other factors, the most apparent is the Peukert Effect, the energy lost due to the battery chemistry's inability to handle high amp loads. FLA suffer the most, AGM much less and Lithium very little. If we use an average load of 60A against a 200Ah bank, most FLA batteries will only deliver less than 60% of their rated capacity to 100% depth of discharge, AGM can deliver about 83% of their rated capacity and Lithium more than 96% of their rated capacity. Smaller packs with lower Ah ratings exaggerate these differences." For these calculataions, I used the following generic Peukert exponents: FLA=1.3, AGM=1.1 and Lithium = 1.02. Specific brands of batteries have different P exponents, so while your results may vary, this does give an idea of the relative differences between battery chemistries.

Eric


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## dtbaker (Jan 5, 2008)

Biostudent said:


> With lithium batteries will I get the same range if the ah is the same or I should get more because of reduced weight?


compared to 'floodies', I think you will find that a Li pack at the same nominal voltage and 100ah capacity cells will produce slightly more usable amp-hours, and much better performace due to less voltage sag, and far less weight. You'll want to get at least 100ah cells so that your max draw under acceleration won't require more than 3C (3*100) amps for very long.

You MIGHT be able to bump up your voltage a little in the same space, which might require a different dc-dc convertor, and will for sure require a different charger with a 'lithium-friendly' charge curve.


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## Biostudent (Jul 13, 2010)

Thanks for all the info. 

Is it possible to have lifepo4 batteries lets say 32 volts running parrelle with 32v FLA batteries? I would charge them separately to avoid issues there. I could add a switch for cutting the batteries off before I charge them so it won't try to flow through to the other series. Is something like this worth it?

I would gain the amps or no? Would it simply go off the weaker amp FLA batteries? I would save about 190-210 pounds in weight though. That's nothing to cough at.

Adding 24-32 volts is more in my realistic price range for now. Just curious if anyone has done this. It would be a good way to slowly add lifepo4 if its doable. 

But I imagine its not otherwise everyone would be..


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## CFreeman54 (Jan 14, 2009)

Biostudent: I'm running a hybrid LiFePO4/PbA AGM pack...similar concept to what you are asking about. I just got the car running, and it is at a higher voltage: [email protected]/[email protected] nominal...all seems to be working ok so far. It does add complexity. You can see my battery wiring arrangement here: http://www.diyelectriccar.com/forums/showthread.php?t=30722&page=14&highlight=Fiero+Build

If you decide to try it, make a spreadsheet of your nominal voltage, fully charged voltage, and cutoff voltage for both battery types, and play with cell numbers until you get the numbers for both battery types as close as possible. 

I personally would not try this without a BMS so I could see what the batteries are doing, but perhaps others who have higher risk tolerance would.

I know of a couple other people who are running hybrid LiFePO4/PbA parallel packs successfully, which is why I was willing to go ahead and try this. It is a compromise in weight, Ah capacity, and cost. As I said before, it does add complexity though.


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## pgt400 (Jul 12, 2008)

JRP3 said:


> Or not  I run with no BMS. Though it hasn't happened, I can kill and replace a lot of cells before it adds up to $1000-$2000. I do see the value in having overvoltage signalling to shut off a charger.


I've been using the Signalab V2.5 BMS I'm not a big believer in the need to top end balance either....but you must have HV & LV cutoff (or watch your cells like a hawk which I don't card to do). The Signalab is only around $80 and so far has worked great. The Mini BMS also looks very nice....if Morrison didn't screw me outa my cells I would have gotten the Mini BMS units because they look high quality and mount nicely to prismatics.


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## pgt400 (Jul 12, 2008)

Interesting...I thought of tring this with my bike...putting A123 cells in parallel with the 35v AGM's. Thought was the A123's would supply burst current needed for the 3-4 seconds during accel...the charge back on the flats.



CFreeman54 said:


> Biostudent: I'm running a hybrid LiFePO4/PbA AGM pack...similar concept to what you are asking about. I just got the car running, and it is at a higher voltage: [email protected]/[email protected] nominal...all seems to be working ok so far. It does add complexity. You can see my battery wiring arrangement here: http://www.diyelectriccar.com/forums/showthread.php?t=30722&page=14&highlight=Fiero+Build
> 
> If you decide to try it, make a spreadsheet of your nominal voltage, fully charged voltage, and cutoff voltage for both battery types, and play with cell numbers until you get the numbers for both battery types as close as possible.
> 
> ...


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## CFreeman54 (Jan 14, 2009)

PGT400: There is somebody doing that with a bike...I can't remember where I read it....on the Zenn forum I think. They seemed very pleased with the results.


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## Biostudent (Jul 13, 2010)

CFreeman54 said:


> Biostudent: I'm running a hybrid LiFePO4/PbA AGM pack...similar concept to what you are asking about. I just got the car running, and it is at a higher voltage: [email protected]/[email protected] nominal...all seems to be working ok so far. It does add complexity. You can see my battery wiring arrangement here: http://www.diyelectriccar.com/forums/showthread.php?t=30722&page=14&highlight=Fiero+Build
> 
> If you decide to try it, make a spreadsheet of your nominal voltage, fully charged voltage, and cutoff voltage for both battery types, and play with cell numbers until you get the numbers for both battery types as close as possible.
> 
> ...


Thanks! I will defiantly read your post. This may prove to be a good choice for now.

Would AGM be a better choice over lifepo4? Does the lower amp lifepo4 make sense? Or should I try to use the maximum I can get my hands on...even if its not that many?


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## CFreeman54 (Jan 14, 2009)

Well, it is very easy to run an AGM pack parallel to a flooded PbA pack since they are the same chemistry, running at the same voltage, etc. ....you could even get the same Ah capacity batteries, but because of the lower internal resistance the the AGM and floodeds will charge and discharge at different rates.... a BMS with shunts will still give you a better chance of equalizing charge/discharge. The people who are saying you do not need a BMS are talking about running all the same kind of cells, with similar charge, discharge, and internal resistance properties. AGMs will add a lot of weight to your car, that would be the primary drawback. You may get away with it since you are talking about a low voltage pack, so the weight may still be manageable, but you need to look at that.

I would not go with LiFePO4 less than 100 Ah for a car, since you should only draw about 3C (3 x 100 = 300 A) continuous out of them if you want them to "live" a normal life span. Theoretically you could use smaller capacity cells and parallel them to get around 100 Ah, but unless you want high performance and need to use A123 or Headway cells to get high current discharge there is no reason to add this complexity to your battery pack.


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## dmac257 (Jun 30, 2010)

CFreeman54 said:


> Range will be a little better than PbA (for same Ah capacity pack) both because of reduced weight and less Peukert effect...but remember regarding Ah capacity PbA's are usually rated at the 20 hr rate and Li Ion at the 1 hr rate. A 100 Ah (@20 hr rate) PbA battery may only be 50Ah at EV current draw rates.


So you are saying that FLA 100Ah = 5a for 20hours and Li 100Ah = 100a for one hour? I see charts that show 1C , 3C , 5C , 20C numbers .. is this 3C = 3X(rated current) ie: 300a for the Li battery?


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## dtbaker (Jan 5, 2008)

Allow me to jump in and attempt to clarify... My current batteries are us battery 8v floodies; great batteries with a 20hr capacity rating of 185+ ah. EV use however is much more demanding as you are dumping the energy in roughly an hour if you are at constant load driving around. Capacity curves are available, and for Pba the capacity goes WAY down at higher rates of discharge.... sometimes you can get a feel for this by how many minutes they say the battery is good for at 75amps. In my case the 185ah battery at a 20 hour rate can only kick out a steady 100amps for about an hour...

so, you have to try to determine a lead battery's 1-hour capacity to compare to Li. The 1-hr capacity is not always easy to find in battery specs.


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## dmac257 (Jun 30, 2010)

dtbaker said:


> compared to 'floodies', I think you will find that a Li pack at the same nominal voltage and 100ah capacity cells will produce slightly more usable amp-hours, and much better performace due to less voltage sag, and far less weight. You'll want to get at least 100ah cells so that your max draw under acceleration won't require more than 3C (3*100) amps for very long.
> 
> You MIGHT be able to bump up your voltage a little in the same space, which might require a different dc-dc convertor, and will for sure require a different charger with a 'lithium-friendly' charge curve.


I was wondering how 72V of T-145 6v 260Ah(20hour rate) FLA would compare to 72V of Thundersky 3.2v 100Ah(1hour rate) LiFePo4.

I dont know what "normal" amp draw from a traction pack is so I can't tell what I need for batteries. Even a gross estimate would be better than what I don't know now. Crusing at 35mph how much are you guys pulling from a pack? People say "a 100Ah FLA pack will be only about 50Ah at EV currents". But I still don't have a feel for what currents we are talking about?

Controllers limit current to motor to say 500amps .. but that isnt pack current. Throw me some ball park numbers please?


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## dtbaker (Jan 5, 2008)

dmac257 said:


> I was wondering how 72V of T-145 6v 260Ah(20hour rate) FLA would compare to 72V of Thundersky 3.2v 100Ah(1hour rate) LiFePo4.


pure guess would be that 6v FLA with 260ah 20 hr rating would be pretty close to 120ah at 1 hr rate, maybe as high as 180ah.



dmac257 said:


> I dont know what "normal" amp draw from a traction pack is so I can't tell what I need for batteries.


totally depends on vehicle... but at that low a voltage, you will pull lots more amps, (like about 150amps at 35mph I bet) which is hard on everything. You are much better to design w/ as high voltage as you can to match motor and controller, up to 120v for most 8" motors, and 144v for the 9" motors is very safe.


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## ewdysar (Jun 15, 2010)

dmac257 said:


> I was wondering how 72V of T-145 6v 260Ah(20hour rate) FLA would compare to 72V of Thundersky 3.2v 100Ah(1hour rate) LiFePo4.





dtbaker said:


> pure guess would be that 6v FLA with 260ah 20 hr rating would be pretty close to 120ah at 1 hr rate, maybe as high as 180ah.
> 
> 
> totally depends on vehicle... but at that low a voltage, you will pull lots more amps, (like about 150amps at 35mph I bet) which is hard on everything. You are much better to design w/ as high voltage as you can to match motor and controller, up to 120v for most 8" motors, and 144v for the 9" motors is very safe.


Since you don't know the amps that you would pull, using the 1 hour rating will probably get us close enough.

Using a Peukert exponent of 1.25 for the T-145, the 1 hour rate would be 143A or 55% of the 20hr rated capacity. At 72V, the T-145s have a capacity of 10,300Wh to 100% depth of discharge.

Likewise, using a Peukert exponent of 1.04 (It might be better than this) for the TS 100Ah cells, the 1 hour rate is 89A or 89% of the 20hr rated capacity. At 72V, the TS100Ah have a capacity of 6400Wh to 100% depth of discharge.

More importantly it would take 160Ah of TS cells to deliver the same 143A 1hr rate (10,300Wh) as the 260Ah T-145 batteries.

Factoring in Lithium's higher tolerance for deeper discharges, the TS pack has a greater effective range, meaning that you should get a little more range with the same effective capacity as the flooded cells in daily use.

Does this make sense to you?

Eric


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## JRP3 (Mar 7, 2008)

ewdysar said:


> Likewise, using a Peukert exponent of 1.04 (It might be better than this) for the TS 100Ah cells, the 1 hour rate is 89A or 89% of the 20hr rated capacity. At 72V, the TS100Ah have a capacity of 6400Wh to 100% depth of discharge.
> 
> Eric


1C with lithium should give you it's full rating. It's a 100Ah cell, probably more in reality. My SE 100 cells all came in at 110 or higher capacity. Peukert isn't much of an issue with lithium in normal usage.


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