which are the best 3.7V lithium cells

[karotto]

Hi there,
I would like to build two 12V lithium battery packs from 3.7V lithium cells to power my electric scooter. I want the cells to be light and extremely durable (1000 of charge cycles). The reason why I want to build this myself is frankly out of distrust. Most of the pack purchased don't last very long. I am thinking that if I buy automotive cells and charge them right (I will also build a charger and power management unit) then they should last for many years. It also is cheaper to solder together your own pack so I am thinking. Can anyone please recommend which lithium cells (close to AA size, I think, with tabs for soldering) are the best quality, longest lasting ones and lightest around? Your input is much appreciated. Karotto

 

[SamTexas]

3.7V nominal, 3.0 to 4.2V operating are LiCo and LiMn.
3.3V nominal, 2.5 to 3.6V operating is LiFePO4

Without knowing the power (watt to achieve certain speed) and expected range (miles or kms) requirements of your scooter, recommendations are hard to make.

 

[karotto]

Thank you for your reply. LifePO4 are out due to weight.LiCo and LiMn. Not sure. whatever fits my requirements. I will need about 10AH total capacity and I will string 2 12V packs together to supply 24V to my 350W motor. Max output current of each cell are irrelevant as the I will be using many cells and the output current multiplies so even the cell with the lowest max output current rating should suffice once strung together. I don't care about the output voltage of each cell. I can just string together as many as needed to supply 12.8V. I just thought they are 3.7V. thank you

 

[SamTexas]

18650 format is about the closest to the AA size you want. 18mm diameter and 65mm long.

LiCo has the highest energy density but low discharge rate. These are the same cells used in laptop computers. The best ones are made be Panasonic. NCR18650B is the lastest with 3400mah capacity. Earlier versions ranges from 2200mah to 3100mah.

LiMN has much lower energy density but higher discharge rate. I have used cells made by Moli but was not completely happy with it so no recommendation from me.

Unfortunately, neither LiCo nor LiMn will satisfy your "1000 of cycles" requirements. Panasonic cells are supposed to retain 70 to 80% of original capacity after 300 to 400 cycles.

I don't know why you're dismissing LiFePO4 cells. A123 has the best lithium cells in term of performance, longevity and safety.

Good luck and have fun with your project.

 

[karotto]

300 cycles!?! That's nothing. That means that after 1 year I only have 80% capacity left. Is this really the sad state of battery technology affairs? How do you know the Panasonic is high quality? Are there actually published figures around? thank you

 

[Hillhater]

300 cycles!?! That's nothing. That means that after 1 year I only have 80% capacity left. Is this really the sad state of battery technology affairs?

Yes ..sad, isnt it !.. maybe you would prefer to use Pb batt's instead ?
OR ..as Sam said , reconsider the A123 LiFePo4 option.
This is real life, with real batteries, so nothing is as perfect as we would wish it to be. Everything is a compromise even the latest technology, so find your best option between capacity, weight, cycle life, output, cost, etc etc.

.. How do you know the Panasonic is high quality? Are there actually published figures around? thank you

A little "Self help" with Google would reveal all you want to know, and why major players like Tesla use the Panasonics.

 

[SamTexas]

Is this really the sad state of battery technology affairs?

Actually no. This is a very "happy state of battery technology affairs". The lithium batteries we have today are the stuff we were dreaming about just 10 years ago.

How do you know the Panasonic is high quality? Are there actually published figures around? thank you

Big customers like HP, Dell or Tesla (as Hillhater mentioned earlier) spent thousands of hours evaluating different brands of batteries before they sign a multi-year, multi-million dollars contract. They could do us a big favor by sharing their findings, but they don't. So I went around disassembling hundreds of genuine HP and Dell laptop batteries in order to find out which brand they use the most. Panasonic.

 

[iperov]

currently best of technologies:

lifepo4 - 20ah pouches
lipo - super low cost in hard case from hobbyking
li-ion - fetch best from this comparator http://lygte-info.dk/review/batteries2012/Common18650comparator.php , currently LG 18650 D1 - available on taobao, through agent taobaotrends.com

 

[amberwolf]

If you can afford them, you could get prismatic NMC cells liek those used in the Zero (EIG) and the Leaf (LG Chem); IIRC from the spec sheets on their sites, these are 20Ah cells about the size of a very thin trade paperback. Light enough, but also capable of higher number of cycles. (EIG datasheet shows at least 1000).

There's a thread in the for sale section about the EIG cells, I think.

I don't know of any of the commonly available cylindrical cells that are capable of that, at least not at a C-rate or depth of discharge that makes them useful to us on EVs.

 

[dogman dan]

Hmmm, or there could be a reason for the stampede for 4s 5 ah hard case lico (RC lipo) packs when they go on sale very cheap at Hobby King.

Nahh, most of us just like to waste our battery budget.

Sorry, I just couldn't resist some sarcasm this time, another one that wants what we all wish exists, (1000 cycles from lico) and wants it cheaper too. Don't we all bro.

Seriously now, Two or three decent options exist.

First of all, if your scooter only draws 10-15 amps, get a 10 ah pingbattery or similar lifepo4 pack. Pouch cells, they are not that heavy not all that huge. 18650 cell monstrosities take up tons of extra space and weigh more. You aren't going to get a small light battery with 150 18650 cans. Go with 1/3 the number of small light pouches.

If your scoot needs more amps, then you want A123's. Again lifepo4, again potentially 1000 full discharge cycles rather than the 500 expected from lico and limn. 20 ah pouches exist, but if you want a smaller pack and use round cells at least find the 26650 type. Just too many connections to make with 18650's.

But though you'd need to replace it more often, there really really are good reasons for the poplularity of the cheap 4s 20c rate hardcase RC lipo packs. Sure, it's not quite as stable as lifepo4, but you get huge amps on the cheap. With such a high c rate, you can pull big fat hotrod controllers with just 10 ah of this stuff.

EDIT, Backreading after the coffee kicked in, I see now you just need to run a small scooter. 6s RC lipo is a good voltage for you then. 25v fully charged. Two 6s 5000 mha packs is all you need. Paralell them for 10 ah of "24v". About the size of three sticks of margarine, and 3 pounds or so. Plus a charger, and a cheap volts display or a low voltage alarm and you are set. Oh, and a bit of wire and some connectors to make a harness to paralell connect two packs.

Two of these for example, no hard cases in 6s. If this isn't cheap enough for you, we simply can't help you. You'd have to dumpster dive for recycled power tool packs to get cheaper.
http://www.hobbyking.com/hobbyking/store/__9176__Turnigy_5000mAh_6S_20C_Lipo_Pack.html

 

[SamTexas]

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[circuitsmith]

If you're interested I've developed a 24V battery system around the HobbyKing Turnigy 3s and 4s lipo packs.
My custom BMS has LVC, individual cell voltage monitoring and cell balancing with every charge and no need to disconnect wires for charge vs discharge.

http://endless-sphere.com/forums/viewtopic.php?f=14&t=44100

 

[karotto]

Thanks everyone for your input. I have one question though. Right now my system runs on lead acid batteries which have 12.8 V fully charged making 25.6 V for two batteries. Should I be using 6S or 7S batteries to replace the lead acid? Which would give me the correct voltage? Thank you again

 

[SamTexas]

7s for LiCo or LiMn. That will give you a max of 29.4V which is tolerable to most systems designed for 24V.
6s (again for LiCo and LiMn) will run too but you'd most likely hit the controller's LVC (low voltage cutoff) way before your battery is depleted.

 

[DAND214]

Thanks everyone for your input. I have one question though. Right now my system runs on lead acid batteries which have 12.8 V fully charged making 25.6 V for two batteries. Should I be using 6S or 7S batteries to replace the lead acid? Which would give me the correct voltage? Thank you again

12.8 ISN'T NEAR CHARGED!

I'd go with the 6s2p. that's 25v 10Ah if you use the 5000Mah packs. Like Dogma says, small and light. Probably smaller than you 1 12pb battery.

LiFePo's are more plug and play which is more for the ones who just want to plug and ride. Can't do that all the time with LiPo's.

Dan

 

[neptronix]

Oh man, misinformation central here. Hey OP, how about you read some spec sheets and decide or yourself what's best for your application? There's no such thing as 1 cell that does it all or is the best. If that were the case, there wouldn't be dozens of cell manufacturers and lines of cells out there

18650's can be heavy, light, wimpy, powerful, safe, or unsafe.. same can be said about pouch cells.. it all depends on the chemistry and formula of battery.

 

[amberwolf]

Thanks everyone for your input. I have one question though. Right now my system runs on lead acid batteries which have 12.8 V fully charged making 25.6 V for two batteries. Should I be using 6S or 7S batteries to replace the lead acid? Which would give me the correct voltage? Thank you again

13.6V would be more likely fully charged for each SLA; typically 14.something during charging, sometimes up to 28.8V total durng charging for 2 SLA in series (such as on my powerchairs).

12.8V is pretty close to dead, almost fully discharged (or at least, as far discahrged as you typically would want to ever go for SLA if you want them to have a long life). 12V is as far down as I would recommend going.

If you charge up LiCo to only 3.86V/cell, then 7s would be about right for a fully charged 2-battery SLA replacement, at 27V fully charged. But you won't get much power out of them that way, as they are nearly discharged like that.

If you charge up 6s to 4.2V/cell, which is completely full, you get 25.2V, which would let your system operate, but it might not reach the same top speed it did with SLA, depending on the voltage sag you got with SLA.

Then again, at 6s, you would probably want the system LVC at 22.2V for 3.7V/cell; that will net you most of the energy out of them. I dont know what the LVC of your SLA 24V controller is (if it has one); sometimes they are as high as 23-24V. If so, you won't be able to use all fo the energy from the LiCo pack, and will have to carry a larger pack than necessary just to have the same amoutn of usable Wh. If your controller LVC is 22.2V or lower, then you can use all the energy, but if it's lower, you will want a separate LVC system to protect the cells.


If you wire up JST-XH balance taps to the cells, you can use Methods' LVC/HVC system (see for sale section) for worst-case protection, at least, though you wouldn't want to use it as an everyday cutoff. . Other monitoring systems are also avaialble from various places if you want them.

 

[dogman dan]

Lies? Go way back and read some of the threads about 18650 lifepo4 packs from about 2008. Granted, some of those packs were made from totally shit cells.

Well, all I know is that large packs of 18650's have so many connections in them that they tend to get unreliable. Don't even think of dropping a huge 18650 pack. You'll pop the spot welds if you drop an 18650 pack. I've had my ping pack fly 20 feet over my head and land in the dirt and come up fine. Double the number of copper strips to connect them (18650) compared to 26650 cells. Sure, there are quality 18650 cells. But I'd personally much prefer a powerfull high c rate pouch cell. ESPECIALLY SINCE THEY ARE CHEAP AT HOBBY KING.

Now if you have an even cheaper source for 18650 cells, or even free, then go for it by all means. If a pile of 18650's landed in my lap, of course I'd use em.

As for cycles, lifepo4 rates the best, so 26650 A123 cells are a great choice if you use a round cell.

Back in 2008, a 36v 20 ah lifepo4 pack of pouch cells weighed 15 pounds. A 36v 20 ah 18650 pack weighed 20 pounds. That's a LOT heavier. But a 10 ah pack of 18650's should still weigh in at only 10 pounds, which will seem quite light compared to your current 20 or so pounds of lead. This is all lifepo4 numbers.

But a 7s 10 ah pouch pack of lico (RC lipo) will be less than 8 pounds. Lifepo4 pouch will be about 8 pounds. Both good choices, but more cycles from lifepo4. On the other hand the lico pack is very very powerfull and quite affordable. So buy 5 packs and use 4 to insure a good quality running pack.

Get your voltmeter and see what your low voltage cutoff really is. You really don't want to discharge below 3.5v per cell with RC lipo. So if your cutoff is 21 or 22v, you are good to go with 6s. If it's 24v, then you might need to use 7s. Unfortunately 7s packs are hard to come by, so you'd likely have to use 3s packs combined with 4s packs, and spend more money than just going with 6s.

Or if it's affordable to you, just get a 24v 10 ah lifepo4 from pingbattery. Lifepo4 pouch is not that heavy, still under 10 pounds, and should last more cycles than lico. The easiest thing would be just to go buy the ping which comes complete with a battery manangement system and charger. So all you do with the ping is add the connectors that match your scoot.

 

[dogman dan]

BTW, you can buy just one 6s lipo pack and a cheap charger, and experiment with the RC lipo very very cheaply. Under $100, mayb just a tad more including shipping. Just one 5 ah pack will likely have nearly the same range as your lead did.

If you are going to make battery contruction a hobby with any cells, an RC charger that does lico, lilo, lifepo4, pb, nicad, etc is a good investment anyway. You'll find it quite handy during the cell sorting stage of building an 18650 pack.

 

[SamTexas]

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[migueralliart]

You know I think this is my take on 18650 cells

You have konions which are basically a good application for ebikes as demonstrated by the DOC and several members. The other 18650 is LiCo from laptop battery packs which HAS been demonstrated to work too but you have to have at least 2x the capacity that you need in order to not stress them. I gotta say thou for flat areas and speeds below 25mph they might be a really good solution for long distances at a constant current draw.

In the end it all comes down to your application.

Race Bike with small range- Go lipo, A123 26650 or go Home
Typical ebike with 25 a controller and low speed below 25mph - Go with a 18650 pack(with oversized capacity), lipo, or pretty much everything else.
You want both high speed and range - So far lipo is the king in this region because of the C rate.

 

[circuitsmith]

12.8V is pretty close to dead, almost fully discharged (or at least, as far discahrged as you typically would want to ever go for SLA if you want them to have a long life).

Not true. Left off the charger a few hours a full battery is 12.7-12.8V, empty is 11.9V.
The voltage will be higher right off the charger, but falling with time and therefore ambiguous.
Here are some references:

http://www.scubaengineer.com/documents/lead_acid_battery_charging_graphs.pdf
http://batteryuniversity.com/learn/article/how_to_measure_state_of_charge
http://www.energymatters.com.au/renewable-energy/batteries/battery-voltage-discharge.php

As I posted earlier I have a solution for the OP and others with a 24V system.
It's 7s and up to 3p (15AH in my case).
I can build up a BMS board the Turnigy packs plug into and Anderson connectors for the charger and output.

 

[neptronix]

18650's and 26550's can make for very hardy packs if you use spacers ( sorta like the allcell pack for example ), and a decent cell ( like say, Samsung 5C NMC, some of the higher discharge Panasonics, etc ).

There are some 18650 cells that can last into the thousands of cycles.
There are some pouch cells that will only last into the hundreds of cycles.

The format of the cell really has less to do with it's properties as a battery and more to do with it's physical attributes.

It's true that a lot of the better cells for our application are in pouch format, but there are some good 18650's and 26550's out there, you jut have to look around.

 

[dogman dan]

Ok sam, tell him where to get lico that lasts 1000 100% dod cycles. We'd all like that web address.

Lifepo4 at least does theoretically last 1000 100% dod cycles. Albeit, typically in a low discharge rate test of a single cell.

10 ah of 18650 cells does weigh more than 10 ah of pouch cells of the same chemistry. Foil just weighs less than cans. That's no lie.

 

[SamTexas]

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