[PStechPaul]

I was looking for some LiFePO4 18650 cells to replace the dead NiCDs in a Ryobi 18V portable drill, and I found some 2500 mWh NiZn AA cells for $32 for 16 pieces.

http://www.ebay.com/itm/16-pcs-2500...Rechargeable-Battery-PowerGenix-/151256251587

This works out to about $0.80/Wh. The LiFePO4 18650 cells of which I have four were labeled as 3600 mAh but tested out as about 1100. The ones I found recently are labeled 1800 mAh are $25/6 pieces which is $0.72/Wh but if they test at 1100 (which I expect) they would be $1.18/Wh.

I also found some AA (14500) LiFePO4 cells rated 700 mAh at $15/8, which is $0.84/Wh, but they are probably also over-rated:
http://www.ebay.com/itm/360874869583

The four LiFePO4 cells I bought almost two years ago still read about 3.78 volts so they seem to be good quality, but just lower than stated capacity. The NiZn cells supposedly have a much higher self-discharge especially after about 30 charge cycles, but otherwise their characteristics are pretty good, according to Wiki:
http://en.wikipedia.org/wiki/Nickel–zinc_battery

I only need 18V for my drill and I have found that it will work satisfactorily down to 12V, so the four LiFePO4 cells I have should work OK down to 3VpC. I plan to incorporate my most recent design of a BMS/shunt charging circuit in the drill to prove its features. I can modify the circuit for the 1.6V nominal NiZn cells, by assuming two in series will remain well balanced enough, or I can add an extra voltage sensor and shunt element.

A major plus for the NiZn cell is low toxicity, recycleability, and inherent safety, as no flammable materials are used.

I will only need 12 of these cells for my 18V drill, so I will have 4 left over as spares, or for use in other equipment that uses AA cells.

Does anyone have any recent experience with these? Thanks.

 

[arklan]

ill be doing something similar with a dewalt 18v cordless drill (20v in america)

http://www.ebay.com/itm/161266812520?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

get 8 18650 cells for $13 rated to 6000mah
5 or 6 of these will give the correct voltage to put in to the dewalt drill and reuse the bms that the batteries come with, im hoping its fine
if not ill just stick a 1 dollar voltmeter on it that will be on all the time so my dad knows when to stop using it and charge

any of the cells with 'fire' in the name are most likely pretty dodgy so ill be doing a 1c ah test when i get them (these are ultrafire)

when we get our batteries we can compare notes

 

[PStechPaul]

You will almost certainly find that these cells are nowhere near 6000 mAh. The ones I bought were advertised as 3600 mAh (which is a lot higher than most of the known good brands of similar size), yet they barely reached 1000 mAh:

There is a website with test results for a wide range of cells, including yours:
http://lygte-info.dk/review/batteries2012/UltraFire SJ18650 6000mAh (Black) UK.html

Note that the internal resistance was measured at 0.5 ohms. So at the typical 4A draw of a drill, the terminal voltage will drop to less than 2 volts. Mine measured about 0.25 ohms at end of discharge, and 0.2 ohms in charged condition (as received).

My LiFePO4 cells measured 0.05 ohms internal resistance, so the terminal voltage only dropped from 3.34V as received to 3.25V at 1.67A. With a measured capacity of 1.11 Ah I figure they should last for about 15 minutes of actual use, which seems acceptable to me. The discharge test:

 

[Karter2]

I assume you know that there is no such thing as an 18650 cell of more than 3400 mAhr capacity commercially available ?
And that any decent 18650 cell able to work at 3C or above will only be around 2500 mAhr and LiCo.
No good cell should have more than 0.01 ohm IR.
There are few cells available that can work at this power level..Sony VTC5, Samsung 25R, LG HE2, etc to name a few, but there are an awful lot of cells that are just junk.....many with fake labels and old recycled lap top cells that have been re wrapped... iE dud cells.!
Some cells even have tiny 3 volt "button" cells inside and packed out with sand or flour for weight !...so beware.
It's really best to only buy from reliable , proven, sources, even if you think you are buying a known branded product.
Top quality, new, 18650, cells capable of 15 Amp discharge can be bought for as little as $5 each in quantity ($0.55 per Whr), so there is really no need to mess with mystery products.

 

[arklan]

Top quality, new, 18650, cells capable of 15 Amp discharge can be bought for as little as $5 each in quantity ($0.55 per Whr), so there is really no need to mess with mystery products.

got a link?

 

[Karter2]

Good place to start...
http://endless-sphere.com/forums/viewtopic.php?f=14&t=57291

 

[PStechPaul]

I couldn't find a really reliable (and recent) source for 18650 size LiFePO4 in smaller quantities such as what I need, for anything near the $0.55/Wh price. The 18650 cells seem to be normally 1250 to 1500 mAh and those are usually about $5-$10 each (new). Those would be 4.0 to 4.8 Wh and would need to be less than $3 each for the $0.55/Wh figure.

I found 24 packs of used 1250 mAh cells for $64 or $2.67 each which is $0.67/Wh.
http://www.ebay.com/itm/351134576340

The same vendor is selling the same cells new for $6.40 each:
http://www.ebay.com/itm/10-NEW-K2-E...able-1250mah-Batteries-3-2V-LFP-/191269568540

I'd be very happy to buy good name brand cells for $0.55/Wh or even as much as $1/Wh. And it seems very doubtful that could be found with low (or free) shipping. I think I'll place an order for these.

 

[Karter2]

Why are you focusing on LiFeP04 cells ?
Most 18650 cells are LiCo with considerably more capacity and discharge capability than those K2's.
Most current spec 18650 LiCo cells are 2600 - 3400 mAhr and 3.8 V nominal , so around 10Whr capacity.
You need some fairly "stiff" cells for use in a portable drill, most of them use the top spec SonyVT or Samsung cell with high C rating.
The best DeWalt packs used A123M1 cells for power, durability and long life..but low capacity !
..which are still available ..http://www.buya123batteries.com/APR18650M1A_Lithium_Ion_Cylindrical_Cell_p/300030-001.htm


EDIT:
Those K2 18650P cells you found have some big claims made by their manufacturer ..IE:- 21Amp continuous discharge !
However , there is little independent test data to support this, and few comments of anyone using them .
The only test i could find was this from ZEVA..
http://zeva.com.au/Research/K2Testing/
which would suggest that whilst they are able to discharge at high rates ( 10C tested), they did suffer from fast heating which would imply a rather high IR most likely. ?

 

[PStechPaul]

The testing that you provided a link to, seems to confirm that the 18650P cells meet or exceed their specifications for 1C, 3C, 5C, and even 10C. It shows a drop to about 2.7V into 0.25 ohms or 10.8A, from 3.2V, which indicates an ESR of about 0.05 ohms. The drill takes about 2 amps no load and 4 amps under moderate load, so if I use six cells there will be about 20V fully charged, dropping to 19.4V no load and 18.8V normal load. They should be able to provide 10 amps at 14V which is 140W.

The E version cells seem to have higher capacity at lower cost, but performance seemed not quite as good. I offered $60 for the 24 cells and I think it will be good for use in my drills and other tools for this battery pack. I will also be using this project as a test for the BMS I have designed, which will handle charging by means of shunting and continuous battery voltage monitoring using low duty cycle blinking LEDs and also possibly a Bluetooth connection for more detailed status.

The LiCo cells probably have higher capacity at lower cost, but may not be as safe. The BMS can be configured for LiCo, and also for 2S NiZn or 3S NiMH, and a similar circuit will be used for my 12V Lead-Acid pack for the EV tractor. If it works well enough, I might be able to offer it as a product for larger EV use.

I may hold off on the NiZn cells for now. They would have been (probably) a better deal than other LiFePO4 cells at about $5 each, but this deal at half that price is hard to resist.