Greg Covey

Kokam has released their latest Lithium cell which was built upon the incredible 340mAh, 20C discharge design. A 2AH (2000mAh) cell that delivers 20C or 20 times the capacity in current. This is an incredible 40amps!

The cell is designed for medium and high current applications. An extra plate is used in the design that allows for such high current delivery but also adds some extra weight. The 2AH cell weighs 66 grams.

Shown in the photos are the pre-factory 4s packs that are being used for third party verification of the specifications. The cell can deliver 2220mAh at 0.5C discharge and 2000mAh at a 20C discharge rate. The cell retains an incredible 98.5% capacity at 20C!

An extra connector on the pack permits access to each cell for a new cell-balancer module or just periodic monitoring with a voltmeter. The new cell-balancer will work for packs up to a 4s5p configuration.

Kokam USA will soon release these cells for sale in various pack configurations. Additionally, the new "unitized" packs will be available for people that prefer a single pack configuration instead of using the Kokam Parallel Connector Modules.

The second photo shows a 4-cell 2AH Kokam pack next to my 14-cell Sanyo 2600mAh NiMH pack. While these packs have a similar operating voltage and both can deliver 40amps, the 14-cell pack weighs 32.5oz and the Kokam pack weighs a mere 11.0oz. This is an incredible 65% drop in weight for a 40amp application with only a 25% drop in flight duration!

Put two of these 4-cell packs together in a 4s2p configuration and you can deliver up to 80amps, increase the flight duration by a factor of 1.5, and still have a weight drop of over 30%.

The e-flight revolution continues with incredible innovation from Kokam!

bassmanh

My Feedback: (5)

morning Greg,

any idea what the cost of these new packs will be ?

and can i use these packs like my current nicad and nimhs in seires EX... 2 of these kokam packs in series to make 14.8volts with about the same MAH rating of my nicad at 2400 and expect the same run times ? or would they not be able to give the amps i need for the AXI motors ? these sound like great packs how long before they will be on the FMA website ?

thanks for the update.


bassman

Greg Covey

Hi Mitch,

I am not aware of the pricing structure but believe that these new cells may be more cost effective because you'll need far less of them in parallel for outstanding current delivery.

What we'll see is a similar effect on flight duration as we did with the Kokam 340 cells. Do to their high current delivery, the effective flight duration is much higher than you would expect from the given capacity. I remember first flying my T3D with a Kokam 340 pack and was very surprised at the duration. This was because I was only drawing up to 2amps on a cell that can deliver 6amps.

Once people return from the Toledo show, we should get some answers to your delivery questions. I'll keep you posted on the verification results.

frederm

66grams for a 2000mah cell is heavy.

our 2600mah Lipo cell is 52 grams.

hmmm..

Mike

jonnyjetprop

My Feedback: (3)

Greg,

The big question I have is when will we see better than 1C charge capibility. A 20C discharge does not help me because I lose flying time before I have to recharge. This will still take 1 hour plus, so I'm back to having several packs to keep my planes flying. This in addition to the increased per cell cost is a problem.

John

Greg Covey

Mike,

As previously mentioned, an extra plate is used in the design that allows for such high current delivery but also adds some extra weight. Can your 2600mAh Lipos deliver 40amps continuous like these cells? What would be the cost and weight of doubling up your 2600 cells to deliver 40amps?

I think the freedom to drop over 60% of your battery weight in certain 40amp applications is revolutionary!

John,

It's a good question but it is not limited to Lithium cells. Like running a motor beyond its current limit, we trade off longevity for performance. This can be done on battery cells as well. Since the Kokam 340mAh 20C cells can be charged at 2C or even 3C, perhaps these scaled-up versions can be too.

Repeated charging at rates beyond 1C will slightly decrease cycle life and usable capacity over time, the recommended charge rate is 1C although occasional 2C and 3C fast charge rates are ok on the 340mAh cells.

cafplanekid

My Feedback: (1)

So, how much do they cost?

Untill they find a way to drop the weight some more, they dont look all that attractive to me, over the Irate cells. I believe the newest irates come with much thicker tabs to allow even the 2600 cells to deliver 15 c without melting the tabs. If this is true they will deliver 39 amps burst, 26 amps continuous. A 2p pack of 2200 cells will weight harldy any more than a 1p pack of these new cells, will handle quite a few more amps burst, and will have much greater capacity. Plus, you can use a 1p pack of 2200 cells for smaller electics, or parallel a few packs for larger electrics, much greater versatility.


'I think the freedom to drop over 60% of your battery weight in certain 40amp applications is revolutionary!'

Im going from 10cell CP2400 packs to 3s2p Irate 2200 packs, and have a 50% drop in weight.

genebond

It all comes down to energy and material density...

These new cells, like the 340's, are heavy for the mah. But, when compared to 10C cells for high current applications, they start to look good.

Just some off the cuff numbers, but the 2000/20C cells are about the same weight as what a 3500/10C cell would be, if it existed... Much like the 340/20C cells are similar in weight to a 500/10C, or thereabouts.

So, it comes down to how your plane needs to be powered. If you want a 6 minute full throttle flight, you need the 20C cells. If you want a 15-30 minute flight, the 10C cells are the right ticket.

Hopefully they will continue to manufacture both the 10C, and the 20C cells, so we have this choice. I'd hate to lose the lighter cell choice, for a plane that doesn't require 20C.

As far as the speed of charging goes, seems the new Astro 109 has the current hot ticket. With the new algorith they have developed, they seem to be able to charge at 1C in about 1.5 hrs, vs 2 hrs for a CC/CV charger. I wonder what it would do at 1.5-2C...

cafplanekid

My Feedback: (1)

Good point. These are a good solution if you just need to drop alot of weight over nicads and dont mind giving up alot of flight time. I still want to know the price, though, i have a feeling it aint pretty

Greg Covey

Assuming our specs on the Irate and Kokam cells are correct, to replace a 10-cell CP2400 pack, you can use a 3s2p Irate 2200 pack, and have a 50% drop in weight with bursts to 39amps or use a 3-cell Kokam 2AH pack for a 60% drop in weight and have 40amps continuous current delivery. You can start to see where the 20C cells may have a prefered application in high performance or competition models. It will be interesting to compare pricing.

Here is another setup upgrade that I am testing on my new Skylark e-conversion from Hobby Lobby. The 16-cell, 27oz, CP1700 NiCd pack is replaced with a 22oz, 4s2p Kokam 2AH pack that provides a 4AH capacity at 80amps.

This is a 5oz drop in weight which provides over twice the flight time at a voltage that will remain relatively constant over the flight. My Skylark draws only about 40amps at full power so it should be and interesting test. What we have seen with the 340mAh 20C cells is that the actual duration is much longer than expected when not coming near the current delivery limit.

genebond

I agree 100%, Greg. Your examples are textbook for these cells.

The issue of the 340's at lower draws, I think has been found to be a little shy of the same weight 8-10C cells. I could be mistaken, but I remember an early debate on wether thay were actually 500mah cells relabled. I don't recall anyone able to show anything over maybe 380-400mah at 5-10C, but 320-340mah at 15-20C.

I think it may be like the chemistry changes in Lead Acid... A cranking battery has very high current delivery capacity, but not a super low draw capacity, where a deep cycle battery of similar size and weight has no where near the cranking amp capability, but a much higher mah capacity.

Seem like a good analogy?

Greg Covey

It sounds like a reasonable analogy.

Another perspective is to look at the scope of things. Just like there are motors for a particular sub-set of power range, batteries can cater to a specific application. Other factors like favorite brand, cost, and complexity muddle the choices for the user.

I am testing a 4s2p configuration of these new cells in my Hobby Lobby Skylark which uses an AXI 4120 motor. I am also testing a single 4-cell Kokam 2AH pack in my new Graupner Extra 300S that uses the same motor. This may prove to be the first .60-size plane flown on only 4 Lithium cells. It should be an interesting test flight.

bassmanh

My Feedback: (5)

hey all,

well was just at FMA directs website and they have almost all there cells on sale. so im hoping that means the new cells and packs will be out before to long. cant wait to see the new packs and what the prices will be.

Greg,

also please let me know right away after you fly your planes with these new packs and how they performed for you. as you know my set-up is alot like yours and if they work for you they should work for me to . any news about the prices yet ?

keep us up to date on these new cells.


bassman

Greg Covey

Mitch,

There has been some suggested pricing set but we'll have to wait a bit before posting it.

I did, however make some measurements on both planes using the new Kokam 2AH packs. On my Extra 300S that uses on a single 4s pack, the voltage dropped so that I was only drawing about 30amps on my AXI 4120/14 using an APC 14x7 e-prop. This was due in part to the load on the cells and in part due to the lower voltage compared to my 16-cell CP2400 pack measurements. My fix was to compensate for the drop in voltage by increasing the prop size to a 15x10 e-prop. This drew about 40amps and feels like it is worth a test flight to see how it performs.

My empty Extra 300S weighs 74oz so it is RTF at 85oz with the 11oz, 4s pack. This is 5oz under the glow flying weight!

My Skylark measured about 40amps using the original 14x7 e-prop so I will test fly it as is. The voltage of the 4s2p Kokam 4AH pack was slightly less than the original 16-cell, CP2400 pack but I do not feel that I have sufficient ground clearance to increase the prop size. I will likely bring a spare 15x8 prop to the field just in case...

Matt Kirsch

My Feedback: (21)

I am waiting for you and your test pilot to get that Extra in the air. If you can fly a 5+ pound .40-size plane with any authority on a 4S1P pack of LiPolys, electric has truly arrived! Heck, even if you have to go to a 5S1P...

Greg Covey

Matt,

Check out my thread in the glow conversion forum for my CG issues on the Graupner Extra 300S using a 4-cell Kokam 2AH (20C) pack.

We flew the Skylark several times yesterday using an AXI 4120/14, 14x7 e-prop, and a 4s2p configuration of Kokam 2AH cells for a 4AH duration. We never did fly it until the pack dropped power level as we came back in around the 10-minute mark. As anticipated, these 2AH cells seem to provide real lasting power when not delivering their full rated current...much like the smaller 340mAh cells. My setup was drawing about 40amps at full throttle which was needed only for short bursts of 10 seconds.

I felt the packs after each flight as I cautiously test them and they were just barely warm. These packs are made to take 40 amps. The tabs on these cells are huge! The tabs are at least three times the thickness of the Kokam 1500 cell and measure almost 0.5" X 0.5 ". Plus, Kokam USA is using a compression interconnect that takes adavantage of that full 1/2 square and has no interconnect longer than about 3/8 max and that is 1/2" wide , made out of 4-ounce/ft square phosphor bronze.

Greg Covey

Here is a photo of the new Kokam 2AH cell tabs. The tabs measure 1/2" wide and almost 1/2" long. The new compression technique requires no soldering to take the pack apart or change from a 4-cell to a 3-cell configuration...very cool!

The 50amp burst current specification is still being verified but I have pulled 43amps on my AXI 4120 motor without seeing any drop for about 10 seconds.

Greg Covey

A 3-cell, Kokam 2AH pack provides my Pigi 3D with 174 watts/lb using the stock PJS100 motor and APC 11x4.7 SF prop.

The setup draws 32amps (320w). The plane weighs 20.4oz and the pack is 9oz so I am RTF at 29.4oz (1.84lbs). That's 174 watts per pound! [X(]

cafplanekid

My Feedback: (1)

Cool. What is the 'compression method'? Are you just clamping the tabs together?
Jonathan

Greg Covey

Jonathan,

The cell tabs are compressed between pieces of circuit board material and 3 plastic screws and nuts. The phospohor-bronze clips can have crimp fittings on them for the sense wires and the pins for the connector can be crimped. No soldering, instant custom assembly, and a pack that is rugged as a brick.

My version is a pre-factory style that will be replaced with stuff molded in a lightweight, phenolic balloon-filled plastic. The factory process will improve upon the technique shown here but you'll have the same flexibility and end up with a pack meant to sustain 40-50amps.

Peter Khor

My Feedback: (4)

Any updates on when the packs will be available to public? iirc they were suppose to be out on the market already a few weeks ago? Also, in your testing have they held up to their 20C claims (long term test are out of the question I suppose)? Not too happy with some KoKams that I got previously, in which they de-rated the cell's discharge capability AFTER getting onto the market.

c-grain

Gerg, I was wondering if it's possible to get on a waiting list somewhere to buy these cells since I have a bad feeling they will sell-out all over the net very fast. As with all the really good stuff almost always does. Also have you heard about the possibility of a 1AH cell@20c in the near future. It seems an illogical jump to go from 340 to 2000 in 1 step. The lighter cells and lower current requirements of a vast amound of craft out there today seems to call for somthing like this.
PS: Whats up w/Hobby Lobby creating a monopoly on AXI---I feel like boycotting them -the greedy bas@#%$!*** Rich K.

Matt Kirsch

My Feedback: (21)

c-grain, I can guarantee you that if a waiting list exists, it's already so long that the initial shipment won't be able to cover it. However, I am more inclined to bet that FMA will put their initial stock up on a first-come, first-serve basis, and there is no waiting list.

As far as Hobby Lobby, I suspect that they simply got the exclusive distributorship in the USA on AXi motors. If you boycott every company that has an exclusive distributorship on a product in the USA, you may as well give up the hobby! Nearly all hobby products are imported by a company that has exclusive distributorship rights on that product or brand.

Exclusive distributorship is NOT a monopoly. It simply means that if you want to sell AXi motors, you have to buy them through Hobby Lobby. For dealers, it's advantageous. They don't have to deal with the rigamarole of importing them directly from the Czech republic.

c-grain

Matt, Please don't misunderstand me, It's not the exclusive that bothers me-it's the fact that they raised their prices across the board on all AXI motors by a BUNCH! This may set an example to the other dealers to raise their's as well. In the 40 or so years I've been in this hobby, I dont remember any of the big Companies doing this--usually the prices remain fairly constant or come down in time.

Greg Covey

My guess is that Model Motors in the Czech Republic raised their prices and it just trickled down to us. Hobby Lobby is the exclusive US distributor for the motors but you can purchase them at many places like Atlanta Hobbies, New Creations, and NorthEast Sailplanes to mention a few.

No word on either the testing results or availability. I'll post back here when I hear anything and keep doing my "real world" testing to see if any quirks show up.

I was pretty happy to draw 43amps on my single 4-cell pack. Unfortunitly, I could not easily balance my Graupner Extra 300S so I abondoned that effort and used a 4s2p configuration which added another 11oz. My Pigi 3D setup draws in the low 30s for current so that will be a good test for how the duration holds up on my 3-cell pack. The 4s2p pack on my AXI 4120/14 with an APC 15x10 prop draws around 50amps on my Extra 300S. I'll shoot some new video when I test fly it.