LiPoly Cutoff & Jeti Controllers
 

Hi,

I don't know if this question has come up somewhere in this forum, or not, but I think it should. It concerns the Jeti Advance motor controllers distributed by Hobby Lobby & their use with Thunder Power packs. I have both the 8 & 4 amp Jeti versions & according to the specs with each, the units sense the cell count upon power up & then calculate the cut off voltage as a multiple of 2.56 volts per cell. Most of the preceding discussions here & elsewhere , including FMA's instructions, have identified 2.56 V/Cell as a safe cutoff threshold & I have manually programmed that value with Castle Creations controllers in the past. Along come the Thunder Power cells, also sold by Hobby Lobby & others. In the accompanying instructions is found a very emphatic statement that they must never be discharged below 3.0 volts under load. They say that 2.9 is no good, only 3.0 & above or the sky will fall! In response, I have swapped out the Jeti units in favor of the CCs & programmed them for 3.0 volts/cell. (i.e. 6 & 9 volts for the 2 & 3 cell packs I use.) Happily, I have found that the Thunder Power batteries have a low enough voltage sag under high current discharge that even with the higher cutoff threshold I still get a satisfying motor run. The FMA 340s & 700s seem to require the lower cutoff setting to give a decent run but are not supposed to be endangered by it.

I assumed that the chemistries of the different brands of Li Polys are pretty much the same, so why the different cutoff requirements?? Must I only use the Jeti Advances with batteries other than TP or is the TP warning just a legal bluff to ward off potential liability. Also, if the TP position is correct, how can Hobby Lobby, in good conscience, sell combinations of batteries, motors & controllers which clearly violate the TP instructions??

Richard

RE: LiPoly Cutoff & Jeti Controllers
 

This whole business of lipo cut off voltages is very vague it seems to me.I have also come across this 2.5v/3.0v contadiction
As I understand it,3.0v is the cut off value for no (or very little) current. 2.5v allows for the voltage drop under high (motor on)current conditions,to allow for the voltage drop caused by the I R of the cell.(I stand ,and am prepared, to be corrected)
Actually I have not found this to be a problem,as by the time 3.o v is reached ,the power,and perfomance drop off encourages one to land anyway.

RE: LiPoly Cutoff & Jeti Controllers
 

Apogee says it will not warrenty any of their batteries when used with the Jeti 08 controller. They also have a list of approved chargers on their website. Apparently their batteries are not as robust as others in terms of LVC.

RE: LiPoly Cutoff & Jeti Controllers
 

As with ANYTHING, you will get contradictory information. Early on in the LiPoly revolution, 2.5 Volts per cell was considered to be the minimum voltage to which you could safely draw your LiPoly packs. This was found to be untrue. While you could get away with it MOST of the time, the extra stress put on the batteries caused some cells to become damaged, leading to unbalanced packs, overcharged cells, and battery fires.

With this experience, the limit was raised to 3.0 Volts per cell. Since then, the number of fires and problems with LiPolys seems to have steadily decreased, even though LiPoly use has grown by leaps and bounds.

The 3.0 Volt per cell limit is actually a hard limit. The cell should never go below 3.0 Volts, whether sitting idle or under full load. If you have to reduce the cutoff voltage to get decent flight times, you're overstressing the battery, and asking for trouble. Get a more capable battery, or reduce the load on the existing battery.

RE: LiPoly Cutoff & Jeti Controllers
 

Several of the Jeti controllers have min cutoff voltages below 3 volts per cell, including the Jeti 08. If the 3 volts is a hard limit, everyone with a Jeti controller should trash it and get a different one. Also, if the 3 volts is a hard limit never to exceed, you should add some margin for error in voltage measurement . A rule of thumb is that the measurement error is one-half the scale readibility and assuming the voltage is measured to the nearest 0.1 volt, then the low voltage cutoff limit should be set to 3.05 voltage to ensure the voltage cutoff is 3.05 volts +/- 0.05 90% of the time. Similar margins should be set for the high voltage during charging.

RE: LiPoly Cutoff & Jeti Controllers
 

Hi,

I notice that Hobby Lobby has just introduced a newer version of their Jeti controllers which appear only enhanced by a separate programmer capability. They don't mention any revision of cutoff voltage/cell. Once again I ask: How can they sell these in conjunction with Thunder Power packs???????

CFIMEI

cc: My Attorney

RE: LiPoly Cutoff & Jeti Controllers
 

Hobby-Lobby said the new programming module for the Jeti controllers would allow a high voltage cutoff of 3 volts per cell or a low voltage cutoff of 2.7 volts per cell.

RE: LiPoly Cutoff & Jeti Controllers
 

Hi,

That's great but when I mailed them about returning the Jeti o4A that I got about 5 weeks ago it was "tough luck, chump" time. No mention of trading my dangerous version for a new one & charging me their cost. The new versions can, I think, only be set to the alternate higher cutoff by means of that stupid little programmer that you also have to buy! I would prefer to buy American where possible & will standardize on the CC's which have been good.

CFIMEI

RE: LiPoly Cutoff & Jeti Controllers
 

You do not have to give up on your Jeti controller or any other ESC for Li Pos. See [www.fmadirect.com] Enter AVC1AIR in the Inventory Search Box to review the new universal cut off that connects in line from your receiver throttle channel to your ESC. AVC1AIR can be programmed to interrupt the throttle channel to force motor cut off at whatever you program up to 3.3V/ cell.

Now, lets remove the confusion about setting cut off. I can not vouch for any other cells, but Kokam cells in loaded dynamic, state can be used safely down to 2.5V cut off and will recharge OK under those conditions. However, here is the important thing: if you do that regularly, then you have a greater chance of the pack developing unbalance. If cells are discharged at low rates, taking them down to 2.5V in that more or less, "static state", (vs a dynamic fast discharge) is deadly. Please read the section of the FMA Direct App Note under Support at the above URL and check out the graph called "Let them live". Now, remember and heed one other item truly important to those who don't like throwing away money (and that includes old watoshi!) If you charge the pack to no more than 90% capacity (i.e., 4.15V instead of 4.2V) and cut off at < 80% depth of discharge (approximately 3.3V you will make your Li Pos cost about half as much to use over a period of time as you nearly double life cycle! You will still have available almost 90 % of the capacity for useful power. Go to the next bigger cell capacity if you need more flight time instead of pushing the cell harder. I think the old adage is "penny wise, pound foolish".

One other tid bit: if you have discharged a Li Po below 2.5 V, watch to see if it rebounds to >3.3V when the load is removed. If it does, no harm has been done, though it may lose a little capacity. If the cell rebounds to < 2.7V , sayonara! Learn how to unseal, soak in salt water and dispose of it as described in the Ap Note.

Finally, a new product we call Cell Pro will arrive in Feb/Mar that monitors each individual cell in a pack to cut off motor power when the first cell in a pack reaches a cut off voltage that you can set. Under those circumstances, no cell in the pack can get driven below 2.5V and the 2.5V cut off can always be used.