Glow to Electric Conversions Discuss glow/gas conversion to electric here.

So let's see if I've got this right.

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Old 01-05-2005, 07:04 PM
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kf6gub
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Default So let's see if I've got this right.

Let's say you have six lipol batteries hooked in a series. That increases voltage 3.7 times 6 cells. Right? Now, if each cell has 2000mah, that number is not multiplied by six, the number of batteries. Right? Yet 3.7 x 6 cells = 22.2 volts. According to Matt Kirsch, one of the moderators, the 22.2 volts pushes out more amps. Right? So 6 lipo cells hooked in series still push no more than 2 amps x10c (12c, 14c, etc.). Right? Therefore 22.2 volts push out the amps but no more than 2 amps x 10c, which equals 444 watts. How's my thinking? Thanx. Jim
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Old 01-06-2005, 01:24 AM
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Default RE: So let's see if I've got this right.

If you change nothing else, increasing the voltage to a motor will cause the current draw in increase.

A 6s pack is 22.2 volts (3.7x6 is 22.2)

Putting any number of cells in series does not increase the capacity of the pack. It's still 2000 mah in your example.

The pack can support a certain amp draw depending on the type of battery. If its a 10C pack, then it can support 20 amps, 15C then 30 amps, 20C then 40 amps.

Power is current x amps. 22.2x20 would be 444 watts. 22.2x30 would be 666 watts. 22.2x40 would be 888 watts. BUT........(and there's always a but) power is the voltage the motor sees. When you increase the current draw of a lipo, it tends to hold about 3.5 volts per cell then drops down at the maximum current draw of the battery. Therefore, if you hook up a wattmeter, you tend to see slightly lower power outputs.

Hope this helps.

John
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Old 01-06-2005, 10:48 AM
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Default RE: So let's see if I've got this right.

John, it does help. Thanx again. Jim
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Old 01-06-2005, 01:20 PM
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Greg Covey
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Default RE: So let's see if I've got this right.

Series and Parallel Cells:

As with NiCd or NiMH cells, Lithium batteries can be wired in series to increase voltage. Lithium cells can also be wired in parallel to increase capacity and current delivery. A combination of series and parallel Lithium cells can be used to create a more capable flight pack for powering larger motors or increasing flight duration. A common designation seen to describe the configuration of multiple cell Lithium packs is the XsYp label where X is the number of cells in (s)eries and Y is the number of cells in (p)arallel.

The voltage of a single Lithium cell is about equal to three NiCd cells. A Lithium cell measures about 4.2 volts unloaded when fully charged and about 3.6 volts when loaded. The cells should not be discharged under 3 volts although it is ok for them to go below this level for short durations which may typically happen at the end of a flight until the ESC’s Low-Voltage Cut-off (LVC) or disconnect circuitry turns off the motor. The capacity of a cell is often referred to as β€œC”. For example, a Kokam 145mAh Lithium cell has a 1C capacity of 145mAh.

The series/parallel diagram shows one pack of four Lithium cells in series and one pack of four Lithium cells in parallel. When the cells are placed in series, the voltage per cell is simply added. The four cell Lithium pack would measure 4*4.2v or 16.8 volts unloaded. Its pack designation would be 4s1p or simply 4s. When the cells are placed in parallel, the voltage remains the same but the capacity and current delivery are added. If the four cells are the Kokam 145mAh Lithium cells then the resulting pack would measure 4.2v unloaded and have a capacity of 145*4 = 580mAh (or 4C). The current delivery capability would also jump from about 700mA continuous current per cell to about 2800mA continuous current for the 4-cell combination. The short term peak current delivery for this particular cell is about double that so our 4-cell combination would deliver about 5600mA for 10-20 second bursts. Its pack designation would be 1s4p or just 4p.

The current delivery capability of Lithium cells is different for each type of cell and manufacturer. It is often a competitive debate among R/Cers and vendors. Like the cell capacity β€œC”, the current delivery capability is also refered to in β€œC”. For our example of the Kokam 145mAh Lithium cell, the continuous current capability is about 5C and the peak current capability is about 10C. This cell was actually among the first high performance cells on the market several years ago.
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Old 01-06-2005, 02:06 PM
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Default RE: So let's see if I've got this right.

So, Greg, let's say that Kokam set you described (1s4p) is doubled (2s4p) in series and no change with parallel issues, how is the amperage affected?
Thank you for your thorough responses. Jim
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Old 01-06-2005, 04:04 PM
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Greg Covey
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Default RE: So let's see if I've got this right.

Jim,

From the Lithium pack point of view, the amperage that it can deliver as a 2s4p pack is the same as the 1s4p pack.

The voltage is different and therefore the power level is different. Remember that Power = volts x amps so that if I double the voltage, I can double the power (or watts).

Ideally, you need to work backwards from the motor specifications. If we have a motor that can handle 10amps of current at a voltage of 10volts, it can handle 10x10 or 100 watts input power. You then select an ESC that can easily deliver 10amps like a 15amp or 20amp ESC. You could use a 10amp ESC if weight was critical and you never really need to use full throttle. In general, it is best not to stress your power system parts.

Finally, we need a Lithium pack that can deliver 10amps of current. Again, we should really design for a continuous 10amp delivery so we don't ever stress the cells. For our example of the Kokam 145 cell above that can deliver 5C continuous current or 725mA, we would need a 14p configuration to supply 10amps.

For 10v to the motor, we can use a 3-cell series Lithium configuration. Our final pack design is then 3s14p when using the Kokam 145 cell.
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Old 01-06-2005, 04:07 PM
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Greg Covey
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Default RE: So let's see if I've got this right.

With the cells available today, it is often possible to select a single Lithium cell that can deliver the continuous current we need for our application. It is then merely a decision on how many cells the motor can handle in series.
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Old 01-06-2005, 04:36 PM
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Default RE: So let's see if I've got this right.

you can get 1000watts in two ways... say you got 6li-po cells 3000mAh each...

6s1p = 22volts and 3000mAh 1000watts can be accived by drawing 45 Amps. Prefered way to go, specially on outrunners

or you can put the 6 cells as a 3s2p pack... 11volts and 90amps needed for the same 1000watts. Less good way to get 1000watts... a geard system handles the amps better, still you want to keep the amps down
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