PDA

View Full Version : Battery Voltage and Current Draw


raognv
09-30-2005, 02:42 PM
If we increase the number of cells in a battery pack how would that affect the current drawn by the motor. Does the current draw increase are decrease? Can a 7 cell Ni-Cd battery pack be replaced with a 7 cell Ni-MH battery of the same Ah capacity without any loss of performance?

rcers
09-30-2005, 02:48 PM
Yes more cells = more power draw.

That is when you don't change anything else, prop or gearing.

NiCads usually offer just a touch more voltage than NiMh, but that gap is about gone with good high quality NiMh cells. For example the old 500 AR nicads can't match the voltage of the new GP Nimh 1100 cells. So twice the capacity at higher voltage! A good thing.

NiMh usually are a tad lighter too.

So buy good high quality NiMh cells are and you good to go with the same number of cells.

Mike

raognv
09-30-2005, 03:16 PM
Thanks Mike.
Could you please tell me what is the safe discharge current of a 7 cell 2000mAh, 4/5 SC , Ni-MH battery pack. I intend to use it in a 600 size motor powered Electric Sailplane. Is it a good choice or not.Please advice

Rao.

rcers
09-30-2005, 03:17 PM
It all really depends on the name brand of the cells. Do you know what they are, or where you got the pack from?

Mike

raognv
09-30-2005, 03:38 PM
It is a Great Planes 8.4 volt, 2000 mAh, Ni-MH custom pack I purchased from Tower Hobbies and the model is Great Planes Spectra EP Sailplane powered by Great Planes S-600 motor direct drive with a 30 Amp ESC.

Rao

rcers
09-30-2005, 03:54 PM
I don't know who's cells they use, but they are likely pretty good.

I suspect you will be fine with at least 25amps likely more....

Mike

GeraldRosebery
09-30-2005, 09:38 PM
If we increase the number of cells in a battery pack how would that affect the current drawn by the motor. Does the current draw increase are decrease? Can a 7 cell Ni-Cd battery pack be replaced with a 7 cell Ni-MH battery of the same Ah capacity without any loss of performance?

Think of it like a pipe and water. Increasing the pressure (voltage) and the flow through then pipe increases too. BUT - with electric motors you can also manage flow (current) by modifying the prop. Unlike glow engines which go ever faster if you reduce the load, the electric motor limits rotational speed depending on the internal wiring. SO - if the motor has a specific KV (rpm's per volt) it will never turn faster than that even with no load. What does this do for you? WELL - practically the effect is that you can increase voltage and get the same current if you reduce the load on the motor. For the same voltage, the current will be less say for a 9x5 than for a 10x6. The 10x6 increases the load on the motor and increases current draw. You can use an oversize motor for a model and keep power under control (that is limit current) by changing to a smaller prop. Sounds wierd, but that's the way it works.

raognv
10-02-2005, 08:23 AM
Thank you guys for your response.

Rao

hoppy
10-02-2005, 01:12 PM
Check the battery performance graphs at this site. You'll get some idea of battery current delivery capabilities.
http://flydma.com/flydmahome.asp