6 and 7 cell batteries

[jgezing]

Hey,

Will an 8.4 v 550mah battery give a longer flight time than a 7.2v 550mah battery or just more power? What about charge time? Since they both have the same capacity, will charge time be the same? Both of these packs are NiMh.

Thanks.

[flyingbuddy]

Depends. If the increase in power more than offsets the increase in weight, then you may get longer run times, if you throttle back because you increased total capacity in watt/minutes by 17%. If you use all of the power available all of the time, then no. The increase in current for 7 cells versus 6 cells will decrease the run time for a given capacity (550mAh.)
If your charger is a constant current type, like the Sirius Charge 100 (http://www.siriuselectronics.com/sc100.htm) or Bob Selman charger (http://bsd.domainvalet.com/) then the charge rate and time are the same for different cell counts.
If it is not a constant current type, then the pack with more cells will take longer.

I hope this is not too confusing.

[jimwalker]

John,

Please explain more about extra cells increasing current. I know that more cells in series give you more volts, but I thought the current was determined by the internal resistance of the cells and the impedence of the motor.

Are you saying that the increased voltage (pressure) allows more amps (current) to flow through the circuit?

Thanks, Jim

[genebond]

The current draw will be higher running 'flat out' on 7 cells (than 6), due to the fact that the motor (thus prop) speed will be higher. The power required by the prop (amps) is exponentially proportional to speed (cube law), while the velocity is a flatter curve (square law).

Impressive, huh?

Really, just remember that as the speed goes up, the power required goes up faster.

So what John is saying is that if you are running the same speed, the total amount of energy in the pack is higher with 7, therefore, it will last longer. But, if you run faster with the 7 cells, chances are good the flight time will be less... :eek:

[jimwalker]

Duh.....

I reread my post and I pretty much answered my own question. More voltage creates more current and more RPM which drains the pack faster. What was I thinking?

Thanks Gene.