Quick question
 

I want to increase my run time by using two batteries, can't remember if I need to connect them in parallel or series?

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You need to connect them in parallel to increase the capacity, and voltage will remain the same.

Yes, in parallel. But, that will not necessarily double your flight time. The increased weight will have some deleterious effect; more sluggish response, faster speed required on landings etc.

Just to clarify, if you are placing two battery packs in parallel, they absolutely must have the same cell count. Example, connection of a two cell LiPo in parallel with a three cell LiPo will cause damage, smoke, and perhaps worse results.

Putting two battery packs in series could burn everything up in your model, such as the speed control and motor. That's similar to connecting two 12 volt lead acid batteries in series in your car, running it on 24 Volts DC. That would be quite expensive to fix all the resulting damage.

It depends a lot on the plane. Something like a Telemaster Electro going from 3S3300 to 3S2P3300 you will definetly double you flight time. With the same ESC/motor/prop you will cut the discharge rate in half for each pack letting you get a bit more capacity out of them.

Computer programs such as www.motocalc.com will provide a lot of useful information about trying the different batteries. This program takes into account the weight of the complete power system, model weight and everything else. If the model weight results in the wing loading getting to high, motocalc will warn you.

Motocalc is free for 30 days, then $39.

Double battery weight & capacity (vs norm in a typical sport model) tends to give 75% more flight time.

Not a bad deal.

The added weight is often helpful for dealing with gusty winds. But it can adversely affect aerobatics and glide ratio.

Note also the added weight can cause the wing to need reinforcing. That extra weight X 10 G (very common) to 100 G (has been exceeded by models!) can snap a wing off in a snap-roll.

That is a very good point about the added weight of the battery pack versus the strength of the model and it's wings.

As for the number of "G's" pulled by your models, I did some testing on that very thing a few years ago. And found a simple loop on a giant scale model was pulling nearly 10 G's. Didn't try any violent acrobatics, that model was not designed for it.

Take a look:
http://www.wattflyer.com/forums/showthread.php?t=47288

AMA Pattern Aerobatics competitors put an Eagletree logger in with G sensor and exceeded its 38 G max.... about 10 years ago.
Pattern is not as stressful on the aircraft as IMAA/IMAC competition can be.

Calculations on "Dynamic soaring" models say they commonly exceed 100 G... for a sailplane. There is no sensor I know of that has been made that can fit in common model aircraft and can measure this yet.

Yup, there are. Sensors for 100 G's are readily available for $$$$. The bad news, is you have to design your own display for it. Or design a digital recorder for it. I've done this, it's a lot of microcontroller software work.

Take a look:
http://www.mouser.com/Sensors/Motion...xgd7?P=1z0jl3t

If higher currents are needed and larger cells with increased ampere-hour (Ah) ratings are not available or the design has constraints, one or more cells are connected in parallel. Most chemistries allow parallel configurations with little side effect. Figure 3 illustrates four cells connected in parallel. The voltage of the illustrated pack remains at 1.2V, but the current handling and runtime have increased fourfold.

No. it doesn't quite work that way. the power needed to keep a plane in the air is directly proportional to the weight of the plane. If you double the size of the battery you might add (for example) 25% extra weight. That means you need to use 25% extra power to fly the plane.

So in this hypothetical case you increase battery capacity by 100% but you increase rate of discharge by 25%. The net result is an increase in flight time of 75%. It may be a little better than that in practice because lower discharge rate means the batteries deliver a few more mah, but that rarely makes up for the added weight. Then you have the negative effects of decreased performance and handling caused by the added weight.

Well... power to maintain flight is not quite linear with weight. A lot of the drag of an aircraft has nothing to do with the wing.

In my stating that doubling battery tends to give 75% more flight time... the total aircraft weight tends to have been increased by somewhat more than 25%.
The results actually vary a lot. Something slow gets less of a % increase in flight time than something fast since the faster plane is dealing with a higher squared "power needed to increase speed" factor.

I think you will find that it is. This is because the plane needs to fly faster, which increases all of the drag from sources.

Quick note:
1.2 Volts per cell suggests either the Nicads or Nickel Hydride cells. IMHO, it's not a good idea to directly parallel connect these types of cells. They have what's called "Negative Temperature Coefficient" which is where as the temperature of the battery increases, its output voltage drops. That's how those peak charger work with Nih chemistry.

Problem is, these cells can discharge into each other. Before retiring, we've had customers put our 24 VDC Nicads directly in parallel, and had them go into thermal runaway. Destroying both $$$$ batteries.

We had to deny their warranty claims, making for very unhappy customers.

We typically don't fly at the minimum speed required to maintain level flight... Usually a significant amount faster.

If my failing memory is correct, doubling the speed of the model requires 8 times more horses. And three times faster is 27 times more HP.

Guess that's why those $$$$ wet turbine models that can hit 200 MPH guzzle jet fuel at a higher rate than that big old motor home we used to have that got 7.5 MPH on the highway.

I agree, but if we look at the most efficient airspeed for the model (where it uses least power) it will be faster for the heavier model.

But you have a point....if you fly around at full throttle all the time then I agree, power consumption will be very similar.