a088008

I would like to have the ability of adding cells to my plane in sets. Sometime I want to use 10 cells, and other times I want to use 20 cells to extend flight time.

I'm considering putting two packs in parallel. Will this work well, or should I not even bother.

What about voltage differences between the packs (assuming the packs have the same number of cells e.g. 2x 12V packs)?

Does the battery types have any influence? I'm considering using two NiMH packs rated at the same capacity and voltage (cells). They will be identical packs from the same manufacturer, but the cells will not be matched accross packs and will not be charged together. I will be charging each pack individually.

Any thoughts/suggestions/comments?

-Q.

Wyohi

My Feedback: (1)

It will work & be worth the effort if the weight difference don't drag you down. Charging seperate packs is the way to go . You get in trouble when you try to charge them together.

Steve Lewin

As wyohi says it works fine provided you can stand the doubling of the weight and provided the batteries are always at the same state of charge when you connect them together. The minimal voltage differences in fully charged batteries cause no problems.

NiMH or NiCd will be fine. The packs MUST be the same number of cells and should ideally be exactly the same battery type and capacity. In fact it will actually work even if you use dissimilar batteries e.g. 10 x 3000mAh + 10 x 1800mAh as they automatically load share fairly well but it's safer to stay with identical cells.

Steve

Geistware

My Feedback: (16)

So long as the two packs have the same number of cells, then you will not have any problems

a088008

Thanx for all your input.

I'm definately going to try it. The extra weight should be no problem as the model has fairly large wing area and light wing loading and fairly well powered. (I should know, I designed it to take some extra weight if there ever was a need for it.)

I'll let you know how this works out when I get my second pack.

By the way, the two packs I'll be using will be identical pre-made packs of 10 cells (1100 mah NiMH).

-Q.

Matt Kirsch

My Feedback: (21)

Don't worry, it'll work fine from a power system standpoint. So long as you've planned ahead for the extra payload, there's nothing to worry about.

john 8750

It has been stated that two battery sources connected in parallel will provide more power than the mathmatical sum of the two. Different amperage is ok, but must be the exact voltage. Have fun.

jgiles-RCU

My Feedback: (1)

From a power standpoint it will work, but you certainly won't double your runtime as it will take more energy to fly the extra cells. Don't underestimate the additional weight of the batterys. It is definitely a big deal. Especially as the battery packs start to die. What was once enough remaining voltage to easily fly the plane will not be enough due to the extra battery weight.

Weight is why sealed lead acid isn't used and why NiCd are being replaced by NiMh.

Steve Lewin

It has been stated that Mickey is a real Mouse too. Probably by the same people .

Steve

john 8750

And I guess you are a real electronic tech. The theory has been proven, believe it or not, it's true.

a088008

I'm guessing here but is it due to the fact that the combined internal resistance of the batteries in parallel is less than the internal resistance of one battery. Am I correct in saying this?

-Q.

Steve Lewin

No, I never was an electronic tech only a power supply designer for some years.

Still I guess it must be some physics they invented after I got my degree. So what exactly is this theory ? Any references ? Don't be frightened that they're too technical. I think I'll be able to understand them.

Of course it does rather depend on an understanding of what "power" means. So exactly which component of power is greater than the mathematical sum of the two ? Current, voltage or some other factor ?

Steve

john 8750

Guys, of corse we cant add 1 and 1 and get 3.


Steve, it must be some other factor, let me know if you find out.

It could be because of internal resistance, or the lower heat produced by the batteries that are used in paralell.

Steve Lewin

I'm always willing to learn something new and there's a lot I don't know about all sorts of things. Where did you hear about this extra power ?

I'm wondering if you're just thinking about the fact that battery voltage drops if you try to drag too much current out of too small a battery. You get more power if you use a correctly sized battery (or potentially 2 in parallel). It's true but it only makes any difference if the original battery was too small for the application.

Steve

jgiles-RCU

My Feedback: (1)

The total ENERGY is exactly the same. 1 + 1 =2. The only difference is that the amp draw from the batteries is essentially cut in half. The internal resistance of the battery therefore is not as big a factor and less of the batteries energy is used to generate heat (energy). For a low amp draw application, you would not see any change as very little energy would be converted to heat.

Power = voltage*current. Again, 2 packs can supply more AMPs than 1 single pack of the same type, so more total power is available at one time.

bmwbob

My Feedback: (1)

I had been considering the two packs in parallel after my initial tests with the new 1700mah 4/5 AUP NIMH cells.
These things are real SWEETHEARTS! BUT, you need to keep the current below 40 amps. Most places conservatively say around 30 amps or so.
I have run an 8 cell pack in my Kyosho T-33. It starts out around 33 amps, and levels out at 30 amps for the rest of the run.
In another application, I was considering making a 10 cell pack of my old favorite zapped RC2400's ( higher power fun fly plane).
I may now go with two 10 cell packs of the 4/5 AUP cells, as my current draw will not exceed 33 amps per parallel pack.
Basic electronics: The motor sees it's battery as part of a SERIES circuit which it is a part of. Current through the entire circuit is equal at all points ( 30 amps through the motor = 30 amps through the battery.
If we make the battery up from two packs in PARALLEL, the 30 amps total current will be divided ( nearly equally ) in half, with 15 amps through each pack.
I noticed on the discharge charts of certain NIMH cells as shown on the battlepack.com website. that the run time can be extended by about 50% when reducing the current levels.
Granted, real world trials may vary slightly, but it certainly looks encouraging!
Let us know how it goes for you!
Bob Cowgill
WB8CLQ
Cocoa, Florida
E-mail:[email protected]

Steve Lewin

More than a single pack sure. But the contention was that 2 packs in parallel will provide more than *double* the power of a single pack. I'm still interested to see how that is supposed to work.

Steve

john 8750

Steve, my electronics instructor experimented greatly with this theory. It was 1970 and I was in school, USAF. I cant remember why or how it works, but was amazed. The two batteries lasted a bit longer. Could this theory, if it works, be of interest to E fliers? Regards and happy flights, John

bmwbob

My Feedback: (1)

Here's a bit of off-topic info: In 1970, I WAS an electronics instructor in the Air Force! :-) Good ol' Keesler AFB in Biloxi, Ms.
As to how two packs could have a longer run than one of double capacity cells, that could well be related to decreased loss from heating due to the lesser current levels through each individual pack.
So, if expressed as watt-hours of "power", I guess this could be right.
Try it and tell us how it REALLY works!
Bob Cowgill
Cocoa, Florida
E-mail:[email protected]

Steve Lewin

Sorry, we're obviously talking at complete cross purposes here. Probably my fault as I tend to forget that people using technical terms often don't use them entirely accurately.

Just for information, POWER is measured in watts, typically V x I. It is an instantaneous measurement, not associated with time. It doesn't "last a bit longer".

If you're saying that, WITH THE SAME LOAD, 2 batteries in parallel will last longer than twice the time one battery lasts then that's completely normal. It's also better ENERGY (or battery capacity) not power. The energy available from a battery is dependent on the load, to a degree. The higher the load the lower the total energy available from the battery. 2 batteries will each have a lower load so each will deliver a higher proportion of their theoretical capacity. If I'm right it should only work for fairly high loads. E.g. if the original battery was going to last 6 days then you wouldn't see this effect. If the original battery was going to last 6 minutes then you might.

Steve

alexandre

Hi,

To connect two batteries in parrallel we shoud use a diode to avoid the highest voltage battery to discharge in the lowest voltage battery.

Hard to find a diode that can handle 50 amp and you will lost 1.6 volt due to the diode.

Is anyone really know a safe and efficient way to connect 2 batteries pack in parrallel ??


(Sorry for my bad English)

PullUpPULLUP!

Would lithium ion polymer batteries also be OK to use in parallel? I bought an E-starter by GWS over the weekend as my first entry into electric flight. The LHS had nice, tiny, incredibly light lithium pack that was rated at 8.4V and 950mAh. The problem was that it was clearly labeled "Do not exceed 4A discharge rate". The E-starter uses a 300C motor with an EP1080 prop which according to the GWS website can draw up to 7.8A at full bore. I figured it wouldn't be safe to use. It didn't occur to me until I got home that if I used 2 of them in parallel it could discharge up to 8A safely in total, would essentially double the capacity, and would still weigh less than a NiCd or NiMH pack.

Would this work?

Thx,
Jack

john 8750

No, you dont need a diode. But the packs should be the same voltage. Should work the same for Lithiums.



Regards from John Smith.

alexandre

Hi,

I heard that lithium batteries are ok for rx and tx but not for electrics motors because if the motor drain too much power, the electronic circuit that come with the lithium battery packs, will cut the power to avoid explosion due to hight temperatures.


It is ok to cut the motor but not the receiver.......!


(sorry for my bad english)

Steve Lewin

Alexandre, I'm afraid you're a little out of date. There are now many Li-ion and Li-poly cells being sold specifically for use as motor batteries. Mostly this is still for relatively small motors (up to about 10A current draw). These cells do not have the protection circuits you are referring to.

Jack you can certainly do what you suggest. It is very common indeed to use Li-poly cells in parallel to get higher current draws. The characteristics of Li cells is such that it is also safe to charge them in parallel. You must always a charger specifically designed for Li cells though. An ordinary NiCd charger will kill them in no time.

Steve