CGRetired

My Feedback: (1)

Greg: That's an excellent description, and thanks for that. It is thorough and precise.

I wish I could get the same explanation of propellor use with various setup's and why one prop works when another won't. It's not as simple as it is with glow engines. For instance, I can fly my Venus II with an OS 1.20 AX engine using a 15-10 prop and get great performance. I can also jump up to a 16-8 and get similar performance at lower RPM, however, such a change in proportion with E props does not, may not, yield the same performance. In fact, it may just drain the battery while I struggle to keep the thing in the air!

Also, if someone would take the time and write up an explanation of props E props vs. E Slow Fly props, and their applications and usage, differences between them using the same motor/esc combination, that would also be greatly appreciated.

Please, guys, forgive me for putting this in this section but I have asked the same question several times in other areas, using different words, and no one has yet answered the question. Perhaps this is to complex? I just want to fly my foamies/flatties and have fun. So far, that's not happening as I would like it to. 4 minute flights do not cut it for me.

DS.

stdun

Ok, I'm starting to get it, sort of. So here's the deal, I'm converting a combat glow plane to electric for my kids to train on (all EPP, crash ready), and the battery I have is a 11.1 volt, 4200 mah, 12 oz monster that has to be mounted left on center of the flying wing. Which will obviously unbalance the plane left to right. So the question is " how do I wire 2 smaller batteries together to get the same output that I now have with one large battery ". 2 smaller batteries could be installed 1 on each side of center and balance the plane better. The cost is about hte same either way. You can see my other post in the "glow to electric forum", Panther etec.

Thank you.

N1EDM

My Feedback: (2)

I've done a similar thing, STDUN in my Funtana 40 (AXi 4120 motor). I have 2 x 2100mAh 4S packs wired in parallel. I made a 'clip whip' though you could also buy something like this comercially (FMA comes to mind). To do the clip whip properly, you have to be set up to do very, very accurate soldering or you could be in trouble.

The packs are wired such that I can disassemble the packs to charge them separately... from what I've heard, parallel packs should not be charged as a single unit. I use Sermos connectors.

If you need, I can provide a photo.

Bob

N1EDM

My Feedback: (2)

OK Stephen,

I got your email. Here's a photo of what I did...

Bob

stdun

So with this set up you have doubled the mah and left the voltage the same ?

Thanks for your help.

N1EDM

My Feedback: (2)

Yes, that's exactly what happens. Plus, I charge each battery individually - its not a good idea to charge them in parallel. I used 12AWG - 13AWG silicone wire, extra flexible. Using that silicone coated extra-flexible heavy guage wire is very important.

The Y-joint was made with crimp terminals (crimped with a proper crimp tool, not mashed with pliers) then soldered for extra insurance.

I hope that answers all the questions... if not, what other info can you use?

Bob

bnrusso

I want to respond to this as no one has.
yes this would be seen as a 3 cell but.......

The picture shows 2 cell in parallel attached to 2 cells in series.
If all the cells were identical then this would offer no greater flight time.
It would also be difficult to charge since with constant current the 2 parallel cells would probably not get fully charged. Each of the parallel cells whould only recieve 1/2 of their current individually. Theoretically one could say that they only need 1/2 the charge current as their discharge rate was half too.

BOTTOM LINE:This is not a good idea.

N1EDM

My Feedback: (2)

Someone brought up a good point about two cells not being equal to each other. I've got two identical cells connected in parallel. But I can see that, as they age, one might retain more capacity than the other, and consequently the good cell might try to 'charge' the bad cell.

This may seem like over-simplification, but a diode (of the correct size) would solve this problem. Has this been tried? Are there any off-the-shelf gadgets that already do this?

Thanks,

Bob

bnrusso

Attaching cells in parallel is a standard procedure to balance out cells before creating a pack.
Putting a diode in to eliminate the discharge would require 2 diodes since you don't know which cell would be weaker.
Now the diodes would either impede charging or discharging the pack.
The cool thing about lipos is that they do not internally loose charge like ni-xx.
If this were a problem then they would not make parallel packs. And the do.
So there really is no problem to solve and no diodes necessary. But you did have me thinking for a moment.

N1EDM

My Feedback: (2)

My bad. I should have said "Two PACKS in parallel..." They are two 4S 2100 mAh 20C packs.

Sorry to make you type that for nothing, bnrusso.

Bob

bnrusso

No problem,
I have had to use the diode idea for my trailer lights.
My blazer has seperate turn signals and stoplights.
ON the trailer end they are common (stop and turn signals same bulb).
4 diodes kept the stoplights and brakelights from interfering with each other but getting to the trailer lights as one.

With the packs, they will charge to the same voltage and not be a problem unless you develop a bad cell within the pack that shorts internally. Diodes would help to protect from the shorted cell but the resulting possible fire would ruin the pack anyway. On the other side if a pack opens it will reduce the output current but not cause a draw on the other pack as all 3 cells would be open as well.

sputnikxxx

ok... ive read through this stuff... and I understand it but I dont... if that makes sense... I want to fully make sure I understand this stuff seeing as I want to make my own lipo packs cause of course its cheaper but, I dont want to put myself in danger or destroy any cells in the process... I have a thunder power lipo 2 cell 2100 mah 7.4v 15c cont 20c burst 31.5a/42a this is what I have learned so far

to get the correct max amp discharge rate I take the 15c and multiply it by the mah or ah for example 15x2100 = 31,500 which would mean 31.5 amps continous or 20c X 2100=4200 or 42Amps and I have been told that to ensure battery life on lipos I should draw 2 amps below the max discharge rate so example on this one 29.5 amps would be safe for continuous draw...

ok I understand that to more voltage I wire in series -batt1+-batt2+ and for more mah or ah I wire in parallel -batt1++batt2- and im guessing In the series setup I run a neg wire off the neg end and a pos wire off the pos end, and for parallel I connect the neg terminals together and the pos terminals together and then run a wire off the neg and a wire off the pos to hook up to my esc...

I guess what im wanting to know is on my thunder power battery 2100 mah 7.4v 2cell 15c is each pack 2100 mah and they wired two 3.7v in series to get the 7.4v or is each cell 7.4v and they are run in parallel to increase the mah?... im assuming that it is in series due to the length of the batteries, if im correct lipo single cells are only 3.7v and if I think..

I am still confused on how do they determine the mah, and the c rating? when you combine cells does the c rating change also? depending on if you do it in series or parallel?

I want to make an 11.1v 3cell 2100 mah and if I want to include a balancer plug how do I wire that up? I notice on my thunder power there are 4 wires going to the plug which im assuming is a pos and neg for each cell?

EDIT: one more question if I have a charger that can charge up to 3 cells, if I had 4 individual cells in a pack and one of the 4 was parallel with another to increase the 'ah would that pack still be considered a 3cell or would that be a 4 cell?

CFII_Jim

I hope I am putting this question in the correct thread/forum...

I have made several test flights with my new Electrifly PBY and have a couple questions about batteries.

I decided to go with two smaller batteries (they suggest one 3s1p 3200mAh 11.1v 20C. This is also with the recommended motor setup of two rimfire 28-30-950 motors. (Actually I think they recomend the 750, I just checked my motors and they are 950's).

With the initial recomendation of a single battery it makes sense to make a splitter cable to go to each esc. (I am using the 25a silver series esc). Under this setup (one 3200mAh 20C battery), is the 20C and the 3200mah being split between the two motors? I think I can answer this one as Yes. But, now go my my current configuration-

Two 1500mAh 20C 3S1P batterise and everything else the same. One more twist- I have connected each battery directly to one esc each. (Right battery to right esc, left battery to left esc). Do I still need 20C packs in this configration or would something like 10 of 15C work properly? I think I understand the mAh consideration as it relates to run time, but I do not understand when I might create a discharge problem for the battery and create a hazard by drawing more from the pack than it can handle?

Is there a problem running one pack to each esc/motor independently? I have made all my flights this way and do not see a downside except that I must plug in one esc at a time and leave some potential for uncontrolled motor running if I plug in the esc NOT connected to the rx first.

I think that is the end of my long question. My purpose here is to reduce the weight a bit by going to a smaller single pack or two smaller packs. My biggest questions is about the C rating. I am guessing that 20C was based on the fact that the original confiuration was a simgle battery feeding to motors, and since I am splitting the load I might be able to use "half the C rating and drop back to two 10C packs". If nothing else I might be able to save some money on the second set of batteries.

Thanks to all!

mrasmm

what's the amp loading on each of those motors with the props you are running?

80% times 1500mAh = 1.2Ah times that by (the 80% is to account for the MFG's liberal ratings on lipo discharge rates, and also reduce the heat from the battery during discharge, lower rate = same amount of energy dissipated over a longer period of time which means lower battery temps)

15C = 18A total max for EACH motor
10C = 12A
20C = 24A

hopefully that makes sense. Also depending on how you want to do it, I dunno if you are running the BEC from the motor ESC, but alot of people recommend that you only hook one of them up to the receiver, that would make it a bit more complicated with your setup, but it might also be something you want to look into.

GL =)

CFII_Jim

I need a load/watt meter to measure the amp loading correct? (I dont have one yet since I am just getting started with electrics).

The props are the stock 8x6 and it seems like it has more than a enough power and I am able to fly at about half throttle or less. Using your table, does the "C" rating on the pack need to be greater than or eaqual to the max amps draw for each motor? This does make some sense but I need more experience with all of it. Is there a generally accepted FAQ or source for this information such as one of the battery or motor manufacturers sites?

It might help me if we go back to part of my initial question- If we assume that the recomended setup of:

two motors (ignore for now what motor and assume they are correct size and draw for the battery and esc)
two esc (again correct size for application)
ONE battery 3200mAh 20C

The question is- If I replace the single battery that powers BOTH motors with TWO smaller batteries are TWO 1500mAh 10C batteries proper for this new configuration? (Can I simply divide the mAh and "C" by a factor of 2 since I am now attaching only ONE motor to a battery?

To your other response about the bec- I am running a single bec from one of the two esc's and have disabled the second by pulling the red wire from the connector of the unused unit.

Thanks for your help-

Jim

mrasmm

the battery C rating should be greater than or equal to the motor draw in that table (since i already figured out the 80% in the table)

about that other q, I would think if you are getting the ESC's to arm correctly and you have the 1 BEC disabled, you should be good to go

You can only divide one or the other if the original single battery is properly sized. So you either divide the mAh rating or the C rating by 2. If you divided them both by 2, then the total cont discharge would be divided by 4

CFII_Jim

That makes sense. Thanks for the help!

One last question- Do chargers that charge through the balance plug actually control the charge amps to each cell?

What is the prefered charger setup that eletric fliers are using? (I have a vision ultrapeak, and I have to charge through the discharge plug and I also use a common sense rc balancer plugged into the balance plug during the charge.)

(I am going to find some more "samples" from arf combo packages that list the motor and battery and other detail and keep looking for more battery and motor manufacturer faqs)

mrasmm

as far as balancing, very few chargers actually treat a 3s pack as three 1s packs. Most just take the current and amperage and divide it by 3, and then apply up to about a 120mA sink between whatever cells are running high.

As far as balancing, it doesn't really matter. I like to do mine while it is charging so that I make sure that the pack gets full and doesn't run into a low voltage and a high voltage cell situation during charging, but if you balance every charge or every few charges, this really should never be a problem =)

Sorry it took a few days to get back to ya

BarryReade

My Feedback: (1)

Great thread, very informative to a non tron kinda guy

Thanks

ACES&8s

putting lipoly packs in parallel means using the same total cells per set. ALSO, the discharge capability I.E., 15C, 20C, 25C, 30c, 40c, 40-50c has to be the same,,, not just the same cells in series. Two packs of equal voltage with different rate of discharge, will heat up, and maybe smoke or burn. The discharge rate has to be matched for all cells in packs.

AlphaOrion

I have been searching for information on the limiting factors for wiring LiPo's in parallel, to no avail.

Wingdinger2

So, this thread has me all confused... Can someone answer this question for me? If I wire two 1800mah 3s packs with 40C output ratings into a parallel pack, I will have an 11.1V 3600mah "pack". By doubling the capacity, do I also double the power output to 80C? I have not been able to figure this one out...

N1EDM

My Feedback: (2)

Hi Wingdinger,

From my knowledge of batteries, you will have that 11.1V (nominal) 3600 mah pack. But the output rate will still be limited to 40C. This assumes that all cells in both packs are at the same voltage, same age (so that a strong, new pack isn't trying to pull along an older pack that has lost some of its punch).

Just my $.02

Bob

[email protected]

[QUOTE=Direwolf;2526357]This'd be a good time to dust off a physics text book if you have one handy. But, here's what you need to know...

A battery is a group of cells.
A cell has a capacity, measured in Amp*Hours (pronounced "Amp Hours" ), abbreviated as "Ah".
A cell has a certain amount of potential to do work, measured in Volts, abbreviated as "V".

When you put cells in series, their voltage is added together. For example:

This battery has 3 cells in series, so this is called a "3S" battery. The total voltage of this battery is 3*3.7V = 11.1V.

When you put cells in parallel, their capacity is added together. For example:

This battery has 3 cells in parallel, so this is called a "3P" battery. The total capacitance of this battery is 3*1.2Ah = 3.6Ah.

You can use combinations of cells in series and in parallel to achieve different levels of Voltage and Capacitance. For example:


Direwolf I made a Lipo pack of 6 5s [ 3 in series then parellel with 3 more in series] but it turned out it was to much for my BBSHD/electric bike.
I was told I need a 14s pack so since I already have 5s I'm thinking on using 4 of them + 2 4s to make my 14s pack BUT I would like to know if what I'm planning is possible. I'm thinking on making a pack as fallow- (keep in mind they are all of the same mah & c rating) 2 5s in parallel + 2 5s in parellel + 2 4s in parellel than connect the three parellel packs into series hoping to make a 14s pack of 51.8 v @3.7per cell / 58.8v "hot of the chart". I HOPE THIS MADE SENSE. Please anyone let me know if I can do this????? THX.