Lipo Talk

[Ryan Thomas]

I have a question?

If one was to use two lipo packs in series, all specs the same but one was rated 30C and the other 20C for example, would this be a problem?

would the cells on 30C pack be depleted before the ons on the 20C pack?

My thinking is they would, but not drastically

 

[Sebbie]

Ryan,
i think you want to wire the two lipo packs in Parallel not serial.
Series wiring increases voltage but NOT amp/hour capacity. ie 2 x 14.8v = 29.6v not good news
Parallel wiring increases capacity NOT but voltage. ie 2 x 20C 4000mAh battery = 160Ah capacity (20 x 4000mA/1000)

You must always work on the smaller sized battery, if you cant get the same size, so you must assume you have two 20C not a 20C and 30C.
This is to reduce any possible damage to the smaller battery as you may run it lower than normal in different sized applications.
The 30C will try to charge the smaller size so the simplist way is ot work on the 20C being your size for both batterys.

So parallel would give you a longer flight time for you to enjoy than a single battery.

I hope this helps

 

[Gary Mortimer]

What Sebbie said

 

[Ryan Thomas]

No! I do want to connect in series, I am repairing a 5 cell lipo (30c) but I only have 20c cells on hand and I want to know practically besides a possible balance problem after a flight, is this a big issue in terms of internal resistance,

Only pulling 30A so even for the 5A 20c cell that's not pushing things.
I have done this by connecting a 4S 5A 30C lipo to a 6S 5A 20C lipo in series and after the flight on my 90E heli both bats were fine.

I am running 5S on my MK and just need to repair one puffed cell.



Ryan

 

[Sebbie]

Ryan,
The choice is yours to repair it, i would place the larger sized cell last in the chain.
The overall resistance will increase a little but youll find youll need to monitor the first charge closely with a digital multi meter and check the lower rated cells as they may get hotter during charge as well.
Heres a video http://youtu.be/t2cJ49EoxDE
Its something i wouldnt do personally as its cheaper to buy a new battery than replace a charger, RC unit or even your burned down house.
Good luck keep us posted

 

[Ryan Thomas]

Thanks in that video link the guy comments that the c rating does not matter?

So to be clear I have a 5000mah 30c pack which I want to replace one of the cells with a 5000mah 20C cell, So as long as I don't exceed what the 20C cell can handle I should be fine!!

 

[Sebbie]

yes correct, please let us know how it goes

 

[Ryan Thomas]

Update on fixed pack (4x5A-30C cells and 1x5A-20C cell)

For the most part this works fine, it seems that the 20C cell is depleted +/-.06V less than the 30C cells after a flight and charges a little slower.

This could just be because it's a new cell, but anyway the pack takes slightly longer to balance but that's all, otherwise perfect.

 

[Sebbie]

Ryan glad your ok and the little extra time to balance allows you to enjoy that cuppa tea longer

 

[DucktileMedia]

Battery voltage question. Why are lipos rated at 3.7v/cell if they charge to 4.2v/cell? It seems that 3.7v is where you want to stop using them. Am I being ignorant here? Confused by this.

 

[Droider]

I have seen a video some were which explains all there is to know about lipos... Ill try find it

Dave

 

[Ryan Thomas]

3,7 is what they call nominal voltage, and 4.2 is peak voltage,
as soon as you draw current the voltage drops below 4,2, so you don't calculate rpm of motors using the peak you use the nominal voltage,

It might be that if you have a 20c 5000mah lipo that can deliver 100A, if you had to measure the voltage while drawing 100A they would most likely be 3,7v per cell.

just a guess