Connecting Li-ion cells in series

[Lesimo]

I am running a device that take 10.5 V and 4000 mAp and it take 5 to 6 hours of use to become discharge. I am building an additinal power source using 3 flat Li-ion batteries rate at 3.5 V and 2800 mAp each. I soldering them in serie which would total 10.5 V and 8400 mAP. First question is the fact that the mAp is double, does it mean that the device would run twice as long?
If solder 3 sets as as explained above in parallel I would keep the voltage at 10.5 and my mAP would be 25300. Does it means that my device would run 3 times longer?
Does the higher Amperage damage my device in any way or form?
What should be the spec of my protection board for the three set together?
Anf finally, what board should I use to recgarge the whole package?
Thank you for your help

 

[KMoffett]

"...3.5 V and 2800 mAp each. I soldering them in serie which would total 10.5 V and 8400 mAP. First question is the fact that the mAp is double, does it mean that the device would run twice as long?"
No. It's 10.5 V/2800mA. You increased the total voltage, but the current available for the series wired batteries stays the same.

"If solder 3 sets as as explained above in parallel I would keep the voltage at 10.5 and my mAP would be 25300. Does it means that my device would run 3 times longer?"
No. In a perfect world you might have 10.5V/8400mA...but paralleling batteries is a very, very, very bad idea. If you need that much current/time, get a bigger battery

"Does the higher Amperage damage my device in any way or form?"
No. Circuits only draw the current they need, not the maximum of what the batteries are capable of delivering.

"What should be the spec of my protection board for the three set together?"
? DON'T DO THAT!

"Anf finally, what board should I use to recgarge the whole package?"
Li-ion batteries require special chargers. You would need a charger designed for the specific battery chemistry, voltage, and current rating. Over-charging and over-discharging can cause Li-ion batteries to explode and start fires.

Get batteries/charger that will meet your need. In this case don't try to kluge together something with something you "happen to have laying around"

Ken

 

[Externet]

...but paralleling batteries is a very, very, very bad idea.
Ken

Why ?

 

[Gryd3]

Why ?

Li-ion batteries are very picky about the charge/discharge rates and values.
If you discharge a li-ion too low, it will die... and if you charge it too hard or too high, it will die.
Larger battery packs that need to charge networks of li-ion batteries are riddled with additional circuitry to ensure that the cells in the pack charge and discharge at equal rates, and to ensure (or help) that no single cell in the battery pack is subject to conditions outside of the normal safe operating parameters.
Something else to consider when using any cells in parallel, is that if one cell is faulty, it could short out the other cells it is in parallel with. It's not impossible... it's just not worth the risk unless you take the time to carefully build a proper power pack and charge circuit.

Edit: Here is an extreme example:
http://www.teslamotorsclub.com/showthread.php/34934-Pics-Info-Inside-the-battery-pack

 

[KrisBlueNZ]

Larger battery packs that need to charge networks of li-ion batteries are riddled with additional circuitry to ensure that the cells in the pack charge and discharge at equal rates, and to ensure (or help) that no single cell in the battery pack is subject to conditions outside of the normal safe operating parameters.

Yes, that's right... charge balancing is needed... when the cells are in series!

I'm not saying you're wrong about connecting cells in parallel. I don't know. I'm just saying that I don't find that a very convincing explanation.

Something else to consider when using any cells in parallel, is that if one cell is faulty, it could short out the other cells it is in parallel with. It's not impossible... it's just not worth the risk unless you take the time to carefully build a proper power pack and charge circuit.

That could be a valid reason.

Is there a really good reason why you shouldn't parallel cells of the same type and capacity? Is it just because they will inevitably have different characteristics and over time one cell will become more charged than the other because it draws more current? I'm having trouble visualising how this would become a problem.

 

[Gryd3]

Yes, that's right... charge balancing is needed... when the cells are in series!

I'm not saying you're wrong about connecting cells in parallel. I don't know. I'm just saying that I don't find that a very convincing explanation.

That could be a valid reason.

Is there a really good reason why you shouldn't parallel cells of the same type and capacity? Is it just because they will inevitably have different characteristics and over time one cell will become more charged than the other because it draws more current? I'm having trouble visualising how this would become a problem.

The only reason I have for avoiding parallel connections of rechargeable cells is if one is faulty or dies prematurely... but that very same reason could be applied to almost anything though...
If I were to start adding cells in parallel and series though for a rechargeable pack I would most likely over-engineer it... if they were disposable batteries I would not care in the slightest.

 

[KrisBlueNZ]

The only reason I have for avoiding parallel connections of rechargeable cells is if one is faulty or dies prematurely... but that very same reason could be applied to almost anything though...

Does anyone else have any good reason for not paralleling two or more identical cells?

 

[BobK]

It seems to me that, as long as the two cells are in the same state of charge when paralleled, then always connected together, it should not cause problems. Connecting a fully discharged cell to a fully charged cell, however, would be a major problem.

Bob

 

[Externet]

My two laptops battery packs have factory paralleled 2 Li-ion cells times 3 series; My electric drill has paralleled 2 Li-ion cells times 4 series for 16 V; My ATV has four paralleled Li-ion times 3 for 12V instead of lead acid.
They do not have a hiccup. All 18650 cells.
----> https://www.google.com/search?q=lap...v&sa=X&ei=CDtiVLeWB4axogTG4IGQDQ&ved=0CB8QsAQ

My emergency portable radio has now 3 discarded cell phone Li-ion cells inside; one instead of 3X1.5v AA; one instead of 3.6V NiCd for its solar panel; one instead of its supercapacitor for the cranking generator. I ended paralleling them all with diodes from the different sources (AC adapter, solar panel, crank) So any source recharges them all.