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limitting to discarge current for li-po battery

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Recep

New Member
Hi my friends,
I want to take your opinion of this circuit. I have a lithuim polymer battery. You know this battery can send big currents if you demand from it. I have an 2.2Kw dc electric engine. If i do not limit discarge current of li-po batteries, it will be fire and explode. How can i do it?
And also how this batteries charge with 1.2Kw source. This source unregulated 24-48V Dc and 0-50A.
Thank you.
 

Chippie

Member
First off, what is the voltage of your battery and what is its Ah rating?

Lipo batteries can be destroyed quite easily if not managed correctly.

Single cell lipos are generally charged to a termial voltage of 4.2volts...overcharging will cause them to explode.
Overdischarging will also render them useless. Do not allow the voltage to drop below 3v.

Low voltage cut off detection circuitry provides the protection of the cell/pack to pevent over discharge. This is usually incorporated into the motor controller.

Discharge current depends on the 'C' rating of the cell or pack.

Example, a 1500mah 3s pack has a terminal voltage nominally of 11.1volts, with a 20C rating a dishcarge current of upto 30 amps can be achieved. However it is not good practice to discharge packs to the 3v/cell limit...70% is a good ball park figure to aim for.
There are differing C rated packs available so its a case of horses for courses....

Charging requires a dedicated charger with a cccv regime with a charge termination of 4.2v per cell. Multiple cell packs also require balancing to ensure that all cells within the pack have a voltage equal to one another within 20mV or so...If a pack becomes unbalanced there is the danger of over charging a pack and eventually damage will occur...fire or explosion or personal injury.

Balancing is achieved using a dedicated balancer that connects to the battery pack via a dedicated 'pigtail' connector. This actually connects to each of the individual cells and shunts excess voltage during the charging process.


Where does all this knowledge come from?

I fly electric powered R/C aircraft powerd by lithium polymer battery packs...
 
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Recep

New Member
Thank you very much for your answer. I have a 11.1V 2200mA li-po battery. I bought it from R/C aircraft item seller :) I use two of them with swift connection. So, 22.2V 2200mA i use. But i will change it because our compettion allow 200W battery for 20C. Also can you advice me something ?
 

Recep

New Member
with 22-24V li-po battery how much current i choose for 200W 20C. I dont know how to calculate it 20C value.
 

Chippie

Member
P=V*I....

What is the use of your battery packs?

Are they in planes/boats?

If its an aeroplane then the current is determined by the motor propellor combination....large diameter/large pitch props require lots of power...reduce either or both and the load current goes down for the the same rotational speed...
 

Recep

New Member
I will use it for dc motor which has 2.2Kw power. We have a group of hydrogen energy team. We have a car which has hydrogen fuel cell. Our fuel cell output unregulated 24-48V Dc and current is between 0-50A. And it has 1.2Kw power. So, i want to use li-po battery for gain an extra energy.

p=v*i , i know. But li-po batteries C20 value is diffrent to other batteries. Our compettion rules said that Li-po batteries C1 value will multiplication 1.04 and their C20 value calculate. Is it means 2200mA li-po batteries value 2200*1.04= 2288mA and its power 2288*22.2= 50,793W. Am i true on this calculation ?
 

Chippie

Member
Your lipo battery pack has a terminal voltage of 22.2v with a capacity of 2200mah....intrinsic power for 1 hour is that product....22.2*2.2A Watts assuming your load current is 2.2A....As load current goes up time goes down....

The 20C rating means that the maximum load current you can take from the pack is 20*2200....44 amps but it wont last an hour!....the pack will be 'emptied' in a matter of minutes...
 
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Chippie

Member
Where can i found 200W li-po batteries ? i found max. 22.2V 5000mah.

There arent any batteries that are 200W......The power is a function of current draw.


Are you saying that you are power limited to 200W....?
 

Chippie

Member
I think you will struggle to find a 6 cell (6S, 22.2v) pack that has a current capability low enough to meet the 200 W limit...

What you may need to do is series up some 3s packs that have lo mah capacities....
 
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Recep

New Member
I don't know meaning of cell number difference. I found 22.2V 5Ah battery this is 6 cell as you said. Do you have technical information about li-po battery?
 
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Chippie

Member
I don't know meaning of cell number difference. I found 22.2V 5Ah battery this is 6 cell as you said. Do you have technical information about li-po battery?

What technical information do you need?

Lipo batteries come in different configurations as do normal nicad batteries...

Typically a lipo cell has a nominal voltage of 3.7v, fully charged would read 4.2v.

A 2s pack is made from from 2 cells connected in series. The mah rating of the pack is determined by the cell's physical, but the voltage is increased by a factor of two....fully charged would be 8.4v

A 3s pack is made form 3 cells connected in series. The mah rating of the pack is determined by the cell's physical size, but the voltage is increased by a factor of three.....fully charged would be 12.6v

and so on....your 22.2v pack is made from 6 cells connected in series..that is its nominal voltage, fully charged it will be slightly higher....6*4.2...25.2v
 

Recep

New Member
Thank you very much. I understood. I asked this when i said "Do you have information about li-po battery?".

If we turn my first question, which circuit i will use for limit discarge current for ex. 30A for dc motor.
 
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