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Question about Voltage used to charge batteries - 11/5/2012 11:04 PM   
Gogu


 

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Status: offline 		Hello,

I don't fully understand the following. What does voltage play in the charging of lipo batts.

So for example if I have a theoretical 6V 10amp charger and I want to charge a 2cell 7.4V 5000Mah lipo that supports 2C charging, I could charge it at 2C since my charger does 10A but what about voltage. Must I charge it a 7.4V ? Will I break it if I don't give it 7.4V or will it just charge slower ?

And i've seen that some higher end chargers support even 30V, but this is useless without an expensive power source rated to output that much voltage. That charger would probably need somewhere around 900W powersource (30Vx30A) if it has 30A capability and you want to use them all.

Thanks,
Mike

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RE: Question about Voltage used to charge batteries - 11/5/2012 11:10 PM   
guver


 

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Status: online 		The voltage follows the battery's voltage plus whatever it takes to do the set current until the CV stage. Then it stays at 4.2 per cell and ramps current down to maintain the 4.2 volts.

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RE: Question about Voltage used to charge batteries - 11/5/2012 11:32 PM   
flyinwalenda


 

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Status: online 		Most chargers operate with a voltage source between 10 to 18 volts DC. I'm sure there are others that operate at lower or higher voltages too.
If your charger can operate on 6 volts dc and that is the input power source you have available then it will take a longer time to charge a 7.2volt lipo pack than if you were using a 12 volt dc input source.


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RE: Question about Voltage used to charge batteries - 11/5/2012 11:41 PM   
guver


 

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Status: online 		Oh, Was OP asking about input voltage? or output? It sounds like maybe both.

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RE: Question about Voltage used to charge batteries - 11/6/2012 9:52 AM   
Gogu


 

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Status: offline 		Hey guiys thanks for the posts. I was just trying to figure out what would happen to a battery if I charge it below its voltage and why would it charge slower at a lower voltage if I still give it 1C or even more charge.

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RE: Question about Voltage used to charge batteries - 11/6/2012 10:08 AM   
Goldenduff


 

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Status: offline 		It will not charge if the voltage does not mach. In your example your chargers max poer is 6vx10amps= 60watts.

If you tried to charge a 7.4 lipo the charger would have to increase the voltage so 60watts/7.4= (about) 8amps. So you only have 8amps (not 10) because some of the energy is going in to boosting the voltage.

It gets worse if you try to charge a 11.1volt because now 60watts/11.1= (about) 5amps. So you can only charge at max rate of 5amps.


In reality this is even less becuase some energy is wasted during the conversion as heat. Usuful equation;

Watts= Volts x amps

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RE: Question about Voltage used to charge batteries - 11/6/2012 12:55 PM   
flyinwalenda


 

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quote:

ORIGINAL: Gogu

Hey guiys thanks for the posts. I was just trying to figure out what would happen to a battery if I charge it below its voltage and why would it charge slower at a lower voltage if I still give it 1C or even more charge.


OK, Well the only way you can charge a lipo below it's rated voltage is to remove the battery from the charger before it has completed its charge cycle. You should be using the "LIPO" chemistry setting that is typically set at 3.7v per cell.
What kind of a charger are you using ?



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RE: Question about Voltage used to charge batteries - 11/6/2012 2:10 PM   
trax de max


 

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Status: offline 		I'm confused.
A 2s 5000mah needs 42W at 1C, at 2C it requires 84W.
A 6V 10A charger has 60W
This is where i'm confused, Why does the input voltage need to be the same or higher than the output voltage, If the power is already higher, or have i missed something somewhere.

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RE: Question about Voltage used to charge batteries - 11/6/2012 3:24 PM   
Goldenduff


 

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Status: offline 		A 2cell (7.4) 5000mah (5amp) needs 37watts to be charged at 1c.

The input voltage needs to be higher than the output voltage otherwise the charger will use some of its amp allocation to increase the output voltage. So if the input voltage is lower than the required output voltage you will not be able to charge at the charger's maximum amp out put....

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RE: Question about Voltage used to charge batteries - 11/6/2012 4:01 PM   
Gogu


 

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Status: offline 		Thanks that was what I was trying to find out. Though I'm not sure of there is a charger such as this that can put out a voltage beyond its factory spec.

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RE: Question about Voltage used to charge batteries - 11/6/2012 7:45 PM   
Goldenduff


 

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RE: Question about Voltage used to charge batteries - 11/6/2012 8:45 PM   
rgburrill



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Status: offline 		There is confusion here because the OP still has not fully defined wht he means by a "theoretical 6V 10amp charger ".  If, in fact, he is saying his charge will only put out 6 volts then he won't be able to charge a 7.4V battery at all and could destroy the charger.  As he correctly infers, a battery is charged by current, not voltage.  However, the voltage output has to be at least the maximum voltage the battery can go to or he will wind up very quickly discharging the battery.  And if a LiPo he will probably destroy it.
If you have ever put a voltmeter on a battery as you charge it you would see that the battery and the charger are at the same voltage.  And if you measured the current you would see it going from the charger to the battery.  Most battery chargers control the current outout and let the voltage float to the battery level.  TO A POINT.  If the maximum output of the charger is less than the battery you would see the current flow back to the charger and that would not be a good thing at all.
So, quite simply, if your "theoretical" charge is really only a 6V charge DON"T try to charge your 7.4V battery.

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RE: Question about Voltage used to charge batteries - 11/6/2012 10:24 PM   
guver


 

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quote:

ORIGINAL: Gogu

Hey guiys thanks for the posts. I was just trying to figure out what would happen to a battery if I charge it below its voltage and why would it charge slower at a lower voltage if I still give it 1C or even more charge.


I'm still trying to guess if you're asking about charger input voltage or charger output voltage.

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RE: Question about Voltage used to charge batteries - 11/7/2012 7:20 PM   
Gogu


 

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It seems I haven't understood anything.
What I'm trying to understand reading about Lipo batteries is what would happen If I would charge a 2S 7.4V pack (well actually 8.4V when fully charged) at a lower voltage and would this be possible. As Rgburrill states probably not. It seems that I would damage both the charger and the battery and that current would flow backwards since the difference in potential is not high enough to overwhelm that of the battery.
But then Goldenduff said the following:


quote:

ORIGINAL: Goldenduff

A 2cell (7.4) 5000mah (5amp) needs 37watts to be charged at 1c.

The input voltage needs to be higher than the output voltage otherwise the charger will use some of its amp allocation to increase the output voltage. So if the input voltage is lower than the required output voltage you will not be able to charge at the charger's maximum amp out put....


So what am I getting from this is that a 6V 10A charger would actually be able to charge this 7.4V pack by increasing its nominal voltage to 7.4V but sacrificing amps in the process. So I would not be able to charge it at 1C but maybe 0.6C.

Also I'm not sure I understand what you guys mean from input and output voltages. I assume input means the voltage that is fed into a charger from a powersource (like a car battery) or directly from the wall outlet. output voltage would be what the charger is actually capable of delivering to the battery ?

So to understand what Goldenduff is saying it would seem that the wall outlet would need to supply a higher voltage to the charger than what it is capable of delivering to the battery else it will need to increase its voltage to output the necessary amount needed to charge a pack ?



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RE: Question about Voltage used to charge batteries - 11/7/2012 8:09 PM   
trax de max


 

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Status: offline 		Yes, Input voltage comes from the power supply/batterey that is fed to the charger.
Output voltage is outputted from the charger.

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RE: Question about Voltage used to charge batteries - 11/7/2012 10:18 PM   
guver


 

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Status: online 		Yes, two different voltages. Some chargers are able to boost the output voltage to a value much higher than the input , but some are not and actually need a higher input than output in order for the charge to go the right way. Most all chargers have a very specific range of input voltage AND output voltage. Going outside that range many times results in an error.

In every case the total power (minus loss) must be equal between input and output wattage. Let's use your example (which is pretty uncommon because most chargers use 12 volts input) of 6 volts and 10 amps = 60 watts if the charger can do it. You'll be able to do a 50-60 watt job on the output side. For a 2s pack that is about 60 watts / 8.4 volts = about 7 amps MAX if your charger can do it. You should be able to charge your 2sx5.0 ah pack at about 7 amps max.

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RE: Question about Voltage used to charge batteries - 11/7/2012 10:49 PM   
trax de max


 

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Status: offline 		that makes sense to me, I just couldn't get to grips with some of the above.
I was born blonde, that's my excuse ha ha.

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RE: Question about Voltage used to charge batteries - 11/8/2012 8:34 PM   
Gogu


 

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quote:

ORIGINAL: guver

Yes, two different voltages. Some chargers are able to boost the output voltage to a value much higher than the input , but some are not and actually need a higher input than output in order for the charge to go the right way. Most all chargers have a very specific range of input voltage AND output voltage. Going outside that range many times results in an error.

In every case the total power (minus loss) must be equal between input and output wattage. Let's use your example (which is pretty uncommon because most chargers use 12 volts input) of 6 volts and 10 amps = 60 watts if the charger can do it. You'll be able to do a 50-60 watt job on the output side. For a 2s pack that is about 60 watts / 8.4 volts = about 7 amps MAX if your charger can do it. You should be able to charge your 2sx5.0 ah pack at about 7 amps max.


Yes that is if my charger could change its voltage from 6v to the 8.4v needed to charge a 2s lipo. It would then even be able to charge a 3s pack at about 4.7A right ?

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RE: Question about Voltage used to charge batteries - 11/8/2012 9:44 PM   
trax de max


 

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RE: Question about Voltage used to charge batteries - 11/8/2012 11:58 PM   
guver


 

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quote:

ORIGINAL: Gogu


quote:

ORIGINAL: guver

Yes, two different voltages. Some chargers are able to boost the output voltage to a value much higher than the input , but some are not and actually need a higher input than output in order for the charge to go the right way. Most all chargers have a very specific range of input voltage AND output voltage. Going outside that range many times results in an error.

In every case the total power (minus loss) must be equal between input and output wattage. Let's use your example (which is pretty uncommon because most chargers use 12 volts input) of 6 volts and 10 amps = 60 watts if the charger can do it. You'll be able to do a 50-60 watt job on the output side. For a 2s pack that is about 60 watts / 8.4 volts = about 7 amps MAX if your charger can do it. You should be able to charge your 2sx5.0 ah pack at about 7 amps max.


Yes that is if my charger could change its voltage from 6v to the 8.4v needed to charge a 2s lipo. It would then even be able to charge a 3s pack at about 4.7A right ?


Yes, that's right. about 4.7 amps IF it can boost voltage. I know you have a "theoretical" charger , but I doubt any charger that is rated at 6 volts input can output anything over 6 volts. In fact I can;t say I can recall any Lipo charger with a limit of 6 volts on the input.

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