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What does "C" mean, in relation to batteries?


MtB

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I sometimes see the parameter "C" mentioned in lead acid battery discussions, but never any explanation of what C means. for example "charge at 0.15C".

 

Does it just mean capacity? And charge at 0.15C means charge a 100Ah battery at 15A?

 

If so, the units are wrong ninja.gif

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I sometimes see the parameter "C" mentioned in lead acid battery discussions, but never any explanation of what C means. for example "charge at 0.15C".

 

Does it just mean capacity? And charge at 0.15C means charge a 100Ah battery at 15A?

 

If so, the units are wrong ninja.gif

It's the capacity at a certain discharge rate. The C stands for Coulomb. See this link :- http://all-about-lead-acid-batteries.capnfatz.com/all-about-lead-acid-batteries/lead-acid-battery-fundamentals/what-does-battery-c-rate-mean/

Edited by Flyboy
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Thanks! That half explains it.

 

That web page starts off saying "The Battery C-Rate refers to the rate at which a battery is charged or discharged."

 

Then goes on the contradict itself with "The letter C in those expressions in more easily understood if you think of it as simply the value of the Amp Hour rating."

 

Then goes on the say it is something else again with "In truth, it stands for Coulomb (as in Charles Coulomb), but if you think of it in those terms, you’ll get yourself pretty confused as it really has little to do with the electrical unit known as a coulomb in the context of modern battery discussions."

 

So now I'm really confused!

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To quote that link:

 

In truth, it stands for Coulomb (as in Charles Coulomb), but if you think of it in those terms, you’ll get yourself pretty confused as it really has little to do with the electrical unit known as a coulomb in the context of modern battery discussions.

 

I would go further than that and say that it doesn't stand for Coulomb at all, either the person or the unit. It's just a shorthand for "capacity".

 

Since it's so common to want to express things like "Charge at a current in amps that is half the capacity of the battery in amp hours", this has been notated as "charge at 0.5C".

Edited by Giant
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on that basis if 0.5C means C0.5 it would imply the half-hour rate (i.e. 200A) ?? ohmy.pngohmy.png .

 

 

I think C is used in two different ways. I still haven't figured it out other than sometimes the coefficient is before the C, other times it is after.

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So with a 120Ah battery, I think 3C could mean three times the magnitude of the battery capacity I.E. 360A.

 

And C3 means the current which draws one third of the stated magnitude of the battery capacity I.E. 40A

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C -the current required to charge the battery from empty to full in one hour. It's a theoretical figure -at least because you cannot charge a battery at constant current to full in exactly any time (they need a tapered charge).

 

Numbers before the C such as 5C are over unity multipliers 5C means five times the one hour rate

 

Numbers after the C such as C0.5 are under unity multipliers, C 0.5 means half of the one hour rate.

Sometimes C/5 is used to indicate the five hour rate.

 

Also the system assumes that charging is 100% efficient -which we know it is not. BUT the C measurement scale lets people refer to "the general battery" rather than specifying a rate for one particular size of battery.

Edited by Arthur Brown
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My understanding is the C rating defines the ratio of your charge or discharge current relative to the battery capacity in amp hours

So if you have a 100 Ah battery and you charge/ discharge it at 10 Amps that's 0.1C or C/10 in theory you would charge / discharge it fully in 10hours, but of course it aint that simple.

I use the C rating as a gude to how much current I can put in or take out its also a guide to battery quality, better batteries have a higher C rating.

Most manufacturers quote a maximum charge and discharge rate in C. They also quote the capacity at a specific rate normally C/20 ie at a rate that would discharge the battery in 20hrs.

For example on my model aeroplane batteries I know that one rated at 50C is going to be better at delivering high currents without the voltage sagging than one rated 20C. When charging the higher C battery can be charged faster .

 

However there are lies, damn lies and battery specs.

 

So for a liveabord who needs to charge his batteries quickly the batteries with a higher C rating are likely to be a better bet.

 

Top Cat

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C -the current required to charge the battery from empty to full in one hour. It's a theoretical figure -at least because you cannot charge a battery at constant current to full in exactly any time (they need a tapered charge).

 

 

Ah, I see. In the mythical 'perfect' battery the value of this charge current would equal the value of the capacity in Amp hours. Thanks!

 

So in a real battery C is a derivative, calculated by taking the capacity in Ah and dividing it by 1hr.

 

Numbers before the C such as 5C are over unity multipliers 5C means five times the one hour rate

 

Numbers after the C such as C0.5 are under unity multipliers, C 0.5 means half of the one hour rate.

 

 

So this is just a convention then? A multiplier bigger than unity goes before the C and a smaller multiplier is place after?

 

 

 

Sometimes C/5 is used to indicate the five hour rate.

 

 

I see, as the five hour rate for a perfect battery would be one fifth of the one hour rate, or one fifth of C.

 

Thanks! Greenie.

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My understanding is the C rating defines the ratio of your charge or discharge current relative to the battery capacity in amp hours

 

 

Hmmmm this supports what I suggested earlier. It appears C can mean two different things.

 

1) "C" is an absolute value, being the capacity of the battery in Ah divided by one hour.

 

2) "C-rating" is something else, being what Top cat is talking about and is related to the maximum rate at which the manufacturer says you you can suck out the leccy, or put it back into a specific, real life battery.

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Ok then, given the conflicting meanings and uses of C posted so far, can anyone help unpick this statement, from the lead crystal battery FAQ?

 

8. Do Lead Crystal® batteries charge faster than lead acid, lead gel or AGM batteries?

Yes, Lead Crystal® batteries can be charged up to 3C without any impact on their cycle life. This means they can be charged 2-3 faster than other batteries.

 

Let's take a nominal 100Ah battery. C is therefore 100amps.

 

1) They seem to be saying a 100Ah lead crystal battery can be charged at 300A, yes?

 

2) They seem to be saying a 100Ah lead acid battery can be charged at 100A to 150A in comparison. Yet the recommended charge rate for a generic lead acid is C/10, or 10A for a 100Ah lead acid, yes?

 

How can we resolve the apparent conflict between these two statements?

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I wouldn't pay much attention to what the lead crystal people say about other types of battery, best look up the specs of your own batteries.

 

Charging at twice the rate won't halve your charge time because you are only putting that current in until the batteries hit their charge voltage. Once they do that you are limited by the current the batteries can take at that voltage which slowly tails off as the batteries approach full charge and that stays the same what ever rate you start at.

Also you would need a very big charger with cables to match to shove in 100's of amps. So lets assume a 400Ah bank if you wanted to charge at 3C thats 1200A serious stuff.

 

Top Cat

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I understand it depends on how it is written, if charged by C/1 you will charge it in one hour, but C/8 it is charged in 8 hours, or C.125 but it all depends on how the writer is thinking and don't have to be correct, but it all come down to how many amp's hour's that can be charged by the hour before 20.00, that's 8 so C/8 meaning the charge must start 8 hours before 8, so 20-8= 12 amps, I am positive. :-)

 

some times (more serious) it state SOC but then also SODC

AGM can be charged and discharged faster then most L/A

then newer LiPo of different kinds, can be charged even faster, but tend to cost the double of AGM, but weight half.

LiPo needs a complicated battery management system though.

Edited by Dalslandia
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