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

[MtB]

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.

 

 

Thanks but I'm trying to learn and understand the principles at work here, before I start looking up individual battery specs.

 

Until I properly understand what C means and how it is used, the battery specs are meaningless.

[Top cat]

 

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.

Very true Lipo batteries are fragile thingscand have a tendency to catch fire if abused, so quite sophisticated chargers are required to charge each battery individually. Lipo also get unhappy if

 

The are charged too fast or just after being discharged, or while too hot or too cold.

They don't like operating if too hot or cold

They dont like being stored fully charged especially if they are in a warm place

They die very quickly if over discharged even once

Things get very interesting if they get overcharged or damaged in any way.

 

They are fine in small sizes for toy aeroplanes and phones but given the rigours of a boat engine room I reckon you would have to be very careful indeed in your system design and battery location. Defo not a drop in replacement.

[Dalslandia]

 

 

Thanks but I'm trying to learn and understand the principles at work here, before I start looking up individual battery specs.

 

Until I properly understand what C means and how it is used, the battery specs are meaningless.

 

Simply C means (to me) capacity, and then you charge or decharge at a % of that or fraction. and dept of charge to a % of the capacity.

[MtB]

 

Simply C means (to me) capacity, and then you charge or decharge at a % of that or fraction. and dept of charge to a % of the capacity.

 

 

How does that fit in with Top cat's post number 16? Where he says:

 

"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."

[Murflynn]

It's the speed of light

is that measured in metres per second or watt-hours per centistoke?

[Top cat]

What I was trying to express was that a battery with a higher C rating could be charged at a higher current than one with a lower C rating.

So if you have a battery with a max charge rate of 1C and one with a max of 3C then the 3C should be capable of being recharged faster, but not 3x mmaybe 1.5x if youbare lucky because of the tailing off as the charge completes. All this assumes you have a charger capable of delivering sufficient current which ain't trivial.

However a battery with a higher C rating is likley to have a lower internal resistance so even with a charger that cannot get to the high currents for multiple C charging you should be able to stuff more in before it hits V max.

 

Top Cat

[dmr]

 

 

How does that fit in with Top cat's post number 16? Where he says:

 

"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."

 

Because the manufacturer says you can charge them faster (though I have never seen anyone actually use the term "C rating").

I think Trojan say to charge at 13% or 0.13C but the manufacturers figures don't really matter because we all charge by as much as our alternator will let us.

I can only manage about 0.17C but that 'cus I now got a lot of batteries.

I think accepted boaty wisdom is then 0.25C is a good maximum.

I think NickNorman manages 0.39 but he's from Scotland and they are brave people up there.

 

.............Dave

[Justme]

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

 

 

I am sure that you have that the wrong way round.

 

I am positive that my 5C rated forklift bats are far better than a 20C rated battery of the same capacity.

 

IE I can draw the full capacity out in 5 hours whilst the 20C battery could have only supplied 25% of the total energy.

 

If a battery was rated at 100ah at 5C it would likely have a 120ah capacity at 20C so battery makers / sellers use the higher C rating so the bats look bigger.

 

So in your aeroplane example the 50C battery would actually take 50 hours to give up 100% of its energy & the 20C one just 20 hours. IE a higher discharge rate with a lower C rating.

 

So a live aboard needs a low C rating not a high one.

[Top cat]

No I've got it right my 50C aeroplane battery in theory should be able to survive being discharged (once) in 1/50th of an hour (72 seconds) Now that's silly rate but it should be and indication thats its better than one rated at 20C which should survive being discharged in 1/20th of an hour (3 minutes) . Even though in practice both would be discharged at an average of about 4C giving a flight time of 15 minutes. The higher C battery would give more power during peak demand than the lower C battery because it has a lower internal resistance. You pay for that with a bigger heavier battery.

 

I think the confusion comes from the marketing hype and downright lies that the battery manufacturers resort to to con us that their products are superior.

Top Cat

[mayalld]

is that measured in metres per second or watt-hours per centistoke?

 

Can we not simply adopt FFF Units for all canal related purposes?

 

The required movement for CCers would then be 1kfur/ftn

 

Lunch hours would become 3 mftn long, giving an extra 24 μftn in every lunch break, which ought to be long enough for 3 mfkn of a really nice ale

[Justme]

No I've got it right my 50C aeroplane battery in theory should be able to survive being discharged (once) in 1/50th of an hour (72 seconds) Now that's silly rate but it should be and indication thats its better than one rated at 20C which should survive being discharged in 1/20th of an hour (3 minutes) . Even though in practice both would be discharged at an average of about 4C giving a flight time of 15 minutes. The higher C battery would give more power during peak demand than the lower C battery because it has a lower internal resistance. You pay for that with a bigger heavier battery.

 

I think the confusion comes from the marketing hype and downright lies that the battery manufacturers resort to to con us that their products are superior.

Top Cat

 

 

Thats not what 50C means or at least not in the FLA battery world. Think about forklift bats.

 

They discharge at huge currents & have a 5C rating.

 

50C means to discharged from full to empty in 50 hours. IE the 50 hour rate.

 

Thats why a 100ah 5c rated battery will give out more total energy at a higher C rating.

 

Just been looking at Lithium bats & they quote a discharge rating in C as you describe.

IE 1C is a discharge of 1 x the capacity.

 

It seems that there are two uses for C.

 

One as you describe for the discharge / charge maximums & the other to work out the batteries usable capacity at a give discharge rate.

 

On a battery you often see 100ah at 20C rate. That would be a current of 5 amps for 20 hours. The same bat at the 5c rate would prob only have an 80ah capacity with a 16amp current.

[MtB]

 

 

Thats not what 50C means or at least not in the FLA battery world. Think about forklift bats.

 

They discharge at huge currents & have a 5C rating.

 

50C means to discharged from full to empty in 50 hours. IE the 50 hour rate.

 

Thats why a 100ah 5c rated battery will give out more total energy at a higher C rating.

 

Just been looking at Lithium bats & they quote a discharge rating in C as you describe.

IE 1C is a discharge of 1 x the capacity.

 

It seems that there are two uses for C.

 

One as you describe for the discharge / charge maximums & the other to work out the batteries usable capacity at a give discharge rate.

 

On a battery you often see 100ah at 20C rate. That would be a current of 5 amps for 20 hours. The same bat at the 5c rate would prob only have an 80ah capacity with a 16amp current.

 

 

Thank you for explaining all that. It became clear early in the thread there were two different uses of the term "C", these being "C" with a multiplier, and the "C-rating", which also has a multiplier.

 

Very confusing, and I'm still not sure if they are the same thing or different...

[Giant]

Thank you for explaining all that. It became clear early in the thread there were two different uses of the term "C", these being "C" with a multiplier, and the "C-rating", which also has a multiplier.

 

Very confusing, and I'm still not sure if they are the same thing or different...

 

If you see a multiplier or divisor for C, e.g. 10C, 0.5C, C/2, C/10, take it to mean a current, defined relative to the battery capacity.

 

The other bits of associated notation are generally to do with the fact that there is no single value for C for a given battery. Capacity in amp hours can only really be defined for a given discharge profile. So a battery will have one C value if discharged at one rate, and another if discharged at a different rate.

 

This is often denoted as a subscript to the C value, so e.g. C₂₀ would usually mean the capacity the battery has over a 20 hour discharge, or you might even see something like C_5A meaning the capacity given a constant 5A load, or what have you.

 

Finally, where the writer cannot or does not know to apply a subscript, sometimes you will see things like C20 to mean the same as the above example.

 

The key thing to bear in mind is that all of these notations describe a current - either a value of C for some conditions, or some multiple of a value of C.

 

If you have a battery that can provide 100Ah over 20 hours, then C₂₀ is a current of 100A. 0.5C₂₀ is a current of 50A. C/2 may also be a current of 50A, if one assumes that an unsubscripted C means a 20-hour capacity.

 

The rest of the discussion in this thread, about "C-ratings" and so on, is about what those currents mean in relation to the battery, which is a separate issue from what the value of the current is.

 

So you may see a battery described as having a max discharge rate of 20C, and a max charge rate of 2C. If its nominal capacity is given as 2Ah, that means the manufacturer is saying it can be discharged at 40A and charged at 4A.

 

Often, particularly for things like lithium polymer packs used for model aircraft, this is all abbreviated to simply sticking "20C" in big letters on the pack with no further explanation.

 

A "C-rating" is just a current rating, that happens to be defined in terms of C.

Edited March 21, 2016 by Giant

[Loafer]

In relation to batteries, C means the design capacity in Ah. (To answer your question Mike.)

[MtB]

In relation to batteries, C means the design capacity in Ah. (To answer your question Mike.)

 

Only for those who don't observe dimensional rigour, it seems...!

[WotEver]

 

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

Yes, it's pretty common scientific notation. Resistance values for example are often quoted using 'r' instead of the Omega symbol and 20r = 20 ohms whereas r20 = 0.2 ohms.

 

Tony

 

Or 4kr7 = 4.7 kilohms

[MtB]

Yes, it's pretty common scientific notation. Resistance values for example are often quoted using 'r' instead of the Omega symbol and 20r = 20 ohms whereas r20 = 0.2 ohms.

 

Tony

 

Or 4kr7 = 4.7 kilohms

 

 

Ah but that's different. The use of C I was noting is where the number after C is smaller than unity, e.g. C 0.2.

 

Are you saying C 0.2 means the same as C2? This seems unlikely...

[by'eck]

 

 

Ah but that's different. The use of C I was noting is where the number after C is smaller than unity, e.g. C 0.2.

 

Are you saying C 0.2 means the same as C2? This seems unlikely...

 

If in this instance C was used with a divisor when multiples below unity were involved it would be easier on the eye/brain. i.e. 5C, 2C, 1C, C/3, C/6, C/10 etc.

[Giant]

Ah but that's different. The use of C I was noting is where the number after C is smaller than unity, e.g. C 0.2.

 

Are you saying C 0.2 means the same as C2? This seems unlikely...

 

"C 0.2" is just completely ambiguous, and I say that having worked with this stuff for ten years including all sorts of sloppy usage.

 

I could mean 0.2 times C, C over an 0.2 hour discharge, C at a 0.2C discharge rate, C at an 0.2A discharge... it's just completely unclear. Maybe there's some clue in the context.

[dmr]

Yes, it's pretty common scientific notation. Resistance values for example are often quoted using 'r' instead of the Omega symbol and 20r = 20 ohms whereas r20 = 0.2 ohms.

 

Tony

 

Or 4kr7 = 4.7 kilohms

 

4k7 is standard notation but I have never seen 4kr7, 2r2 would also be standard for 2.2ohm. r2 would thus mean 0.2ohm but I have never seen this this done, engineers would more likely say 0.2ohm to avoid ambiguity.

 

.................Dave

[Giant]

4k7 is standard notation but I have never seen 4kr7, 2r2 would also be standard for 2.2ohm. r2 would thus mean 0.2ohm but I have never seen this this done, engineers would more likely say 0.2ohm to avoid ambiguity.

 

That whole notation is mostly only used for resistor and capacitor values. The idea is basically to use the SI prefix for the unit multiplier (p, n, μ, m, k, M) in place of the decimal point, and omit the unit, except where there's no multiplier, in which case you use either an actual decimal point, or the symbol for the unit. Except that often this is done in software that can't do Greek letters so R gets substituted for Ω.

 

So e.g. 2.2kΩ becomes 2k2, with the fact it's a resistance implied by context - usually the fact it's next to a resistor symbol in a circuit diagram or a component listed as "R7" or whatever in a bill of materials. Similarly capacitors may be 220p (for 220pF) or 4u7 (for 4.7μF) and so on.

 

It's not that consistent - a resistor of 220 ohms might be simply "220", or "220R". And one of 2.2 ohms could be "2.2" or "2R2". One of 0.22 ohms could be "0.22", "0R22", or "220m". It would not be given as "R22", because that would normally indicate a component identifier, i.e. resistor number 22 in the circuit.

 

You occasionally see it for voltages too, particularly in schematic & PCB layout software where you need to name the nets in the circuit, but the software will often only take a simple combination of letters and numbers. So the 3.3V rail may get named 3V3, and so on.

 

I am glad to report that all this is gradually starting to be seen as archaic and is slowly going away.

Edited March 22, 2016 by Giant