What is C/8, C/10 ??

Rngr275 · July 2012

Sorry if this is a stupid question but is see C/whatever in a lot of posts and think it has to do with batteyr charging but I am not clear on exactly what it is and how it is used.

Thanks
McD

Cariboocoot · July 2012

Re: What is C/8, C/10 ??

It's the theoretical peak charge rate. "C" is the capacity of the battery bank in Amp hours (at the 20 hour rate).
So C/8 for a 220 Amp hour battery is 220/8 = 27.5 Amps.
Multiply by 100 and divide by the capacity and you get the percentage rate: 27.5 * 100 = 2750 / 220 = 12.5%
Thus C/8 corresponds roughly to the 13% practical upper limit for charging most lead-acid batteries.
C/10 is the easy one to work with.

Rngr275 · July 2012

Re: What is C/8, C/10 ??

Ok we are almost there

. So how do you use it? Say you have 370Ah battery bank, 48v system, 85amp charger, etc. Is it a specified number somewhere or a guideline for the 8-13% charging of the batteries.

Type slow for me if you would...;)

ggunn · July 2012

Re: What is C/8, C/10 ??

Cariboocoot wrote: »

It's the theoretical peak charge rate. "C" is the capacity of the battery bank in Amp hours (at the 20 hour rate).
So C/8 for a 220 Amp hour battery is 220/8 = 27.5 Amps.
Multiply by 100 and divide by the capacity and you get the percentage rate: 27.5 * 100 = 2750 / 220 = 12.5%

Actually, it's simpler than that. 1/8 is 12.5%.

ggunn · July 2012

Re: What is C/8, C/10 ??

Rngr275 wrote: »

Ok we are almost there. So how do you use it? Say you have 370Ah battery bank, 48v system, 85amp charger, etc. Is it a specified number somewhere or a guideline for the 8-13% charging of the batteries.

Type slow for me if you would...;)

If you want to charge a 370Ah (20 hour discharge rating) battery at C/8, you'll need 370/8 = 46.25A of charging current.

Rngr275 · July 2012

Re: What is C/8, C/10 ??

Where do you want to charge at (rate) and is that rate the same no matter the battery bank Ah capacity?

Cariboocoot · July 2012

Re: What is C/8, C/10 ??

Well I use it for calculating the size of a solar array. Like this:

370 Amp hour 48 Volt battery. 37 Amps (10%) * 48 Volts (minimum Voltage - the time when peak current will occur) = 1776 Watts. Since panels need to be derated (their average output is less than the STC number) this is divided by the derating (typically 77%) and you get the need for an array of 2306 Watts in order to be able to supply that peak potential current.

Keep in mind that solar charging is not a constant current event. You may never see or need that level of current, but if you have the potential it will be there. Works well with 25% DOD limit.

If you're going for sizing a plug-in battery charger or setting the current limit on an inverter-charger the principle is the same. It does not have to be exact either. Because the current isn't constant, a slightly higher current for a short period of time won't hurt anything and a lower current will be present most of the time (as the batteries charge their internal resistance increases and the current goes down).

You would want to avoid putting 85 Amps to 370 Amp hours as that would be a 23% rate and would create a lot of heat in the batteries. Heat is the enemy of all things electrical; it shortens life.

Cariboocoot · July 2012

Re: What is C/8, C/10 ??

Rngr275 wrote: »

Where do you want to charge at (rate) and is that rate the same no matter the battery bank Ah capacity?

The battery manufacturer should have a recommended rate, but too often they don't. The rule-of-thumb for flooded cells is 5% to 13% as this is the most practical range. Also that is a net rate: any loads drawing current while the charging is happening will reduce the actual current going to the battery. I pick 10% because it is easy to work with and generally will assure the minimum 5% even with loads on. A good "ballpark" number.

When expressed either as a percentage (10%) or fraction (C/10) the expression works for any size battery battery bank @ Voltage. As in:
220 Amps hours charges at 22 Amps maximum whether it is 12, 24, or 48 Volt system; the Voltage multiplies which increases the Watts needed to achieve the current desired.

ggunn; my formula given works with any set of numbers for finding percentage.

Rngr275 · July 2012

Re: What is C/8, C/10 ??

It is starting to click:

So for a 740Ah battery bank @ 48v= 3552W, derated would = ~4612W of panels to supply peak potential current.

85amps to 740 Ah would be OK (`11%)

But if I had 3500W of panels (derated) for that 740 battery bank it would be OK just take longer to charge the battery bank... correct? But seems that the 3500w panels would be kind of low amperage for batteries (30amps from panels) that had 740AH. (4%)

Correct?

Photowhit · July 2012

Re: What is C/8, C/10 ??

Just to confuse things, C/ is also a rate of discharge, a battery sizing tool based on rate of discharge.

All batteries have different capacities depending on the rate of discharge. Trojan's 'new' RE (renewable energy) line now most often uses a 100 hour discharge rate to describe them, So an L16 which typically have a 325-330Ah (C/20 rate) now carry a a 360-370Ah (C/100) rate, their capacity when discharged over 100 hours. I just realized that this is an added factor in Trojan's wanting a charge rate of 10% since they have artificially 'boasted' their capacity.

Forklift batteries are typically described at a c/6( or C/8 ) rate since they are often discharged that quickly, but discharged over 20 hours the capacity is about 50% greater. So my Fork lift battery which is 510Ah at C/6 is 804Ah at C/20.

Normally, Deep Cycle battery capacities use to be described at a C/20 discharge rate. Now Trojan likes to use 100hr rate, and Sun extender doesn't even give a 20hr rate. Time for a post on 'What's the real capacity of your battery'.

Cariboocoot · July 2012

Re: What is C/8, C/10 ??

Yes; you do not want to go below 5% charge rate, and again that is the net charge rate. If you put 37 Amps in (5%) and the loads are pulling 3 Amps for themselves the batteries only get 34 Amps - less than 5%.

As a rule I would only go with a 5% calculated rate if it were a "nights only" power use or a "weekend only" (let it charge all week) application.

How long it takes to recharge a battery is dependent both on the rate of charge and the depth of discharge. The trick is to get the whole charge in during the hours of available light. If you deeply discharge the batteries (50%), have a low charge rate (5%), add loads (-1%), and short daylight (4 hours) it will not work. The batteries will be chronically deficit charged and will fail long before they should.

vtmaps · July 2012

Re: What is C/8, C/10 ??

Rngr275 wrote: »

It is starting to click

Oh yeah? Well how about this: The battery capacity in AH is often rated by the number of hours of discharge. Thus a battery of 370 AH at the 20 hour discharge rate may be said to have a C20 capacity of 370 AH. The same battery when discharged at the 100 hour rate (C100) may have a capacity of 400 AH.

Now if you charge that battery with 37 amps you are charging at 10% of the C20 rate (or C20/10) which is the same as 9.25% of the C100 rate (or C100/10.81).

Hope that's not too unhelpful

--vtMaps

Cariboocoot · July 2012

Re: What is C/8, C/10 ??

Photowhit wrote: »

Time for a post on 'What's the real capacity of your battery'.

No, I don't think newcomers could handle that. The minute you mention "Peukart Effect" and try to explain that the faster you discharge the lower the actual capacity, brains shut down in self-defense.

Next you'll want to explain that as batteries age their capacity inevitably goes down (even when maintained properly) and that temperature also changes the actual Amp hours. In fact, it changes moment to moment!

We rely heavily on "fudge factors" to get systems to work due to the overwhelming number of variables. Knowing which way to round which numbers is important. I've said it before but it bears repeating: people who try to run close to the edge often fall off it.

Cariboocoot · July 2012

Re: What is C/8, C/10 ??

You know guys it does not help to throw all the complex stuff and rule exceptions et cetera at people who are just starting out and have not yet grasped the basics. It's like trying to teach arithmetic using calculus. :roll:

vtmaps · July 2012

Re: What is C/8, C/10 ??

Cariboocoot wrote: »

You know guys it does not help to throw all the complex stuff and rule exceptions et cetera at people who are just starting out and have not yet grasped the basics. It's like trying to teach arithmetic using calculus. :roll:

I mostly agree with you. But I am close enough to being a newby that I can still remember some of the things that tripped me up. The difference between C20 and C/20 was one of those things, and that's why I brought it up. For me (but not necessarily others) the nomenclature was more difficult than the concepts.
--vtMaps

BB. · July 2012

Re: What is C/8, C/10 ??

Just to be clear... C/20 is the 20 Hour Rate of discharge:

370 AH (at 20 hour rate) / 20 hours = 18.5 amps discharge for 20 hours

We use rules of thumbs based on 20 Hour Rate... I have seen others than use slightly different "fudge factors" based on C/6 or C/10 rates (but convert pretty closely to the 20 Hour Rate fudge factors we use)...

The whole idea behind rules of thumbs is like Marc/Cariboocoot explains... We here are trying to design a system that will meet your needs not only when new, but 5-10 years down the road when the batteries are getting old, panels are aging/covered dust, people added a few more small loads because they now have I²Pads & X²Boxes, etc...

And, even then, we here use slightly different variables for rules of thumb... for example a 370 AH battery bank should be charged around 5-13% rate of charge (I do agree with Marc that 10% is a pretty good starting number), but:

370 AH * 48 volts battery bank * 1/0.77 panel+charger derating * 0.10 rate of charge = 2,306 watt array "nominal"
370 AH * 59 volts battery charging * 1/0.77 panel+charger derating * 0.10 rate of charge = 2,835 watt array "nominal"

Both will adequately recharge the battery bank, but one array is ~23% larger than the other.

Either will work fine. My numbers I tend to use as a derating for the maximum array you can put on a YY Amp MPPT charge controller... Above YY amp rating, the MPPT controllers will "limit" the maximum current and you will be "wasting" solar output from the array.

Ideally, the batteries are nearly fully charged (59 volt Bulk/Absorb) and it will only be clipping on cool/clear/sunny days and only around noon time... So the wasted energy is not worth buying a larger charge controller and since solar panels are so reasonably priced--having a 10-23% larger array is not going to be too hard on your pocket book (i.e., "worth it" for cloudy weather, a bit more loads than expected, etc.).

Batteries are the "heart" of an off grid power system... And you need to treat them right or they will die an early and expen$ive death... The rest of the hardware is fairly tolerant to poor operation conditions--So having somewhat under/over-sized hardware vs loads is usually not a big issue (watch +/- DC polarity and maximum operation voltage ratings--Those are several "user errors" that hardware usually does not recover from).

-Bill

Photowhit · July 2012

Re: What is C/8, C/10 ??

Thanks for the Edit Bill! I was just coming back, after having added it to my archives.

Sorry for confusing things, surprised I hadn't noticed the Trojan thing before, I'm going to go through their info and see if they note % of what...

I'll go sit in the corner now...

Rngr275 · July 2012

Re: What is C/8, C/10 ??

Ok clear as slightly muddy water.

Would a chrge rate of 8% be too low/bad any of the above?
I ask because one of the proposals I have been given is for a 3.8Kw in PV and 740Ah in batteries.
Running BB's numbers I come up with ~8%.

I have no more bugget so if this is not good I will nneed to cut back on the batteries (1 series only in stead of 2 series in parrallel) which puta me at 16%.

WHich one to go with is the question.

Cariboocoot · July 2012

Re: What is C/8, C/10 ??

Nothing wrong with an 8% charge rate. Just watch out for loads eating in to that and taking it below 5%. Remember the inverter is going to be active and will have a demand on the current as well.

Given the size of the array (3800 Watts) I'll hazard the system Voltage is 48.

BB. · July 2012

Re: What is C/8, C/10 ??

You are welcome Photowhit... I too make the mistake of a 8_) turning (remove "_") into a 8) all the time.

The C/20 hour rate seems to meet the "typical" usage of the off grid Solar PV system sizing we normally do here (2 days no sun, 50% max discharge, 10% rate of recharging, etc.)... The C/100 rate would keep take the batteries below ~75% state of charge for too many days and run the risk of early sulfation. The C/20 allows 5 hours of use per night (two days of discharge to 50%, then recharge quickly back to >90% state of charge for long battery life).

A huge battery bank either take a huge a array to quickly recharge and/or takes many days to recharge... Why several of us here keep pushing for "balanced" system design (nominal battery bank, nominal array, nominal inverter/loads). Seems to give the best bang for the buck. Over-sized systems are expensive to install and maintain--And battery bank replacement time is a real hit to the wallet.

That is what backup generators are good for (some fuel costs in winter/during cloudy weather).

-Bill

vtmaps · July 2012

Re: What is C/8, C/10 ??

BB. wrote: »

Just to be clear... C/20 is the 20 Hour Rate of discharge

Throughout this thread C/20 has referred to the 5% charge rate. Is it also used as a discharge rate? --vtMaps

BB. · July 2012

Re: What is C/8, C/10 ??

Happens to be "the same"...

With lead acid batteries (and any non-ideal energy source/sink), the details complicate the picture.

For Lead Acid batteries, they happen to be 80-90% energy efficient but very near 100% Amp*Hour Efficient (note the equalization/bubbling a battery is near zero percent efficient--Turning energy/electricity into hydrogen and oxygen gases, and into heat).

So, when discharging 80 AH from your lead acid battery, it takes about 80 Amp*Hours to return to 100% state of charge.

However, energy wise, you discharged the battery at an average of 12.3 volts (to pick a number: 12.7 volts to 11.5 volt discharge range), but recharged it at 14.0 volts (13.5 to 14.5 volts) or so--So the energy efficiency would be:

14.0 volts / 12.3 = 1.14 = 114% more energy to recharge the battery bank than was used during discharge.

At higher discharge and charging rates, the voltage difference will be larger (higher charging voltages, lower discharging voltages). The details are horribly more complex than I have in this example--But it is "close enough" to see the high level effects on the use of energy when cycling a battery bank.

-Bill

ggunn · July 2012

Re: What is C/8, C/10 ??

BB. wrote: »

Happens to be "the same"...

With lead acid batteries (and any non-ideal energy source/sink), the details complicate the picture.

For Lead Acid batteries, they happen to be 80-90% energy efficient but very near 100% Amp*Hour Efficient (note the equalization/bubbling a battery is near zero percent efficient--Turning energy/electricity into hydrogen and oxygen gases, and into heat).

So, when discharging 80 AH from your lead acid battery, it takes about 80 Amp*Hours to return to 100% state of charge.

However, energy wise, you discharged the battery at an average of 12.3 volts (to pick a number: 12.7 volts to 11.5 volt discharge range), but recharged it at 14.0 volts (13.5 to 14.5 volts) or so--So the energy efficiency would be:
14.0 volts / 12.3 = 1.14 = 114% more energy to recharge the battery bank than was used during discharge.

At higher discharge and charging rates, the voltage difference will be larger (higher charging voltages, lower discharging voltages). The details are horribly more complex than I have in this example--But it is "close enough" to see the high level effects on the use of energy when cycling a battery bank.

-Bill

Amps going in are at a higher voltage than amps coming out. Watts is amps X volts, so Watts out < Watts in for the same number of amps. Watts is what counts.

Cariboocoot · July 2012

Re: What is C/8, C/10 ??

ggunn wrote: »

Amps going in are at a higher voltage than amps coming out. Watts is amps X volts, so Watts out < Watts in for the same number of amps. Watts is what counts.

Unless you're talking about a battery and the effect of different discharge/charge rates on its health. That is Amps only, and it's across the board; same rule for any system Voltage and the variation in actual vs. nominal Voltage is of no particular consequence.

nsaspook · July 2012

Re: What is C/8, C/10 ??

Cariboocoot wrote: »

Unless you're talking about a battery and the effect of different discharge/charge rates on its health. That is Amps only, and it's across the board; same rule for any system Voltage and the variation in actual vs. nominal Voltage is of no particular consequence.

For engineers :-) voltage always has consequence because Amps/Ah in/out is just a measure of the movement of charge and charge is not "electricity/electrical energy" in a technical sense. The true electrical power/energy efficiency of a system can only be expressed in energy units like Watts. For batteries, knowing the exact voltages with the current while charging and discharging can tell us a huge amount of information that can pin-point problems quicker than just current monitors alone.

Cariboocoot · July 2012

Re: What is C/8, C/10 ??

nsaspook wrote: »

For engineers :-) voltage always has consequence because Amps/Ah in/out is just a measure of the movement of charge and charge is not "electricity/electrical energy" in a technical sense. The true electrical power/energy efficiency of a system can only be expressed in energy units like Watts. For batteries, knowing the exact voltages with the current while charging and discharging can tell us a huge amount of information that can pin-point problems quicker than just current monitors alone.

*sigh*
Except we're not talking about that either.

nsaspook · July 2012

Re: What is C/8, C/10 ??

Cariboocoot wrote: »

*sigh*
Except we're not talking about that either.

Maybe not directly and I understand the intent of your statement as those C numbers assume constant rates.

I just think saying -> "Unless you're talking about a battery and the effect of different discharge/charge rates on its health. That is Amps only" goes a little far in simplifying things when solar is concerned as varying currents during that 1 hour might result in the same Ah but battery could be in a different state of charge than a pure Ah only calculation would show.

BB. · July 2012

Re: What is C/8, C/10 ??

And the type of loads will affect battery discharge too...

Filament lamps (standard bulbs) tend to be constant power devices.

Motors (really varies a lot) and AC inverters tend to be constant power devices (Power=Voltage*Current). As voltage falls, current increases.

Standard deep cycle batteries are rated in Amp*Hours and I have seen some AGM/GEL batteries intended for UPS/backup power rated in Watt*Hours instead (for constant power discharging).

-Bill

ggunn · July 2012

Re: What is C/8, C/10 ??

Cariboocoot wrote: »

*sigh*
Except we're not talking about that either.

Maybe it's not what you are talking about...

What is C/8, C/10 ??

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