# Battery Capacity C Ratings

nigtomdaw
Solar Expert Posts:

**705**
Hi, Im a little confused what is the best rating C to use for off grid homes in calculation of capacity and charging abilities.

It seems in Spain most manufactures or dealers are using the C100 rate to advertise there batteries as a example Im using Rolls

A Series 4000 FLA 6 volt 3 cell unit (S460) rated at C100 460 ah and C20 350 ah.

Questions for a typical off grid home using propane for heating water and minimising electrical use as one does, two questions & for simplcity lets say we have two batteries in a 12 volt system.

Which C rating is most appropiate

Which Max charge current can i apply assuming a target recharge rate of 10% can I charge max 46 amps or max 35 amps

Thanks for your replies Nigel

It seems in Spain most manufactures or dealers are using the C100 rate to advertise there batteries as a example Im using Rolls

A Series 4000 FLA 6 volt 3 cell unit (S460) rated at C100 460 ah and C20 350 ah.

Questions for a typical off grid home using propane for heating water and minimising electrical use as one does, two questions & for simplcity lets say we have two batteries in a 12 volt system.

Which C rating is most appropiate

Which Max charge current can i apply assuming a target recharge rate of 10% can I charge max 46 amps or max 35 amps

Thanks for your replies Nigel

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## Comments

1,164✭✭The C20 or C24 is probably the most accurate for how systems are used. C100 makes them look better than they are.

1,959✭✭✭I assume you mean C10 not C100?

The higher the number, the faster the charge and discharge in amps in the lower the efficiency of the battery. Slower charge/discharge rates , the better the efficiency is so C10 would have better overall numbers

10,311✭✭✭✭sorry sg as you got that one backwards. it is c100 as c10 is a faster rate and c100 the slower rate. slower rates equal better capacity.

705Rolls Batterys:

6 volt

Capacity: 460 AH

Series 4000 - 6 volt (3 cells) Polypropylene container

Type:

S 460

Capacity AH @ C100:

460AH

Capacity AH @ C20:

350AH

Dimensions (LWH in mm):

311 X 181 X 425

Weight (kg):

dry:

40.8 kg

filled:

53.1 kg

These are Rolls specs copied and pasted

705I inderstood the C rating was the time it took to discharge the battery from full to a set level, thus the lower the timing ie C10 against say C20 the power draw would be about double at C10 than C20, thats why a much smaller draw at the C100 rate is kinder on the efficiency levels and allows a high ah rating for the battery.

What about charging rates again are we best sticking to C20 rather that C100

334✭✭✭✭✭✭Nigel,

I think that you can play with the numbers a lot of different ways regarding sizing the system, as long as you know which numbers you're using. In other words don't use the 100hour rating for a system with heavy loads, etc. For an off grid system that has a large reserve capacity, I think that the C/50 or C/100 numbers are actually quite helpful since typical discharge currents should be much lower than a typical C/20 rate.

Regarding max charging currents those are to be based on the C/20 rating. The tech support guys I've talked to at Surrette for those very batteries felt that you could do much higher charge currents, I think one said as high as 23% (if memory serves correct). I'm not advocating that high of a charge current, as even if the batteries can take it that would probably not be the best for long life.

It would seem to me though that 46 amps would still be well within tolerable limits, just depends on what the local system needs/demands are. I kind of think that "ideal" charging currents probably depend a bit on what typical discharges are, with available charging times weighed in as well. If you can count on fully charging the batteries with 35amps being the max current, so much the better; but if the discharges are deeper, and the charging window narrow, then I'd be inclined to up the current to 40-45 amps.

HB

p.s. You are correct about the C ratings; C=capacity, so C/20 = Capacity divided by 20.

264✭✭✭✭✭✭From the Rolls web site, the recommended charging current is 0.25 x C20 and max at 0.35 x C20 (the link seems to indicate this is for AGM but probably ball-park for FLA). So, for 350AH at C20, bulk charge current is 87.5A and 122.5A max

GP

938✭✭✭I suspect that what people really mean when they talk about C ratings,

should be shown as a fraction, like

i.e. C/20 (C over twenty) (1/20th of the batteries Amp-Hour capacity)

That can be confusing.

EDIT. I just noticed that Greener Power already mentioned this.

boB

705So the C rating is very much like a MPG rating on a Motor Vehicle drive frugal and economically get C100 rate aka as Manufacturers Max MPG at 56 mph, C20 rate would be the Urban cycle and C5 rate would be street racer, yes;)

246✭✭✭✭✭good description

Most FLA manufactures seem to recommend 5% > 13% of 20 hr as a min/max charge current,

Have a good one

Tim

10,311✭✭✭✭it is the hour rate a battery has. c100 is 100hrs and what i said stands true that the battery will deliver more ah over 100hrs than at 20hrs and is what windsun was referring to as few of us will charge/discharge a battery at that slow of a rate. the problem with that is we'd need really big battery banks to reap this small extra benefit.

as to the high end charging, i have said this many times that it varies by manufacturer. think of the 13% as a minimum maximum where you are not sure of its actual ability. many fla batteries can go into the 20+% range, but will also need more maintenance due to the high boiling of the electrolyte. most of us aren't feeding our batteries this high rate anyway due to the costs involved in buying the pv power to do so. fast charging isn't as good as the slower c/20 rate, but so many elect to do it because of the limited hours of sun available in a day allowing batteries to reach full charge (or at least closer to it) every day making sulfation a lesser problem.

334✭✭✭✭✭✭I think we're all in agreement on the "C ratings". The only difference I see is that I do actually find the 100 hour rating to be helpful in a stand alone system. For those S-460's, we have two parallel strings which would amount to the following discharge rates and capacity:

9.32amp load = 932AH capacity (C/100)

12.3 amp load = 882 AH capacity (C/72)

16.7amp load = 834AH capacity (C/50)

35amp load = 700AH capacity (C/20)

In our case a typical sustained load is somewhere between 8-15amps, meaning that we're typically discharging in the C/100 or C/72 rates and sometimes up to the C/50 rates. When designing a battery bank, if we were to go say 30% smaller or started to add on some additional loads (not that anyone ever does that), we can easily end up with less capacity than we think due to the fact that we're now in a comparetively higher discharge rate. I think that when sizing up a battery bank it's handy to also consider checking that the typical loads match up to the C/100 to C/50 rate for a battery bank with sufficient reserve capacity.

Of course as I said in my post earlier, I think that you can compare any like ratings as long as you have an idea what that represents in terms of both charging and discharging currents, as well as the overall affect on capacity and lifecycle.

705Hillbilly a good reply we need bases to found our calculations you have given me one opps all of us one to start from.

1,571✭The C rating to use will depend on the loads, the number of autonomous days and the DoD. I've noticed that some Spanish installers recommend 5 day batteries discharged 70% (i.e. there's 30% left in the battery). So assuming ideal conditions and 0 losses:

100Ah load per day, 5 days autonomy, 70% DoD:

Battery size = (100 * 5) / 0.7 = 714Ah. So the battery would be 70% discharged after 5 days = 120 hours which means it will be 100% discharged after 170 hours. So C170.

100Ah load per day, 3 days autonomy, 50% DoD:

Battery size = (100 * 3) / 0.5 = 600Ah. After 3 days the battery should discharged to 50%, so after 6 days it will be 100% = C144.

If you factor in the losses the C values should get closer to C100. So in _theory_ for typical deep cycle batteries sized for 3-5 days using < 70% DoD, C100 seems like a good number to use.

175✭✭✭✭✭What is being considered is the Peukert effect. For estimating realized energy capacity, you want to match your loads (power draw) to the energy capacity of the battery at the current that will be pulled from it to supply that load.

Using the C/xx approach is needed for battery banks because the wiring of the bank is going to determine the current drawn from a specific battery to provide an overall power from the bank.

Note that C/xx is a rule of thumb and not a measure. It is a means to normalize current draw by battery size. As a contrast, the most common energy capacity ratings for batteries tend to involve time rather than current, i.e. the 20 hour rate being most common. For a 100 AH battery, the 20 hour rate implies a 5 amp 12v or 60 watt load.

For the Peukert coefficient usually found in lead acid batteries (1.2 to 1.3 or so), a change in current draw means a change in battery capacity of about 15% over the typical useful range.

For the topic here, the first thing to do is to figure out your use profile. What is the average power draw and how uniform is it? What is the peak draw and what is the median and well as the mean. These statistics will give you a target for your battery bank. You want it to supply peak loads without significant loss and you want it to be most efficient at the median load. From that, figure out the current you will be taking from a battery in various bank configurations and use that current to determine the effective capacity of the battery from the C/xx ratings that most closely match your loads.