Math is Hard-- C Rate ?

[BFL13]

I am confused about how many amps in a C rate, if anyone can explain that again. Thanks.

After getting the SiO2s late last year, I charged them to as full as they would get, which was at 14.6v with amps tapered to 0.5a /100AH same as ordinary AGMs spec. The SiO2 battery spec charging graph says they get down to "0.02CA" and then after more time to "0.01CA" as Full.

Does that "0.01CA" for Full mean 1 amp per 100AH? If so, that is twice the 0.5 amps per 100 with ordinary AGMs.

Meanwhile, after floating them at 13.3v--I picked that voltage as being not a full float in case I was not supposed to be floating them at all--still not sure about that whole thing--

They got charged yesterday (as a spot check) at 14.6v and amps tapered on the two in parallel (200AH) to 0.2 amps and stayed at that for a time, so I went back to float at 13.3 for now.

That is 0.1 amp each as their Full. Could "0.01CA" be a typo? When I had ordinary AGMs, I would see lower than 0.5a/100AH on them too.

(I assume that time floating did them some good, since they now taper to 0.1 each instead of that 0.5 when I first got them. I think I will keep floating them between trips, never mind what Trojan says.)

[BFL13]

This is hard to see, but their charging graph uses "C" where they mean C20 and use a percentage of capacity for the amps.

https://www.google.ca/search?source=univ&tbm=isch&q=ESG+6GFM250&sa=X&ved=2ahUKEwjo9piq1cvuAhWYs54KHa...

It all works if you already know what they are trying to say, but we really need a better way to "name" what exactly we are talking about.

I have been using "charging rate" for that, and then having to explain that means the "initial charging amps" wrt battery bank in AH. This is related to the "battery acceptance rate" in amps, which changes with SOC and voltage and type of battery.

As the battery voltage rises with its SOC, the acceptance rate falls at a constant charging voltage. If you raise the voltage, then at the same SOC it will accept more amps (up to a point of course but "generally speaking") Graph makers want a way to show the amps along the way during a charging profile.

So nothing to do with discharge rates, but something to do with charging rates. Looks like C-Rate for charging only applies to Lithium batts, since the other types reduce their acceptance rates as SOC rises, so the definition they give won't work for the time it takes to charge the battery.

Trojan is doing it "right", but it takes too much typing to fit on those graphs, so it gets muddled.

This is worse than velocity and acceleration!

[StirCrazy]

BFL13 wrote:
I see now, that "C-Rate" has a more specific meaning to many, and also that graph makers and the rest of us are looking for a short-cut way to express current wrt the AH size of the battery bank. Looks like "C-Rate" has been taken, so we can't use that?

Trojan in their guide uses the form "C20" for the capacity at the 20 hr rate. (5 amps per 100AH)

(for AGMs) "as the current slowly declines. The
absorption phase ends when the current stabilizes at a low value of approximately 0.005 x C20"

We say that AGMs need to be charged at 14.x volts until amps taper to
"0.5 amps per 100 AH of battery bank". Trojan says that is "0.005 x C20", while that other guide uses those CA numbers , so they would say that is "0.005CA"

When my actual battery bank of 200AH settled at accepting 0.2 amp at 14.6v, ( 0.1 amp per 100AH) Trojan would call that 0.001 x C20, 1/10th of that "0.01CA, so their graph does not get low enough for long enough in time I guess to reach True Full.

So what is "C/5" really? Batt U. says C-rate of 0.2C = C/5, the five hour discharge rate. ISTR LFP specs saying they can be charged at 1C or like BB, recommended at 0.5C.

it almost looks like your trying to combine two things that are totaly different to come up with some sort of rule. mind you I am not sure if you are asking a question or trying to reason somthing here either.. but 1C, .5C and such have nothing realy to do with C20, C5 or that. well they kinda do but are not relatable. both deal with the internal resistance of the batgtery and how it can change as the charg level and temp of a battery change. (I may be leaving some stuff out but hopefully I am remembering the basics)

the first one 0.5C, 1C and such is an overall measure of the charging curent a battery will accept. it isnt nessasarly for 100ah unless that is the size of the battery it is on. a 1C on a 25AH battery means you can completly charge it from dead using a 25amp charger in one hour. on the same battery a .5C means you could charge it with a 12.5amp charger in 2 hours and no faster a 2C means you could use a 50amp charger and charg it in 1/2 an hour. so that directly relates to the rate of charge for the totaly capacity of the battery. if you had a 2000AH bank of li with a 1C rate you could charge it in 1 hour with a 2000amp charger if you had or could afford one. most of the time this number wont mean anything to the average camper as your camper charger wont put out enough amps to reach the charging limit during bulk. , unless your running a single SiO2 as we have recently found out.

now the other number C20 is basicly how long the battery will last at a constant output over a period of time so C20 is discharging the total battery over a 20 hour period at a constant draw. C5 is over a 5 hour period, C10 is over a 10 hour and I think I have seen C40 also. this number has all to do with the Peukert effect and as far as for us is a convenient way to compar capacitys of batteries on an even playing field . We use the C20 capacity to compare one battery to another. if this info wasnt published then battery makers could pick the best rate publish that as the AH capacity and not tell us which one they used.

so people dont have to look it up Peukert effect says that the capacity of a battery decreases as the rate of draw increases and its a different rate of increase vs. decrease depending on battery chemistry and such. so lets pick 100Ah battery to be easy at a C20 rate its capacity is 100AH, at a C5 rate that same battery might only have a capacity of 72AH and at a C40 rate it might be 120AH.

so you can change the C20 rate like you are trying to do, it is a overall measurment at a constant rate of discharge,

for charging a three stage charger doesnt realy apply to a 1C or 0.5C rate as it is an optimized method of charging that increases battery length and I would have to look it up but I think that rate is just for bulk , or its and average for the whole charge. three stage chargers are a relitively new thing when you think about it. so it may very well be that the #C rate only applies to constant output chargers. so when you notice the charge voltage and curent decreasing as it goes from bulk to adsorption to float that is just an optimization to gently bring the battery to 100% and reduce battery sulfication and such. this is why the adsorption phase is longer on a conventional battery and short (around 1 to 3 min) on a Li battery.

Steve

[BFL13]

I see now, that "C-Rate" has a more specific meaning to many, and also that graph makers and the rest of us are looking for a short-cut way to express current wrt the AH size of the battery bank. Looks like "C-Rate" has been taken, so we can't use that?

Trojan in their guide uses the form "C20" for the capacity at the 20 hr rate. (5 amps per 100AH)

(for AGMs) "as the current slowly declines. The
absorption phase ends when the current stabilizes at a low value of approximately 0.005 x C20"

We say that AGMs need to be charged at 14.x volts until amps taper to
"0.5 amps per 100 AH of battery bank". Trojan says that is "0.005 x C20", while that other guide uses those CA numbers , so they would say that is "0.005CA"

When my actual battery bank of 200AH settled at accepting 0.2 amp at 14.6v, ( 0.1 amp per 100AH) Trojan would call that 0.001 x C20, 1/10th of that "0.01CA, so their graph does not get low enough for long enough in time I guess to reach True Full.

So what is "C/5" really? Batt U. says C-rate of 0.2C = C/5, the five hour discharge rate. ISTR LFP specs saying they can be charged at 1C or like BB, recommended at 0.5C.

[StirCrazy]

here.

itrelates to the charge or dischare rate in relation to its total capacity. a 1C charge rate means you can put the entire capacity of the batter back into it in 1 hour. same as a 1C discharge means you can run a load bigh enough to discharge that entire battery in 1 hour with out hurting it. so in a 100 amp battery if it has a 1C charge rate you can use a 100amp charger on it with out dammage , as for discharge you can put a 100amp load on it with out dammaging it.

li has a higher than 1C rate but most BMS will limit to 1C for even longer life and extra insurance.

Steve

[red31]

CA clearly means charging current as seen in the charging graphs.
Capacity is shown in % (% of C), makes graph generic.

the math has never been correct since a % of ah is not equal to amps.

these folks introduce CA = % of C in amps (charging current)

Which means .2C (amps) = .2CA (amps)

.2CA = 20% of C (capacity)in amps

Nothing new here

[BFL13]

Here is how these guys use "CA" (which means "capacity" I think/hope)
See the cycle charging graphs down a ways. Hit the + sign at the top so you can read the numbers. Note they do use "C" in the top discharge info.

It might not load right away or you have to hit refresh but it does appear:

https://soneil.com/wp-content/uploads/2018/07/12-EFSN-100.pdf

[wopachop]

Hmm maybe i have C rating wrong for how it relates to trailer batteries. Ive always encountered C rating more as a measurement of performance. Not capacity.

Lets say you have a 10ah battery with a safe discharge rate of 2c. That means the battery manufacture has told the consumer they can safely draw 20amp from that 10ah battery without excessive voltage sag or excessive heat. Its not talking about the size of the fuel talk or how long you can draw 20amp for. Its more a measurement of performance.

That would obviously be the maximum discharge and charge rates. You could also use C rating to describe any type of charge or discharge rate. That same 10ah battery charging at 0.5C would be getting a 5a charge.

[wa8yxm]

C is the amp hours at the 20 hour rate.
So if you can draw 5 amps off a battery for 20 hours before it hits ZERO. that's a 100 amp hour battery.. WARNING not all batteries display their 20 hour rate (Example follows)

So for a 100 amp battery 1% C is 1 amp. that simple.

Now the warning
I know of some companies (Walmart) that use the 1 amp discharge rate.
How long, at one amp. till toast (Zero) This gives their batteries much higher amp hours on the label.

How to tell (Lead acid batteries other than Optma)
Group 24 is around 70-75AH
Group 27 just under 100
Group 29 Just over 100.. NOTE this may vary brand to brand
Group 31 about 130
GC-2 (Golf car) Varies but 220 give or take 15
4D or 8D are about the same as GC-2 but 12 volt (Hernia bate if you try to lift one.. Well perhaps not. I could lift one.. I think)

If you see a Group 27 with say 150 amp hours.. It's likely at the 1 hr rate.

Optional reading and I can't believe I got it on the first search.

If you want to know more about why the lower discharge rate gives higher capacity

Click on this Wikepedia link

[wopachop]

BFL13 wrote:
The "C-Rating" is the term I used here (perhaps wrongly) to describe the "CA" number in the battery spec. Thanks for confirming "0.01CA" is 1 amp on a 100AH battery. (It is not the charging rate, but is what the amps will be down to after tapering down to "Full")

I think you're using the term correctly.
In a way that 0.01CA is the C charging rate. Its just not the "maximum c charging rate."

The C rating is useful and makes things more simple because battery capacity gets included.

You could use c rating to talk about different things.
1. What's the max discharge c rating?
2. What is the recommended float charge c rating?

Makes it easier than discussing that in amps and then needing to also specify the aH of your battery bank.

[time2roll]

I believe charging some of these chemistries will separate the electrolyte or plate chemistry.
I know with lithium for example too fast will move the actual metallic lithium in the cell instead of just the ions.
I assume SiO2 would have similar breakdown for moving things too fast.
Otherwise we could charge an electric vehicle or your phone in 10 seconds for just applying more power.

[BFL13]

The "C-Rating" is the term I used here (perhaps wrongly) to describe the "CA" number in the battery spec. Thanks for confirming "0.01CA" is 1 amp on a 100AH battery. (It is not the charging rate, but is what the amps will be down to after tapering down to "Full") These batteries can be charged to a max of 27 amps per 100 AH or about "0.27C" or "0.27CA" if I got that right.

I would like to know more about why discharge rates are sometimes seen as way higher than spec charging rates. Why can't the batts accept as fast as they can discharge like Wets can (where the chemical change is just reversed between charging and discharging--same speed of change)?

[wopachop]

C rating is a way to describe how much voltage drop and heat a battery will suffer. 

This example is opposite of RV batteries, but might help to understand. Some of those radio control drones use a tiny little 1amp battery with a 100c discharge rate. That means you can pull 100amps of burst power out of that super small little 1amp battery. 
There will also be a recommended charge C rating. Lets say the battery has a 5c max charge rate. For a 1amp battery you would want to limit charge to 5a or less. 
For your RV the math looks a bit differnt. Since your batteries are 100s of amps and it appears the charge C rating is a fraction like you posted. 0.01C rating for a 100a battery would be 1amp. 
Often the discharge C rating is much higher than the charging C rating.

[time2roll]

Once the curve goes flat you know they are full charge. I would spend minimum time at 0.2 amps and cut the voltage sooner than later.

For storage (or even during use) you might even stop sooner such as 2 amps. I don't think you really need to cram them all the way to the top.

Double check with the manufacturer or supplier.

[roam1]

my hobby batteries have a charge C rate and a discharge C rate
one C is the amp hour capacity of your battery in amps it takes to charge in one hour
for example a 4 amp hour battery rated for 1 C charge can be charged at 4 amps and it should take about one hour

a 4 amp hour battery rated for charge at .5 C charge can be charged at (.5 x4 = 2 amps) and it will take about 2 hours