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BFL13's avatar
BFL13
Explorer II
Jan 31, 2021

Math is Hard-- C Rate ?

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.)
  • roam1's avatar
    roam1
    Explorer
    Jan 31, 2021
    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
  • time2roll's avatar
    time2roll
    Nomad
    Jan 31, 2021
    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.
  • wopachop's avatar
    wopachop
    Explorer
    Jan 31, 2021
    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.
  • BFL13's avatar
    BFL13
    Explorer II
    Jan 31, 2021
    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)?
  • time2roll's avatar
    time2roll
    Nomad
    Jan 31, 2021
    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.
  • wopachop's avatar
    wopachop
    Explorer
    Jan 31, 2021
    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.
  • wa8yxm's avatar
    wa8yxm
    Explorer III
    Jan 31, 2021
    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's avatar
    wopachop
    Explorer
    Feb 01, 2021
    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.
  • red31's avatar
    red31
    Explorer
    Feb 02, 2021
    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

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