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Thread: mAh or C. Which is more important?

  1. #11

    Re: mAh or C. Which is more important?

    From this thread, it seems that mAH is more important especially when C is over 20. Am I right? Second, in the past, I read that higher C batteries mean they run out quickly due to larger discharge rate. So, this is incorrect?
    The specific C rating you need depends on your load. What is your load? If you're building a small robot that draws at most 10 Amps, then any C rating of 10 or higher will work fine for any battery of capacity 1000 mAh or more.
    If you're building a quadcopter that needs to draw 100 Amps at times, and you can only allow the weight of 2000 mAh, then you'd need a 50C battery.

    Batteries do not "push" current; motors (and other loads) "pull" current, and the C rating is a measurement of how much current you can attempt to "pull" out of the battery before the internal losses start to dominate, cause overheating, too much voltage sag, etc.

    So, again, the process I suggested will lead you right:

    1. How many amps do I need to draw at peak?
    2. How much weight can I afford to carry?
    3. Choose the biggest mAh battery of the right voltage that weighs less than 2. and has a C rating that achieves 1.

    For example:

    1. I need to draw 120 A for an EDF based jump jet mech
    2. I can afford to carry 210 grams
    3. 210 grams lets me choose 2000 mAh batteries at 11.1V but not more. 120 A / 2 A means I need a C rating of 60 or higher.

    Another example:

    1. I need to draw 25 A to drive four 5A motors and a mini-ITX computer for a rover
    2. I can afford to carry 800 grams on this rover
    3. 800 grams at 14.8V lets me choose up to 7000 mAh. If I choose a 7000 mAh battery, 25 A / 7 A means a C rating of 4 is enough. However, if I save weight (and money) and go with a 2000 mAh battery at 14.8 V weighing 280 grams, I'd need a C rating of 25 A / 2 A == 13 C or better.
    Last edited by jwatte; 08-09-2015 at 01:02 PM.

  2. Re: mAh or C. Which is more important?

    Hi jwatte, Thanks for the examples. In your two examples, how come 210g lets you choose 2000mAh batteries at 11.1V and 800g at 14.8V lets you choose up to 7000mAh?

    Let's check my understanding with an example.

    A 5000mAh battery provides 5A of current for an hour. If I have a motor that draws twice the current (10A), the battery can last for 30 minutes. If I use the same battery to drive two such motors (the total required current becomes 20A), it lasts only 30/2=15 minutes. Since such motor draws only 10A, I need a 5000mAh battery with at least 2C (5x2=10) to handle one motor. If I want to use a battery to drive two motors, I need a 5000mAh battery that has at least 4C (5*4=20). In each respective case, a C value above 2C and 4C means less heat from the battery and it is save to use the battery. Some batteries have very high C value (e.g. 30,50C). It does not matter in this case since the motor draws only 10A and the battery is 5000mAh. Am I right? I recall reading that higher C provides more current when the motor is under load. What 30C and 50C mean in this case? I
    Last edited by Snoopy; 08-10-2015 at 09:13 AM.

  3. #13

    Re: mAh or C. Which is more important?

    Yes, your description is correct.

    A higher C value means a lower source impedance (resistance) in the battery, although there is not a 1:1 relation between "C" and Ohms, so it's not possible to exactly calculate the voltage drop at a particular current draw. You have to measure it on the specific battery type. Although, in general, if everything else is the same, higher C will mean "less voltage sag under load" when compared to lower C from the same manufacturer. (It's harder to compare between manufacturers, as they may rate differently and have slightly different chemistries.)

    how come 210g lets you choose 2000mAh batteries at 11.1V and 800g at 14.8V lets you choose up to 7000mAh?
    Because those are the approximate weights of batteries I found weights for on a hobby website :-)
    Energy capcity is volt times amp-hours, so 22.2 Wh in the first battery, or 105.7 Wh/kg, and 103.6 Wh, or 129.5 Wh/kg, in the second. I assume the lesser efficiency on the smaller battery is because the wiring/connectors/case adds disproportionately more weight to smaller batteries.

  4. Re: mAh or C. Which is more important?

    Thank you all very much for checking.

    In my example, do higher C values (e.g. 30C, 50C) has any effect?

    Am I right that I should compare batteries basked on mAh, size and weight rather than mAh*C?

    What kind of charger do you guys recommend? I am considering Zippy and Turnigy batteries. Which brand produces better products?

    I plan to use 2S or 3S batteries now. Later, I may use higher S batteries and more mAh ones.

    HobbyKing recommended the following. A few got bad reviews on the net.

    1. HobbyKing ECO6 50W 5A Balancer/Charger w/ accessories
    HobbyKing Power Supply 100~240v 5A

    2. Turnigy 12v 2-3S Basic Balance Charger

    3. Turnigy A-6-10 200W Balance charger & discharger

    4. Turnigy Accucel-6 50W 6A Balancer Charger LiPo LiFe NiMh NiCd JST-XH w/ MANUAL!

    5. HobbyKing™ X200 AC/DC Touch Screen Smart Battery Charger 10A 6S (US Plug)

    6. Turnigy E3 Compact 2S/3S Lipo Charger 100-240v (US Plug)
    Last edited by Snoopy; 08-10-2015 at 07:29 PM.

  5. #15
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    Re: mAh or C. Which is more important?

    I personally use #2. Just about any of those will work, it just depends on what options you want. (ie, touchscreen/no touchscreen, fast charge/slow charge)

    #2 is a slow charger. Mine takes about an hour/hour and a half to charge a 2200mAh 3 cell.

  6. Re: mAh or C. Which is more important?

    I would prefer one that can do fast charge and can charge 2S to 3S batteries. Currently there is no plan to use 4S but who knows. Most likely I will use Zippy or Turnigy. I don't care too much about touchscreen/no touchscreen. Simple to use and fast charging are more important. Anything better than that ones I listed?

    It seems that there are quite a few videos on Zippy catching fire. Are Turnigy batteries more reliable?

  7. #17

    Re: mAh or C. Which is more important?

    The only ones that do fast charge are #3 and #5. I think both #2 and #6 are "slow" chargers, as they have no individual settings.
    #1 and #3 are "medium" chargers -- at 50W, they'll charge a 3S battery at about 4A max, which is a 2C charging rate for a 2000 mAh battery, which is about as fast as you'll want to go at 10C batteries, but you could likely go faster with 20C and up batteries.
    I have a couple of 50W chargers, and they're fine. I don't need ultra-fast charging. They are all largely interchangeable; I think they all come out of the same firmware/design in China.

    Note that some of them may need a separate 12V power adapter, which needs to be higher-rated than the wattage. So a 12V/60W power adapter might be enough for a 50W charger, but I'd rather go with 12V/70W or higher.
    Personally, I use a bench power supply to power the chargers. Also, a slightly higher input voltage, like 15V, will proportionally reduce the amp draw of the charger, as long as it's within the rated input voltage.

  8. #18
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    Re: mAh or C. Which is more important?

    I've have an older SkyRC iMAX B6AC (50W with built in AC-DC power supply) that I got from pololu several years ago, and trossen sells the non-AC input version. They can be a bit expensive, but genuine SkyRC chargers are worth the cost (hobbyking sells both genuine and clones/knockoffs, but at least they price and describe each version accordingly). Still takes at least 5 hours to fully charge megabatt (4S 20Ah LiFePO4) at the B6AC's 3.79A max charge rate for 4S LiFePO4, but the smaller 4S 2Ah batteries can charge in less than an hour. When charging megabatt, I keep a spare 80mm computer fan blowing on the charger so it can keep charging at the max current instead of being auto-throttled to prevent overheating.

    Pretty sure Zippy and Turnigy are both brands produced by/for HobbyKing, so only differences between the two are likely to be packaging and C ratings.
    Last edited by tician; 08-11-2015 at 07:51 PM.
    Please pardon the pedantry... and the profanity... and the convoluted speech pattern...
    "You have failed me, Brain!"
    bleh

  9. Re: mAh or C. Which is more important?

    I would prefer a genuine charger that can charge quickly. As I read, cheaper ones could cause the LIPO to die sooner or cause fire.

    Is HobbyKing a reputable store? Are Zippy and Turnigy reliable? What other brand do you guys recommend? Is it better to get a charger the same brand as that of the batteries?

  10. #20

    Re: mAh or C. Which is more important?

    HobbyKing is about as reputable as Radio Shack or Wal-Mart. If you buy from their US warehouse (and make SURE it says US warehouse on the actual order) shipping is reasonably fast. If you end up with the Hong Kong or some other warehouse, not so fast.

    I have a few Hobby King batteries and gear, and the batteries have actually been better than the gear. The cameras/transmitters are finicky. The only battery I had problems with was one that I ran down to zero through my own damn fault. Drained LiPo batteries do not take a charge, and trying to do it may cause fire.

    A charger that blows up may cause battery fire, true. However, I'd expect that any fire or damage attributed to a nominally working charger, is more likely user error. Unless there are horrible firmware bugs, chargers are chargers -- they provide a somewhat-regulated voltage and limited current to the battery, according to a certain profile.

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