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Snoopy
08-05-2015, 07:50 PM
Hello, given two batteries: Battery A has more mAh (e.g. 2000) than Battery B. However, its C value is lower. The mAh*C values are almost the same. Which battery do you recommend and why? Thanks.

r3n33
08-05-2015, 09:40 PM
mAh tells you the capacity of the battery and the C rating or continuous discharge rating tells you how much current the battery will be able to supply to your project.

jwatte
08-05-2015, 09:43 PM
If the C rating of battery A is good enough for your application, then the higher capacity is going to give you more runtime.

However, I would expect battery A to weigh more, which may or may not be a consideration for you.

Snoopy
08-06-2015, 12:09 AM
Thank you. How can I tell if the C rating is good enough or not? I recall somebody mentioning that it is better to get a battery with higher mAh*C value. Is that right?

Zenta
08-06-2015, 02:50 AM
It depends on how much current you need. Like jwatte said, batt A will give you longer runtime since it has more energy. But if weight is important batt B might be a better choice. In general it's better to have a battery that can deliver more current.

jodell
08-06-2015, 11:15 AM
If I recall, 1C is the milliamps that the battery can put out in 1 hour. For example, if you had a 1000mAh battery then 1C is 1000mA or 1 amp. If you had a 2000mAh battery then the same 1C rating would provide 2 amps of continuous current. The C rating by itself is meaningless without also factoring in the mAh rating.

You need to determine how many amps your project needs at max load and then get a battery that has at C rating that will provide at least that much power. As long as the C rating is high enough, going for a C rating higher than you need doesn't give you anything extra. When choosing a battery, I usually determine how much current I need then then find the largest battery based on mAh that will fit in the available space.

jwatte
08-06-2015, 11:50 AM
How can I tell if the C rating is good enough or not?

How long do you need to run? Rate the battery for enough mAh that it can sustain the average current for at least that long.

How much current will your motors/servos/controllers/equipment draw? On average? At peak?
Rate the battery so that the C rating times mAh capacity divided by 1 hour is at least twice the average draw, and higher than the peak draw.

Now, you probably realize that the battery is too heavy, so you have to compromise on runtime, and re-do the math (to reduce weight, lower the mAh, increase the C to compensate for draw.)

For most robots, 10 C is a fine rating, and most hobby/RC batteries you get these days are more than that anyway. High C ratings are mainly for very high peak draw applications, like big brushless motors in RC cars, or quadcopter fliers.

ArduTank
08-06-2015, 01:25 PM
Quick example: HS Walker ran on a 20C 2200mAh battery, and only killed it because of being left on :(

However, my twin engine RC aircraft requires at minimum a 2200mAh 30C LiPoly (2.2Ah X 30C= 66A continuous draw)
The twin runs 30A power packs (motor+ESC) the 5000mAh I put in it is just bonus cap so I can fly for more than 7-10 min at a time.

The batt for HS Walker only cost me \$20. (Will give link if asked)

Snoopy
08-08-2015, 08:33 PM
Thanks for the replies. I have two confusions. First, should I choose a battery with higher mAh or higher mAh*C. 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?

Zenta
08-08-2015, 09:50 PM
No, two batteries with the same mAh but different C will run out at the same time under the same discharge current. The only difference is that the one with higher C can deliver even more current. Still they will have the same amount of energy (wattage).

jwatte
08-09-2015, 11:59 AM
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.

Snoopy
08-10-2015, 06:51 AM
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

jwatte
08-10-2015, 10:58 AM

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.

Snoopy
08-10-2015, 05:59 PM
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. HobbyKingâ„˘ X200 AC/DC Touch Screen Smart Battery Charger 10A 6S (US Plug)

6. Turnigy E3 Compact 2S/3S Lipo Charger 100-240v (US Plug)

ArduTank
08-11-2015, 05:58 AM
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.

Snoopy
08-11-2015, 11:03 AM
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?

jwatte
08-11-2015, 01:28 PM
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.

tician
08-11-2015, 06:49 PM
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.

Snoopy
08-11-2015, 08:12 PM
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?

jwatte
08-11-2015, 08:45 PM
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.

Snoopy
08-13-2015, 07:38 AM
Thank you. Hobby King only allows two payment methods: PayDollar (No Paypal) + 2.93% and Paypal Pay by VISA. I heard lots of horrible things about Paypal when troubles come. Never heard of PayDollar. Could be even worse. Which payment method is better?

For charger, how is the G6P compared with the IMAX B6AC V2 for charging batteries of over 4000mAh? The former does has more Watt.

http://www.gforce-hobby.jp/products-en/G0025.html

https://www.hobbyking.com/hobbyking/store/__58285__IMAX_B6AC_V2_Professional_Balance_Charger _Discharger.html

CreedyNZ
08-13-2015, 07:52 AM
Hi,
My experience with PayPal has only been good so far. I had a package sent from a supplier in China that went missing and PayPal refunded no problem.

jwatte
08-13-2015, 11:28 AM
PayPal is fine-ish these days, especially for buyers. Certainly better than no-name foreign payment methods...

Snoopy
08-14-2015, 07:13 PM
Thanks for all.