C rate help please

[ghost1001]

Hi All,

I am new here and to ebikes in general and need help with C rates. I have read a lot including the battery FAQ and searched around but still don't get it.

If I am using 4 of the following packs, paired in series and then those pairs in parallel for a total of 44.4v and 20ah what will the c rate be in amps?

If you can also explain to me how you get that figure it would be greatly appreciated so i can look at other packs for my build. I plan to use an astro motor and have been told I need 200 amps in bursts.

Spec. Turigy nano-tech
Capacity: 5000mAh
Voltage: 6S1P / 6 Cell / 22.2V
Discharge: 25C Constant / 50C Burst
Weight: 769g (including wire, plug & case)
Dimensions: 163x49x48mm
Balance Plug: JST-XH
Discharge Plug: HXT4mm

Thanks
Kevin

 

[Ykick]

Your Ah calculation needs some work. If I understand your pack configuration correctly you're assembling 12S2P which would be 10Ah labelled capacity.

Current handling capability is doubled for every parallel connection of these bricks. I tend to use "brick" referring to what you call packs. This way we can reserve the term "pack" for reference to the total bike pack.

Focus on "S" and "P" indications to decode the eBike pack configurations.

 

[gwhy!1]

Hi All,

I am new here and to ebikes in general and need help with C rates. I have read a lot including the battery FAQ and searched around but still don't get it.

If I am using 4 of the following packs, paired in series and then those pairs in parallel for a total of 44.4v and 20ah what will the c rate be in amps?

If you can also explain to me how you get that figure it would be greatly appreciated so i can look at other packs for my build. I plan to use an astro motor and have been told I need 200 amps in bursts.

Spec. Turigy nano-tech
Capacity: 5000mAh
Voltage: 6S1P / 6 Cell / 22.2V
Discharge: 25C Constant / 50C Burst
Weight: 769g (including wire, plug & case)
Dimensions: 163x49x48mm
Balance Plug: JST-XH
Discharge Plug: HXT4mm

Thanks
Kevin

dont series the batteries to 44.4v then P them ( this is bad , other may disagree if so I would love to here there reasons !) P two 22.2v first them S them to make up the voltage to 44.4v. the C rating is the multiplier of the total Ah's of the total battery, if you are doing a pack with just 4 x 22.2v 5000mah batteries then the total pack Ah's will be 10Ah's not 20Ah's so the total C rating of a 10Ah pack will be C rating x total Ah's and this will give you the total discharge ratings for the pack. The batteries you have linked to configured as a 44.4v 10Ah pack the max discharge rates work out as 10 x 25 = 250A const and 10 x 50 = 500A burst.
hope this helps

 

[teslanv]

Another explanation of "C" rating is this:

The batteries you specify are rated for 25C Continuous and 50C Max. This means that for every Amp-hour of capacity they can output 25 Amps of current continuously and 50 Amps of current peak. - so each 5Ah "pack" can output 5X25 = 125Amps of current continuous. For Most ebike applications, this is WAY overkill. My BBS02 Mid Drive kit only requires 25 Amps max current.

The batteries you spec are perfectly fine (with proper safety precautions & charging/discharging procedures) to use for an ebike. They are just gratuitously powerful.

Where you can run into trouble with "C" ratings is when using LiFePO4 batteries, as they are usually only rated for 1C-2C continuous and 3C-4C max. So you need a really BIG LiFePO4 battery to supply 25A to a motor. That being said, LiFePO4 batteries are some of the longest-lasting batteries and a great value when you cost-average them over their lifespan. HobbyKing LiPo, not so much...

 

[ghost1001]

OK, I see how my total AH capacity was off, because I doubled the voltage from 22.2 to 44.4 so therefore it stays at 5Ah..... and now i get the C rate part.....I think

So C rate is based on the original brick unit? And regardless of configuration, 2 bricks 4 bricks, serial parallel, etc. the C rate remains the same as the original single "brick" that was selected?

Using the same example, regardless of if I use 4,6,8 of these the discharge rate will always be 125A constant and 250A burst, correct? Why don't they just say that?

Capacity: 5000mAh
Voltage: 6S1P / 6 Cell / 22.2V
Discharge: 25C Constant / 50C Burst


As for the configuration you are saying make 2 big 22.2v bricks wired parallel and then put them together in series to make 44.4v as opposed to series first then parallel. What is the reason for this?

Kevin

 

[teslanv]

Using the same example, regardless of if I use 4,6,8 of these the discharge rate will always be 125A constant and 250A burst, correct? Why don't they just say that?

Not quite. the "Rate" will be the same, but Amps supplied will vary, depending on how big of a battery you build. So for 25C continuous rated packs here are your Available Current outputs:

5 Amp-hour = 125 Amps Continuous
10 Amp-hour = 250 Amps Continuous
20 Amp-hour = 500 Amps Continuous

Make sense?

 

[Ykick]

Lacking proper parallel cables and/or adapter boards many of us have done serial 1st and then parallel those strings. There's little performance difference but in the event a cell in one series string dies and shorts (common cell failure, btw) you would then effectively have 12S paralleled with 11S which carries the potential of overcharging the 11S series string.

Shorted cells in parallel can and will probably cause balance wires to melt/burn but at least you'd know there's a problem before things get too far out of hand.

 

[ghost1001]

C rate and pack config.....Got it!

Thanks guys, i'm sure I will have more questions....

Kevin

 

[ghost1001]

OK, one more thing, what if i use the brick above to build my pack so I would have 10Ah and 250 amps continuos on tap......but what if I don't draw that much? Is there a disadvantage to the higher c rate?

The main reason I ask is that most of the bricks I am looking at say 25c-50c.

Kevin

 

[teslanv]

Just higher cost.

Otherwise like driving a 5.0L engine at 15 miles per hour.

These 4S Hard Case packs are the current cheapest per watt-hour LiPo packs.

http://www.hobbyking.com/hobbyking/...P_14_8v_20C_hardcase_pack_USA_Warehouse_.html

If these were in stock, I would say pick up two, and you'd be set:

http://www.hobbyking.com/hobbyking/...0mAh_Multi_Rotor_Lipo_Pack_US_Warehouse_.html

 

[gwhy!1]

Lacking proper parallel cables and/or adapter boards many of us have done serial 1st and then parallel those strings. There's little performance difference but in the event a cell in one series string dies and shorts (common cell failure, btw) you would then effectively have 12S paralleled with 11S which carries the potential of overcharging the 11S series string.

Shorted cells in parallel can and will probably cause balance wires to melt/burn but at least you'd know there's a problem before things get too far out of hand.

I would just like to add to this : also it is less likely should 1 cell fail in one pack/block of 22.2v (6s2p) that it will take out the whole 12s2p pack , so only 2 of your 4 batteries will be damaged instead of the whole 4 . also good practice is to always check cell levels with a cell level meter before putting them on charge and again before you actually start to use them on the bike.

by using a much higher C rate than you really need also helps with the usefully life span of the battery ( gives it a bit of a easier time )