DIY Electric Car Forums banner
Cancel

Calculating the C rating and Amp output of a battery, Im learning

TeZla
1008 3
Im looking at the Westart NCM120A battery module to build my battery pack with.
Im trying to calculate some of the values of the pack, But im learning as I go, So im trying to understand.
Perhaps someone could tell me if im going wrong and explain it like im 5.

On the spec sheet for the module the nominal capacity is listed as 120Ah @ 0.3C discharge
A little further down the discharging current is listed as 2C continuous.
On the website of the seller they list the burst discharge as 5C

Does that mean that at 2C, the amps discharged in a continuous situation are approximately 790A?
If the module discharges at 120A at 0.3C, then 120A multiplied by 3.3 (0.99C) would be 396A, then multiplied by 2 (1.98C) equals 792A
If I am correct in my calculation, then 792A draw would be an acceptable discharge current, with a short burst at 5C being 1,980A

Is this calculated correctly? Or am I very wrong in my approach?

If this is right, then at 360v (nominal pack voltage with 100 modules) and 790A, 284kw of power is what the inverter would be putting through the motor, which is basically full power for a Tesla SDU with the inverter never really needing to draw any more than 2C of battery output, Which should give the pack a long and healthy lifespan.
Even at my smallest pack size, (88 modules) the nominal pack voltage would be 317v and 790A, for 250kw of power. Still a very healthy figure and well within the operating specs of the motor.

My other question is, From this, how do I calculate the total kWh of the pack? from that I should be able to work out a rough expected range.

Attachments

Save Share
Reply Quote
1 - 4 of 4 Posts
J

· Registered
Joined
·
1,080 Posts
#2 ·
Very wrong

There is no way to state Ah capacity without giving the discharge rate of the test used. So 120Ah at 0.3C might be 110Ah at 2C, or 90Ah at 5C. See Peukert's Law.

Assume min/max voltage spec'd as the boundaries as well.

At 120Ah, 0.3C is 36A.

So if 110Ah, 2C becomes 220A

And if 90A 5C means 450A

Amps * Volts gives Watts

Ah *V = Wh

divide by 1000 to get kWh

For range only use 80% of capacity, and realise that will get reduced maybe 20% per year, accumulatively compounding.
 
TeZla

· Registered
Joined
·
124 Posts
Discussion Starter · #3 · (Edited)
Cheers John,

Still learning lol, I did kind of have a feeling I had it wrong, Im glad I asked here

When I redo my calculations using your info, I get a 22kWh pack, with an estimated range of about 80kms in ideal conditions.
Looks like im planning the battery from scratch again...
 
1 - 4 of 4 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
About this Discussion
3 Replies
3 Participants
Last post:
remy_martian
DIY Electric Car Forums
A forum community dedicated to DIY electric car owners and enthusiasts. Come join the discussion about electric vehicle conversions, builds, performance, modifications, classifieds, troubleshooting, maintenance, and more!
Full Forum Listing
Explore Our Forums
All EV Conversions and Builds Batteries and Charging EVDL List Technical Discussion eBay EV-Related Auctions
Top