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Old 09-10-2020, 11:24 PM   #1
MyCousinVinnie
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Default nissan leaf amp hours and watt hours?

i did a leaf 14s2p conversion. my bms wants to know my amp hours and watt hours. seller measured the amp hours at 120ah. 120ah 4.2 voltage (the max) times 14 cells equals watt hours of 7056? or do i not use the maximum volts to calculate watt hours? thanks.
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Old 09-11-2020, 06:49 AM   #2
DaveTM
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Default Re: nissan leaf amp hours and watt hours?

Watt\Hr rating is AH x V = WH

So AH=120. V=(4.2x14=58.. So 120x58.8= 7,056.

Why the BMS wants or needs this info I'm not sure.

But IMHO the key "number" to watch is the pack voltage. (And let the BMS "worry" about each individual cell's voltage.)

When my pack drops to 50V, it's time to charge as there's not much "safe juice" is left to go anywhere.
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Old 09-11-2020, 07:37 AM   #3
MyCousinVinnie
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Default Re: nissan leaf amp hours and watt hours?

it needs to know in order to calibrate the state of charge. i tried posting that elsewhere - https://buggiesgonewild.com/showthread.php?t=168847 but didn’t get any response. this all came about from upgrading bms firmware.

what’s strange is that chargery says i should use 3.85 instead of 4.2 “because battery is discharged at 3.85”. i don’t understand that.

i’m also wondering how accurate the seller’s measurement is but at this point given that it’s installed i’m not sure i want to pull it out and measure it.

i’m hoping someone with more electrical knowledge can explain
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Old 09-11-2020, 08:21 AM   #4
PingEye3
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Default Re: nissan leaf amp hours and watt hours?

I'm with DaveTM on this one, I refuse to deal with trying to use an SOC calculation when NONE are reliable. I use the Digital Voltmeter as the gas gauge, no calculations need there or wondering if it is set up to get correct readings. 27 Leaf packs completed to date, and only two with Zeva BMS which has SOC and I have never cared whether they were accurate or not.
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Old 09-11-2020, 08:22 AM   #5
Sergio
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Default Re: nissan leaf amp hours and watt hours?

Typically WH is calculated based on the battery "nominal voltage" which for the LEAF is 3.6v/cell.
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Old 09-11-2020, 08:43 AM   #6
Volt_Ampere
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Default Re: nissan leaf amp hours and watt hours?

It makes no sense to use max voltage in calculating watt hours because the battery is only at that max voltage at the start of discharge. It rapidly drops some and then continues to drop during discharge. As Sergio said, you should use the nominal voltage to calculate watt hours. It is only a spec anyway - not all that useful. If the BMS does coulomb counting to determine SOC then knowing approximate watt hours is useful. In a golf cart application where current is all over the place, coulomb counting is never very accurate.
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Old 09-11-2020, 11:01 AM   #7
MyCousinVinnie
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Default Re: nissan leaf amp hours and watt hours?

So, I suppose my next questions are: (1) is 120 amp hours correct? (2) should i use the 3.85 that chargery told me to use (I told them it was a Leaf battery) or the 3.6?, (3) what, if anything should I do about SOC once this BMS calibrates it? My options are set it at zero, or set it at some other percentage. The manufacturer's recommendation is set it at 20%. However, that makes no sense to me if I am going to raise level of the lowest cell discharge and lower the level of the highest charge.

My plan was to set at 4.05 and 3.25 after calibration. Lots of folks on here have set there BMS to limit the range of cell discharge in a Leaf to those levels in order to preserve the battery. However, if I do that, and I leave the battery at 20% SOC, then my cart will cut off well before any cell reaches 3.25 because it "thinks" 3.25 is 0%. isn't that right?

Thanks so much.
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Old 09-11-2020, 01:41 PM   #8
Volt_Ampere
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Default Re: nissan leaf amp hours and watt hours?

cutoff based on SOC is different than a cell voltage cutoff. The BMS will cut off if ANY cell gets below the cell minimum. SOC is the total pack. It's best for the battery life to run no lower than 20% SOC and charge to no higher than 90%. This reduces range but extends battery life significantly.
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Old 09-11-2020, 03:26 PM   #9
MyCousinVinnie
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Default Re: nissan leaf amp hours and watt hours?

Understood, but if I change the cell maximum to 4.05 and the cell min. to 3.2, doesn't that change my state of charge, assuming they stay fairly balanced? Or does it not? If it does, I'm going to cut off well above 3.2. Also, still need to figure out which figures to use for amp hours and watt hours.

I've looked back at the original specs of a Nissan Leaf battery, first generation. It had a max of 4.2volts per cell. Min of 2.5. Nominal voltage was 3.75. Amp hours 66. So, the math is

I have 14s2p so the math is 3.75v times 66aH x 14 cells x 2 parallel = 6930.

Should I just use that?
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Old 09-11-2020, 11:12 PM   #10
Volt_Ampere
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Default Re: nissan leaf amp hours and watt hours?

That seems reasonable but I do not know how your BMS works. My self designed BMS does SOC based on the Leaf discharge curve (SOC vs voltage). This is probably good to 5% over the range that I use it.
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