Re: [EVDL] LiFePO4 efficiency; WAS: amusing problem

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Re: [EVDL] LiFePO4 efficiency; WAS: amusing problem

Hi Guys (and Gals),

Blast from the past I know, however,
Bruce has been busy doing some round trip efficiency measurements on his EV:

energy supplied from powerpoint to energy delivered to motorcontroller is
83%.
The battery itself is ~92% efficient.

On my EV (a motorbike, hence the low wh/km) I am measuring an average of
80wh/km (averaged over the past 10'000km) measured at the powerpoint.

My cycle Analyst has allowed me to measure an average of 70wh/km discharged
from the battery, giving an efficiency for battery and charger of ~87.5%.

I have 44 x TS 60Ah cells and an Elcon 1500W charger.

discharge is mostly between 50A and 70A, as I spend much time at 100kmh.

For curiosity, I bought a infra-red temperature gun,
On the bus bars I could measure 1-2 deg C temp gain, but the cells
themselves experienced almost no heat up.

Matt

----- Original Message -----
From: "Lee Hart" <[email protected]>
To: "Electric Vehicle Discussion List" <[email protected]>
Sent: Sunday, August 22, 2010 1:26 AM
Subject: Re: [EVDL] amusing problem

>>> The only way to get >95% power efficiency on a lithium is to charge and
>>> discharge it at negligible currents, below anything usable in an EV.
>
> Matt Lacey wrote:
>> I disagree. I have measured just 0.1v worth of sag at 1C on my Thundersky
>> cells at full charge (at 1C I can hold 60mph). At ~80% DOD that increases
>> to 0.15v.
>>
>> Voltage increase when charging at 0.1C... is around 0.02v until you get
>> to 90% SOC.
>
> You need to compute efficiency over the entire range of charge that you
> are using, not just a peak at one point. Basically, it watthours out
> from driving divided by watthours in during charging.
>
> If you have a 0.1v sag on a 3.2v cell, that is over a 3% loss right
> there. If you get another 0.02v rise during charging, that's another
> percent. Then the current efficiency is not 100%, either. Another couple
> percent loss in coulombmetric efficiency and you're at that 5% loss I
> mentioned.
>
> And, I suspect these are your best-case numbers; not what it averages
> over the whole charge-discharge cycle.
>
> Do you have an E-meter or other instrumentation that is measuring
> watthours in/out? What does it say?
>
> Heat is also the "bullshit detector" of efficiency claims. What is the
> temperature rise of the pack after a charge/discharge cycle?
>
>> (not many people can charger faster than that)
>
> 0.1C is only 6 amps on a 60ah cell. Even on a 240vdc pack that's only
> 1440 watts, which an ordinary 120vac 15amp outlet can provide.
>
>> I have measured a round trip efficiency of 95% for my LiFePO4 pack (44 x
>> 60Ah cells).
>
> --
> Lee A. Hart | Ring the bells that still can ring
> 814 8th Ave N | Forget the perfect offering
> Sartell MN 56377 | There is a crack in everything
> leeahart earthlink.net | That's how the light gets in -- Leonard Cohen
>
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