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Old 10-11-2009, 11:45 PM
mxmtech mxmtech is offline
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Default I get the Peukert Effect

Electricity is a byproduct of the chemical reaction which goes on inside of a battery.

If I go as fast as I can I use about 15 amps and go about 30 Km/hour but I can only travel about 4 kilometers. If I maintain a steady current of about 5 amps I travel about 20 km per hour and I can go 22 kilometers.

My battery pack was pretty dead and it took .26 KWH to charge it.

KWH = (AH x Volts) / 1000
So therefore .26 = (10.8333 x 24) /1000

I am satisfied that my 24 volt 12 AH SLA battery pack is working properly.

But it does bring up another issue that causes me great dissatisfaction. The temperature here was 2 degrees above freezing today and my weaker battery pack gave out substantially sooner than expected. I knew that lead acid batteries lose their power faster in the cold, I have read that NiMh batteries do the same and I've seen here on this site today that the Lion batteries are also subject to this flaw.
That leaves NiCad's as the only option for sub-zero temperatures.

It's going to snow within the next couple of weeks here where I live and I'll have to put the studded snow tires on my e-bicycle.

PS I would be really surprised if other battery technologies did not suffer from the Peukert Effect also.
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Old 10-12-2009, 01:42 AM
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IamIan IamIan is offline
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Default Re: I get the Peukert Effect

all chemical reactions are effected by the cold... thus any chemical battery of any chemistry type will be effected by the cold.

Now some are more effected than others... some manufactures list cold temperature operations , but not all ... if not you could test your batteries by doing some discharge tests on them in a freezer or something that lets you regulate them to a specific colder temperature... I would expect you would only have to test one battery of a given specific model and type to get the general effective operating performance under those conditions... So if you want a usable 5kwh... it isn't the rating of the battery ... it is how much it gives you under the conditions you will put it in ... that includes the temperature as well as the discharge rate.

Other energy storage devices that do not depend on chemical reactions ( like capacitors ) will not be effected by the cold like chemical energy cells are... but people use chemical energy cells becuase to-date they offer more kwh of usable energy per $... per kg ... or per L... in most applications.

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The Peukert effect comes mainly from two different things that happen at the same time.

Some battery companies ( but not many ) will publish the Peukert k value for a specific battery model and type... if they give you this than you can get a good idea of how the battery will react under those current rates... don't forget to also include losses due to temperature effects.

#1>
the power dissipated in watts = The Current in Amp squared * The Resistance.... W = I*I*R ... notice that voltage is not a varable ...

this means at 10 amps and 1 ohm you dissipate/loose ~100Watts due to resistance.... but at 20 amps and 1 ohm you dissipate / loose ~400Watts due to resistance.... so at the same voltage you increased your power output from the batteries by 2x but you increased the resistive losses by 4x... and so on... this power is gone.

This is the reason people use higher voltage systems to reduce resistive losses... 10V * 10 A = 100 Watts .... but 100 V * 1 A = 100 Watts... so you can get the same power output at higher voltages while not loosing as much to resistive losses.

#2>
Chemical reactions are slower than electrons.

Kind of makes sense... while it is the chemical reaction that causes the electrical current ... each chemical reaction is far slower than the rate at which you can drain the battery.

This is why after you 'park' a EV for a little while it 'grows' amps ... this is just the chemical reactions catching up with the electrical discharge.

This effect can also be measured... put a battery under a load... then graph the voltage of the cell over time from the moment you disconnected the load... ideally use a multi-meter that has a PC interface so you can log and view it easily on the PC...

You will see the Voltage rise up quickly once the load is removed... then it slowly continues to rise in voltage... the slow rise in voltage tends to be a logarithmic graph ... which just means that the rate of change gets smaller and smaller over time as it approaches its 100% maximum value... of the highest value it reaches before the voltage begins to drop again due to self discharge.

By looking at the logarithmic graph you plotted from a test cell you can use it to determine the yet unused remaining capacity in the battery cell ... and you can estimate how long it will be before you can use how much of it... because it is logarithmic the last 10% takes much longer than the first 20% and the last 1% takes much long than the first 10%.

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If you just want to know the usable service life under some specific conditions like at ___ Amps and ___ Temperature.... If estimates are not enough ... then the easiest way to know if to test the cell under those conditions ... or close to it .... you want to test the cold ... but the battery in the freezer until a thermometer reads that it is as cold as you want to test the performance at... then discharge it at the kind of current rates you are expecting .... you don't need to test your whole traction pack ... as long as they are all the same specific battery model and type ... you only need to determine the performance of one under those conditions to then be able to apply that to the whole traction pack together.

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I've heard it said that at freezing temperatures a PbA EV battery pack looses about ~1/2 its usable capacity.... I don't know how accurate that is for specific models and types... that would require those models and type to be tested... it is just what I've been told by some of the people I know who run on PbA EV battery packs...

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Remember the battery pack has a good amount of thermal mass... if you insulate it even bellow zero temperatures will not hit you nearly as much.

Just don't forget about that insulation come those hot summer days.
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