how to choose battery, its capacity

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how to choose battery, its capacity

Postby nikita » Sat May 03, 2014 11:10 am

hii everyone! this is my first post.
i am a student making electric car for racing purpose.
can anyone help me to decide how to choose a battery for vehicle? what are the parametera we should see for selecting a battery?

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Re: how to choose battery, its capacity

Postby timpootle » Sat May 03, 2014 9:09 pm

The numbers tell their story. First you need to know what voltage your motor/controller will run at. Then you need to know what current it will draw, at continuous and peak power. Finally how far or how long do you need to go on one charge?

A 48v 100Ah battery pack has 4.8kWh of stored energy ( using the basic law Power= Volt x Amps)

If your 48v motor draw 10 amps, your pack will feed it for 10 hours. If the motor draws 100 amps it will last for 1 hour. This theoretical capacity drops dramatically if you are using lead-acid batteries which suffer from the Peukert effect at high current draws. Lithium batteries do not suffer in the same way.

Tell us more about what you are hoping to achieve, and we should be able to advise more.
Tim Crumpton

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Re: how to choose battery, its capacity

Postby Grumpy-b » Sun May 04, 2014 8:30 am

As well as how much stored energy you want, you need to understand at what rate you need to extract it.
If you need to draw 200amps for a few seconds then you would probably need 3to 5 times the battery capaity with lead acid than you would with the latest Lifepo4 cells. That could equate to hundreds of Kg in weight difference. The latest Lifepo4 can readily deliver 5c and 10c for a few seconds. Lead acid will struggle to give a couple of C with any decent life span.
Voltage sag is important, and while lead acid can drop on a continuous curve and keep going down, Lifepo4 will drop and stay virtually constant before finally droppingoff a cliff. Like using a battery drill, they work great then virtually just stop. Thats the lithium type of curve. But your torch with older cells in it will get dimmer and dimmer and so on right from the start.

So what are you trying to achieve, top speed, acceleration, how long for, what sort of vehicle. Budget, AC DC, what system voltage are you considering.
Has it to keep working for years or a couple of runs and thats it done.


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Re: how to choose battery, its capacity

Postby GregsGarage » Mon May 05, 2014 9:16 am

Basically you will need to know how much fuel you require for the race and what the duration of the race will be. The fuel is measured in watt/hours and ideally your pack will be sized for the amount of Wh or kWh you are going to use. The duration of the race will tell you what the average discharge of the pack is going to be. A 1 hour race will discharge the pack at 1C, a 15 minute race will discharge the pack at 4C on average. But your peak discharge (and maybe charging if using regen) will be much higher depending on what rate of acceleration (and deceleration) you wish to achieve. So your batteries need to be able to handle the average discharge for the duration of the race and also peak discharges during acceleration and peak charging if you have any sort of regenerative braking putting energy back into the pack when slowing for corners. Sadly you get into a bit of a chicken and egg problem, you need a pack to test the vehicle to see what size pack you need to buy. If you can borrow a pack for testing or even get some used car starting batteries from a local car breakers that may help you get the data you need to size your pack.

Greg Fordyce

Daewoo Matiz

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