Readout for battery?

[aardvark5]

Somebody must have come across this before and solved it.
I want to buy another 250 watt Cyclotricity kit (I like them) but I'm not bothered about a computer or pedal assist so basically motor, bottle battery and throttle.
How would I go about displaying an accurate readout for the battery?
I read somewhere that an average of 20 watts are used per mile so it would be great if I could read how many watts are in the battery.

Thanks

I've just found a couple of You Tube videos but any help/advice appreciated.

 

[Alan Quay]

What you need is a watt meter:
http://pages.ebay.com/link/?nav=item.view&id=390932345418&alt=web

It won't tell you how much is in there, but it will let you know how much has been taken out.

 

[aardvark5]

I'm confused Mike.
I've done a bit of reading and that meter would go between the controller and battery so surely when you turn the battery on it would show how many Watts/Volts you have in your battery and it would steadily decrease over time?

 

[Alan Quay]

There's no accurate way of determining how much capacity a battery has just from volts (the only thing you can measure while it's idle).

If you know (because you've measured it previously) how much capacity it has, and you know how much you've used, then you know how much remains.

Another way to think about it is that you have a tank full of stuff. You know how high the stuff is from a dip stick, or by measuring pressure (e.g Volts), but don't know what shape the tank is.

Batteries are all different shapes when new. After you use them, the shape changes a bit more.

All of this means that 'dipping the tank' is a very in-accurate way to determine how much is in the thing.

 

[aardvark5]

Still a bit confused.
Is there a way to see how many watts there are in it with that device, perhaps when I'm moving?
I read somewhere that to calculate your watts you multiply the volts by the Ah so in my case 36v x 9Ah = 324 watts.
An average wattage usage per mile is 20 watts so I would get 16.2 miles which isn't far off to be honest but I've never ran it so it has no power.
It would be great to see how many watts I have left so I can make a judgment on recharging.
Obviously I realise there might not be a great accuracy and I would have to experiment with the figures.

 

[Deleted member 4366]

You can calculate how many wh there should be. You can then see how many there actually is by running your battery right down with the wattmeter connected. After that, you use the wattmeter to show how much you've used, so you can see how much you have left.

 

[flecc]

Still a bit confused.
Is there a way to see how many watts there are in it with that device, perhaps when I'm moving?

To help you to understand, when a battery is charged, the electricity is changed into a chemical state which gives the potential to convert back to electricity when we need it. Therefore batteries are chemical devices, not electrical, so an accurate immediate electrical readout of what charge is in a battery isn't possible.

For example, in the days when we used to top up lead acid battery electrolyte with distilled water, the charge in a battery could be measured with a hydrometer. That drew out some of the electrolyte to show its specific gravity, which was a chemical measurement.

Electrically, we can only measure the charge in a battery accurately by using it and measuring it as it comes out.
.

 

[aardvark5]

All coming clearer now so thanks.
I'll get one of those meters that Mike linked so I can have a play.