# Batttery life

1. Nov 5, 2004

### darkar

Hello !

Anyone have any idea how to test the total avaible energy can be drawn by battery?

Thx

2. Nov 5, 2004

### Chi Meson

Yes, but only by draining the battery, and then it would be "dead."

One way: have the battery connected to a heating element. Put the element in a known amount of water (contained in an insulated cup) and find the change in temperature of the water after the battery is "used up."

There is an equation to translate the chnage in temperature of water into the amount of energy transferred.

3. Nov 5, 2004

### darkar

this means u cant get the result until u wait for a long time?
Is there any faster way?

Well, i suppose the formula is Q = mc(theta) right?

4. Nov 5, 2004

You can't see how much energy it will produce without draining it. If you want a 'quick' result draw a large current.

5. Nov 5, 2004

### Integral

Staff Emeritus
Batteries have a life rating, it is given in AHr (Amp hours) or mA hr (milliAmp hours) this it exactly what it sounds like. A 10 A Hr battery can draw 10 Amps for 1 hr or 1 Amp for 10 hr. This rating should be available for any battery.

6. Nov 9, 2004

### darkar

What if u want to test whether the given value is correct?

7. Nov 9, 2004

### Integral

Staff Emeritus
Assuming that you are testing commercially available batteries you would want to test several, set up a known load with a good voltmeter and/or an ammeter. Drain several and see what you get. You should be able to see a range of values around the manufacture specs. Check the web page of the manufacturer for detailed specs.

8. Nov 9, 2004

### rcgldr

There are battery dischargers / chargers that measure this. The most common ones to find are those in hobby shops, which are used for radio control models (cars, helicopters and planes).

The capacity rating of batteries is based on a typical load. Higher loads can cause the batteries to have less capacity (at least with nicads), probably because of the amount of heat generated.

These loads can get extreme on certain types of models. A 27 cell F5B LMR (Limited Run Motor) "glider" will draw about 80 amps, the motor producing a bit over 2 1/2 hp for the short periods it's on (a few seconds at a time), and the batteries get very hot. The motor doesn't need to run for long because a vertical climb at 120+mph gets the model to barely visible altitude quite quickly (for most of the time the motor is off, and the glider is flying at high speeds).

9. Nov 9, 2004

### kilowatthour

If u could calculate one or a few batteries and map out its voltage as time passes you could make a table / equation and then be able to calculate the expected life left of other batteries with equal expected ratings. There are equations / tables out there for this kind of thing relating types of batteries and capasities and life, I wonder if they are findable and available on the internet for free? some manufacturors might provide this information online but many I bet would not as many companies dont provide very detailed specs on things they make unless they have to.

Last edited: Nov 9, 2004
10. Nov 9, 2004

### snbose

The testing procedure will be a cumbersome one. You will need to measure the Voltage across the load and Current at regular intervals, take their products and plot it against time. Then measure the area under the curve. You may use a Wattmeter so you don't have to measure V and i and multiply.

Ampere hour will not provide any insight into the energy deliverable by the battery, since the Potential diff reduces with draining of battery (actually the internal resistance grows).

11. Nov 9, 2004

### Gokul43201

Staff Emeritus
You'll find a bunch of datasheets here.

12. Nov 9, 2004

### Staff: Mentor

snbose, a decent multimeter (or two) will output voltage and/or current at regular intervals, straight to a spreadsheet. Its a peice of cake. In any case, a wattmeter is just a voltmeter and ammeter put together.