how to discharge battery

[turbogt16v]

hy
how to discharge battery 1.2v rechargeable ,with 3mm diode (what resistor)
to check for ah capacity (to see if its fake Chinese)

 

[Harald Kapp]

No diode needed. Use a resistor that draws a current between AH*0.1 and AH*1 (e.g. nominal AH=1200mAH -> use 1.2 A), measure the time from full voltage (freshly charged battery) to "empty" battery. The exact voltage for the "empty" battery depends on the chemistry (lead, NiCd, NiMh, Lithium...).

Read more e.g. here.

 

[turbogt16v]

ok i am a little noob here.resistors measure in ohms ,right
and its hr6/nimh 1.2v/2500 mah duracell

 

[Harald Kapp]

resistors measure in ohms ,right

Right.

its hr6/nimh 1.2v/2500 mah

Which means the discharge currrent should be between 250 mA and 2.5 A. The better the discharge current matches the real current when the battery is used in the real life application, the more precise will be the determined capacity. Battery capacity is not a fixed value but depends on parameters like currrent drawn temperature, age etc.)

 

[turbogt16v]

ok i have loot of different resistors.what value in oms do i need (ohms)

 

[Harald Kapp]

R = V/I = 1.2 V / 2.5 A ~ 0.5 Ω
Note that the resistor will dissipate P = V*I = 1.2 V * 2.5 A = 3 W!
3 W requires a power resistor or multiple low power resistors in parallel (or series).
Using 250 mA instead results in 5 Ω and 300 mW, much more suitable, but discharge time will be approx. tenfold.

 

[turbogt16v]

ok so i need 5 Ω ,and it will discharge in about 1 hour

 

[BobK]

No, at 5Ω it will discharge in 10 hours. Assuming the capacity is correct.

Bob

 

[turbogt16v]

ok i put 0.5 omh and it is to hot to touch,
is there easy way to see the capacity on these batt

 

[BobK]

Depends on how long you want it to take. Use 5Ω and it will not get so hot, but the test will take 10 hours instead of 1.

Edited to add: Anything you do to drain the battery in 1 hour is going to produce the same amount of heat, unless it is doing some actual mechanical work, like lifting a weight.

Bob

 

[Bluejets]

I'm curious about what you intend to make comparisons with to get a result you claim.

 

[Harald Kapp]

is there easy way to see the capacity on these batt

Measuring the capacity of a battery is tricky at best. If there were an easy way, it could be used to reliably indicate remaining battery capacity when in use. As you may well know, battery level indicators are not so reliable at all.

Read the information in the link I gave. Imho you can be satisfied with a +-10 % error on your test result.

 

[Colin Mitchell]

Here is what you need:
It's and automatic tester :

RECHARGEABLE BATTERY CAPACITY
This simple circuit tests the capacity of a rechargeable cell.
Connect a 4R7 (yellow-purple-gold-gold) resistor across the terminals of a clock mechanism and fit a fully charged rechargeable cell. Set the hands to 12 O'Clock and the clock will let you know how long the cell lasted until the voltage reached about 0.8v.
Now fit another cell and see how long it lasts. You cannot work out the exact capacity of a cell but you can compare one cell with another. The initial current is about 250mA for a 1.2v cell.

 

[(*steve*)]

For capacity testing, a constant current load will allow you to measure in mAh, and a constant power load will allow you to measure in in Wh.

A constant power load is rather more complex and it wouldn't be my first recommendation.

In either case you need to specify an end point. For rechargeable batteries that is unlikely to be 0 volts as this may damage the battery. For lithium ion or lithium polymer etc, you might choose something like 2.8V.