how to discharge battery

Discussion in 'Power Electronics' started by turbogt16v, Sep 21, 2016.

1. turbogt16v

176
4
Mar 27, 2015
hy
how to discharge battery 1.2v rechargeable ,with 3mm diode (what resistor)
to check for ah capacity (to see if its fake Chinese)

2. Harald KappModeratorModerator

11,803
2,749
Nov 17, 2011
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...).

3. turbogt16v

176
4
Mar 27, 2015
ok i am a little noob here.resistors measure in ohms ,right
and its hr6/nimh 1.2v/2500 mah duracell

4. Harald KappModeratorModerator

11,803
2,749
Nov 17, 2011
Right.
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.)

5. turbogt16v

176
4
Mar 27, 2015
ok i have loot of different resistors.what value in oms do i need (ohms)

6. Harald KappModeratorModerator

11,803
2,749
Nov 17, 2011
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.

7. turbogt16v

176
4
Mar 27, 2015
ok so i need 5 Ω ,and it will discharge in about 1 hour

8. BobK

7,682
1,688
Jan 5, 2010
No, at 5Ω it will discharge in 10 hours. Assuming the capacity is correct.

Bob

9. turbogt16v

176
4
Mar 27, 2015
ok i put 0.5 omh and it is to hot to touch,
is there easy way to see the capacity on these batt

10. BobK

7,682
1,688
Jan 5, 2010
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

11. Bluejets

5,141
1,075
Oct 5, 2014
I'm curious about what you intend to make comparisons with to get a result you claim.

12. Harald KappModeratorModerator

11,803
2,749
Nov 17, 2011
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.

13. Colin Mitchell

1,416
314
Aug 31, 2014
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.

14. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,500
2,840
Jan 21, 2010
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.