Anyone knows how to design a battery discharging circuit?

[SNAG]

Hi there, I have this small little idea..

Some of the chargers out there do not have a "re-condition" feature, and hence users have to physically "discharge" the batts by draining them up in some gadgets or something...

But one thing I notice is that it is very taxing to actually discharge the battery FULLY (one would need a very long time to do this, as you have to play around with the features or lights, or whatever)

I was reading one of the articles (The Battery Shootout by some site) posted in this forum, and the test rig used a "virtual load" by using resistors as the load..

I was thinking whether is was feasible to actually design a simple discharging circuit (with resistors) to drain out the juice of the battery completely, so that one can then re-charge it again...

As I have very limited physics background, can some engineers or someone devise such a circuit?

Just my 2 cents... But do let me know if this is not feasible too..

 

[Zoomer]

Go to sls, get some resistors ~500 ohm.

The lower the ohms, faster discharge but also risk of batt burst if it heats up too fast.

P=ir
=0.003 * 500 = 1.5w


1.5watt of heat produced...

 

[roygoh]

Originally posted by Zoomer
Go to sls, get some resistors ~500 ohm.

The lower the ohms, faster discharge but also risk of batt burst if it heats up too fast.

P=ir
=0.003 * 500 = 1.5w


1.5watt of heat produced...

Your equations are not correct.

V=I*R
P= V*I = I*I*R = V*V/R

If you connect 500 ohms across a 1.2V battery, the current you get is 2.4mA. The power dissipation in the resistor is less than 3mW. To discharge a fully charged 750mAH AA NiCd battery, it will take more than 300 hours.

SNAG,

If you are using NiMH batteries, you should not need to re-condition them, as far as I know. For NiCd batteries, to minimise the memory effect, they should be re-conditioned from time to time.

If you connect the batteries in series for discharging, then the voltage is the sum of the individual battery voltages, which is 1.2V each. A discharge current of 300mA should be safe to use.

The resistor value is thus:

N*1.2/0.3 ohms, where N is the number of battereis connected in series.

The power dissipated in the resistor is N*1.2*0.3 Watts

Say you have 4 batteries and you connect them in series for discharging, the total voltage is 4.8V, and the resistor value should be 16 ohms, and the power dissipated in the resistor is 1.44 Watts.

In this case, get a 16 ohm resistor that is at least 3W rated.

The discharge time for fully charged 750mAH NiCd batteries will be a little more than 2 hours. Since you normally would not start a discharge cycle with fully charged batteries, it will take less time.

 

[Zoomer]

Oops, sorry. Was too sleepy last night.

If you are planning to make a circuit, you might be interested to add a led into it. This will tell you when the batts have neared the end of their discharge cycle.

Use a 5v led, for the example above.

You can choose the colour, but the cheapest would be red/green/yellow. Check if the led really is 5v, cos there are 3.3v versions commonly avaliable too.

 

[SNAG]

Originally posted by Zoomer
Oops, sorry. Was too sleepy last night.

If you are planning to make a circuit, you might be interested to add a led into it. This will tell you when the batts have neared the end of their discharge cycle.

Use a 5v led, for the example above.

You can choose the colour, but the cheapest would be red/green/yellow. Check if the led really is 5v, cos there are 3.3v versions commonly avaliable too.

? 5v LED?
If you are using 5v LED, how can the thing light up?
The emf of all the batteries is only 4.8v...
Or will it really light up?
Please comment..

I know that NiMH do not need to be re-conditioned.. But all my batteries were really screwed up by the previous charger...
"fully charged" voltage (i measured it to be ard 1.2v), but it lasts shorter than my alkalines!!
That's why I was thinking whether I needed to discharge all the batts before re-charging them again..

And is there such a thing as a breadboard, where I can solder all these onto a board, so that its not so messy?

 

[Zoomer]

Go to sls, ask them.
Basically, its a unprinted circuit board with holes in them.
You then solder the components on and use wire to link up. An alternative is veroboard, which has pre-printed paths but its overkill for this.

But it isn't really necessary, unless you carve neatness.

And yes, 4.8v will definately light up a 5v led. Most likely, 4v will also work.

If you can measure the voltage of your batts, I assume u have a meter or multimeter of some kind. Then, you can forgo the led or put it in for a rough gauge of the progress of discharge. Cos NiMH batts have a sudden drop off in voltage when they are almost exhausted, this will result in the led dimming significantly. When the led is off, the batts will be almost completely discharged. The minimum operating voltage of leds depend on the individual led.

Btw, if you have lithium ion batts, do not discharge them fully. Its life and capacity is shortened considerable.

If you find the resister heating up too much, get a cheap metal heatsink from slt. The <$1 ones will do.

 

[SNAG]

Originally posted by Zoomer
Go to sls, ask them.
Basically, its a unprinted circuit board with holes in them.
You then solder the components on and use wire to link up. An alternative is veroboard, which has pre-printed paths but its overkill for this.

But it isn't really necessary, unless you carve neatness.

And yes, 4.8v will definately light up a 5v led. Most likely, 4v will also work.

If you can measure the voltage of your batts, I assume u have a meter or multimeter of some kind. Then, you can forgo the led or put it in for a rough gauge of the progress of discharge. Cos NiMH batts have a sudden drop off in voltage when they are almost exhausted, this will result in the led dimming significantly. When the led is off, the batts will be almost completely discharged. The minimum operating voltage of leds depend on the individual led.

Btw, if you have lithium ion batts, do not discharge them fully. Its life and capacity is shortened considerable.

If you find the resister heating up too much, get a cheap metal heatsink from slt. The <$1 ones will do.

Thanks a lot!
I'll definitely go get these stuffs when I go to SLS next time...

 

[Zoomer]

Originally posted by SNAG


Thanks a lot!
I'll definitely go get these stuffs when I go to SLS next time...

Wait...its SLT...not sls..3rd floor, or basement.
Maybe I should start sleeping earlier.

 

[SNAG]

Originally posted by Zoomer


Wait...its SLT...not sls..3rd floor, or basement.
Maybe I should start sleeping earlier.

Oh, my mistake..
What I meant was that I'll pop into SLT when I go to SLS...
:bsmilie:

By the way, is there any battery holder which connects batteries "serially"?
Should have right?
Some of the battery holders connect the batts in parallel..
Then how do you count the total voltage?

Remember that resistance is (1/R)power-1 for parallel... But not for voltage right?

 

[misato]

Actually.. the common LED you can buy in SLT needs a minimum of 0.7V to light up. But at 0.7V, the light is rather weak. The main reason why circuit designer add LED in for power circuit is to use it as a visual indicater of the power.

BTW.. Power != voltage

The standard unit for DC battery is mAh, which stands for mA Hour.

For simple understanding, mAh and voltage references in battery.

Using a GP 1600 mAh,1.2Volt battery as example.
It means it can output 1600mA of power @ 1.2V for up to an hour.
A regular Energizer batt, have a rated of 720mAh @ 1.4Volt.

Lets say a given radio uses 1.2V and 100mA of current.
GP 1600 will be able to last (technically) 16hours,
where the Energizer might last 8-9 hours.

For use in flash.. Lets say a Flash unit needs 3200mA to charge up. So a GP 1600mAh battery will charge up the unit faster as its max output is 1600mA. The regular energizer will do so in a much longer time. Using the same example, the GP batt should last 30minutes where the energizer might last only under 15 minutes.

For actual calculation

Max Power output = rating x 1 hr
How long a batt last = (Max power usage / Max power output) x 1hr

Thats the main reason why high capacity re-charagble NiMH last longer for application such as digital camera, which uses more current than Voltage.

 

[Ian]

Actually with NiCad's the recommended discharge point is around 0.9V unless the battery hasbeen overcharged. Thus a simple resistive discharger such as hooking up a resistor over the cells annode and cathode is quite dangerous as it can over discharge the cell. What's needed is a discharge circuit that switches the discharge load off when the cell voltage equals 0.9V and that folks means a voltage detector and transistor or preferably mosfet switching method.

There are a couple of circuits online if you do a search for them, or you can obtain a circuit from the ARRL handbook amongst others.

Misato:

For the benefit of those reading this thread who don't have electronics knowledge you really should have specified that mA = milliamperes or 1/1000th or an ampere (amp).

I really should spend a bit of time ploughing through some datasheets as I seem to recall that NiCads have a far lower internal resistance than NiMH and deliver a far greater peak current value which is why NiCads are actually superior to NiMH for flash units, though they don't get as many discharge cycles per charge they charge the flash faster.

 

[misato]

Actually. To prevent over discharge, especially if there is a limit like what Ian mention. Once could implement a Zener diode in the application of the simple resister network. However, the Zener diode limits voltage. Thus V*V / R way of wasting power will not be effect as V is now effectively 0.9V.

However, the Zener diode does not affect the ampere(depends on specs) out, only voltage. So you can run a few of those resister network to drain more current. Using the calculation

V = I*R

Since V is fix at 0.9V, so you would like to increase mA drainage.

But as Ian mentioned, using a properly designed transistor with a cut-off voltage at 0.9V would require far less component and thus a simpler design. But the trade off would be that transistor circuit would be harder to design and implement.

 

[misato]

As for how battery should be discharged, this i not know, but for simplicity, doing so in a serial manner is much faster. I think a battery holder for 4 AA battery cost around $1.50-$2 in SLT.

Just make sure to design your discharger to accept 4x the value of 1 battery specs.

 

[SNAG]

Hmmm..
So that means that the cut-off voltage will be at 0.9v for NiCd right?

What about nimh?
What should be its cut-off voltage?
What would be the voltage of the nimh when it is 100% discharged?

Many thanks..

 

[Ian]

Discharge Circuit.

Circuit 1