Automatically Battery Disconnect

I think I saw some power stuff that did that on the TI (texas instruments) website.

Cris

Varun Ramesh wrote:

 

It is recommended that for optimal operation a battery (laptop, phone etc.) must be maintained at a charge between 40$-80% never more, never less.

am trying to build a circuit that hooks on to either my laptop charger or my phone charger.....and turns on or turns off the charger based on if the present charge is less that 40% or more than 80%.

Battery life is extended if you don't fully discharge the battery, but if you build a circuit that connects between your charger and the laptop, then if you use your laptop plugged-in for a day, your circuit will make the laptop to run off the battery because it will disconnect the power cord, then it will charge it, then disconnect the power cord again to cause the laptop to run off the battery again, etc. This could be worse than letting your laptop internally stop the battery charging through it's internal circuitry while it continues to be powered from the power cord.

Also, apparently Li-Ion batteries have a limited life even if you don't use them. They don't last longer than a year in my laptop, and maybe 12-18months in my mobile. Apple manages a longer time in their products apparently (I don't know) but I'm guessing they're doing more sophisticated things maybe on a per-cell level.

I Think a good approach would be to monitor the current being consumed by the existing battery charger.  Battery chargers for lithium batteries are constant current source. The current should drop sharply when the battery becomes fully charged.  Breaking the circuit at that point would stop any top off charges tha follow. 

This method also makes construction easy since a small circuit can be installed into a short cords receptical.  A torrid and an op amp can detect the current drawn.  A relay driven by a 2N2222A2N2222A transistor can be controlled by a 7474 flip flop.  Then the dropping current when the battery gets fully charged will generate a pulse that can be used to change the state of the flip flop and de-energize the charger that is plugged into the cord.   To reset the cord could be unplugged and plugged into the wall again.

Thanks for your reply Jay, but as I said I want to disconnect it at 80% of its full charge not at 100% and turn it on back again at 40%. This is what is recommended for optimum battery life to maintain the charge of the battery between 40%-80% of its full. You should be able to get the present charge status from the PC through a USB with an appropriate script.

Varun

Varun Ramesh wrote:

 

This is what is recommended for optimum battery life to maintain the charge of the battery between 40%-80% of its full. You should be able to get the present charge status from the PC through a USB with an appropriate script.

 

Varun

Not if you're using it! If you are querying the battery status via USB then the PC is on.. So why chew up battery charge cycles for no reason?

It's one thing having a battery charged to 40-80% of capacity, but another thing actually running the laptop until the battery drops to 40% and then charging it again.. You're using battery cycles for no reason.

Better to actually let the PC stop charging the battery and let the PC run off the mains adapter.

Or if you're still worried, just physically detach the battery and run your laptop off the mains power.

Well my laptop has a primitive battery control system and does not turn off the power when its fully charged.

At the end of the day its a trade off right? If I imporve my battery life by maintaining charge between 40-80% even though I use up battery cylces I still emerge a winner as compared to if I do nothing about it.

I'm too worried about my battery, its just something I would be interested to do and see if it actually makes any difference.

Varun

Hi Varun,

Have you considered using a 555 timer chip?  They have a nice analog input with an adjustable threshold trigger.  The timer output can be used to turn charging on and off.

There a lot of 555 battery charger circuits on the web.  I am sure you can find one to meet your needs.

Just a thought,

DAB

IIRC most devices have this already built in. If you added this 80/40 limit in again, you would be running the battery between approximately 65% and 56% charge.  Thats only 9% of its actual range!

You can detect a battery's charge level with op amps (operational amplifiers)

By putting their threshold at the top and bottom levels, you could control your charging.

That said, as others have mentioned, the designers have probably already done this in your device.