I have installed a Galaxy Tab 2 7.0 in my car as a permanent fixture. It can be seen here. It is hooked up through a cigarette lighter socket that powers on with ignition.

What I've noticed today is that my tablet is draining faster than it's charging. So instead of having it only charge when the car is on (how it's set up now) I will make something to have it full charge when car is on, trickle charge when car is off. I picked up a specific car charger that the tablet registers as AC charging, like from a wall outlet. So what I think I want to do, is make the power supply circuit for it to AC charge when the car is on, and trickle charge it when the car is off, like if it was hooked up to a USB outlet. This shouldn't effect my battery so I've been told, and will let me keep using the power state flags that I use with an app to automate my tablet.

I drew up a circuit that I think will work, however I have a feeling I'm missing some fundamental knowledge. Here's is the circuit:



For AC charging, the tablet uses 5v 2a. For USB (trickle) charging, it uses 5v 500mA.

The car charger I bought is however I don't know what it's internals are.

I "think" using the correct resistor where I have it in the picture would allow me to provide the power difference to kick the tablet between USB and AC charging, but I'm not sure. That's where I think I'm not remembering something important.

Can you straighten me out and point me in the right direction? Any help or criticism is appreciated.

 

If I understand your requirements correctly:
- you have 2 chargers
- you want to select which one is charging the tablet based on whether the ignition is on

You can use the relay to switch the 12V power to the two chargers based on ignition key position. You would not need any resistor for that.

However, you might consider whether you want the tablet charging fully (via the 2A adapter) with the accessories on but engine not running/charging. E.g. for watching movies or listening to music while parked, without having to turn the ignition on (only windows and radio).

Am I close?
Lou

 

Close. I only have 1 charger. With the tablet being mounted in dash, I can't change what charger is plugged in every time. My goal is to provide 2 different currents to the same charger depending on what the car is doing. One 2A, one 500 mA.

 

Alright, I might just be dense tonight.
I want to make sure I understand correctly what the charging situation is.

Currently, you have the tablet connected via a 0.5A charger (I assume one of those universal car USB chargers).
And you have a second charger that can supply 2A. What is its input voltage? You mentioned A/C charger so does it require 110VAC?
You have only one charging port on the tablet, i.e. the two chargers cannot be plugged in at once?

Providing 2 different currents to a charger is not usually an option since they need full voltage and full supply current to operate properly.

Thanks for your patience.
Lou

 

I rewrote my original post like 5 times trying to make it readable....then I just gave up cause I had to leave the house lol. You're right now, 1 charger, 1 port on the tablet. The charger I linked from Amazon in my original post is what is hooked up. It's the only charger I have for this tablet, besides the wall one in the house. This charger registers on my tablet as an AC charger(it's plugged into the cigarette lighter hidden in the dash), so I must be putting out close to 2 amps. My goal here is to have it running full power when the key is on, but restricting it to .5 amps when the car is off. Is it going to hurt anything if I "try" restricting the current? And I was guessing around a 6 ohm resistor would work? Assuming battery voltage while sitting is like 12.5ish?

 

Ok, now I get it. Thanks for the clarification. No worries, I too sometimes have to rewrite things multiple times to make them understandable to the outside world.

I will have to disappoint you but you won't be able to use a simple resistor to limit the charging current. Devices nowadays do not use analog voltages or charging currents. The charger probably runs PWM circuitry inside so putting a restrictive resistor on the input or output would only limit the voltage which would in turn not let it charge.

Pardon my ignorant question but if your remove charging power from the tablet, won't it go into standby mode and conserve power? Would that work for you? Charge it while ignition is on, not charge when ignition is off.

Lou

 

That's how it's set up now. When the key is on, it's charging, and when key is off, tablet goes to sleep. The problem I ran into today though, is that as I was driving around today, the tablet was draining faster than it was charging. This is most likely due to how high I have the brightness of the screen. I was looking for a way to keep it charged in the car, instead of having to take it out every few days, which is what it looks like I'll have to do.

 

I see, I get it now. (took me long enough) The charger does not provide enough current to catch up with the tablet's draw.
Is there a stronger charger that can supply enough current to the tablet to prevent it from discharging while in use?
Also, have you verified how much current the charger draws from the battery when fully charging and when the tablet's battery is full? The charger should not draw full 2 amps once it is done charging the tablet, it should only trickle.

 

I would doubt that little cigarette charger can supply the 2A the tab requires for full operation.

As an experiment, can you get hold of a 12v DC - 120v AC inverter, plug your wall socket tablet charger into that, and see if it keeps up?

 

I have a power inverter installed in the car. I will try that tomorrow, and see what happens.

 

Yes, a wall charger would be a great test.
For the cigarette lighter socket, it is rated at 20 amps so it can provide 2 amps without problems. Now whether the little 5V adapter can provide 2 amps, that's definitely a good question. Another important question would be how much current the tablet requires in full-performance mode without having to discharge the built-in battery.

 

Yea, these are questions I haven't been able to find solid answers too. I use an app on my tablet called Battery Monitor Widget. It is what's been telling me if I'm USB or AC charging. I'm not sure what some of the info the app is telling me actually is though. Here is a screen I took today with the car ON...



Not sure if you can garner any info from that. My probes on my DVOM aren't small enough to get in the plug, and I don't want to damage it since it's proprietary. Maybe after work tomorrow I can try and fab some smaller ones.

 

Does the tablet charger have the current rating printed on the side in tiny little print like they usually do? Assume that it's capable of supporting full-functions when plugged in.

So happens my wife has a Galaxy tab 7" too, I'll see if I can get some info when I get home.