Question for computer literate
 

I would like to break the + voltage circuit of a usb cable to control charging of my tablet. Does anyone see an issue outside of adding a resistor to keep the input voltage at 5 volts? Reason being, I would like to be able to stop the charge cycle when fully charged. It may not be necessary to implement but it's something that I'm interested in doing.

You're going to have major issues with that. You need a proper voltage regulator if you're trying to output 5v over USB.

For your specific case, you'd probably be best off wiring a new 12v cigarette lighter socket and semi-permanently (tape!) shove a USB charger in there with a USB cord coming out of the dash somewhere.

Good point,I can modify that answer to create the alternate power source. If I remove (cut) the + wire out of the usb cable, I still have a ground and the +,- for data so I should retain full functionality correct?

What you want is a buck converter like this: http://a.co/b4t2WUZ

I would NOT trust amazon for this specific type of thing. I have bought quite a few different DC-DC converters on amazon and about 90% of them were completely out of spec.

If you really need to buy one off amazon at least look for someone who's a reputable parts supplier.

Last thing i'll really say on this subject. Buying electronic components on amazon is a lot like buying car parts off ebay. You have absolutely no idea what you're actually getting.

What my goal is... is to remove the charging duty from the grom interface all together. I may be trying to do something completely unnecessary but I don't want to turn the car on to charge the tablet. I'm also not in the car long enough to significantly charge while driving. This is in an 8 that I don't drive. I just start it once a week to keep the battery up. I have two others that I drive regularly. Once I get the kinks out of this design, I'm going to repeat it for my daily driver.

This is probably a pretty bad idea. Your iPad can pull 2.5-3A continuously. Plus whatever losses you get from the buck converter.

If you forgot to unplug it at the wrong time your car won't start.

What some car manufacturers have done is add a relay to the switched power circuit that shuts itself off when the battery voltage reaches a threshold. This makes the project about 10 times more complicated though.

This may be the reason it hasn't been posted here. I'm not smart enough to give up on the idea until I'm convinced it's not worth the effort.

I've had the opposite experience.

Honestly I've only had issues with hardware that was related to power regulation or conditioning.

I have at least a dozen breadboard power supplies (buck converters) that are outputting between 4 and 5v. The last one I bought was supposed to be dual 3.3 and 5v. The 5v line output 6v and the 3.3 was giving me 2.1.

None of the converters i bought from a well known company ever exhibited these issues.

I'm totally confused with what you are trying to achieve here:
1. By cutting the 5V line on a USB cable the USB connectivity wont function.
2. All mobile devices have built in over charge protection.
3. If you don't want to have the mobile device charging when the car is off (to protect against potential car battery drain) then just wire the charging adapter/voltage regulator to a switched +12V source.

Hooking 12v up to your phone's 5v USB port is a disaster waiting to happen. DO NOT DO THIS. Overcharging is not the same as overvolting. Charging protection circuitry is to protect the battery, not your phone. They DO have voltage regulators but do not have the heatsinks required to make this even a remotely good idea. The regulators in your phone are there to smooth out out of specification power supplies. Not act as a replacement for a power supply.

I know that, and that is why I clearly stated in part 3 to use a charging adapter/voltage regulator.

I think you miss the OP's original question, he is asking about cutting the +ve voltage wire on the USB cable (since USB is 5V that would be done after any voltage regulator/charging adapter), he isn't talking about wiring +12V directly to his phone and neither was I. Anyone with any ounce of common sense would understand that everything being discussed here is after the voltage has been stepped down from the car's 12V to the 5V supply to the phone/tablet. In fact the Grom kit he mentions he is using already takes care of a 5V output (via USB).

I think what the OP is trying to achieve is to be able to have USB data connectivity to his mobile device without actually charging it (I assume to save wear and tear from charging cycles/continuously charging the device). Problem is that USB data connectivity relies on that 5V line being connected (along with the ground and data +/-) and what he is trying to protect against is a moot point since you can't overcharge current mobile devices and current li-ion tech in these devices don't suffer from battery fatigue that he is trying to protect against.

He is trying to create a solution to a problem that doesn't exist.

You can pick up a cable from Amazon that will take the output from 12v to 5v.

I have picked up a few of these type things from Amazon. Yes they fail. All metal cables fail. If you have concerns, pick up a Volt - o - meter. They make them built in to cables and even fancier. Up change the voltage on things all the time. Output of pwr connector on MB is only 5v. fans run better at 9 /12v. but any time you customize something, expect the unexpected. Have spare parts and ways to measure and verify what happened.

I think a better solution to your problem is getting the proper charger from the manufacturer. The newer batteries and devices self regulate.