Easy & low cost way to limit input current from car battery?

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Sunny55

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I need a simple way to limit the input current fron the car battery? Can anyone give good suggestions to achieve this please?
Thanks
 

Fuses.

Input current to what, anyway?
 

godfreyl said:
Fuses.

Input current to what, anyway?
Click to expand...

Its actually to control/limit input current to a usb charger operated by car battery. How many amps of fuse i need ? The battery is 40Ah i guess.
 

The usb charger should have built-in current limiting.
 

godfreyl said:
The usb charger should have built-in current limiting.
Click to expand...

Yes but its only for output current to load only. I have another circuit in another thread. When connected to small solar panel(low current), it can charged with no problem but connected to a car battery, usb charger does not charge at all and its shifts between charging to discharging modes very fast.
 

The USB charger is probably switching between charging and not charging very fast - that is, current limiting. Perhaps the reason it does not switch like that when powered by your solar array is that the solar array produces less voltage than the car battery and so it can charge continuously without exceeding the current limit for the target device. That fast pulse charging you observe when connected to the car batter may be doing its job just fine. Does the target device get charged appropriately?
 

Re: Easy & low cost way to limit input current from car battery?


Hi, do you mean that this current limiting process that causes phone to shift from charging to discharging modes for some time is SAFE for the phone? Today, i tried charging with the phone off, it does charge but does not show the battery charging indicator animation that shows on the phone when charging with the supplied wall charger.

- - - Updated - - -


Hi, do you mean that this current limiting process that causes phone to shift from charging to discharging modes for some time is SAFE for the phone? Today, i tried charging with the phone off, it does charge but does not show the battery charging indicator animation that shows on the phone when charging with the supplied wall charger.
 

Re: Easy & low cost way to limit input current from car battery?

I didn't realize it was the phone that was telling you to was switching like that. Yes, it is probably safe, but no, it may not charge the phone properly. Is this USB charger designed to work off of a 12v battery? If so, then it should work, unless that battery is very low. I would now measure the voltage from the solar array during charging and compare that to the voltage from the car battery to find out why the charger behaves differently when connected to the two sources.
 

A phone has a charger circuit inside that is designed to be powered from 5V (wrongly called a "charger").
The 13.8V or more in a car will cause severe overheating so the charger circuit in the phone protects itself by limiting what it does.

You can use a 5V linear voltage regulator IC to drop the 13.8V or more down to the 5V that the phone is designed to use but the regulator will get very hot and will need a pretty big heatsink. Or use a switching voltage regulator circuit that does not get hot.
 
Yes, i have a 5v regulator to step down voltage but in my case it heats up n got very hot. Besides of using heatsink, what can i do ? Use a resistor to limit input current to prevent heating of ic? Its going to work?
 

Hi, let's get back to main problem of limiting current into a circuit. Basically i think most circuit connected to car battery will heat up. Can i use a resistor in series at vin to limit current ?
 

A car battery is 13.8V or more. Then your regulator has at least 8.8V from input to output. If the load draws 1A then the poor little regulator must dissipate 8.8W of heat. It will need a pretty big heatsink that is in the open air.

If a resistor is added in series with the regulator then the current is the same but the resistor simply shares the heat.

The car battery does not FORCE a high current to a load. The load draws as much current as it needs. A clock does not heat up because it draws a very small current.
An ordinary car amplifier IC TDA2005 draws only 120mA MAXIMUM when it is not blasting. Then it heats with only 1.7W.
But when it is blasting 18W into a 4 ohm speaker it heats with 11W.
 

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