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What is the output current (limit) of a typical USB car charger???? Answered

I need to power 2 hard disks in the field. Power requirement: 900 mA each. For the 5V connection, I want to convert the 12V power input with one or more car USB chargers. I need to know if one or more chargers in parallel (how many??) are needed. I have sufficient 12 V juice (small lead acid battery). Additional: for surges in power consumption, do I need to add a capacitor? What would be the rating? Do I have to wire it in series or across? (Sorry I'm a layman in electronics...)


Typically most USB car chargers are going to put out around 500ma. There are a few that will put out 1amp (1000ma) you could just get a pair of those. You can get them very cheap here:
If you have two of them you could use on for each HD, since each has one port this isn't a big deal.

I need to limit output to 10ma what should I do?

Any help please?


As I recall, the USB spec defines the output from a USB port to be 5V @ 1A I suppose it all depends on the charger(s) you purchase. I do not believe they are all the same. A pair of powered USB hubs might be more inline with what you want. They usually run off a 12V wall warts, so can easily be modified (by lopping off the wart and wiring up to your source) to achieve the end you desire

Note, I say pair because I'm assuming that sharing a single USB connection for two drives might not be what you want or may not be viable.

Finally, I found that I have to install 3 chargers: One was still in its wrapping (usually I just rip things out and throw the wrapping in the trash). It actually had specs on it: 400 mA!! It seems the surge during startup = 2 X 600 mA, so 3 chargers in parallel should work....

I thought they had to negotiate to get 1A, otherwise you are stuck with 100mA ? Steve

That's my understanding as well...Configuration stage provides 100mA, then negotiable. But that's the problem of the hard-drive and it's host, not the end-user's. Having said that....Now that I checked my documents, I find that I was incorrect in the 1A claim. USB 1 and 2 both specify 500mA max for a high current load, not 1A.. So frankly, I'm not sure how one powers a 900mA hard drive, outside providing an external 5V regulator/power supply and ties common to the USB common.

Yup. In total agreement on the substance, but he does want to run it off batteries, and he has two harddrives, (field installed...ie, remote) so technically he should be looking for a 2A+ DC/DC. A couple of the beefy versions of the 7805 would do the trick, or a more energy economical switching regulator like this 5V, 2A TI switching regulator, which only needs a 100uF cap on the output to gitup and go.

For overcurrent protection, an inline circuit breaker, fuse or a polyresettable fuse, but I'd think that the charger would protect the harddrive, so the overcurrent protection would go between the 12V source and USB supply.


8 years ago

This is what I wanted to know! Today I'm going to wire it , to see if it holds up. It seems that the 5V lead is involved in moving the arm? That would mean little power consumption, since I will use the HDs as gyroscopes.