I need to lose 500mV in 4 hours...

[Primathon]

Sorry for the rushed nature of this post, but I'm quite short on time and in need of a quick and dirty solution... Here's the deal: In about 5 hours, I'm getting on a Greyhound bus for 16 hours. I expect this to be mind-numbingly boring. To help me cope, I've loaded up my Motorola Droid with a bunch of videos that I've been meaning to watch... The problem is, I don't have 16 hours of battery life (and I need my phone when I get there). What I do have, however, is the following:

12v 7.2 Ah lead-acid battery, fully charged **broken link removed**
12v to USB automotive charger **broken link removed**

So, my plan is to wire the these two together to give me a big fat battery to plug my phone into (it charges via MicroUSB). The problem is that when I do this, the USB charger outputs 5.5v instead of the nominal 5.0v -- when I plug it in, my phone "wakes up", but doesn't start charging. If I use a **broken link removed**, it outputs a steady 5.0v and charges just fine. I'm guessing the extra 0.5v is causing me problems.

That's my problem in a nutshell. I need to somehow put something inline (I have a spare USB extender cable that I can chop up) that will drop the voltage from 5.5v to 5.0v. The bus leaves in 4 hours and 39 minutes.

Any ideas? I'm all about the quick and dirty, and don't particularly care if it generates too much heat, as I can keep that part isolated.

Thanks guys

 

[Boncuk]

Just google how to loose 100KV within 1/1000sec.

500mV won't do any harm, and should be within tolerances.

Boncuk

 

[dougy83]

I need to lose 500mV in 4 hours...

Sounds like a request for some fad diet

you can try a diode (e.g. 1n4001 which will drop around 0.5-0.7V, 1n5818 which will drop something in the vicinity of 0.2-0.4V) in series with the supply.

 

[Primathon]

500mV won't do any harm, and should be within tolerances.

I though so too, but the phone didn't want to play ball. It'd "wake up", but wouldn't charge.

I ended up just picking up a new 12v to 2xUSB adapter at Best Buy, as I was a bit too short on time to work with the old one. I'll still give the diode idea a shot, as this contraption will certainly prove useful in the future. Thanks for the replies

 

[dougy83]

If you're going to open up the converter anyway, you can usually change a single resistor to change the output voltage (you may need some finding which resistor though).