Best way to reduce DC voltage for a LiPo Charger

[Steve Joblin]

I have a small 1S 3.7 volt 110mAh Li-Po battery that uses a charger that runs off of four AA batteries... I'm not the biggest fan of using batteries to charge a batttery, so I wanted to modify the charger to accept DC power from a AC Wallwart.  I plan on using a Power Jack w/Switch so that it will use batteries when the wallwart power plug is unplugged and cut off the battery supply when the wallwart power plug is plugged into the charger.

I got a 6v 1A wallwart that I ordered off ebay for a whopping $3 (shipping included!). Great deal huh? Well I used my meter to confirm which wire was the positive, and to my surprise, the voltage was 9.75 volts! I'm gonna fry the little sucker or expode my battery with that kind of voltage! I was thinking that I could either use a voltage divider by putting two resistors in parallel or using a bunch of diodes in series to drop the voltage to 6v.

1. Am I correct in that 9.75 volts is too much and I need to drop the voltage?

2. What is the best method to drop the voltage?
a) a voltage divider using two 1/8 watt resistors
b) a bunch of diodes in series
c) something else I am unaware of

I've aslo heard that some cheap wallwarts put out larger voltages until a something is connected, then it settles down to it's stated voltage output.  Is this true?  If so, should I just connect a resistor and LED inline to get the voltage down?

[hudbrog]

I got a 6v 1A wallwart that I ordered off ebay for a whopping $3 (shipping included!). Great deal huh? Well I used my meter to confirm which wire was the positive, and to my surprise, the voltage was 9.75 volts!

Judging by that statement you have non-stabilized power supply (erh.. don't really know how it is called in english. correct me if i'm wrong). That means voltage out of it will greatly depend on load. I.e. without load you get 9.7 V, with load of 0.5 you will get smthing like 7-8V, and so on.
If you LiPo chrager is "dumb" it won't be easy to modify it.

[ArcMan]

I choose c) Something else you're not aware of.

What you're not aware of is that you can't just pump some current into a Li-Po cell to charge it (unless you want ruin your battery and the other things that it may set fire to).  You need a circuit purpose-built to charge that Li-Po battery so that it doesn't overcharge.

BTW, you also need a circuit on your project to protect the battery from over-discharging, or the battery will be ruined.

[Steve Joblin]

Yes ArcMan... I am quite aware that you can't charge a LiPo directly.  That is why I have a charger... it is just that the charger uses 4 AA batteries (whereas most chargers plug into the wall... I am looking for a way to use my wallwart in place of the four batteries.  The wallwart will not be attached directly to the battery; the wallwart will be used to power the circuitry in the charger that will power the battery.

[Trumpkin]

Couldn't you just use a regulator to bring down the voltage?

[Steve Joblin]

I guess I could... unfortunately they don't make 6 volt regulators... do they?

[ArcMan]

Yes ArcMan... I am quite aware that you can't charge a LiPo directly.  That is why I have a charger... it is just that the charger uses 4 AA batteries (whereas most chargers plug into the wall... I am looking for a way to use my wallwart in place of the four batteries.  The wallwart will not be attached directly to the battery; the wallwart will be used to power the circuitry in the charger that will power the battery.

OK.  I see that I misread your OP. 

[ArcMan]

I guess I could... unfortunately they don't make 6 volt regulators... do they?

They do.  The 7806, for example.  I've used it to power small servos @ 6V.

[ArcMan]

BTW, did you measure the output of the wall wart under load?  Those are unregulated supplies, so the voltage might drop to around 6V under load.

[Commanderbob]

You can always use an adjustable regulator to get almost any voltage.

[Steve Joblin]

I tried it under load (I hooked up a small DC motor to it) and the voltage was 9V across the motor

Thanks for the ideas on using a V-Reg... I think that is probably the best way to go!

[Admin]

I noticed that no one mentioned what the required input voltage or current to the charger is

I'm pretty sure it already has a regulator built in . . . does it say anything on the charger? You can also open it up and look up the datasheets on the chips you see inside.

[Steve Joblin]

Thanks for the additional insight Admin!

The charger has no labels on it.  The circuit board is the tiniest thing I've ever seen... it has two caps on one side and a big black blob on the other side... I assume the SMD's are under the big black blob