how to make 12 volt battery charger from 12 volt AC adapter?

[robotmir]

for my robot I use 12 volt battery,,,what I want to know is can I directly charge the  ??if possible, can anyone tell me how the connection should be??

[ArcMan]

To answer the question in your subject line; You can't.  You will need more than a 12VDC supply to charge a 12VDC battery.  To make a simple constant current charger, you can use a 16VDC supply with an LM317 wired as a current regulator.  Look at the LM317 data sheet to see how.  For the simple charger, just don't charge your batteries at a rate of greater than C/10.

I'm assuming your charging an "easy" battery like NiMH, NiCd or lead acid.

[benji]

there is a current charger in the lm317 datasheet which injects 20 mamps of current into the battery
,,i was just wondering,, can this charde batteries with any voltage?> any batteries?

[ArcMan]

Yes.  But...

Your supply voltage must be 3 or 4 volts above the battery voltage so that the LM317 can attain the current for charging.
Of course, you must not exceed the maximum ratings of the LM317.

I use my little home-made LM317 charger to charge all of my NiMH battery packs - anywhere from 4.8V to 14.4V.  I have the current set to 170 mA so that I won't exceed C/10 for my AA packs (1850 mAH batteries).

[benji]

people hook adapters into batteries to charge em
it works,, howcome? i mean with no resistors in between

a 7 volts adapter into a 7 or less volts battery

[Admin]

Actually you can. Although your adapter says '12V', its usually a volt or two higher. Just measure it with your multimeter.

I use a '6V' adapter that gets my NiMH battery up to about 6.5V in about a day. Slow, but it works.

[ArcMan]

people hook adapters into batteries to charge em
it works,, howcome? i mean with no resistors in between

a 7 volts adapter into a 7 or less volts battery

If you don't control the current charging your batteries, just make sure you have a fire extinguisher handy.

[benji]

if we connect the adapter directly into the battery then the charging current should be very high
howcome it takes a day?

[dunk]

if we connect the adapter directly into the battery then the charging current should be very high
howcome it takes a day?

for the sake of this metaphor try thinking of power like plumbing...

voltage is like water pressure.
amperage is like the pipe's diameter.
there's no point having a wide pipe capable of carrying a lot of water if there is no pressure to push the water.

if your power supply is the same voltage as the battery it is like there being no difference in pressure between the 2 supplies.
it would be like connecting 2 taps with the same pressure this a bit of pipe. no water would flow even though the pipe is capable of carrying a lot of water.

dunk.

[Admin]

if we connect the adapter directly into the battery then the charging current should be very high
howcome it takes a day?

Mathematically looking at it . . .

Power = Voltage * Current

wall adapter:
Power = (6.5V-6V) * .2A = .1W

battery charger:
Power = (7.5V-6V) * 1A = 1.5W

Using the typical battery charger, it'll charge x15 faster than the wall adapter.

[benji]

yea guys i totally understand

but ...
 a battery charger wont inject currents like 1 amp

and an adapter of 0.5 volts above battery voltage is going to produce about 0.5 amps
so if the batter amh is 7 you should get your battery fully charged in about 14 hours

now its understandable

i still believe (and it should be) an adapter is much faster than current injector and specially the one in the lm317 datasheet (20 ma)

[ArcMan]

i still believe (and it should be) an adapter is much faster than current injector and specially the one in the lm317 datasheet (20 ma)

The data sheet just gives an example of a 20mA charger.  You can set the current to whatever you want (up to the chip's limit) by changing the series resistor.

[Admin]

a battery charger wont inject currents like 1 amp

Yes it can   I've seen some that can do 2 amps.
However, some battery types just can't absorb energy that fast.


The adapters are usually 'trickle chargers'.

[robotmir]

anyone can answer my initial question??? im new in this robot stuff.....can you guys show me how to create a battery charger using ac-dc adapter....? i want to recycle my unused adapter....maybe it would reduce the cost for my robot...

[Admin]

Scroll halfway down this page until you see an adapter:
http://www.societyofrobots.com/step_by_step_robot_step3A.shtml