GETTING 9 VOLTS FROM YOUR ONBOARD 12 V BATTERY

[bobhenry]

There are several neat little clocks and radios that are 110 ac with 9 volt backup. Is there a quick and dirty way to drop the 12 v onboard battery voltage to 9 volts with a dropping resistor to allow continuious operation whether plugged into shore power or not. What ohms and how to due schematics are welcome.

[TPMcGinty]

There are several neat little clocks and radios that are 110 ac with 9 volt backup. Is there a quick and dirty way to drop the 12 v onboard battery voltage to 9 volts with a dropping resistor to allow continuious operation whether plugged into shore power or not. What ohms and how to due schematics are welcome.

Aren't the 9 volt backup batteries just to maintain the time internally in case the power goes out? If I remember right, my clock will stop displaying the time when the power goes out but will remember the time for when the power comes back. Plus the time seems to go faster on the 9 volt battery. It always seems to be 5-10 minutes fast after the power is restored.

[bobhenry]

OOPS I MAY NOT HAVE THOUGHT THIS OUT TOO WELL !!!

[madjack]

There are several neat little clocks and radios that are 110 ac with 9 volt backup. Is there a quick and dirty way to drop the 12 v onboard battery voltage to 9 volts with a dropping resistor to allow continuious operation whether plugged into shore power or not. What ohms and how to due schematics are welcome.

Aren't the 9 volt backup batteries just to maintain the time internally in case the power goes out? If I remember right, my clock will stop displaying the time when the power goes out but will remember the time for when the power comes back. Plus the time seems to go faster on the 9 volt battery. It always seems to be 5-10 minutes fast after the power is restored.

Tim(and Bob), that has been my experience as well.............................

[jmullan99]

Why not just get one of the smallest inverters available ( 75W ? ). Draw will be minimal and offer best of both worlds.

[bobhenry]

I found a tiny 1" x4" digital 120volt clock and was thinking of building it in but every time you are off of shore power it would need to be reset. I don't know why I am worried about time while camping but I thought it would be nice as a shining little widget. Well it was a dumb thought and now I have an extra alarm clock for the house for $1.50 . Got one of those projector alarm clocks now that resets itsself automatically from atomic time whatever. It would look cool shining into the trees thru the lexan roof. I'm just gonna trade up and put it into the tear and put the bargain in my bedroom !!!

[wlooper89]

That sounds really cool. It should reset itself with shore power too.

Bill

[Larwyn]

Why not get a wind up clock. No drain on any system other than the energy to wind it up..........

[COMP]

Why not get a wind up clock. No drain on any system other than the energy to wind it up..........

[wlooper89]

I rather like the idea of fitting every possible modern convenience in a TTT, and at the same time enjoying the great outdoors. The best of both worlds!

Bill

[BrwBier]

Well, back in the 60s if you wanted to put a 12v starter and lights in your car and not change everything else you just turned a screw diwn through the tar top and taped 6v from the middle of the battery. So just go 3/4 of the way and get 9v.
brwbier

[Alphacarina]

Well, back in the 60s if you wanted to put a 12v starter and lights in your car and not change everything else you just turned a screw diwn through the tar top and taped 6v from the middle of the battery. So just go 3/4 of the way and get 9v.
brwbier

A 12 volt battery is made up of six 2 volt cells - So you can get 6 volts 'from the middle of the battery' by tapping 3 of the 2 volt cells

But you're never going to get 9 volts from '3/4 of the way' - 8 volts sure or 10 if you like, but never 9

Don

[brian_bp]

Well, back in the 60s if you wanted to put a 12v starter and lights in your car and not change everything else you just turned a screw diwn through the tar top and taped 6v from the middle of the battery. So just go 3/4 of the way and get 9v.
brwbier

A 12 volt battery is made up of six 2 volt cells - So you can get 6 volts 'from the middle of the battery' by tapping 3 of the 2 volt cells

But you're never going to get 9 volts from '3/4 of the way' - 8 volts sure or 10 if you like, but never 9

Don

I get that this is not a serious suggestion, but if it were...

8 volts nominal might be enough, since it would really be closer to 9V when the battery is well charged.

10 volts might work okay, too, depending on the device.

Neither is a good idea, since the cells which supply the 9V power would get run down more than the other cells, but all cells get the same charging current. Eventually, after enough cycles, either the cells being used for the "9V" would not get adequately charged, or the other cells would get overcharged.

[BrwBier]

You are right, it was not a serious suggestion. Besides, it only worked it tar top batteries. You could just screw into the tar and check for the crrect voltage, and yes it was quite crude, but it worked and was quite common.
Brwbier

[eamarquardt]

Here is a site that has a schematic for a simple 9 volt regulator. The parts should be avail from any mail order house.

Cheers,

Gus