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Voltage Tripler

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crunch53

New Member
Is there a way to charge 3 1.2 volt rechargeable batteries in parallel and then come off of them in series to get 3.6 volts? I have attached a file to kinda show what I want to do but don't know how to do it or if it is even possible. Maybe with diodes or something?

Mike
 

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audioguru

Well-Known Member
Most Helpful Member
Battery cells should not be charged in parallel. One or two will overcharge and one or two will not be fully charged and it gets worse with each charge and discharge.

My solar garden lights use a small solar panel to charge a single 1.2V rechargeable battery cell.
Then it uses a simple voltage stepup circuit to light a 3.5V white or multi-coloured LED.
 

Hero999

Banned
Yes, in theory, but as audioguru stated it isn't practical.

What to you want to do?

If it's just triple the voltage, you're better off with a charge pump (basically charges two capacitors in parallel then puts them in series with the power supply to get triple the voltage) or a boost converter.
 

crunch53

New Member
What I have is a solar garden light and it seems to be putting out 1.2 volts or whatever the battery is charged at. What I did was cut out the led and hooked up a Joules Thief to it. Then I have 4 led's hooked up to that.
As I was doing this I was thinking 1. why not use 2 or 3 batteries in parallel so it would last longer at night when there was no light and 2. I was wondering if there was a way to come off those same batteries to get 2.4 or 3.6 volts for other projects.

Mike
 

Sceadwian

Banned
If you don't expect it to last too long that exact charge/discharge scheme would work. Better if the cells were closely matched, and you did not mind if the system spontaneously decided to stop working when a cell grew week and inverted. Not ideal for sure, but it would work.
 

audioguru

Well-Known Member
Most Helpful Member
What I have is a solar garden light and it seems to be putting out 1.2 volts or whatever the battery is charged at. What I did was cut out the led and hooked up a Joules Thief to it. Then I have 4 led's hooked up to that.
LEDs do not light with only 1.2V. The garden light already has a "Joule-Thief" circuit that triples the voltage.
The circuit is designed to drive only one LED, not 4.

1. why not use 2 or 3 batteries in parallel so it would last longer at night when there was no light
The cheap little solar panel is designed to charge only one battery cell, not 2 or 3.

2. I was wondering if there was a way to come off those same batteries to get 2.4 or 3.6 volts for other projects.
The garden light already has a voltage stepup circuit. Its output current is low at about only 10mA. It has a fair amount of ultrasonic ripple. If the battery cell is an old poisonous Ni-Cad then a charge does not have much capacity. The cheap little solar panel is too small to charge a higher capacity Ni-MH cell.
The charging works only outdoors in bright sunlight in summer.
 

Hero999

Banned
Use a C or D cell if you want it to last longer.

Be careful with rechargeables, some D cells are AA cells with extra padding, make sure the capacity is ≥4000mAh for C cells and ≥8000mAh for D cells (assuming NiMH chemistry).
 
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Sceadwian

Banned
I think audioguru is right hero, I didn't think of the solar cell itself. They don't even usually fully charge the cheap nicads they put in them during the day. As far as lawn lights like that go I'd always wire them, then they're full brightness all the time, and look so much better.
 
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