Building battery charger, can't figure out where pwm fet should be??

Thread Starter

rudyauction8

Joined Jan 27, 2012
250
I've been trying to figure this out for hours. I attached 2 pictures of 2 different circuits showing why I can't use either one. Any advice?
 

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Thread Starter

rudyauction8

Joined Jan 27, 2012
250
I can't measure the battery voltage without turning the mosfet all the way on to get a ground reference, which if the lithium battery is full or too small to handle the current from the 20 volt power supply could be dangerous. Any ideas here? I'm using a pic12f675 and using the adc to measure the voltage.
 

Thread Starter

rudyauction8

Joined Jan 27, 2012
250
You are just gong to have to do some level shifting with normal transistors to get a 5 volt signal to control a 20 volt supply.
Thanks. I'll start working on it today. However doesn't the fet require 6-7 volts between gate and source? If so my power supply needs to be that much higher than the highest battery voltage?

Why are you using PWM to charge a battery?

bob
Because pumping 20 volts at 15 amps into a 300mah 3s lipo will cause it to explode and I need the 15 amps for my larger packs. PWM will be used to limit current and manage the constant voltage portion of the charge.
 

BobTPH

Joined Jun 5, 2013
2,725
PWM will not limit the current to the batteries. That is why I asked. It will limit the average current, but the peak current will be the same as if there was no PWM, and you will probably start a fire. Batteries must be charged by a constant current.

Edit: PWM is usable in the context of a buck converter that controls a constant current source.

Bob
 

Thread Starter

rudyauction8

Joined Jan 27, 2012
250
PWM will not limit the current to the batteries. That is why I asked. It will limit the average current, but the peak current will be the same as if there was no PWM, and you will probably start a fire. Batteries must be charged by a constant current.

Bob
output filter capacitors will smooth out the pwm. using twin 470uf and single 0.1uf tantalum with pwm freq of 20khz.
 

Thread Starter

rudyauction8

Joined Jan 27, 2012
250
PWM will not limit the current to the batteries. That is why I asked. It will limit the average current, but the peak current will be the same as if there was no PWM, and you will probably start a fire. Batteries must be charged by a constant current.

Edit: PWM is usable in the context of a buck converter that controls a constant current source.

Bob
You ever heard of switching power supplies? They use pwm and filter caps and their output is very smooth, the voltage spikes that make it past the caps are not powerful enough to harm batteries or the circuits they drive.

Just saw your edit. Thats exactly how I will use pwm, a buck converter that limits current into the battery, the amount of current let through based on the batteries size and charge level.
 

Papabravo

Joined Feb 24, 2006
14,836
If you came up with those circuits yourself we have every reason to be concerned. If you got them from somewhere else you need to find better sources of circuit ideas. People can and do get hurt doing silly things with voltage and current and chemistry. There is even the posthumous Darwin Award for scientist/engineer/inventors who like to tinker with stuff. BTW if we manage to save your life then I would judge that to be very helpful, even if a bit harsh.

http://en.wikipedia.org/wiki/Darwin_Awards
 

#12

Joined Nov 30, 2010
18,216
doesn't the fet require 6-7 volts between gate and source? If so my power supply needs to be that much higher than the highest battery voltage?
Only if you insist on using an N channel mosfet...which again brings up the question; If you can't tell the difference between an N-channel and a P-channel transistor, why do you think you can do this safely?
 

Thread Starter

rudyauction8

Joined Jan 27, 2012
250
If you came up with those circuits yourself we have every reason to be concerned. If you got them from somewhere else you need to find better sources of circuit ideas. People can and do get hurt doing silly things with voltage and current and chemistry. There is even the posthumous Darwin Award for scientist/engineer/inventors who like to tinker with stuff.

http://en.wikipedia.org/wiki/Darwin_Awards

Those circuits are not my actual design. I made them in less than 5 minutes to visually explain the problem I'm trying to figure out. Think of them as a very basic idea, with only the actual components I'm thinking about and none of the rest of the circuit.
 

Papabravo

Joined Feb 24, 2006
14,836
So it might be more efficient to research actual circuit designs rather than trying all possible combinations of hooking up components in the vain hope that you will stumble on one that works. You might want to start with a basic principle of N-channel FET operation that Vgs must be greater than Vgsth, the gate threshold voltage, in order to turn the device on. That one fact will prevent you from doing some things that have no chance of working.
 

Thread Starter

rudyauction8

Joined Jan 27, 2012
250
So it might be more efficient to research actual circuit designs rather than trying all possible combinations of hooking up components in the vain hope that you will stumble on one that works. You might want to start with a basic principle of N-channel FET operation that Vgs must be greater than Vgsth, the gate threshold voltage, in order to turn the device on. That one fact will prevent you from doing some things that have no chance of working.

Maybe you don't understand. I already knew neither way would work without even trying them. I even said so on the circuit diagrams. I'm trying to figure out what will work, and if I can't find help here I will be trying every combination until one works!
 
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