will this battery charger work?

[dirty_d]

would this circuit provide a constant but adjustable voltage to charge a battery? i know there wouldnt be any current limiting but if i keep the voltage reasonable it shouldnt draw too much. i would need a heatsink on the mosfet too.

 

[fechter]

The +18v would tend to sag under load, causing the output to drift.

Ideally you want to use a zener or some other more stable voltage reference for the op-amp. This feedback from the output is usually divided by resistors to match the reference voltage.

 

[dirty_d]

but other than that it looks like it will work? i dont really mid if it sags during the beginning of charging just that at the end the voltage will be just right to float the battery, the charger i have now puts out about 16V so i have to disconnect it once its charged, id like it be able to just hook it up and leave it on without worrying about it gassing. or i can use a LM317 to keep a constant adjustable voltage at the opamp's input right?

 

[tomv]

Float voltage for 12V battery normally would be higher than 12V. My SLA charger floats at about 13.6V (chargers at up to 14.3). 12V is midpoint of battery voltage, meaning that when battery is at 12V it's half empty (very aproximately)

For voltage regulation you might pick a one-chip voltage regulator. LM317 is typical (LM350 for more amps). For fixed 12V you should be able to find one chip that will do the job without external components.

 

[xyster]

12V is midpoint of battery voltage, meaning that when battery is at 12V it's half empty (very aproximately)

SLAs I've had, and others I've read reports about, have very little capacity left by 12.0 volts.

 

[dirty_d]

yea i know thats why im going with at least a 18V supply so i can dial it in to 13.something volts and when the voltage sags during the highest current draw the opamp will drive the fet gate higher to keep the same output voltage as the LM317 inputs to the +input.

 

[fechter]

Yes, you could use a LM317 as a voltage reference. It could do the whole job if you don't need high current.

One other possible problem, check what the allowable gate voltage is. The op-amp will drive the gate almost to the supply rails. I guess most are rated for around 20v max, but will turn full on with only 10v.

 

[dirty_d]

i dont really get what you mean, shouldnt the opamp just output whatever voltage to the gate is needed to keep the voltage at the FET source the same as the output of the LM317?

 

[fechter]

Right, but the op-amp could exceed the maximum gate voltage and blow the FET. It would be safer if the FET gate drive was limited to around 12v.