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thank you very much audioguru, I have bought the LM339 IC and almost ready to test. but first I want to add a feature to auto stop charging battery once the LED goes on. it doesn't have to keep monitoring it would be satisfactory for it to completely shut off until reactivated manually via a push button my thoughts there are 2 ways to do this 1. tap into the computer supply voltage regulator and connect the output to the adjuster 2. have it modular design: have the output swich on a N/C relay. and add a diode to prevent battery from powering the circuit once switched off i prefer 2. as then PSU can supply function as generic PSU when not being used as a charger drawing up a schematic now for 2. will post shortly

Thank you audioguru, I have gone back and done some calculations with the inputs, and have dramatically reduced the current. I don't understand the part about positive feedback but I am reading the datasheets now and wikipedia. my interpretation of that is: and I'm not sure on what value that resistor should have, doing more reading - Schmitt trigger? edit: Ok so I did more reading and so for positive feedback I need to make the INPUT more (1 - 10mV more) so that it acts fast and sharp to prevent oscillation.... now I currently thinking how I can do that

Thanks Audioguru, I have switched LM311 with LM339 which has a voltage range 2-36V: http://www.national.com/mpf/LM/LM339.html#Overview The 15V is regulated by a computer PSU. Is that what you mean? I have added a pullup resistor on LM339 output - value as per datasheet

Hi, I have designed, based on another car battery charger (http://www.aaroncake.net/circuits/charger1.asp?showcomments=all), a circuit to charge a lead-acid battery to about 14.4V then auto-shut off and light an LED. I think it may need some work but will the concept work?? Am I crazy? The supply voltage will be from a computer PSU regulated at 15V Your thoughts and comments and suggestions are very welcome Please forgive the diagram, KTechLab doesn't have relays as an option. Version 0.2 is here: I am stuck on how to get it to switch off the power to the battery, but still monitor it so it float charges the battery, so when the supply voltage drops again it begins to charge, and so on.. How it works: The HIGH output on the comparator keeps the relay ON, then as the supply voltage increases due to the increased resistance in the battery (decreased current flow), which once gets higher than the reference voltage, the voltage comparator output is then LOW, switching on the LED indicator