12V LM350T Lead Acid battery charger question - Page 1
Forum section: electronics / projects

author: khatus
author_rank: Regular Contributor
timestamp: October 22, 2024, 05:46:21 pm
content: Hello friends, I want to make this lead acid battery charger.Here, I want to use 1000pF polyester film capacitor instead of the electrolytic capacitor C3? My first question is can I use 1000pF polyester film capacitor instead of 1000pF electrolytic capacitor?if i use the DPDT Self-Locking Push-Button Switch, in the place of the S1 Start pushbutton, will there be any problem?

author: Benta
author_rank: Super Contributor
timestamp: October 22, 2024, 06:27:06 pm
content: It's not an electrolytic, the "+" is missing.Yes, use what you show.

author: floobydust
author_rank: Super Contributor
timestamp: October 22, 2024, 06:44:50 pm
content: Yes- a film or ceramic cap is fine for the 1,000pF part C3. The original circuit is from Radio-Electronics I think.Note the charging circuit has a few weaknesses - if you connect a battery backwards, it is kill.If power is off or unplugged and you connect a battery, backfeed into LM350 it does not like as well.

author: khatus
author_rank: Regular Contributor
timestamp: October 23, 2024, 03:03:18 am
content: What is the purpose of the push button in this circuit?

author: floobydust
author_rank: Super Contributor
timestamp: October 23, 2024, 04:52:31 am
content: The charging circuit has two states controlled by the op-amp. The momentary pushbutton takes the BALANCE/COMP pin 1 low to GND, and forces the op-amp output high. I think this sets the output voltage to the higher setpoint where it stays in CC mode until charging current drops off.Note output pin 6 is missing from the schematic.LM301 datasheetdoes not ID pin 1 but NS one does.more discussion on the same old circuit:https://www.edaboard.com/threads/using-1n4001-diode-over-1n457-diode-in-a-battery-charging-circuit.411240https://maker.pro/forums/threads/12v-battery-charger-schematic.267632

author: khatus
author_rank: Regular Contributor
timestamp: October 23, 2024, 08:02:00 am
content: Will there be a problem if I use 15 ohm and 230 ohm 1/4 watt resistors? The value of these two resistors must be high wattage or low wattage? If it is high wattage then how much wattage should I use?

author: floobydust
author_rank: Super Contributor
timestamp: October 23, 2024, 09:41:59 pm
content: The resistors can all be small 1/4W parts except the current-sense resistor 0.22R needs to be 1-2W. The LM350 needs a big heatsink.Tony van Roon did his take on the same charger circuit, adding trimpot to adjust the output voltage. This is surely necessary.https://www.learningelectronics.net/VA3AVR/gadgets/labc2.htmHe says the op-amp output is low for "charge" and high for "float" which opposite to what I said was going on with the pushbutton switch so I might be wrong.

author: khatus
author_rank: Regular Contributor
timestamp: November 03, 2024, 05:57:54 pm
content: Hello friends,I have completed soldering of the circuit on perfboard according to this Lead acid battery charger diagram, and got the exact output according to the formula given in the datasheet.Here is my resistor value,R2+R3 = 239.6 k ΩR1 = 3.276 k ΩAccording to the formula given in the LM350T datasheet,Vout = 1.25×(1+ 3276/239.6) + 50×10-6×3276Vout = 18.34+0.1638vout = 18.50and I have got exactly this output voltage.Now my question is ,1) charging a 12 volt lead-acid battery with this 18.5v output voltage cause any damage to the battery?2) What is the optimum output voltage to charge the 12V lead acid battery if it is not charged at 18.5 volt.3) What is the purpose of the 15KΩ resistor in the circuit?4) If I make the output voltage slightly higher than 12 volts (say 13.5 volts) will the LM301A OPAMP would work properly? Because it requires +/-15 volts at pin 7,4.

author: khatus
author_rank: Regular Contributor
timestamp: November 03, 2024, 06:09:39 pm
content: Here is my circuit,

author: SteveThackery
author_rank: Super Contributor
timestamp: November 03, 2024, 06:36:34 pm
content: Quote from: khatus on November 03, 2024, 05:57:54 pm18.5V is far too high for a 12V lead acid. In fact I think you'd need a LOT of current going into the battery to achieve 18.5V across the terminals. I'm pretty sure it would do some damage due to gassing.Lots of vehicles charge their batteries at 14.4V. Vehicles with "smart" charging will sometimes push the battery voltage up to 15V or so immediately after starting, in order to push as much charge into the battery as quickly as possible. After a few minutes they drop the voltage to around 14.4V.

author: SteveThackery
author_rank: Super Contributor
timestamp: November 03, 2024, 06:41:09 pm
content: Quote from: khatus on October 22, 2024, 05:46:21 pmDoes anyone know why the schematic uses the symbol for electrolytic capacitors everywhere?OP: I think the symbol is wrong - I don't think you can get a 1000pF (1nF) electrolytic.