Why can i not slow charge a lead acid battery?

[Paul__B]

Why can I not just limit the voltage to 14.2volts and the current to a 20th of the battery capacity right from the beginning? Then I don't have to change a thing? For the bulk charge it will charge at 300mA at 14.2volts and the topping charge will charge at 14.2volts at 300mA? Why can this not be done?

That is precisely what you (need to) do!  And how it is virtually always done in practice, no more complex than that.

Actually, I would make it a little lower, 14 or 13.8V, but a current limit to about a tenth of the AH capacity plus that required continuously by your circuit.

One other trick - you have to arrange your charging circuit such that there is no "back-feed" through the regulator when the mains fails - generally as simple as a diode, but the voltage drop of that diode needs to be factored into the regulated voltage.

[calvingloster]

Why can I not just limit the voltage to 14.2volts and the current to a 20th of the battery capacity right from the beginning? Then I don't have to change a thing? For the bulk charge it will charge at 300mA at 14.2volts and the topping charge will charge at 14.2volts at 300mA? Why can this not be done?

That is precisely what you (need to) do!  And how it is virtually always done in practice, no more complex than that.

Actually, I would make it a little lower, 14 or 13.8V, but a current limit to about a tenth of the AH capacity plus that required continuously by your circuit.

One other trick - you have to arrange your charging circuit such that there is no "back-feed" through the regulator when the mains fails - generally as simple as a diode, but the voltage drop of that diode needs to be factored into the regulated voltage.

So can I use two LM317 voltage regulators to do this? One will limit current and one will limit voltage? All in one circuit?

[Chagrin]

This is completely untested and I'm too lazy to figure out the proper resistor values. Basically it acts as a constant current charger during bulk charging and a constant voltage charger at float charging.

R10 and R9 should be sized to set the LM317 to 14.4V (bulk charge). When the transistor (any small signal transistor) is set on the resistance of R8 will be in parallel with R9 and cause the output voltage to drop, and would be set to the float of 13.2V. Should have labeled that "set float charge".

R7 sets charge current, but that current regulation would be negligible when the previous LM317 is set to the float voltage. R11 would be 22K and R10 at 10K to act as a voltage divider so you can read the battery voltage with the Arduino.

[Paul__B]

So can I use two LM317 voltage regulators to do this? One will limit current and one will limit voltage? All in one circuit?

Actually, it is easier than that if your charger does not need to have the same negative rail as the battery, you only need one regulator.

You put a resistor calculated to drop 0.65V at the current limit, in series with the negative of the battery, and connect an NPN transistor with a 1k series base resistor, across this resistor, its collector going to Vref on the LM317.  This transistor switches on as the current comes up to the calculated value, and pulls down the regulated voltage.