Battery Charging

[rednwhite]

I am trying to charge 2 12V batteries (car batteries) but having read a lot of articles I am getting confused with terminal and charging voltage, what voltage would I need to charge them and what kind of currents?

Cheers

 

[MikeMl]

In a nutshell:

A proper charger for lead-acid batteries does three steps:

(1) limit the initial charging current to a safe level specified by the battery maker, (and possibly to protect the charger itself)

(2) allow the battery voltage to rise to ~14.5V (2.42V/cell), hold it there until the charging current naturally decreases to ~10% of the initial charging current used in step 1, (could take hours to days, depending on state-of charge, the current used in step 1, and the A-H rating of the battery).

(3) then switch to the float voltage of ~13.2V (2.20V/cell, must be very accurately set, should be temperature compensated).


is the straight dope:

I do take issue with the following statement in that article:

"During the topping charge in Stage 2 that follows, the charge current is gradually reduced as the cell is being saturated."

Actually, all the charger needs to do is keep the voltage fixed at ~14.5V, while monitoring the current acceptance of the battery. The time to step from 14.5V to ~13.2V is after the current during step 2 drops below a few tens to hundreds of mA, depending on battery size.