Thanks for the great circuits. I am looking forward to start putting one together for hibernating my RVs battery over the winter. However, can I exchange the transformer + diode bridge with the +15V DC power output from an old PC power supply, i.e a switched power supply? I don't see any reasons why not, but don't know too much about the charging restrictions for 12V Lead Acid Batteries.
I think I'll be moving down the path with a switching power supply that is rated for 5A max current. However, I am wondering if I can charge 2 batteries at the same time. I have an older VW camper that has an auxiliary battery as well as a starter battery. Over the winter I'd like to keep both batteries happy and your schematic seems to be promising to achieve that. The batteries are not connected to each other when the car is off.
Do you think it is possible to use only one power supply, but two NE555 schematics to achieve this? I am thinking that I could use one NE555 schematic per battery, probing for voltage levels and controlling individually when each battery is charged. I am also thinking to put a diode into the current path to the battery so that, when both batteries are charging, the current can never flow from one battery to the other.
According to the spec sheet, the 44 Ah auxiliary battery that I am going to buy has a max charging current of 12A. The other battery should have about 75Ah capacity. My interpretation of those values is that both batteries can handle the full 5A current when only one is charged. If both are charged simultaneously, they'd simply take longer and current will distribute itself according to the voltage levels of the battery.
Obviously I am trying to prevent buying two switching supplies (the PC power supply actually didn't offer 15V when I checked), which would keep the cost to a very interesting level => ~$30 vs. ~$55 for a system with two PS or vs. about $90 for buying two chargers.
Looking forward to your thoughts on this.
The proposed automatic double battery charger circuit from a single power supply shows two identical stages made by using the IC555. These stages are basically responsible for controlling the lower and upper charging thresholds of the connected batteries.
The SMPS which is the common power source for both the 555 stages supplies power to the batteries via the individual diodes and the relay contacts of the respective 555 stages.
The diodes make sure that the power stay well isolated from the two stages.
However the crucial part of the circuits are the two resistors Rx and Ry which are the current limiting resistors for the two stages.
These resistors ensure the correct specified amounts of current to the respective batteries This further ensures that the SMPS is loaded uniformly across the connected batteries.
Rx and Ry should be calculated as per the AH ratings of the batteries with the help of Ohm's law.