12V Lead Acid/Sealed Lead Acid Battery Charger

I designed an SLA charger once, for 12V 6.5 Ah batteries. It used a buck switching regulator and had a maximum charge current of 1.5A. To charge a 100 Ah battery at an equivalent rate you would need a charger with a maximum output current of 23A!

You should decide on your maximum charging current. Then I recommend a buck switching regulator for best efficiency (i.e. reasonable heat dissipation). You can use synchronous rectification to increase efficiency, if your controller IC supports this.

The connection between the buck regulator and the battery should probably include a series diode, and probably a low-value series resistor, which doubles as a current shunt resistor; monitoring the voltage across this resistor will tell you the charge current. This resistance is needed for stability; without it, a tiny adjustment in the regulator output voltage could cause a big change in the charge current, and this would make the regulator hard to control.

Then you need to be able to monitor and control the regulator. I used a microcontroller for this, and a software controller is what I recommend to you as well. If you don't currently use microcontrollers and you don't want to start, another option might be a generic I/O module like the ones made by National Instruments to provide the analogue inputs and outputs to the regulator, and software running on a PC to control it.
You need at least the following analogue signals:

• Analogue output from control device to charger, to set its output voltage (I used PWM in my circuit);
• Analogue input to the control device from the charger, to monitor the charger output current;
• Analogue input to the control device from the battery, to monitor the battery's terminal voltage.

My circuit also monitored the charger's output voltage, and had a charger ON/OFF control.

To start charging, you ramp up the charger's output voltage until current starts to flow. Then you adjust the charger's output voltage according to the control strategy. The simplest control strategy is constant current / constant voltage - while the battery terminal voltage is less than the target voltage, regulate the charge current; then reduce the charge current to keep the voltage constant.

You regulate the charge current by watching the current flowing through the current shunt that's between the charger regulator output and the battery. If the current is too low, increase the regulator output voltage, to force the battery to draw more current, and vice versa.

You may want some kind of supervisory process as well, to detect when the battery voltage has been at target for a certain length of time and stop charging, if that feature is required. Alternatively, the battery can be left float-charging with a trickle charge current.

It could also abort charging if the battery is taking too long to reach the target voltage.

 

Could you not just use a bridge rectifier to solve that problem then?