UPS - battery charger

Original post date:05/12/2015

This is a simple Lead Acid Battery Charger based on TI's UC2906. VCC is 18V from my AC adaptor.

I based it on Figure 1 "Dual Level Float Charger" from the datasheet. There is an Excel spreadsheet that I used to calculate the values on my Github.

The charge current is set to 500mA by R1//R2. I also have a NiCd battery and charger with a much faster charge rate to take care of short term outages. The charge current is limited by what my AC adaptor can spare. For my 8A*Hr battery, that should take 16 - 20 hours to fully recharge. This is one area that a microcontroller can provide some flexibility by allocating a higher charge current based on the amount of current available. The charge algorithm for lead acid is actually very similar to Li-ion batteries.

Figure 2 in the datasheet shows the equation to arrive at the values while figure shows the charging states. The charger is temperature compensated for proper charging.

The circuit was constructed on a single side PCB that was etched with toner transfer. You want to keep the charger chip near the battery as it sense ambient temperature. Ideally you want to place the transistor on a heatsink away from the charger chip and run wires. There isn't much heat generated in float charge stage where the battery spend most of its time, so the heating effects is minimal.

3D rendering of the PCB.

This is the NiCd Charger I used. The charging algorithm of Ni type battery is hard as it depends on a small voltage and rate of change. I used a BQ2002 NiCd/NiMH Charger Controller.

It is optional as this type of battery are harder to find these day. X3, C4, D4, D5, and C7 are not part of the charger circuit and they should always be there. +UB is the the backup supply rail. Additional battery packs etc should be made with Or'ing diode to +UB.

Most of the design details are covered by the datasheet and the app notes.

I have selected C/2 for the charge rate by connecting TM pin to mid rail voltage with R5 & R6.

The CC control signal is level translated by Q3 to the Constant Current ciurcuit. Q5 provides the negative feedback by sensing the voltage across R16 hence regulating the current. In this case, the current is about 0.7A which is roughly 1/2 C for th battery pack I have. Q4 selection is non-critical as almost any P-MOSFET would do.

Voltage divider R13, R14 is used to attenuate the battery voltage. I have N=10 cells, so I pick R13 to be roughly 9X the value of R14. The BQ2002 voltage is regulated by a 5V Zener diode (D3). Q6 detects when the battery pack is connected and use it to power cycle the BQ2002.

I use a bicolour LED for the charge status on the front panel: Red = Charging, Green = Battery Full and Off = No batteries.

J1 is the battery connector. Due to high current involved, a 4-wire connections is used to measure the battery voltage.