If you own a motorcycle, you already know the pain. I was little out of station for a couple of weeks and when I came home and gave a push to my bike, there was nothing. The battery is dead. Lead-acid batteries self-discharge over time, and if you don't ride often enough, the battery just slowly dies on you. Buying a new battery every few months is not an option. So, I used to charge manually on a workstation which costs me time and money. Even though most of them are using just a transformer with a rectifier, no regulation, no protection, just brute force current being dumped into the battery. But I am an electronics engineer and that’s how I decided to give a try to lead acid battery charging.

The one that actually understands how to charge a battery properly. With trickle charge for deeply discharged batteries, constant current for bulk charging, over-charge for topping it off, and float charge to keep it maintained without overcharging. The whole charging profile, done right. That's where the CN3767 comes in. It's a dedicated 12V lead-acid battery charger controller IC from Consonance Electronics that does everything I just described. I have designed a PCB in EasyEDA and fabricated it from JLCPCB and tested out the final prototype.

Why CN3767?

There are a lot of battery charger ICs in the market, but for 12V lead-acid specifically, the options narrow down quickly. Most charger ICs are designed for Li-ion and Li-Po. The CN3767 is purpose-built for 12V lead-acid batteries, and that makes the design so much cleaner.

Features:

• Complete 4-stage charging profile
• Wide input range: 6.6V to 30V
• Up to 4A charge current
• Solar panel MPPT
• Fixed regulation voltages
• Status indication
• High switching frequency: 300kHz
• Automatic recharge
• Sleep mode

The IC uses a PWM buck (step-down) topology with an external P-channel MOSFET as the switch. So it is clear from the description that this is a controller IC, not a standalone charger. Choose your own MOSFET, diodes and inductor based on your power requirements.

Lead Acid Battery Charging Profile:

Lead-acid batteries have a very specific charging requirement, and getting it wrong either undercharges the battery or overcharges it. Which causes capacity reduction or damaging the battery. The CN3767 handles all four stages automatically.

Stage 1: Trickle Charge

When the battery is deeply discharged and its voltage is below 75% of the over-charge voltage (that's 75% of 14.8V = 11.1V), the CN3767 enters trickle charge mode. In this mode, the charge current is only 17.5% of the programmed constant current. Because dumping full current into a deeply discharged lead-acid battery can damage it. The low trickle current gently brings the battery up to a safe voltage level before the real charging begins.

Stage 2: Constant Current (CC)

Once the battery voltage rises above 11.1V, the charger switches to constant current mode. This is the main charging phase where the battery receives the full programmed current. The charge current is set by the sense resistor using: ICH = 0.12V / RCS.

For my design, I'm using a 0.12 ohm sense resistor, which gives me exactly 1A of charge current. That's a safe and steady rate for a typical motorcycle battery (usually 7-12Ah capacity). During CC mode, the battery voltage steadily rises while the current remains constant. The CHRG LED stays on to indicate active charging.

Stage 3: Over-Charge (CV) - Constant Voltage

When the battery voltage approaches 14.8V (the over-charge regulation voltage), the charger transitions to over-charge mode. Now the voltage is held constant at 14.8V and the current starts to taper down. This is the classic CV (constant voltage) phase. The over-charge continues until the current drops to 38% of the constant charge current. For my 1A charger, that means the over-charge ends when the current falls below 0.38A. At this point, the battery is essentially full.

Stage 4: Float Charge - Maintenance

After over-charge terminates,...