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Battery Charger - Check Voltage

nicolaoscon96

Hello there,

 

I am working on a long-term project and am in need of your help. I have bought a 18650 battery charger that takes 12v in and charges the battery pack boosted to 14.4v. I want to install it in my car as a primary power source for the project so it won't kill the main car battery because it only runs when the car's engine is off. Now I need help with the next one. I want to make a simple circuit (not a microcontroller) that will check the 18650's voltage and if it is less than 10 volts it will start to charge the pack from the main battery by turning a MOSFET or something on and then stop the charge once it fully charges (14.4v). Is it possible to make such a circuit? If yes can anyone help or point me to a website that already did anything similar?

 

Regards

Nikolas

  • Yes, I can help on this one. First, I would say that it can be done either with or without a processor, but in my opinion, it would be far easier with a processor, like an Arduino. But if you're not comfortable with coding, it can be done with a window comparator circuit who's output is connected to the enable on the charger. If you need to isolate the charger from your car battery when it's not charging, either a MOSFET or a relay can be used.

    Before I spend time on the details, please reply and let me know if you (or anyone else) is still interested in this. The reason I say it's easier with a processor is that you would have very little breadboarding to do. But with an analog circuit, you must breadboard it from scratch, probably using a Vectorboard or similar. 

    The comparator circuit can use a low-power, rail-to-rail type, such as a TI, LMV331-Q1 and a suitable reference voltage IC, or, Analog Devices (Linear Technology Division) has an IC with all these functions built in that might cost more, but will save you much in breadboarding and troubleshooting time. That IC is the ADI, P/N: LT6700, which has a dual comparator available in a window-comparator version that will sense your 18650 battery voltage and enable your charger if it's too low and disable it when it gets above the fully-charged voltage. For the isolation from the car battery when the charger is off, you can use the ADI P/N: LTC7003, which is a high-side switch. You must provide your own MOSFET however. They have an evaluation board you can use with that IC on it as well as a MOSFET with a 14.5 AMP rating. The eval board P/N is: DC2749A. I didn't see an eval board for the LTC6700, and it's not available in a DIP package, making it a bit tough to breadboard, but it can be done. As an alternative, you can do a simple PCB layout and you can get up to a 2-layer PCB at 4" X 4" for $2.00 for #5 boards in 24 hours or up to a 4-layer PCB of the same size for $8.00 for #5 boards in 4-days, which is a great deal. 

    If you need more help, just ask and I'll help you as best I can.