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Protection/management for battery-powered application

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nickagian

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I am currently designing a battery-powered application, which involves a MCU and digital sensors. Two Li-ion batteries will provide voltage to the PCB and a step-down DC/DC converter creates the main +3.3V DC power supply. My question is the following:

What protection circuits should I have for the battery pack? I have the following in mind :
-> Overcurrent protection
-> Overvoltage protection
-> Excessiv temperature protection

Are all these necessary? What else should I consider?

Moreover, I intend to use a voltage monitoring IC to check when the battery voltage is really low and create and UVLO state of the system with the help of the MCU.

What about the battery monitoring? Is a simple ADC-measurements based monitoring of the battery voltage enough for determining how much battery capacity still remains? I know that this method is not very accurate, but I am wondering wether a full gas-gauge system is necessary.

Thanks,
Nikos
 
Hello there,

When you are using Li-ion there are a few precautions you have to be aware of like under voltage and over voltage when charging. The nice thing though is that you can estimate the charge remaining (SOC) by measuring the voltage and applying a simple formula...this will get you your gas gauge. This isnt super accurate but it does work and most other systems wont be super accurate anyway.
Your battery charge has to take care of the votlage and max charge current, and limiting load current isnt a bad idea either. Im sure a PIC chip could do this all in one package with say an added current sensor. You could monitor voltage and current and shut down in the event of some problem.
If you are really concerned, build two systems that are electrically powered from two different sources (if possible) and have them both monitor the same quantities. That way if one system fails the other system will pick up the problem.
 
They make "protected" li-ion cells. The small pcb circuit protects against low-high voltage, and short circuit. As an **broken link removed**.
 
Low voltage disconnect is placed at about 2.2-2.5vdc. Charging at cell voltage below about 3 vdc should be done at 0.1C, then it can be raised to higher rate.

If your application doesn't need too high a current draw I would look at just using a low drop out regulator from a single cell. Makes things easier.

At 0.05C discharge rate and lower, the battery is pretty well fully discharged at 3.6 vdc.
 

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Thank you all for your answers and your time. As a general comment, I forgot to say that the batteries that will be used are primary Li-ion batteries. Thus, no charging and no charging circuit will be required. Summarizing I intend to apply unde-rvoltage and over discharge current detection/shut-down.

Regarding the gas gauge, I will in the end stay with the solution of just monitoring the voltage of the battery. I think this is enough accurate for now.

What about thermal protection? Against excessive heat or cold? Is this important?

Thanks again.
 
Hi again,


Thermal monitoring is usually used as a fail safe mechanism, to detect something else that went wrong, but there is no reason why you can not simply monitor the temperature and shut down if something heats up or it gets too cold, etc.
 
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