Objective

 

Our capstone team has been asked to design and build a BMS (Battery Management System), that can receive power from solar modules, wind turbines, generators, and charge the batteries that will be used in the hospital currently being built in Haiti. Although we used 24 volts for testing purposes, this system can manage up to 48 volts, and will be used to power lighting, wheelchairs, DC air-conditioners, fans, computers, and many other appliances. 

 

 

What’s a BMS?

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BMS is basically a device that ensures the safety of any rechargeable battery. It is present in our phones, laptop, literally anything that deals with charging batteries. It protects them from overcharging, over-discharging  overcurrent, high temperature, etc. Without a system like this to protect the batteries, their life cycle would dramatically decrease, as well as increasing the potential risks of fire and explosion.   

 

Reason to build one?

 

The reason our team decided to build one is because most BMS's in the market don’t really accommodate the financial situation of the hospital and country Haiti at the moment, as well as its performance. Their efficiency depends a lot in the conditions of the battery, which plays an important role in this project. BMS can be divided in active balancing and passive balancing. The more popular is the passive balancing, which can be divided mainly into two parts. The first one cuts off the entire battery pack whenever one cell reaches full charge. The second type of passive balancing is more efficient in the way that it allows all the cells to fully be charged, but it can waste energy. When one cell gets fully charged, a resistor is placed in parallel with that cell, allowing it to be slightly discharged and therefore giving time for the other cells to catch up. The best scenario to use passive balancing is by having all new cells, or with equal capacity. That way they can all charge and discharge around the same time, decreasing the resistor time of action. 

 

On the other side there’s the active balancing system, which is more efficient than the passive one, and actually allows the fully charged batteries to share some of their voltage with the lower ones. The team originally wanted to build an active system, but its costs and complexity were a big concern, because our goal was to make something cheap and easy to build, so that the people in Haiti would be able to actually build them and create a business opportunity for themselves. 

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Quick Overview 

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What we created follows our requirements, its cheap and easy to understand. We did the all process from testing in a breadboard, design the circuit in Eagle CAD, print the PCB, and solder all the components on it. Our BMS is a DC–AC–DC, and mainly formed of 5 parts. There's a frequency oscillator, half bridge mosfet converter, inductor, triac system and capacitor. Basically we get the DC from the renewables, and convert to AC to charge the capacitors that are used to charge our batteries. 

 

Each cells has an IC (integrated circuit) voltage monitor that is constantly checking the status of the batteries. If they are bellow 4.2 volts, this IC will send a signal to the MOC driver that will then allow the capacitors to charge the batteries. If they reach 4.2, it sends another signal to cut off the charging for the particular cell that has reached full capacity, allowing the other cells to fully be charged.