This project was discontinued. The idea was to use relay for switching the power instead of transistors to get rid of heat. Also, I wanted it to be more than just a BMS, like main power board & monitoring board.
Hi, I was thinging how could I improve the way people protect their batteries. Currently, most of us use some kind of BMS, but not all the functionality is acctualy needed. If we use 25R cells for example, they are so good quality that they don’t have to be balanced. So the only thing we need then is overcharge, over-discharge and short circuit protection. Then I thought… What is the most effective way? Like something that won’t produce any heat. Do you remember those old mechanical switches called relays? Yes, they’re still used. They can switch hundreds of ams without producing any heat! Well, they produce some heat, but negligibly amount. Compared to mosfets it’s amazing! This can be used as anti-spark switch as well, but about this later. So, the next thing I need to do is to measure the actual voltage of the pack. Yeah, I could use some parts made for this… BUT: why not to use microcontroller? ATTiny are great for that. They have built-in ADC, so you can measure the voltage. All we need is a voltage divider ( 2 resistors ) and one ceramic capacitor ( 330nF should be fine ). And because the microcontroller itself is controlling the relay, we can connect switch to the microcontroller and voala: we have anti-spark switch as well! Nice! What’s next? Well, you can connect a display to the microcontroller ( those 0.97" OLED displays you can buy on ebay are great for that ) and display the voltage or the percents left based on the voltage to percent chart from datasheet. And because we all love to short circuit our stuff, the protection is very important. So let’s add a fuse as well.
So, what this solves?
- Over-charge protection
- Over-discharge protection
- Over-current protection
- Anti-spark switch
- Voltage / percentage monitor
Are there any disadvantages? Well, yes - idle current would be higher than on the most of BMSs.
So… how much would it cost? Shouldn’t be expensive, as I can’t product SMD at home, I’m working with THT components, main components:
- cuprextit ( material used to produce PCBs ): up to 0.20 eur
- input filter cap: 0.05e
- linear voltage regulator ( should be fine for ~10mA of power ): up to 0.15 eur
- microcontroller: 1.00 - 1.50 eur
- relay: 7 eur
- fuse & fuse socket: 2 eur
- resistors ( for voltage divider ) : 0.06 eur
- headers - 0.02 euros
- display - 3 euros If you do the math, you will find out that it will cost at max 10 euros.
Not that bad as it’s small, does a lot and produces virtually no heat. I don’t have a prototype yet, I’m going to draw a scheme tomorrow and share it with you guys. I’m not going to say it’s the best way to do it, it just an idea that I’m going to prototype once I have my pack.
What do you think?