Hey, It’s been a while since I’ve been active in the forums, but spring is finally approaching Finland. Roads are finally starting to loose their ice coatings, the air is warming and the light time outside is getting longer, so I feel like it’s to become active again (3D printing some mudguards for my board atm, so I can get on the roads once the ice melts).
I would first like to have a look-back on my first approach for a BMS for the esk8 community (https://www.electric-skateboard.builders/t/flexibms-0-2-hw-under-work-flexible-configuration-and-charging-bms/46117) and identify the problems with it and how to solve them at least partially with the focus being on developing something functional that can be shipped.
The problem can be summarized as “Trying to do too much cool stuff at once” this caused high component cost, while hamstringing performance of the board as a whole. Even though the BMS was functional it had multiple tripping points on it’s way for mass-production.
So my solution will be to simplify/streamline and cut back features to bring down the component cost, lower assembly costs and make the board more sensible as a whole. I will list below some of the changes and the reasons for them:
Design ethos/focus should be aimed at the layman esk8 hobbyist and battery builders BMS doesn’t need to loaded with all the imaginable features, because the layman esk8 user doesn’t need them or will never in the most cases use them. Most people just want to charge their battery safely and don’t need to have all their components interconnected. They need something simple, flexible (series cell count wise), reliable and affordable.
Remove the whole boost SMPS block It was a cool idea IMO, but now in hindsight it really didn’t make sense. Thermal limitations were the biggest problem, plus it was quite expensive as a block. Easiest solution for this is to externalize this feature to the charger, and even with it, you would still have needed an external power source. Higher charging power ceiling as well due to heat generation being much lower on the board itself. You can get decent battery chargers (CC/CV) from china for good price these days.
Remove extra module connector Save costs, KIS
Cheaper MCU Moving from STM32L433 to STM32L052, lots of unneeded peripherals. Cuts MCU cost roughly in half from 5€ to 2,5€
Cheaper 5V buck SMPS Moving from MAX17502 (1 Amp) to MAX15062 (0.3 Amp). Overkill, price and size.
Higher balance current With more board space available and less heat generation due to no SMPS block, we can switch to bigger package bleed resistor and increase balance current from ~90 mA to ~140 mA.
Cheaper current sense circuitry ISL28022 is a really good IC, but maybe a bit overkill. I’m testing with a INA180 for price cutting.
Full blocking charging switches Problem with the earlier implementation was that battery voltage was always present at the bulk charger connector (flowed through the P-channel body diode). This was a safety concern for me personally. Charging switches are now 2 n-channel mosfets with common drain, controlled by BQ76200, enabling a fully blocking setup.
CAN and USART removed Cheaper MCU doesn’t have CAN support. Maybe USART can have some solder pads, but no connector. Opinions welcome.
So what are the planned BMS’ features?
- 3S-12S series cells support Flexible series cells support
- 140 mA balance current Higher balance current
- 9 A max charging current Higher maximum charging power
- Pack temperature NTC probe for thermal monitoring Solder pads or a dedicated connector.
- USB connectivity for parameter configuration and charging monitoring Configurable parameter: Max charging current, charging end current, max cell voltage, cell balancing behavior, temperature limits.
- Buzzer for audio signaling Interested in opinions on implementation
- On board status LED Interested in opinions on implementation
- Smaller PCB With a lot of components removed, I can shrink the board down further.
I’m planning on splitting the FlexiBMS into a Lite and Basic versions, with Basic being more feature rich and Lite being the no-bells-and-whistles version meant for battery pack builder integration/basic functionality for controlled charging.
Layout WIP picture.
Currently when I’ve been sourcing the components from Digi-Key with the planned amount for 100 boards and the total is standing a bit over 2000€ or 20€ per board, which is MUCH lower than with the earlier approach and is something that even I feel safe to pull a trigger for ordering.
I aim the sell price to be between 40€-60€
And as always I welcome comments and questions from the community and invite everybody to the conversation.