Cheap DIY BMS

Yo guise

im reading the forums since 3 month but didnt register yet so dont worry


im currently developing a cheap and lightweight bms that can be used in combination with HK Lipos and i want to hear your oppinion about it
reasons for me to develope a new bms are that bms systems currently on the market are either too big, too expensive, not expandable or just not programmable
this is a approach to maybe develope a community bms that can be adjusted to everyones needs

Features:
- capacitive + resistive balancing
- buzzer to notice user
- temperature sensor
- 2 Leds (not yet sure what they will show)
- communication between battery packs
- extendable up to ... 10000S in series

Electrical Specs:

- standby current of about 100uA (which is about 7 years on 5000mAh)
- balancing current 1-30mA (depends on voltage difference of cells)


Physical Specs:

- small dimensions: 40x25x8 mm
- 2 connectors for 6s lipos (6s2p made easy)



Functional description:

Balancing:
- bms should be left connected all the time
- if charging is detected and cell voltage > 3.8? balancing starts
- voltages are sent to other bms boards periodically
- all boards target the same voltage on every cell
- balancing ends when all cells are balanced or battery is used

LVC:
- while battery is used bms stays active and monitors all cells individually
- if LVC condition is detected (one cell below 3.0V?) bms starts buzzing
- it is not planned to cut the battery

HVC
- currently not really planned ... maybe add one connection to notice external charger to stop charging?
- starts buzzing

Temperature
- start buzzing if battery gets hotter than 50°C?



Hardware:
- the bms is based on capitative balancing, so no energy gets wasted! (but there is also a resistor as alternative)
- i use 2 analog switches to switch a capacitor from cell to cell and sometimes to the adc to balance and measure
- the boards are connected via optocoupled uart

the balancing idea is based on a schematic i found on a german electronic forum:

its simple and does not need much parts -> cheap

Expected price per 6s Battery:
- parts: ~10$
- pcb: ~5$

means: price for 18sXp = 45$

i guess i will order the first prototype pcbs in 1-2 weeks

there are still some pins unconnected of the Atmega88 so im glad to hear your improvements :lol:



JP10+JP11 = Supercap 0.1mF (not uF!)

dr_lulz said:

- 2 Leds (not yet sure what they will show)

Click to expand...

Why would you design something in that you haven't got a purpose for?

HVC
- currently not really planned

Click to expand...

Bad idea.

If it's a BMS for RC LiPo(lico) then a cell-level HVC is essential; overcharging that stuff is what often causes fires.

dr_lulz said:

- balancing current 1-30mA (depends on voltage difference of cells)
[snip]
- if charging is detected and cell voltage > 3.8? balancing starts

Click to expand...

Balancing at 30mA would take a really long time.
IMHO balance current should be at least 5% of the charge current.

In my own tests balance voltage is too ambiguous until (LiCo) voltage reaches about 4.1V.

Take a look at my BMS:

http://endless-sphere.com/forums/viewtopic.php?f=14&t=44100

so you start balancing at 4.1V? the problem is i usually dont charge them higher than 4.1V
maybe its better to balance after charging?

the balancing current of my design is fairly low but you have to take into account that the bms is connected to the battery all the time and even if it takes 7 days to get initial balance this would be ok. Im currently running without bms and batterys stay more or less "balanced" with very low drift from charge to charge, means once the cells are balanced they should stay balanced even with low balancing current

I already took a look at your design, initially i was thinking to build about the same, the voltage measurement circuit looks very accurate and "high end", but the downside is you have lots of parts on a big pcb while i have almost no free space in my battery box. Thats why im going for the capacitive balancing method based on just 3 parts + uC, the whole pcb is so small i can simply put it ontop of each battery

dr_lulz said:

so you start balancing at 4.1V? the problem is i usually dont charge them higher than 4.1V

Click to expand...

You balance whenever your cells begin reaching their top of charge, whatever you define that by. Stop putting current into the "full" cells, and shunt it over to others one way or another (resistively, capacitively, however you like. Doesn't matter what terminal voltage you want to use.

the balancing current of my design is fairly low but you have to take into account that the bms is connected to the battery all the time and even if it takes 7 days to get initial balance this would be ok. Im currently running without bms and batterys stay more or less "balanced" with very low drift from charge to charge, means once the cells are balanced they should stay balanced even with low balancing current

Click to expand...

Problem is after cells age they dont' stay balanced, and that's when you need the BMS to balance more quickly than yours could--otherwise you might not be able to use the pack for days, or weeks, if it gets out of balance badly because of aged cells with differing Ir and capacity, and you still need to use all of the capacity they have left to go where you need to.

Fast balancing (overnight, at worst) can be critical for a small pack on a bike that gets used to max range every trip. Even more critical for one charged at each stop on a leg of a trip (like for work commutes where it has to be recharged at work to even get back home).

As long as you don't need that, either by oversizing your pack significantly so that as it ages it will still have enough capacity to easily get you where you must go without recharging (or fully), or by replacing cells as they age and begin to require more balancing than your BMS can deal with in the time necessary, then your proposed specs would be fine.



As an example: I have a very light roadbike build (Nishik-E) that has been planned partly on the principle of carrying only enough battery to do my work commute, or to make it to/from the light rail for trips across the city, and require charging at the other end to get back home, for most trips. That way I can still lift it into the bike "racks" on the light rail, whereas I couldn't do that with enough battery to go both ways or more, most of the time.

If I were to use the full capacity of the pack each time, then carrying only the bare minimum plus enough to account for aging-caused capacity loss, some cells will lose more than others and eventually the pack will become badly unbalanced even just during a single ride--rebalancing it would require far too much time with a low-balance-current BMS. Using a much faster balancing scheme would be necessary if I wished to keep them all in balance for each ride.

In practice, I would probably simply use an RC-charger with higher balance-current capability in this particular case, but in other cases I might actually need a balancing BMS of some sort.

Run the balancer all the time (whether charging or running the motor). I built some small ones a while back: http://endless-sphere.com/forums/viewtopic.php?f=14&t=20864

so i finally finished my prototype testing today, everything works as expected

final features are:

- 180µA sleep current
- balancing current about 60mA
- stackable up to 4 boards (software limited)
- 6s per board (7s still possible)
- its possible to sniff the uart to get all voltages (might be possible to show them on CA)
- pcb size 4x2.5cm

installation process:
simply plug them to your packs and never remove them again

functional description
the boards will balance all your cells to the same voltage if cell voltage is above 4.15 V

i am also thinking about adding a "switch board" which can disconnect the battery if HVC/LVC

final price will be about 20$ per board, just wondering if anyone is interested?




pics of my 2 prototypes (i know they look messy, lots of testing and soldering )

I would be interested if:
- there is HVC to prevent overcharging on cell level (no more than 4.25V/cell)
- there is LVC to prevent overdischarging on cell level (no less than 3V/cell)
- there is some visual indication of cell health. Ex: have a small visual display to show all cells are healthy, or if there is something to indicate when the voltage difference between cell is too high

Well done Dr.
Great to see people coming up with DIY solutions.