The 18650 - 13s10p Project - 48V x 34Ah

Solarsail said:

The DG408 idea is interesting PStechPaul. Would a digital isolator be different from an optoisolator?

With 10 bit resolution, that would be 0.005V, assuming the range is 0 to 5V? My feeling is that this may be sufficient resolution. If so, then all optoisolators could be eliminated except for one, when we use one MC per cell. I think an ATmega can be as cheap as $2.00. And there would be no need for precision voltage division. My gut reaction is that this is superior to using a specialized BQ7693 chip. But a nuanced analysis would be required to decide between these several designs.

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Yes if you did one microcontroller per cell you could get away with it, there are some very small/cheap ATtinys, the ATtiny102 is $0.36 each (lower in bulk) and has a 10bit ADC. You need to consider voltage reference, the on board 1.1V bandgap reference is only accurate to a few % and varies over supply voltage and temperature. There are very few reference ICs they have supply current at the uA level so you may want to just use a TL431 or similar but switch it on only when needed.

As for balancing and whether it's needed, yes it's very much needed for an 18650 type pack, please see the conclusion to my experiment here: http://www.diypowerwalls.com/t-Pack-Cycle-Testing

After around 40 charge/discharge cycles at room temperature in a lab environment, even charging only to 4.125V/cell, one cell already got out of balance enough to charge at greater than 4.2V during the charge cycle and I had to end the test. I don't know about you but I don't want to have to crack open my pack once a month to manually balance it. Note this was with a pack balanced to within 30mAH or so also. You can't expect it to be as well balanced as a proper EV battery with all cells made on the same factory line like a Nissan Leaf pack.

Here's what I've got for my board (using a dedicated 12-bit ADC, 8 dividers (switchable using P-FETs) and 8 balancers. Board to board communication is still with an isolator but can make a version with just a divider/level shifter for the packet forward approach:

Board is 3.05x1.25", haven't finished routing, just initial placement.


Schematic for the balancers:



Schematic for the Vsense section:


Solar, if you'd be interested in collaborating on the firmware side I'd be happy to handle hardware design/test/build. I hate writing code but I can do it if I have to

dcb said:

rev0 said:

I'm now looking at trying to make the simplest possible single cell stackable BMS.

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"attiny24"
why do you need a divider though, and a voltage reference in that case?

fwiw, this is a pretty deep rabbit hole though, and you should know how to program to work with microcontrollers effectively. I did sort out an assembly routine for the tiny44 once (same as 24), no hardware uart made assembly the only choice for timing (literally counting instructions). It had majority vote, and I reduced the frame size (below what a typical hardware uart would allow) so it would be good without a crystal oscillator, in daisy chain (tx of cell 0 goes to rx of cell 1, etc). Auto addressing (command packet had counter that each node stored and incremented and forwarded). chain functions, like just give me the max voltage in the string, or the min voltage, or the sum, plus individually addressable voltages, with room for a temperature reading as well (thermistor, with max and min chain commands and apa), plus an led to help troubleshooting (i.e. follow the chain till the led's stop lighting for network problems, or look for the blinking led to identify the battery in question) and sleep/wake modes.

It wasn't one cpu per cell though, but easily modifiable for such, it is literally just the cpu and an opto for bare minimum at that point since the voltage range is acceptable. Though you still may need a calibration step after assembly (and it might need to measure vcc and internal temp to be accurate).

so while "simplest" can be optimized for hardware, it doesn't make the software (the part you are avoiding

) simpler. Siloing hardware from software in microcontrollers while trying to optimize is a bad mix. If you can wash your hands at the programming side, then by the same token the programmer can wash their hands at the hardware side, and just make whatever you give them work, more or less, suboptimally.

Though I'm concerned that you are ignoring very relevant comments regarding methodology, as if you see fortune in this direction, and that is what kills batteries and floods the market with junk. I think you missed that ship by about 8 years though.

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The internal bandgap reference is only good for +/-10% in ideal conditions, clearly a better reference is needed. A divider is needed to get the cell voltage below VCC which would itself be the voltage reference of 2.5V (common reference value below 3V).

Trust me I hear you about the challenge and problems of making a custom board and programming it, I still don't think it's the way to go for my project, this is more a fun academic exercise for me, if others find this work useful I'll continue, otherwise not much has been lost except time.

dcb said:

ah, RC enthusiast! (I like the sailplanes)

Just FYI a lot of the research shows significant degradation in lithium batteries if charged fully to 4.2, i.e. in my leaf I used the timers to set it to %80 charge and keep it out of the red, as has been discussed on the mynissanleaf forum. Also sitting at full charge is bad, and if it is hot then it gets worse, so the low voltage you reported at receiving is actually a reasonable long term storage voltage (%30 SOC IIRC). I charge my leaf cells to ~4.1v. Over charging and discharging reduces cell life exponentially so that you get less total energy out of the cell over its lifespan. I know the RC guys tend to max it out and just replace the pack more often, and cell phones want to maximize their battery claims, but on car scale that gets expensive.

This is part of the battery management domain that you are entering.

I think you missed my point on the cpu though, the attiny24/44 can get reproduce-able results with minimal external hardware, and is cost effective even at the 1 cpu per cell level, but it is a large initial software cost, plus building a test/calibration rig that you validate each unit produced. And you are likely to run into the same problems with any microcontroller, unless you pay a lot for it. The devil is in the details as they say.

So for a "cheap" personal pack, it doesn't make a lot of sense to get fancy with the bms hardware either. If it costs 3x the cost of the battery, then just get 4x the battery instead and simplify the monitoring.

also there are concerns with the stated need for balancing, I think there may be errors in your testing procedure, and you are comparing 1p to 10p). But you are claiming without really backing it up that there is a need, and you have the solution. It is rather circular. Like inventing a bms with built-in imbalance then using that as the reason for adding balancing. Then saying you don't want to "crack open" the pack when you know each cell will have a lead routed externally anyway...

There are a lot of holes in the reasoning here, and in the implementation, and this is for an extra 5 miles range on a leaf? Do you have any idea how to interface with the leaf battery?

I guess the problem is you are saying things as absolutes, but you haven't done all your homework yet or properly backed up those assertions, despite being told to the contrary from numerous experienced folks.

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Understandable about your skepticism of my project, I never really explained the "big picture" anywhere. I bought a Leaf with 100k+ miles on it and a battery at death's door (only 5 capacity bars), which I had replaced with cells from a 2013 just a couple weeks back, I'm pretty well aware how expensive it is (the replacement pack cost me twice what the car did). I originally intended to buy a Leaf with a few bars missing and supplement with lost range with a modular system such that I could add more 1.2kWH packs whenever I had the time and money to. I was inspired to take on an EV project by the likes of Jehu and his video about how a DIY built car got 700+ miles of range using recycled batteries. I'd like my Leaf to be able to scale someday to a couple hundred miles or more if I came across a good deal on batteries, and as battery tech and energy density increases over time as well.

I'm new to the EV world, true, but very familiar with Li-Ion batteries and how they degrade, and how to take care of them to prevent such. The Leaf only charges to a max of 4.125V and discharges to a min of 3.4V, which is already conservative, even at "100%" charge cycle. I've seen most of the implementations of range extenders on Leafs as well, with my favored route being to tap in right after the battery connector (after the battery's internal contactors), and monitor the contactor signals to know when to switch in the extender pack and when to disconnect it. I've got the HV connector (both sides) already to make an in-line tap, and I'm trying to get a hold of the circular connector which contains the CAN bus signals and contactor relay controls.

So for my first 96s2p pack, cell balancing will be relatively important, there's not much in parallel to offset the effects of cell imbalance. As I add more packs in parallel (every cell will be in parallel with the original pack by connecting together the balance leads) the system should get progressively easier to balance. I also am skeptical about how required balancing is, I've done the reading from both sides of the argument, and there just wasn't enough data to convince me one way or another. My preference is to be able to monitor all cells and make the judgment on my own whether balancing is needed or not, but the extra hardware is only a few extra cents when designing a custom board, so why not add it. And as a disclaimer again, I will be using a Nissan Leaf BMS for my project, which already has the automotive grade HW to monitor and balance all 96 cells with proper isolation, and a nice Android graphic interface on top I'm just looking into my own BMS for the fun of it, maybe to use in other projects.

Solarsail said:

I am trying to get my head around this. In one of the other designs linked, there was no need for disabling the voltage reference for balancing. Only one FET for the balancer and the voltage reference appeared to draw current only when voltage exceeded reference?

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Yes, there's a few ways to use voltage references, this would work also if you wanted to only top balance, but you wouldn't have direct control over the balancing if you wanted to balance at a different state of charge. The voltage reference draws a minimum of 40uA for the one I saw, lower power ones exist but are more expensive.

Solarsail said:

Is a FET needed for level shifting communication? Communication is one way from upper to lower board only, and is serial. Could you not use another zener/resistor to drop the voltage so that when out is low, there is no current drain? Assuming daisy chain serial communication, it can be as many bytes as you want, and you want it to be synchronous and periodic I think. If the controller detects no pack charge or discharge, then it would minimize the polling, and eventually put all on standby. A data space of 128 bytes would be sufficient with logarithmic coding of the voltage. No board ID needed in the packet, as position of data is the ID.

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From my calculations you would always need a FET, in the worst case, say you had a divider/zener setup, you set output low on the top MCU, the voltage is then VCC of the bottom board (GND of the top board = VCC of the bottom board), your voltage divider would divide that by say 3.5 to meet the Vil requirements of the bottom MCU, then Vin to the bottom MCU is VCC/3.5. For the high case, you need to exceed Vih requirement of the bottom MCU, with the same 3.5 divider your Vin would not be high enough. There's not a single divider value that meets both criteria over the worst case ranges (cell 1 voltage = 3V, cell 2 voltage = 4.2V, and vice versa). If you want it to work over a narrower range, sure that's fine, but that's a limitation you'd need to be aware of. With a P-FET, the output is true 0 to VCC of the bottom device. The output pin of the MCU however is clamped to its own GND and VCC via internal diodes.

Just so I'm not crazy I threw it in LTspice and it works:

Here's the schematic/layout for the "1s" stackable BMS:




To keep the ATtiny102 (for lower cost and smaller package) and still have a temperature sense function, I added a thermistor which would only be active when the ADC is enabled (supply is ADC_EN pin), and would be read out on the BAL pin, which is normally an output to control the shunt. This has the obvious downside of turning on the shunt when the temperature is below 15 C (depending on the threshold voltage of the shunt N-FET), but you could look at it as a plus of having a free battery heater circuit

The board is 0.425 x 1.175". Could be optimized a bit further, this is just a first pass to connect all the nets together.