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#### dcb

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conceptually, if it is an issue, you can modify what major said, and do a voltage measurement at current A, then a voltage measurement at current B, then another voltage measurement at current A and use the average A voltage. And they should be fairly brief measurements anyway.

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#### Karter2

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If you just compare two discharge curves at known loads (0.5A and 5.0 A shown) you can easily see the delta V at any point in the capacity rating.
Knowing the delta I is constant at 4.5A , thn the DCIR is directly proportional to the delta V.
And in the case of the Pana 18650B shown , it "looks" to vary between 0.3v (67mohm) at 1ah, and 0.4v (88mohm)at 3ah toward the end of the discharge.
This is just a "quick and dirty". Visual calculation, but with a logging charger you could plot DCIR completely over the discharge cycle if necessary.

#### Solarsail

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Thank you PStechPaul, Karter2, dcb. I will look into the Arduino data acquisition. Plan to build the 100 kWh BMS using an Arduino someday. Thank you for the valuable information.

(Trivia: Did you know that on the Stalinesque forum Stack Exchange - Electrical Engineering, you are not allowed to say "thank you" in the comment? Avoid that place like wildfire.)

#### Solarsail

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What devices and functions would you recommend for this project?

Pack is 48V, 34Ah, 1.6 kWh, rated at continuous 1.5C (50A), Peak 2.2C (75A), charge 0.5C (16.7A).

The 13s10p pack will need the following at the minimum. A 800W charger, a balancer-protection, fuse, i/o plugs, ammeter, voltmeter, 4x thermistors.

Anything else?
Should the charger be installed internal to the enclosure? (power supply will be external)
What size fuse? 100A?
What kind of output plug? What kind of input plug?
Shall the voltmeter also measure individual cell groups voltage? If so, it needs to switch between 13 groups. What is the best way to do this? Use analog CMOS switch?
Charge timer?
On/Off switch or contactor?

#### Karter2

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What devices and functions would you recommend for this project?
?
......unless i have missed it, i dont think you have actually said what the intended use is for this pack .
Mobile, ?..Car , bike, boat, aircraft, ?
Stationary, ? ..Solar store, emergency back up, etc ?
Intended use is helpful to know , especially for things like housing , cooling etc.

#### Solarsail

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Good question. This is a generic block that can be used in a variety of applications. Plan is to build 8 of these to augment a Leaf.

Another idea is to build larger packs that would be used in a catamaran with electric drive. About 100 kWh. For this purpose, I think each module will house 2p13s20p or 13s40p (6.4 kWh).

Ultra light aircraft is another possibility. The light weight of the pack is very important here.

#### Solarsail

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Re: The 18650 - 14s10p Project - 48V x 34Ah

1) There has been a design change. The module configuration has been bumped up to 14s10p (from 13s10p). It turns out that 14s balancers-protectors are easy to find. There is a major advantage to this. To arrive at 96s x 4.2V = 403.2V, one can string 7 x 14s = 98s cells, if cells are charged no more than 403.2/(7x14) = 4.12V. This will be a loss of about 2% in capacity, but an increase in charge cycle, which is acceptable. So the pack configuration improves from an unwieldy (7s13s+5s)10p @4.20V to a more design efficient 7s14s10p @4.12V. Another advantage is that a 24V center tap at 7s10p becomes available. I am checking with Torqeedo to see if their Cruise 10FP 10kW 48V Pod Drive controller will accept 14s x 4.2V = 58.8V.

In other words:
12s allows for 96s but not 48V operation
13s allows for 48V but not 96s operation
14s allows for both 48V and 96s operation

2) Does anyone have or know where to find the schematics for a cheap non-ASIC balancer-protection board? I don't have any particular board in mind, but the following link gives an example. It could be any board of such a class of BMS. It does not have to be this board, and I am not using this board. And yes I know that the schematics will be very different from one board to another board. Note that this is not an "intelligent" BMS and I don't believe it contains ASICs. Is there such a thing as a "database of schematics" on the net?

#### rev0

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2) Does anyone have or know where to find the schematics for a cheap non-ASIC balancer-protection board? I don't have any particular board in mind, but the following link gives an example. It could be any board of such a class of BMS. It does not have to be this board, and I am not using this board. And yes I know that the schematics will be very different from one board to another board. Note that this is not an "intelligent" BMS and I don't believe it contains ASICs. Is there such a thing as a "database of schematics" on the net?

There's some videos from this guy on YouTube that explains how these protection boards work, though these ones don't have the balancing function populated: https://youtu.be/Tw-fnea3-gw

I tried designing my own protection board (originally started as a full smart BMS board with balancing) but it was too expensive compared to buying a Leaf BMS and using WolfTronix's custom microcontroller with it (once it's available for sale anyways).

#### Solarsail

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I tried designing my own protection board (originally started as a full smart BMS board with balancing) but it was too expensive compared to buying a Leaf BMS and using WolfTronix's custom microcontroller with it (once it's available for sale anyways).
Fair enough. But now you have to live forever with the quirks of the board, and god forbid if you lose an ASIC or something goes wrong.

Does each Leaf BMS ASIC contain an optocoupler? Is that a must? Why couldn't you not use an ATMega and a voltage divider let's say, to sense the digital signal from the cell directly above (higher in voltage)? One need not duplicate the sophistication of a Leaf BMS - which is designed for commercial operation and one in 100 million fault tolerance? Maybe a board containing 8 ATMega and associated resistors, capacitors, mosfets and zeners. 12x that is still cheap. That should cost very little. These are just random musings, but building from scratch is often faster and easier and certainly more productive and more fun than trying to shoe-horn some existing system with a steep learning curve, resulting in something that is hard to port to other environments or configurations.

I just don't understand when you say it is too costly.

#### rev0

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Fair enough. But now you have to live forever with the quirks of the board, and god forbid if you lose an ASIC or something goes wrong.

Does each Leaf BMS ASIC contain an optocoupler? Is that a must? Why couldn't you not use an ATMega and a voltage divider let's say, to sense the digital signal from the cell directly above (higher in voltage)? One need not duplicate the sophistication of a Leaf BMS - which is designed for commercial operation and one in 100 million fault tolerance? Maybe a board containing 8 ATMega and associated resistors, capacitors, mosfets and zeners. 12x that is still cheap. That should cost very little. These are just random musings, but building from scratch is often faster and easier and certainly more productive and more fun than trying to shoe-horn some existing system with a steep learning curve, resulting in something that is hard to port to other environments or configurations.

I just don't understand when you say it is too costly.
I haven't dived into the Leaf BMS communication between ASICs, but it seems to use something like an open drain sort of bus with a zener diode and voltage dividers to couple the signal between them. There's a detailed schematic of it here: http://www.mynissanleaf.com/viewtopic.php?f=8&t=17470

So I have purchased 2 Leaf BMSes, at \$70 and \$78, and the price of one 8s smart balancer/monitor module for my design was \$15.34 in parts only, not including the PCB, plus assembly time/effort, and I'd need 12 of those for a 96s setup (\$184 extended price).

#### Solarsail

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So I have purchased 2 Leaf BMSes, at \$70 and \$78, and the price of one 8s smart balancer/monitor module for my design was \$15.34 in parts only, not including the PCB, plus assembly time/effort, and I'd need 12 of those for a 96s setup (\$184 extended price).
So it really boils down to time and effort, as neither is a cost issue, and I am not sure which option is higher in labour. The advantage of your design is that it will be portable and extensible, and does not need to interface with a recalcitrant controller. It could for example be used on a 14s or 27s 18650. There is going to be a whole world coming into existence between 48V and 346V. Namely 27s, 40s, 54s, and 80s. It would be great to have an extensible design, which could be programmed to do things that the Leaf could not do.

I may have asked this before, and my apologies if I have, and I am a bit too tied up to do a search at this moment, but is there any chance having your design? I will be happy to take it to production and share the results and software with you.

#### rev0

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So I have purchased 2 Leaf BMSes, at \$70 and \$78, and the price of one 8s smart balancer/monitor module for my design was \$15.34 in parts only, not including the PCB, plus assembly time/effort, and I'd need 12 of those for a 96s setup (\$184 extended price).
So it really boils down to time and effort, as neither is a cost issue, and I am not sure which option is higher in labour. The advantage of your design is that it will be portable and extensible, and does not need to interface with a recalcitrant controller. It could for example be used on a 14s or 27s 18650. There is going to be a whole world coming into existence between 48V and 346V. Namely 27s, 40s, 54s, and 80s. It would be great to have an extensible design, which could be programmed to do things that the Leaf could not do.

I may have asked this before, and my apologies if I have, and I am a bit too tied up to do a search at this moment, but is there any chance having your design? I will be happy to take it to production and share the results and software with you.
I'll be happy to share what I have which at this point is just a parts list in a spreadsheet and a partially finished schematic and layout in Eagle.

#### rev0

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Actually I did come across a good BMS chip in my investigation, it might cost a little more and draw a little more current but it already does all the critical functions (balancing, cell voltage monitoring, pack current monitoring/cutoff, coulomb counting): https://www.digikey.com/product-detail/en/texas-instruments/BQ7693003DBTR/296-39961-1-ND/5177838

Would just need to pair it with a low power optoisolator (also expensive, but there's not really any good alternatives) and have a microcontroller to talk to the various packs: https://www.digikey.com/product-detail/en/analog-devices-inc/ADUM1442ARQZ/ADUM1442ARQZ-ND/4461940

There is already a design out there using this chip: https://github.com/LibreSolar/BMS48V and I think I saw another earlier. I don't think this one has an optoisolator for stacking, but you could build an interposer to handle the isolation or handle it at the main controller.

#### Solarsail

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Thank you rev0 - I will study these ideas. This will be my next project, probably in 6 mos. time for the larger pack (> 12 kWh).

#### Duncan

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Hi
If you are going to make your own BMS be aware that BMS problems have killed more batteries than they have saved

#### Solarsail

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There is already a design out there using this chip: https://github.com/LibreSolar/BMS48V and I think I saw another earlier. I don't think this one has an optoisolator for stacking, but you could build an interposer to handle the isolation or handle it at the main controller.
Interesting. Am I correct to assume that you only need one optoisolator per BMS board (of 8 or 15 cells) as opposed to one optoisolater per cell?

Interposer?

Why not pass down the signal from the bottom of one board to the top of the next board down by a simple voltage divider? Why have to use an optoisolator for "stacking" (not sure what that is) boards? And then the controller gets it off the bottom of the lowest board - no optoisolators needed? So the top ATmega creates a packet, sends it to the next one down which fills up its slot, and then passes down the packet, and so forth, until the packet is completed at the very bottom and passed to the controller? Only if the controller needs to communicate back to the top ATmega would one optoisolator be needed? Does it even have to communicate back?

#### rev0

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Here is the other project I saw, just a basic board for the TI chip: https://github.com/nseidle/BMS

Interposer means a small board to go between two others, in this case which would have an optoisolator to keep the BMS board isolated from the main controller.

You can do the voltage divider method also, if you have your own microcontroller to store/forward the packets like you mentioned.

I wanted to do bidirectional communication in my system so I could manually enable/disable the balancing or trigger ADC reading only when the system was on (with the ADC active the current draw is significantly higher, maybe on the order of a couple mA, so you wouldn't want it on all the time when the car is parked).

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