Low Cost BMS for Home Storage Batteries

non-EV Solar, Wind and other renewable power sources
non-EV batteries and other energy storage stuff
Forum rules
Important!
This forum is for discussion of Non-EV matters.
User avatar
4Springs
Site Admin
Posts: 923
Joined: Thu, 23 Dec 2010, 01:14
Real Name: Christopher Walkden
Location: Selbourne, TAS

Low Cost BMS for Home Storage Batteries

Post by 4Springs »

Looks like I will be getting some of the ex-AVASS cells (as per the the forum topic here: viewtopic.php?f=16&t=5574).
The "Low Cost BMS" is a project I used in my car, and should be ideal for these - low cost but just fancy enough to do all that is required.
As is the tendency for these sorts of projects, there are multiple versions out there. People tend to grab the source files and modify it to fit their own circumstances.
This is Nevilleh's original AEVA forum on the topic: viewtopic.php?f=17&t=2753&hilit=low+cost+BMS
The topic went on a few tangents over the years...

This is a version that I made for my car: https://sourceforge.net/p/low-cost-bms/wiki/Home/
I think this will be fine for home storage.

The design has a single master unit, with a display and two buttons.
Then there is a circuit board on each cell. The celltop module fits onto the terminals, so each type of cell needs a specific board made.

So.....
Despite having too many projects on the go at the moment, I think I'll make a version of the Low Cost BMS for these cells. I'll get one of the circuit board manufacturing companies to make the boards for me, and it will be cheaper per unit to get a larger quantity done.
If you also have these cells, and would like to use the Low Cost BMS, the process would be:
  • Buy the components (I'll be making a list of what to get and from where)
  • Buy a programmer thingi (Again, I'll let you know which one and from where)
  • Buy the circuit boards (Let me know how many you want and I'll get them made - then buy them from me)
  • Assemble the components on the boards
  • Programme the microcontrollers (master board and each module board) using the programmer thing connected to your computer
  • Hook them together and test
This should be possible for someone with no electronics or coding experience, although it would be a bit of a learning curve. The components are surface mount, so they are fiddly to put on the boards, but shouldn't be too bad with only 16 celltop modules (assuming 16 cells in a home storage unit). I bought myself a little oven (cost $100), or you could try doing it with a soldering iron.

Let me know if you are interested.
Especially if you are interested in doing some of the work! (re-design the celltop module for instance, or source the components).
Post a reply here, or send me a private message through this forum, or email me at: secretary@tas.aeva.asn.au
rhills
Site Admin
Posts: 688
Joined: Fri, 25 Jul 2008, 01:57
Real Name: Rob Hills
Location: Waikiki, WA

Re: Low Cost BMS for Home Storage Batteries

Post by rhills »

Hi Christopher,

This looks like a great project. I'd like to help, but the soldering puts me off, I've never been much good at it. I followed your link to the BMS you made for your car and read in the Wiki that another option could be to get the whole board manufactured. Would this add much to the cost? I'd certainly be willing/able to help out with any coding, though I suspect that's all pretty well sorted.
Rob Hills
AEVA Webmaster
  • 2022 Tesla M3 MIC LR
  • 2014 Mitsubishi Outlander Aspire PHEV
    Petrol Usage to last refill: Jul 2014 - Jul 2022
    Total Petrol: 889.8L
    ODO: 88417
    Av Consumption: 1.01 L/100km
User avatar
4Springs
Site Admin
Posts: 923
Joined: Thu, 23 Dec 2010, 01:14
Real Name: Christopher Walkden
Location: Selbourne, TAS

Re: Low Cost BMS for Home Storage Batteries

Post by 4Springs »

rhills wrote: Tue, 26 Jun 2018, 12:14 Hi Christopher,

This looks like a great project. I'd like to help, but the soldering puts me off, I've never been much good at it. I followed your link to the BMS you made for your car and read in the Wiki that another option could be to get the whole board manufactured. Would this add much to the cost? I'd certainly be willing/able to help out with any coding, though I suspect that's all pretty well sorted.
Someone else has suggested getting boards populated, he knows a bloke who does it in Launceston. I think we would get the boards manufactured overseas, then take them to this local company and get them to populate them with components. Definitely worth investigating.
Apparently you can get the microcontollers pre-programmed if you order them from certain suppliers. This would mean that people wouldn't have to buy the programmer if they were happy with the program as-is.

There are a few jobs to do:
- Figure out how to power the BMS Master. Currently runs on 12VDC, because that is what is in a car. Not sure if it will run on 48VDC, it might need a component change.
- We need to figure out what the BMS Master needs to do. How does one interface with an inverter? We may need different input or output options, so we may need to modify the circuit board.
- The coding will need some modification, depending on the outcomes of the previous question. This may be different for different people.
- The parts will need to be sourced. One of the files in the project is a list of parts and suppliers that I used once - well out of date now, but probably a good starting point.
- The celltop board will need to be modified to fit the cells. Hopefully both cell types have the same terminal dimensions.
User avatar
jonescg
Senior Member
Posts: 4715
Joined: Thu, 21 Jan 2010, 23:05
Real Name: Chris Jones
Location: Perth, WA.
Contact:

Re: Low Cost BMS for Home Storage Batteries

Post by jonescg »

4Springs wrote: Tue, 26 Jun 2018, 17:57
There are a few jobs to do:
- Figure out how to power the BMS Master. Currently runs on 12VDC, because that is what is in a car. Not sure if it will run on 48VDC, it might need a component change.
I've found some really nice little DC/DC converters that will put out over half an amp - https://www.digikey.com/product-detail/ ... ND/4477405
Probably good enough to run a BMS but you can't hold contactors open with it.

@rhills - the WA shipment will probably be here in a few weeks time. I think the best thing to do is put them all in parallel and leaver them like that for a week while the all reach the same state of charge. Should make the balancing job a bit easier.
AEVA National President, WA branch director.
User avatar
4Springs
Site Admin
Posts: 923
Joined: Thu, 23 Dec 2010, 01:14
Real Name: Christopher Walkden
Location: Selbourne, TAS

Re: Low Cost BMS for Home Storage Batteries

Post by 4Springs »

jonescg wrote: Sat, 30 Jun 2018, 20:22 I think the best thing to do is put them all in parallel and leaver them like that for a week while the all reach the same state of charge. Should make the balancing job a bit easier.
I did this with the LFP cells for my car. 45 of them. The way I did it was to loosely put bolts in the terminals, line them all up in a row, and run from terminal to terminal with electric fence tape.
This really didn't do much. I worked out later that it was because the LFP chemistry produces such a minimal voltage change over a huge amount of its state of charge.
For instance, say we had a difference between cells of 1kWh. If this is in the middle of the capacity of the battery it is in the flat bit of the charging curve. So the voltage difference might be (perhaps) 0.001V between cells. Let's say our wire is 1 ohm of resistance. I=V/R, so the current that flows would be 0.001A. W=VI, so we have 0.000001W flowing.
1000Wh/0.000001W is approximately 114 thousand years to balance.

So doing this passively is not really going to do much. If you can parallel them all up and charge them, that is a different proposition. Once they get almost full the voltages should balance themselves out.
User avatar
jonescg
Senior Member
Posts: 4715
Joined: Thu, 21 Jan 2010, 23:05
Real Name: Chris Jones
Location: Perth, WA.
Contact:

Re: Low Cost BMS for Home Storage Batteries

Post by jonescg »

This is true - but assuming they were way out of balance it would help. I did that for my scooter batteries however they were lower capacity and I did leave them for a good 3 weeks. Probably the better option is to charge them in this state as a giant 3.2 V cell. That way any individual cell differences will start to make a difference at the upper end of the curve.
AEVA National President, WA branch director.
Nevilleh
Senior Member
Posts: 773
Joined: Thu, 15 Jan 2009, 18:09
Real Name: Neville Harlick
Location: Tauranga NZ

Re: Low Cost BMS for Home Storage Batteries

Post by Nevilleh »

Hi Chris,
I haven't looked on here for years, been busy with other stuff, but i am wondering if this project ever got off the ground? Nice to know my old BMS stuff still has relevance!
Neville
User avatar
4Springs
Site Admin
Posts: 923
Joined: Thu, 23 Dec 2010, 01:14
Real Name: Christopher Walkden
Location: Selbourne, TAS

Re: Low Cost BMS for Home Storage Batteries

Post by 4Springs »

Nevilleh wrote: Sun, 12 Aug 2018, 12:42 Hi Chris,
I haven't looked on here for years, been busy with other stuff, but i am wondering if this project ever got off the ground? Nice to know my old BMS stuff still has relevance!
Neville
Hi Neville, good to hear from you!
Every couple of months I'm at a car show somewhere, showing someone the BMS modules and explaining that I made them from a design by a bloke in New Zealand...

I'm still intending to modify this BMS for these new cells. Haven't been inundated by people wanting to hop on the bandwagon, but I may be a bit slow for some.

From what I've learned so far, it looks like I might want a contact closure as an output. Something to say to an inverter "stop discharging the battery". Or just hook it up to a contactor so it drops the connection. Plus the existing 0-5V output that controls the charger.
Nevilleh
Senior Member
Posts: 773
Joined: Thu, 15 Jan 2009, 18:09
Real Name: Neville Harlick
Location: Tauranga NZ

Re: Low Cost BMS for Home Storage Batteries

Post by Nevilleh »

Oh well,the hard part is usually working out what you want to do rather than doing it. I'm sure you'll get it going once you do that. If you want any help, let me know. I still have all the original files.
Neville
User avatar
4Springs
Site Admin
Posts: 923
Joined: Thu, 23 Dec 2010, 01:14
Real Name: Christopher Walkden
Location: Selbourne, TAS

Re: Low Cost BMS for Home Storage Batteries

Post by 4Springs »

I've done some work on the BMS module board to fit the Brighsun cells. Here is the version I made for my car:
CellTopBMS-SE130AHA_1.jpg
CellTopBMS-SE130AHA_1.jpg (46.65 KiB) Viewed 6568 times
I had some notes on this version:
1. The board was a bit wide at the terminals. Some boards had to be trimmed a bit narrower to sit on top of a certain type of battery connecting lug.
2. The isolated input from Din (Data In) was quite close to the other circuitry. The isolation would benefit from moving things away further.
Here is my updated design for the Brighsun cells:
CellTopBMS-Brighsun_220AH_1.jpg
CellTopBMS-Brighsun_220AH_1.jpg (59.55 KiB) Viewed 6568 times
It is 22mm wide instead of 30mm, plus shorter to suit the Brighsun cells.
I've re-arranged the components somewhat to make some more room. Gained a few vias and probably made a mistake or two.

Here is the circuit diagram - unchanged from Neville's original:
CellTopBMS_1.jpg
CellTopBMS_1.jpg (41.4 KiB) Viewed 6568 times
I did toy with the idea of a differently shaped board to go around the central vent. Here is a cardboard version I made, next to one of the existing BMS boards:
IMG_20180819_080240195.jpg
IMG_20180819_080240195.jpg (100.89 KiB) Viewed 6568 times
This didn't look great, so I had a look at raising the board up over the vent:
IMG_20180819_074737193.jpg
IMG_20180819_074737193.jpg (44.09 KiB) Viewed 6568 times
This looks fine with a spacer like the nut I've put in. The bolts are available in 20, 25 & 30mm lengths - this one is a 25mm one. The one on the right is screwed all the way in to the bottom. There are spacers available, but they are zinc plated rather than stainless steel like the bolts and washers. I guess that wouldn't matter in this application.
doug
Groupie
Posts: 119
Joined: Tue, 14 Apr 2009, 23:37
Real Name: Doug
Location: Lismore

Re: Low Cost BMS for Home Storage Batteries

Post by doug »

Hi,
I just received some welcome news; I am getting my old Imiev battery back! (After much discussion, without going into details!)
What I would like to know is if I can modify this BMS to use with the Imiev batteries, that I think are in blocks of 4 cells (I will know more when I get the battery pack back).
What I want to do is to recycle some of these batteries into 48v packs to use with a Selectronics inverter/charger on my grid-connected home system. (This will give about 10Kw of storage, from my pack that had a range in the car of about 70Kms).

regards, Doug
ps: When you re-design the boards, you could think of putting 2 holes in the long battery leg so the boards could be guillotined then extended/reduced for other battery types by using a wire to connect the 2 sections.
User avatar
4Springs
Site Admin
Posts: 923
Joined: Thu, 23 Dec 2010, 01:14
Real Name: Christopher Walkden
Location: Selbourne, TAS

Re: Low Cost BMS for Home Storage Batteries

Post by 4Springs »

doug wrote: Sat, 06 Oct 2018, 19:40 What I would like to know is if I can modify this BMS to use with the Imiev batteries, that I think are in blocks of 4 cells (I will know more when I get the battery pack back).
Yes, I can't see why not.
They have been used (so far) on LFP cells, I imagine that the iMiEV cells would be a higher voltage? Should be fine as long as we stay within the voltage range of the microcontroller: 1.8 - 5.5V per cell.
The voltage warning values are easily altered to suit the chemistry.
doug wrote: Sat, 06 Oct 2018, 19:40 ps: When you re-design the boards, you could think of putting 2 holes in the long battery leg so the boards could be guillotined then extended/reduced for other battery types by using a wire to connect the 2 sections.
Good idea.
If I also put in two holes, two wires could be soldered in. The wires would end in ring terminals (possibly both in the same ring terminal) to be bolted to the positive terminal. Would mean that any terminal spacing could be accommodated.
nuggetgalore
Groupie
Posts: 365
Joined: Sun, 11 Dec 2016, 18:02
Real Name: Andreas Tobler
Location: Rowville 3178 V
Contact:

Re: Low Cost BMS for Home Storage Batteries

Post by nuggetgalore »

4Springs wrote: Mon, 20 Aug 2018, 19:08 I've done some work on the BMS module board to fit the Brighsun cells. Here is the version I made for my car:
CellTopBMS-SE130AHA_1.jpg
I had some notes on this version:
1. The board was a bit wide at the terminals. Some boards had to be trimmed a bit narrower to sit on top of a certain type of battery connecting lug.
2. The isolated input from Din (Data In) was quite close to the other circuitry. The isolation would benefit from moving things away further.
Here is my updated design for the Brighsun cells:
CellTopBMS-Brighsun_220AH_1.jpg
It is 22mm wide instead of 30mm, plus shorter to suit the Brighsun cells.
I've re-arranged the components somewhat to make some more room. Gained a few vias and probably made a mistake or two.

Here is the circuit diagram - unchanged from Neville's original:
CellTopBMS_1.jpg

I did toy with the idea of a differently shaped board to go around the central vent. Here is a cardboard version I made, next to one of the existing BMS boards:
IMG_20180819_080240195.jpg
This didn't look great, so I had a look at raising the board up over the vent:
IMG_20180819_074737193.jpg
This looks fine with a spacer like the nut I've put in. The bolts are available in 20, 25 & 30mm lengths - this one is a 25mm one. The one on the right is screwed all the way in to the bottom. There are spacers available, but they are zinc plated rather than stainless steel like the bolts and washers. I guess that wouldn't matter in this application.
Hi, just wondering how this project is going.
Have you had success in building these boards? Just in case you have,did you make some spares that would be for sale?
User avatar
4Springs
Site Admin
Posts: 923
Joined: Thu, 23 Dec 2010, 01:14
Real Name: Christopher Walkden
Location: Selbourne, TAS

Re: Low Cost BMS for Home Storage Batteries

Post by 4Springs »

nuggetgalore wrote: Fri, 09 Nov 2018, 08:39 Hi, just wondering how this project is going.
Have you had success in building these boards? Just in case you have,did you make some spares that would be for sale?
I haven't done anything further yet, other projects have taken priority.
I hope to get back to it within a couple of months. Next step is to compile a list of the components. Once I have that I'll get a quote for board manufacture, and I'll be able to work out a price. Then we can order enough for anyone who is interested.
nuggetgalore
Groupie
Posts: 365
Joined: Sun, 11 Dec 2016, 18:02
Real Name: Andreas Tobler
Location: Rowville 3178 V
Contact:

Re: Low Cost BMS for Home Storage Batteries

Post by nuggetgalore »

I haven't done anything further yet, other projects have taken priority.
I hope to get back to it within a couple of months. Next step is to compile a list of the components. Once I have that I'll get a quote for board manufacture, and I'll be able to work out a price. Then we can order enough for anyone who is interested.
Thanks for letting us know.
I have been looking at Batrium and Lithium Battery Systems. Maybe I should get one of these complete systems, as some of the nitty gritty might test my knowledge and skill. Also prefer to get it done soonish,as my preloved supersafes getting week in the legs.....
Cheers
Last edited by rhills on Sat, 10 Nov 2018, 19:49, edited 1 time in total.
Reason: Edited by Admin to fix quote tags
T1 Terry
Senior Member
Posts: 1952
Joined: Thu, 30 Sep 2010, 20:11
Real Name: Terry Covill
Location: Mannum SA

Re: Low Cost BMS for Home Storage Batteries

Post by T1 Terry »

Have you thought about making the board shorter but wider with all the components mounted to either the positive end or the negative end, then a cable to connect to the other terminal? That way the board would fit between the terminal and vent and the cable would go around the vent and make the module adaptable to many different cell configurations. Even better, a cable each end and the board mounted on heatsink so it can sit above the cell to keep the heat out of the cell.
I do prefer the capacitor type cell balancing rather than the controlled short circuit to burn off some of the excess capacity because it generally dumps that heat into the negative terminal block of the cell it is balancing, but a piece of heatsink would probably do the job ok if there was enough air circulation while charging.
I do use the controlled short type boards from EV Works when condition charging cells ready for a house battery pack, but I mount the negative end onto a piece of aluminium 80mm x 80mm x 3mm as a heatsink. They can transfer around 2 amps at full operating condition and by mounting them this way there is a cable each end so I can get the heat away from the cell. It is only 2 amps @ 3.8v, but that 7.6w gets the heat sink too hot to touch if it's that size.

T1 Terry
Green but want to learn
User avatar
4Springs
Site Admin
Posts: 923
Joined: Thu, 23 Dec 2010, 01:14
Real Name: Christopher Walkden
Location: Selbourne, TAS

Re: Low Cost BMS for Home Storage Batteries

Post by 4Springs »

T1 Terry wrote: Sun, 11 Nov 2018, 12:13 Have you thought about making the board shorter but wider with all the components mounted to either the positive end or the negative end, then a cable to connect to the other terminal? That way the board would fit between the terminal and vent and the cable would go around the vent and make the module adaptable to many different cell configurations. Even better, a cable each end and the board mounted on heatsink so it can sit above the cell to keep the heat out of the cell.
I do prefer the capacitor type cell balancing rather than the controlled short circuit to burn off some of the excess capacity because it generally dumps that heat into the negative terminal block of the cell it is balancing, but a piece of heatsink would probably do the job ok if there was enough air circulation while charging.
I do use the controlled short type boards from EV Works when condition charging cells ready for a house battery pack, but I mount the negative end onto a piece of aluminium 80mm x 80mm x 3mm as a heatsink. They can transfer around 2 amps at full operating condition and by mounting them this way there is a cable each end so I can get the heat away from the cell. It is only 2 amps @ 3.8v, but that 7.6w gets the heat sink too hot to touch if it's that size.

T1 Terry
Interesting.
The way you are talking about heat highlights a slight difference in design.
The board shown above has space for various shunting resistors. As assembled in the car, I used two 10 Ohm 1W resistors in series. This means that I am dissipating no more than about 200 mA, or about 700 mW. I used a relatively large surface mount resistor - package 2512. You can feel that the resistors are warm, but definitely not too hot to touch. And nowhere near enough energy to heat up a 5kg cell! Note that the board has a lot of copper just there, and holes to help passive heat dispersion.
This relatively low balancing current has served me well in my vehicle. I turn the charging current down to about 1A when balancing, so the 200mA is a significant fraction of that. My guess is that this will be fine for the home storage battery, as the charger I am planning to use will not charge quickly.
So heat is not a problem.
T1 Terry wrote: Sun, 11 Nov 2018, 12:13 Have you thought about making the board shorter but wider with all the components mounted to either the positive end or the negative end, then a cable to connect to the other terminal?
I actually made this board thinner than my last attempt, as the wider boards interfered with certain cable lugs. On these cells the board will be high above the cable lugs, but I slimmed it down anyway. Look how wide the board would have to be to fit in before the central vent - almost as wide as the current one is long!
There would need to be two cables to the other end, note the two tracks running down the board. If you connected these two and ran just the one cable then the voltage sense circuitry could show different values depending on if we were shunting or not.
User avatar
4Springs
Site Admin
Posts: 923
Joined: Thu, 23 Dec 2010, 01:14
Real Name: Christopher Walkden
Location: Selbourne, TAS

Re: Low Cost BMS for Home Storage Batteries

Post by 4Springs »

Ok, I've finally got around to pricing these.
The master module will be about $100 including the board (about $20) and all components. You need one of these - or two if you want a spare.
The cell-top module will be about $6 each including the board (about $2) and all components. You need one of these per cell, plus any spares.
Spares are a good idea, since you are making them yourself! Very easy to make enough mistakes to decide to throw the board away...
So for a 16 cell home storage battery I'd recommend:
1 x master
21 x module
This would be about $226

The pricing will vary a bit based on how many circuit boards I get manufactured. I've made my estimate above on about 10 systems. I've also added a bit on to my estimates, because I've probably forgotten something.

I'm planning on getting people here to help assemble these boards. If you can't get here then there will also be shipping costs (I'd send the bits to you), plus a PicKit to program the microcontrollers. If doing them here then I can program them with my kit.

If you'd like to order these please let me know by filling in the form here: https://airtable.com/shrEdYHvjoLEaMpz6.
Once I know final numbers I'll be able to calculate final prices. I'll only order enough for people who have placed orders!

If you can program these yourself then things should happen within a couple of months. If you are getting me to program them then don't expect anything to happen very quickly. Before I invite people here I have to get the boards, assemble some, write the code and debug it.
doug
Groupie
Posts: 119
Joined: Tue, 14 Apr 2009, 23:37
Real Name: Doug
Location: Lismore

Re: Low Cost BMS for Home Storage Batteries

Post by doug »

Hi,
I am interested. I have my old Imiev battery, but I need to find a compatible Grid connect inverter/charger.

I looked at the Schneider, but they do not recommend Lithium Ion batteries with their chargers when grid connected. (But OK if charging direct off the panels, which is not how my system is connected).
Victron: seems similar. Which leaves the SP Pro, which I have yet to research.

Any comment?

Once I find an inverter charger, I will need a BMS system.

regards, Doug
User avatar
4Springs
Site Admin
Posts: 923
Joined: Thu, 23 Dec 2010, 01:14
Real Name: Christopher Walkden
Location: Selbourne, TAS

Re: Low Cost BMS for Home Storage Batteries

Post by 4Springs »

doug wrote: Mon, 26 Nov 2018, 16:44 Hi,
I am interested. I have my old Imiev battery, but I need to find a compatible Grid connect inverter/charger.

I looked at the Schneider, but they do not recommend Lithium Ion batteries with their chargers when grid connected. (But OK if charging direct off the panels, which is not how my system is connected).
Victron: seems similar. Which leaves the SP Pro, which I have yet to research.

Any comment?

Once I find an inverter charger, I will need a BMS system.

regards, Doug
Here are some options I've thought of, but I'm no expert to know which are feasible.
1. Use a grid-connect inverter charger that expects a lead-acid battery, these should be relatively plentiful? The LFP cells are a good match to the voltage required for a 48V lead-acid battery, but I'm not sure about the iMiEV cells. Configure the BMS to protect the battery from overcharging or discharging, either using signals into the inverter charger, or just use a contactor to disconnect it completely.
2. Use a dumb grid-connect inverter, and put in a separate charger, controlled by the BMS. Something like a car charger should be suitable. You can use a third-party device such as a Smappee Switch to turn on the charger when you are exporting power. Or just use a timer.
3. Use a really dumb inverter that is not connected to the grid. Fine if you have a single large load such as a car to charge. MUCH simpler to do. This is the way I'll be going. You can buy an inverter from Jaycar, or get a secondhand one - I've purchased a couple of secondhand UPSs.
T1 Terry
Senior Member
Posts: 1952
Joined: Thu, 30 Sep 2010, 20:11
Real Name: Terry Covill
Location: Mannum SA

Re: Low Cost BMS for Home Storage Batteries

Post by T1 Terry »

4Springs wrote: Sun, 18 Nov 2018, 09:03
T1 Terry wrote: Sun, 11 Nov 2018, 12:13 Have you thought about making the board shorter but wider with all the components mounted to either the positive end or the negative end, then a cable to connect to the other terminal? That way the board would fit between the terminal and vent and the cable would go around the vent and make the module adaptable to many different cell configurations. Even better, a cable each end and the board mounted on heatsink so it can sit above the cell to keep the heat out of the cell.
I do prefer the capacitor type cell balancing rather than the controlled short circuit to burn off some of the excess capacity because it generally dumps that heat into the negative terminal block of the cell it is balancing, but a piece of heatsink would probably do the job ok if there was enough air circulation while charging.
I do use the controlled short type boards from EV Works when condition charging cells ready for a house battery pack, but I mount the negative end onto a piece of aluminium 80mm x 80mm x 3mm as a heatsink. They can transfer around 2 amps at full operating condition and by mounting them this way there is a cable each end so I can get the heat away from the cell. It is only 2 amps @ 3.8v, but that 7.6w gets the heat sink too hot to touch if it's that size.

T1 Terry
Interesting.
The way you are talking about heat highlights a slight difference in design.
The board shown above has space for various shunting resistors. As assembled in the car, I used two 10 Ohm 1W resistors in series. This means that I am dissipating no more than about 200 mA, or about 700 mW. I used a relatively large surface mount resistor - package 2512. You can feel that the resistors are warm, but definitely not too hot to touch. And nowhere near enough energy to heat up a 5kg cell! Note that the board has a lot of copper just there, and holes to help passive heat dispersion.
This relatively low balancing current has served me well in my vehicle. I turn the charging current down to about 1A when balancing, so the 200mA is a significant fraction of that. My guess is that this will be fine for the home storage battery, as the charger I am planning to use will not charge quickly.
So heat is not a problem.
T1 Terry wrote: Sun, 11 Nov 2018, 12:13 Have you thought about making the board shorter but wider with all the components mounted to either the positive end or the negative end, then a cable to connect to the other terminal?
I actually made this board thinner than my last attempt, as the wider boards interfered with certain cable lugs. On these cells the board will be high above the cable lugs, but I slimmed it down anyway. Look how wide the board would have to be to fit in before the central vent - almost as wide as the current one is long!
There would need to be two cables to the other end, note the two tracks running down the board. If you connected these two and ran just the one cable then the voltage sense circuitry could show different values depending on if we were shunting or not.
200mA is ok for a small capacity cell but wouldn't do much for a 200Ah or 400Ah cell. This is the real difference in balancing house battery cells against EV battery cells, the house battery is big capacity but low voltage and top balanced rather than bottom balanced.
A lot of current needs to move from cell to cell for effective top balance large capacity cells and if the balancing is via the discharge to heat through a resistor method, that is a lot of heat to get rid of without heating the cell internals. No point in limiting the charge current to less than the board can dissipate as heat if the charge current is going to be less than 2 amps. The sun doesn't shine long enough to top up a few Ah in the low cells at the top of charge where the cell runs away to 3.6v. As soon as the charge stops and the load brings the cell voltage below 3.4v, all the cells will be virtually the same voltage, just not at the same state of charge.
Most people would be surprised to hear a 400Ah 24v battery can get as much as 25Ah out of balance in just a few mths when charged via solar only, simply stopping the charging when any cell reaches 3.6v and switching the charge back on at say 50%SOC only make the problem compound until eventually the low cell gets dragged down to 0v or actually reverse charged yet the state of charge was still better than 50%SOC average across the battery.

T1 Terry
Green but want to learn
francisco.shi
Senior Member
Posts: 826
Joined: Mon, 17 Sep 2018, 16:30
Real Name: Francisco
Location: Brisbane, Strathpine

Re: Low Cost BMS for Home Storage Batteries

Post by francisco.shi »

Are there any balancing circuits with a dc-dc converters instead of resistors that transfers the energy to the other cells?
nuggetgalore
Groupie
Posts: 365
Joined: Sun, 11 Dec 2016, 18:02
Real Name: Andreas Tobler
Location: Rowville 3178 V
Contact:

Re: Low Cost BMS for Home Storage Batteries

Post by nuggetgalore »

francisco.shi wrote: Sat, 01 Dec 2018, 04:18 Are there any balancing circuits with a dc-dc converters instead of resistors that transfers the energy to the other cells?
Interesting question.
Actually I would be interested in a step further, to divert the shunted power to a second device.
It still would charge up the lower cells but not waste the energy in heat near the battery.
But probably not cost efficient as the BMS needs to do a similar job when all cells are up.
francisco.shi
Senior Member
Posts: 826
Joined: Mon, 17 Sep 2018, 16:30
Real Name: Francisco
Location: Brisbane, Strathpine

Re: Low Cost BMS for Home Storage Batteries

Post by francisco.shi »

That is what I was going to do for my BMS but I still don't know how much current to bleed off. I was going to aim at 10A per cell and I will balance during charge and discharge instead of waiting until one cell is fully charged before starting the balancing process.
The main reason is that during discharge I can draw more energy out of the stronger cells so all cells will reach the low voltage threshold at the same time. This way I can get a little more energy out of the pack.
T1 Terry
Senior Member
Posts: 1952
Joined: Thu, 30 Sep 2010, 20:11
Real Name: Terry Covill
Location: Mannum SA

Re: Low Cost BMS for Home Storage Batteries

Post by T1 Terry »

I still feel the best type are the switched capacitor models. Basically, each cell is linked to a capacitor, the circuit then switches to disconnect the capacitor from the cell and links it to the next capacitor in the chain. Now all the capacitor voltages equalise, it then switches back to its cell and either discharges or recharges. Depending on the voltage difference as to the amount moved each time but the ones we use can move up to 5 amps at a time. once the voltages are within 10mV the unit shuts down and doesn't trigger again until it sees a 100mV differential. I think those are the values, haven't actually read the doco recently

T1 Terry
Green but want to learn
Post Reply