Low Cost BMS for Home Storage Batteries

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francisco.shi
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Re: Low Cost BMS for Home Storage Batteries

Post by francisco.shi » Sat, 01 Dec 2018, 07:53

How big are the capacitors?
Do you have a circuit of how the capacitors are switched?

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Re: Low Cost BMS for Home Storage Batteries

Post by T1 Terry » Thu, 06 Dec 2018, 11:14

Sorry, been off the air for a while, daughter is in hospital in Canberra with back problems that required surgery.
The units we use are mass produced some where in China and are designed to balance 4 cell groups. We balance cell groups 1 & 2, 3 & 4, 5 & 6, 7 & 8.Seems to working ok, there is the possibility of the 2 series cell groups within each pair to get out of balance slightly with each other, but combined with the charge limit of 3.6v in any one cell group the drift apart is limited to a max of 3.4v low - 3.6v high in the pair and that has never shown up o the extend over a 12mth log of a 600Ah @ 24v off grid system we monitor closely as our test outfit out in the field while I get our system installed and running. I could drop the high charge cut off to 3.5v to bring the two even closer together, but a differential of 0.02v max at the top end of charge is really neither here nor there.

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Re: Low Cost BMS for Home Storage Batteries

Post by 4Springs » Sat, 26 Jan 2019, 12:28

T1 Terry wrote:
Fri, 30 Nov 2018, 14:30
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.
Thanks Terry, you've given me food for thought.
I was thinking of top balancing, like in the car. But bottom balancing might be more suited to this application. The car's cells are 130Ah by the way, and these home battery storage cells are 220Ah (some people have 460Ah).

In the summer top balancing would be fine. 6 hours out of the 11 hours of sunlight we get would fill my 22kWh battery (assuming a 4kW charger), and top balancing can progress for the remaining 5 hours.
In the winter it is a different story, as the battery might not be filled each day. Indeed there might be many days in a row where it is not filled. But it will be emptied every night, as we seldom use less than 22kWh on a wintery evening.
This does seem to suit bottom balancing. Discharge the pack into the load (i.e. the house) until one cell reaches (say) 2.9V, then isolate the pack from the inverter. Turn on the balancing resistors for every cell that has not reached 2.9V, then turn them off as they hit 2.9. With a balanced pack , all cells would reach 2.9V at some point, and then we'd be ready to charge again the next day. As Terry points out, there might not be much time to top balance, but there should be plenty of time to bottom balance. In the summer we might not balance very often, but we do occasionally use more than 22kWh after the sun goes down (if both cars need charging), so it would happen at least once every couple of weeks.

I'm assuming here that LiFePO4 cells are ok being used at their 'lower end'. In the scenario above each cell would go to 2.9V most nights, then get charged up during the day to a greater or lesser extent.

I think I'll start off with top balancing, because I'm familiar with it (and the code is written). But bottom balancing might be a good option to explore.

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Re: Low Cost BMS for Home Storage Batteries

Post by T1 Terry » Sun, 27 Jan 2019, 10:43

As far as pulling the batteries down to their lowest point, for extended battery life you are better off stopping at the 100% discharged point rather than the 2.9v in any cell point. The cell voltages certainly need to be monitored just in case the Victron BMV looses its way as far as the SOC, but discharging to 2.9v ever time .... your call but I certainly wouldn't advise it if you still want the 10yrs plus battery life.

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Re: Low Cost BMS for Home Storage Batteries

Post by 4Springs » Sun, 27 Jan 2019, 15:03

Good news!
10 people have decided to jump in on the order for these boards, meaning over 200 of the celltop module boards to manufacture. This has dropped the unit price dramatically. So dramatically that it has now become economical to get some of the components pre-assembled.
The new prices are:
Master Board only: $5
Master Board and all components: $25
Module Board only (some components already fitted): $5
Module Board and all components: $10

So a BMS for the typical AVASS battery might be:
2 x Masters = $50
20 x Modules = $200
(this includes some spares just in case)

What does this get you?
  • Module circuit boards are designed to fit the AVASS cells recently purchased by AEVA members. There were two types of cells sold, this suits both types.
  • Circuit boards and components delivered to my place in Northern Tasmania. Shipping costs to you are extra, but can be arranged.
  • Some components are not included - wire to go between the cells, connectors, a box, buttons, LED etc. These things will depend on your situation, so I've left them for you to purchase or make.
  • You need to assemble the boards yourself, or find someone to do it for you, or come to my place where we'll all be assembling them together.
  • The microcontrollers are delivered blank, so these boards will not work until programmed. You'll need a programming tool, or you'll need to come to my place once the boards are assembled (can't programme them until they are assembled in the board).
  • I have not tested this version of the boards. They are almost the same as the version I have working in the car though, so I'm pretty confident there won't be too many faults to fix.
  • The Master is configured with two digital inputs (buttons), and three digital outputs. This is in addition to the LCD display that lets you know voltages etc., and an RS232 output for logging and/or control of some devices. The digital outputs could be connected to relays, chargers, a fault light etc.
  • My code is available to use this system as a vehicle BMS.
  • The Master code will need to be altered if you are using this for a home storage battery (Module is the same). I have not done this yet. I'll write the code for my system, and make it available, but that might not be suitable for yours. I'm happy to tweak it for others - to a certain extent! My initial idea is that the basic code will balance the cells, turn off a relay at high voltage and turn off another relay at low voltage (cell or pack voltages). With a fault LED to gain attention if something has gone wrong.
I have not ordered these components yet, so there is still time to place your order. Go to: https://airtable.com/shrEdYHvjoLEaMpz6
If you've previously put in an order, and want to change it based on the pricing changes (e.g. add more boards), fill the form in again and put a comment that you'd like your previous entry deleted.

In the interests of transparency, I'd like to point out that the prices above are calculated to give me a little bit of money left over. Hopefully enough so that I don't have to change the price when the thing I've forgotten to price turns out to be pricey! But if nothing like that happens I'm pocketing it.

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Re: Low Cost BMS for Home Storage Batteries

Post by rhills » Mon, 28 Jan 2019, 20:35

I guess the circuitry difference between top-balancing and bottom-balancing is too great to be able to include both and switch between the two?
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Re: Low Cost BMS for Home Storage Batteries

Post by 4Springs » Tue, 29 Jan 2019, 13:55

rhills wrote:
Mon, 28 Jan 2019, 20:35
I guess the circuitry difference between top-balancing and bottom-balancing is too great to be able to include both and switch between the two?
No difference in hardware, the difference between top and bottom balancing is all in the software. Nevilleh's original BMS was setup for bottom balancing, but it isn't really suited to a car who's battery is rarely empty. So he changed it to top-balancing before I came along.

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