Low Cost BMS for Home Storage Batteries

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peter_mcc
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Re: Low Cost BMS for Home Storage Batteries

Post by peter_mcc » Sun, 12 May 2019, 07:37

1. I haven't seen holes used like that before. I've seen lots of small vias used to try to thermally link the copper on the two sides - if you do that you could try making the copper area on the bottom as big as possible. You might also want to increase the area of the copper on the top side as much as possible. Using the PCB as a heatsink is largely a matter of how big you can make it. Ordering 2oz copper PCBs also helps as bit (normal is 1oz).
A bit more info on the vias to join the sides thermally:
https://www.edn.com/electronics-blogs/t ... ermal-vias

2. I'd put a pull down on the mosfet as a matter of habit. Thinking about it more since my earlier emails to Chris, the failure mode I'd be concerned about is the micro dying and leaving the mosfet gate undriven - it could then float up to the point where the mosfet turns on and flattens the battery. If someone is away or doesn't monitor their battery regularly (eg for a house) they might not notice things have gone wrong until it's too late and the cell is dead. The micro could fail in other ways too (pulling it high or low) so it isn't a guarantee, just risk mitigation.

However... given you have the holes there for a through hole mosfet it would be possible to fit it later on using a through hole resistor.

3. The cap is just there to stabilise the power supply, the specs shouldn't be too critical. Colleagues who know more than me shy away from Tantalum caps because they can fail "short circuit" (bang) - electrolytic caps tend to fail open circuit. Ceramic caps have improved a lot over the years - why not go for a 10uF 10V 0805 ceramic cap like Seeed 302010158 ? You could put a through hole set of pads (2.5mm pitch) next to it for those who can't easily source the SMD part and want to use a through hole electro. Plus because it's so much smaller you could put the mosfet pull down resistor in :D

This looks like an interesting source of info for the differences:
https://electronics.stackexchange.com/q ... ces-in-use

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4Springs
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Real Name: Christopher Walkden
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Re: Low Cost BMS for Home Storage Batteries

Post by 4Springs » Sun, 12 May 2019, 16:40

Ok, I've placed the order for the modules.
I ended up not putting in an explicit pull-down resistor, although as Peter said, one could be added using the through-hole pads.
I eventually figured out how to properly search the components available through Seeed. The secret is to click on the link to 'Download Library for Eagle and Kincad'. This takes you to SourceForge, where they have the components in a downloadable spreadsheet file. Use your favourite spreadsheet program to search the components.
The result of being able to search well is that found a nest of 10uF aluminium electrolytic capacitors for 3c (US) each. So I'll use them and I've left the C1 pads as-is. A different capacitor could be used here, the pads are a standard 1206 spacing.

I did pour a bit more copper on both sides of the boards. Removed the large holes and replaced with lots of small vias. I left them at 1oz copper as it cost another $50 to go up to 2oz, and I don't intend to run these boards at full power for long.

Total cost was $911 US for 400 boards populated with 8 of the 18 SMD components. Electronics always seems way too cheap.
"I'll have 800 resistors please."
"Certainly Sir, that will be $1.47."

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mikedufty
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Re: Low Cost BMS for Home Storage Batteries

Post by mikedufty » Fri, 24 May 2019, 22:17

Have you got any spares in those 400 boards? I just collected my AVASS batteries and am now thinking about what to do with them.

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4Springs
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Re: Low Cost BMS for Home Storage Batteries

Post by 4Springs » Sat, 25 May 2019, 06:21

mikedufty wrote:
Fri, 24 May 2019, 22:17
Have you got any spares in those 400 boards?
Yes.
Final totals were 352 cell module and 37 master boards ordered by people in the group buy.
There are set order quantities at the board manufacturing place, so I ordered 400 module and 40 master boards.

I've updated the Airtable form: https://airtable.com/shrEdYHvjoLEaMpz6
You can order just the boards, or the boards plus most of the components. Almost all of the components that I ordered had order quantities such that I'll have quite a few spares. But some did not, and I won't have spares at all. So if you order boards+components now, you'll get boards plus all of the components that I have. I'll also give you a list of components with supplier part numbers, so if you are missing some you can see where you can purchase them from.
If you place an order it just sends me an email, it doesn't take any money. So the first thing I'll do is to reply via email to let you know what I have.

By the way I'm very happy to sell all of the spares. I have my own personal spares reserved as part of the 352, so anything above that is just shed clutter as far as I'm concerned.

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