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post #148 of Old 01-18-2019 Thread Starter
Captain Canuck
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Re: Electric Conversion log for Kahleesea

Originally Posted by texlan View Post
any relay running at 5v/100ma is a tiny little relay that doesn't have the spark gap to stop the other 46v + inductance of the whole circuit (Read: You will get a voltage surge when your relay trips, and that will weld the contacts the first time it opens under load)

The only thing your individual cell monitor needs to do is bleed the cell for balancing and alert for under voltage/over voltage. The alert lines should be opto isolated for safety to the main BMS unit, and engineer it to it keep the alert lines at the same relative voltage to each other (since they'll have a lot less insulation on them than the primary conductors). There's no point in putting a relay on each cell since the cells are all in series...any one relay trips you shut the whole bank anyways. You cannot realistically take one cell out of the circuit, since each set of relays to do that would have to be rated at max system current and voltage. That's expensive.

Better and far less expensive to use one big assed fat contactor to shut the bank down and just trigger it on HV or LV events.

The individual cell monitor is probably better off a very simple low power 1.2v digi device with a dedicated AD, or a pure analog device, though pure analog devices are hard to calibrate/maintain calibration over temperature ranges. A commercial solution that has had decades of sound engineering behind it coupled with practical application experience is really the only safe way to go esp. considering your desire to use less forgiving LiIon vs. LiFePO4.

If I remember correctly there are some BMS projects on if you want to look at people's design thoughts on the subject. After you become familiar with the sheer magnitude of what you are trying to do I think you may reconsider either the chemistry or the bms(Though again, i recommend reconsidering both.) I looked into BMS design figuring it can't be that hard. And I'm really familiar with eletronic design, have had my own circuit boards printed, have etched them myself, built transmitters, antenna tuners, antenna tuner control circuits, programmed Microchip PIC and Atmel microcontrollers, I code in a plethora of languages c c++ python etc... and while I enjoyed learning about BMS design, I decided it would be hubris to pursue it.

YMMV but I don't recommend it.

Even if I put one big ass relay in place, I still need to find said big ass relay, and a way to actuate it.

The batteries will be in parallel, not in series. This is why I'm trying to see if it's possible to cut one out if it goes bad while under power, so I only lose some run time instead of stopping the boat dead. I agree that it could get pricey, but so does one charger per battery.

I'm not afraid of difficult tasks. That's how you learn. That being said, looks like I'm being Quixotic in building my own BMS. I didn't think there were any off the shelf commercial models available, but apparently, there are. I'll dig into that and see if I can find something that will work. At least the converter and inverter are relatively straightforward and there's plenty of off the shelf parts available.
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