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Old 06-20-2018, 03:57 PM   #11 (permalink)
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Originally Posted by redpoint5 View Post
Not sure how that would work since starting requires several hundred amps. If I remember correctly, the BMS has a decent bit of phantom power draw too. If I'm undersizing the battery, I can't have large phantom loads.
You'd let the BMS control charging, and lighter discharging loads (like the rest of the car). You'd hook the starter wire direct to the battery's terminal. No worries about needing a big BMS, as long as it can handle the charging (and discharging, if used) amps.

Too many charging amps from the alternator for the BMS? Put in some diodes, so the battery can discharge as much as it wants, and a resistor to limit the charge current going to the BMS. Or a PWM current limiting circuit.

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Old 06-20-2018, 06:16 PM   #12 (permalink)
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For my next lithium iron phosphate battery I will probably use 100 ah cells and include a bms for when solar charging.
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Old 06-21-2018, 08:20 AM   #13 (permalink)
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100ah cells are not suitable for starting a car. Most cells are rated at 1C, but you would be doing 3C. Short life if you do that. Instead, go for 26650 cells which are happy putting out 7C. I use recycled packs from electric buses (ebay) . 96 A123 cells in a 8P12S configuration. Break them into 8P4S packs and reconfigure to 24P4S which at 300A is only about 4C.

Last edited by Crashy; 06-21-2018 at 08:33 AM.. Reason: Bad math
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Old 06-21-2018, 08:27 AM   #14 (permalink)
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Do not use a bms. Use arduino with voltage dividers. Get a heavy duty latching relay which uses zero power unless triggered. Have the arduino turn on a signal relay which then feeds the latching relay if any cell is under 2.5v. If you want you can use more relays to discharge over x volts.
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Old 06-21-2018, 08:37 AM   #15 (permalink)
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Add a temperature probe to the arduino that engages a relay with current limiting resistor to slow charging in cold weather
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Old 06-21-2018, 08:53 AM   #16 (permalink)
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I would not use li-ion. A minor accident to the battery compartment WOULD result in a fire that takes the entire car with it.
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Old 06-21-2018, 08:57 AM   #17 (permalink)
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FWIW, LiFePO4 buddy pack, split in 3 12 cell units each with their own BMS, has been waiting further developments for a couple of years now.
Once a year I check on them, and they charge for a minute or so before they max out.
There is virtually no draw whatsoever from their BMS.

My 12Ah cells are good for 10C (120 Ampere) during 30 seconds.
The BMS limits output to 55 Ampere, and completely cuts off power when the current rises above 55 Amp, the temp sensor on the BMS exceeds 90 Celsius or any cell gets out of range (above 3.85 Volt, below 2.something).
It also has passive top bleeding to 3.65 Volt.

So far I'm very pleased with how it works out.
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Old 09-05-2018, 11:22 PM   #18 (permalink)
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Wanted to put this company on your radar screens. The cost for a starter replacement with BMS looks to be about $1/Wh. How does that compare? Somebody should make a table. Probably me.

https://bioennopower.com/collections/360-to-375-cca

But seriously, the Cruze will need a replacement soon and I'm leaning towards bioenno for now. The specs say it can operate down to -10C.
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Old 09-06-2018, 02:13 AM   #19 (permalink)
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Quote:
Originally Posted by ME_Andy View Post
Wanted to put this company on your radar screens. The cost for a starter replacement with BMS looks to be about $1/Wh. How does that compare? Somebody should make a table. Probably me.
They don't give how many Wh, or any capacity rating of their batteries, so it's not possible to make a table with that info.

Anybody selling a battery without listing the capacity is shady. Price per Wh isn't everything though. If you want that, it's still probably lead acid.
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Old 09-06-2018, 07:28 AM   #20 (permalink)
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Are there any lithium chemistries which would be safe to use in Vermont's climate? I can realistically expect -20F a few times per year, and I understand an alternator trying to charge the battery at that temperature could be very bad.

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