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Unread 09-25-2018, 11:57 PM   #31
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Default Re: Ordered Nissan Leaf batteries

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Originally Posted by todis View Post
Hi All! - I just joined this forum a few minutes ago. I'm going through all of the Lithium conversion posts and there's a lot of good info!

My question on the Leaf batteries is if each CELL in the pack (2 cells in series, 2 parallel) have a certain cut off voltage for charging and discharging before the cell is damaged.. is the BMS smart enough to manage each INDIVIDUAL cell in the pack? Or does it strictly manage the voltage of the total pack at 7.4-8v? My understanding is the BMS needs a lead (wire) to connect to each cell to measure voltage. I don't see how that's possible in the Leaf pack with the way it's built.
There are 3 terminals on the end of each module.

+, -, and a center tap.

Voltage between the + and - is 7.6v nominal.
Voltage between the + and center is 3.8v nominal.
Voltage between the - and center is 3.8v nominal.

Depending on the bms chosen, it typically monitors each cell voltage, balances them to be the same voltage, and monitors each cell and if above or below the limit will trigger the protection circuit. The protection circuit is up to you how to build it. You can use a heavy duty relay and disconnect the battery, or many other ways.

You can get very fancy, or be simple. On the simple end is reusing the original charger and manually checking voltages while charging and unplugging the charger when you get your desired voltage, and using a light bulb to drain any higher voltage cells to manually balance the pack. On the more fancy side is the bms with can bus that allows you to see what is going on and have everything automated.

To charge, you are just charging them all at once. This is the same with these bms setups as it is with a manual setup.

If you really wanted to get fancy, you could get 14 chargers and charge each cell individually. You could do that without a bms. But the cost of 14 chargers would be more than the bms, and the bms does more than balance and high voltage protection.
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Unread 09-26-2018, 09:40 AM   #32
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Default Re: Ordered Nissan Leaf batteries

Quote:
Originally Posted by ag4ever View Post
There are 3 terminals on the end of each module.

+, -, and a center tap.

Voltage between the + and - is 7.6v nominal.
Voltage between the + and center is 3.8v nominal.
Voltage between the - and center is 3.8v nominal.

Depending on the bms chosen, it typically monitors each cell voltage, balances them to be the same voltage, and monitors each cell and if above or below the limit will trigger the protection circuit. The protection circuit is up to you how to build it. You can use a heavy duty relay and disconnect the battery, or many other ways.

You can get very fancy, or be simple. On the simple end is reusing the original charger and manually checking voltages while charging and unplugging the charger when you get your desired voltage, and using a light bulb to drain any higher voltage cells to manually balance the pack. On the more fancy side is the bms with can bus that allows you to see what is going on and have everything automated.

To charge, you are just charging them all at once. This is the same with these bms setups as it is with a manual setup.

If you really wanted to get fancy, you could get 14 chargers and charge each cell individually. You could do that without a bms. But the cost of 14 chargers would be more than the bms, and the bms does more than balance and high voltage protection.
Ag4ever, thank you for all your insight on this subject! I've learned a lot from you in your posts!

I'm trying to figure out which BMS I should go with, to go with my 14 module leaf battery pack and chargery battery charger. Do you have any insight on the chargery products?
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Unread 09-26-2018, 10:18 AM   #33
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Default Re: Ordered Nissan Leaf batteries

I just casually looked at the chargery, but decided on the Zeva/TC Solution just for the monitor.

The bms system I have will work on a 7 module ,14 module, 21 module, 28 module, 35 module, 42 module, 49 module, etc...

The way it would work is:

7 module = 14s2p (14 cells in series, 2 cells in parallel)

14 modules = 14s4p (14 cells in series, 4 cells in parallel)

21 modules = 14s6p (14 cells in series, 6 cells in parallel)

28 modules = 14s8p (14 cells in series, 8 cells in parallel)

Basically, the parallel batteries get bigger and bigger and bigger the more modules you add. The down side to this is the BMS balance circuits can only handle so much current, and as the battery capacity gets larger, the time it would take to correct an out of balance condition takes longer and longer and longer. Now the other side is, the larger the battery, the longer it takes to get out of balance.

Want to see the extreme? 49 module 48v nominal battery (14s14p):

http://www.beginningfromthismorning....teries-part-2/

Now that is a battery! Wonder if my cart would do well pulling a trailer with the battery on it? Nah, never mind.
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Unread 09-26-2018, 09:19 PM   #34
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Join Date: Sep 2018
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Default Re: Ordered Nissan Leaf batteries

ag4ever - Thanks for the detailed reply! I didn't know that. I'm glad there is a way to check the voltage on each cell in the pack.
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