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Unread 04-21-2019, 09:07 AM   #1
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Default Relationship between SOC and voltage?

Sorry for the newbie question but I'm having trouble understanding the relationship between state of charge SOC and voltage.

My Nissan Leaf 7 module pack fully charged is at ~ 57 volts and the SOC reads 100%. After discharge from an 18 hole round on my cart the SOC still reads 99% but the pack voltage reading has dropped to 48.6. Is this normal or is there something wrong with my BMS/meter?
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Unread 04-21-2019, 09:44 AM   #2
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Default Re: Relationship between SOC and voltage?

What BMS are you using? Are you able to change settings in it?
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Unread 04-21-2019, 11:08 AM   #3
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Default Re: Relationship between SOC and voltage?

I'm one of those guys who kind of has the same question. I get it....then I don't.

All I do is monitor my voltage to determine when it's time to come back and recharge.
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Unread 04-21-2019, 11:10 AM   #4
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Default Re: Relationship between SOC and voltage?

In the simplest terms, Voltage is just that, the voltage your pack or battery is at as a result of the cells charge state. The state of charge (SOC) is the usable energy calculation that is available from the battery and lets you keep from over charging or overdrawing (depleting) the pack. It is stated as a voltage level or point.

Remember, going below 50% SOC on FLA's and/or 20% on Lithium's will greatly lessen your batteries life cycle.
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Unread 04-21-2019, 01:47 PM   #5
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Default Re: Relationship between SOC and voltage?

Unfortunately no. Its a cheapie BMS from tech direct. All they say is that its a 14S Smart BMS
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Unread 04-21-2019, 04:31 PM   #6
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Default Re: Relationship between SOC and voltage?

Quote:
Originally Posted by sporty2k View Post
Unfortunately no. Its a cheapie BMS from tech direct. All they say is that its a 14S Smart BMS
If that is the BMS where we modified to by-pass the BMS for the high current and use an LV relay instead, the SOC calculation can no longer measure the discharge current.

The BMS will still provide LV and HV protection for the batteries.
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Unread 04-22-2019, 09:05 AM   #7
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Default Re: Relationship between SOC and voltage?

Yep that's the one! Makes sense now. However it appears that LV on the BMS is set to around 47.6 volts as thats when the relay kicks in and kills all power. Does that number seem right or should I try to lower the LV?
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Unread 04-22-2019, 09:17 AM   #8
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Default Re: Relationship between SOC and voltage?

You can drop the Low Voltage to 3.0v/cell which is 42v pack voltage.

That change (from 3.4v/cell to 3.0v/cell) will only give you about 3AH more to use so it is not going to make a huge difference.
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Unread 04-22-2019, 09:32 PM   #9
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Default Re: Relationship between SOC and voltage?

I estimate SOC from my pack voltage and it is reasonable but not super accurate. It's very nonlinear so I used a lookup table when I coded it up. I used the voltage vs SOC curve for the Leaf batteries that was published online.
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