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Old 01-18-2021, 09:43 AM   #41
Not Yet Wild
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Default Re: building 2 more 72V carts

every time i post here I learn so much. my background is maintenance mechanic and fabrication so this is very different to me. so forgive what might seem like dumb questions. since my controller is 650A would i need this? with the 750A shunt?

Also if I use a cheap BMS what are some good relays for 72V? I know in Sergios post he said stay away from cheap "ice cube" ones found on amazon. where is a good source? and what dictates good and bad?
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Old 01-18-2021, 10:48 AM   #42
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Default Re: building 2 more 72V carts

Volt_Ampere, my comment was intended for folks that are not very familiar with the discharge curve of a lithium cell.

Voltage based SOC are also affected by the discharge rate and depending how long you drive at a continuous high rate of discharge, the SOC meter will drop as the average sample time should not be too large.

The SOC will "bounce" back up when the discharge rate reduces/stops and the voltage reading increases.

A coulomb count meter (AH) will always provide a stable capacity regardless of the discharge rate.

It is also safer to estimate capacity left with AH, voltage based SOC can be very misleading specially if one does not understand that just because the voltage only drop "X" volts one way, it does not mean that you will be able to drive back as it could take "2X" volt drop to use the same capacity as before.

I also only use my voltmeter but I am very aware of the non-linear voltage drop of my pack.

Perhaps I should have said "non-linear" rather than "not accurate".

keepinitreal538, "ice cube" relays are those small 1 inch cube relay that was used on the board that I made the comment. They are not very robust if you are running the charging current through them.

I like these type of relays (48v), but 72v relays are more expensive and not always needed, specially if your BMS requires 12v to operate or You just use a small voltage converter to power the relays, solenoids. contactos, etc.
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Old 01-18-2021, 12:25 PM   #43
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Default Re: building 2 more 72V carts

Coulomb counting is difficult under motor noise conditions. Yes - my SOC changes a couple percent after a drive but it's plenty good for predicting when to charge. I looked into Coulomb counting when designing my stuff but I opted for a simpler approach.
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