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Dual-battery voltage difference (.5V) disengages Stock ECU


knockknuckles
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(@knockknuckles)
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After reading some of the posts of @arjan_1980, I decided to add a second battery to my otherwise stock TC, using a second-hand NIU NQi battery. I wired it up in parallel with the original battery (including a 30A DC circuit breaker, since the stock controller should only pull 40A total) and the first ride was amazing.

Screenshot 2021 12 13 at 16.56.04

Second battery mounted with circuit breaker

However, when I attempted to ride home this afternoon, I couldn't.

The dash showed a blinking charging-icon (which also shows during regen), leading me to believe there was a voltage difference between the original and the second battery, causing the second battery to charge the original.

My measurements did indeed show a .5V voltage difference (the original was at 67.2, while the second was at 67.7), even though the batteries were both at 70.2V when I connected them.

I used 14AWG wiring (55A rated), so I didn't expect the resistance difference to cause any issues here.

Do you have any idea why this happened? Is there a way to circumvent the charging-lockout?

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knockknuckles
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(@knockknuckles)
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A little follow-up. I have been keeping track of the battery voltages after some rides.

This morning, I started with fully charged batteries with equal voltages. After a 18K ride (riding mostly top-speed), the voltage difference between the batteries reached 1V.

This leads me to believe there is, indeed, a significant difference in either a) the internal resistance of the batteries or b) the resistance of the wiring.

I may attempt connecting the secondary battery with thicker wire, but otherwise I'm kind of lost on this issue.

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ElectroChris
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The issue will be due to the 2 battery packs being different configurations, and slightly different capacities.

 

The Super Soco Battery has 157 cells in a 17S9P config and doing a google on the NUI battery it appears it has 170 cells in probably a 17S10P to make them both "60v" batteries. Likely both have different capacity cells in use as they are stated as about the same capacity of 30Ah vs 29Ah. I know the Soco use LG MH1 18650 cells, not sure on the NUI

EDIT - That info was for the TS Battery, the TC Battery is 17S10P but not sure what cells there are in there.

 

This matches with what you are finding, as both batteries drain the one with the smaller capacity will likely end up being at the lower voltage of the pair, I would suspect the NUI is the one with the higher voltage causing the BMS in the Super Soco battery to attempt to charge from it to balance it out sending the info the ECU saying its in charge mode, hence the symbol coming up on the dash.

To run a pair of batterys in parallel you ideally need them to be identical, otherwise if they are mismatched as yours are, when they are both fully charged they will have the same voltage but when they start to discharge their voltages will start to differ as they drain until you get to the point where the BMS in one will see the voltage difference and start to attempt to charge from the other one.

I dont think there is much you will be able to do about this other than switching to a pair of identical batteries.

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ElectroChris
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Another thought is that one thing you could try to get round the issue would be to remove the communication between the battery and the Soco ECU, that way when the Soco battery is in charge mode when it is trying to balance with the NUI battery it wont be able to tell the ECU that it is in charge mode and the ECU cut the power off.

You can do this by lifting the socket on the battery case and unplugging the connector to the BMS communication wires.

Im not entirely sure if doing that will stop the ECU working completely but the battery should still operate as you can run a pair in parallel with only having one connected with the communications cable connected.

The downside if this does work is that you will likely loose the battery charge level readout on the dash.

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