Optimal Regen Braking Settings

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kingjamez   100 W

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Optimal Regen Braking Settings

Post by kingjamez » May 16 2017 3:49pm

I'm trying to determine the optimal way to setup regen on my Mobipus controller. The Mobipus allows you to set a maximum pack voltage for enabling regen, and it allows you to set a "turn back on" voltage after having exceeded the other max. For example: Given a 10 cell pack you can say "If when you startup the voltage is above 40V, then don't enable regen", next "If the voltage drops below 39V regardless of starting voltage, enable regen".

I can also set the maximum regen amperage back into the pack.

For simplicity, let's talk about volts per cell instead of pack voltage.

What I'm after is where the "right" place is in the SOC to start regen. I also don't know how regen's amperage output works.

I "think" it raises it's output voltage until the maximum amp draw is taken in by the batteries. Indeed that seems like the only way that it could "push" a particular current into the batteries.

What I'm a little fuzzy on is how the charge profile of Lithium batteries plays here. I (again) "think" that it's safe to use the above constant current type charging, where the voltage is free to change, all the way up to 4.1V/cell. At 4.1V/cell the method for charging should be constant voltage of 4.2V and the battery takes whatever current is available due to the potential differences between the source voltage and the cell voltage and thus tapers in amperage draw as the battery gets closer to 4.2V.

From these assumptions, it would seem "safe" to me to set my controller to not enable regen if the battery is at greater than 4.1V/Cell. Then, if the battery drops below 4.05V/Cell or so to re-enable regen making the assumption that you won't charge it past 4.1V/Cell.

Do those sound like reasonable limits?

Related question: I believe the constant voltage portion at the end of the charge is there for two reasons: 1. To maintain a particular amperage level, you'd need to increase the supply voltage beyond 4.2V/Cell resulting in an unsafe condition. 2. A lithium cells true resting voltage isn't known during charging, switching to a saturation stage allows the cell's true state to determine the charge rate.

Does that sound right?

-Jim

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amberwolf   100 GW

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Re: Optimal Regen Braking Settings

Post by amberwolf » May 17 2017 2:29am

kingjamez wrote:I also don't know how regen's amperage output works.

I "think" it raises it's output voltage until the maximum amp draw is taken in by the batteries. Indeed that seems like the only way that it could "push" a particular current into the batteries.
I don't know about the Mobipus, but the various generic ebike controllers I've used don't have a set current that they attempt to force into the battery, they simply output whatever voltage is generated by the motor at whatever speed it's spinning at (possibly multiplied by pulsing the FETs synchronously with the motor rotation, depending on controller design), and so the current flow is determined by the difference in voltage between battery and controller (and whatever resistances are in circuit), and decreases as speed decreases.

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