panamamike
Posts: 121
Joined: Sat Jan 25, 2020 10:21 am

Hold state of charge vs. normal US REX operation

Mon Feb 03, 2020 10:30 am

Does anyone know if there is a benefit to running hold state of charge mode over running REX basically at the last minute?
My guess would be it could be more efficient because the motor could operate at maximum charge efficiency and not have to kick up to maximum output due to real time vehicle demand.

Has anyone tried to study/measure the difference?

Mike

symonray
Posts: 70
Joined: Thu Mar 07, 2019 8:52 am

Re: Hold state of charge vs. normal US REX operation

Mon Feb 03, 2020 12:15 pm

Like any ICE unit, I would think the Rex engine would occasionally need to run an extended period of time to fully warm up. After coding my car to activate the hold SOC function, I use the Rex on my work commute in the afternoons (an hour commute covering 30 miles of distance) 3 or 4 times a month to allow the engine to run the entire time. I imagine you could run the Rex to give it a chance to fully warm up without activating the hold SOC function, but it would require running on a low SOC, which may not be the best practice for the high voltage battery.
Timothy Simonds
2017 BMW i3 Rex

panamamike
Posts: 121
Joined: Sat Jan 25, 2020 10:21 am

Re: Hold state of charge vs. normal US REX operation

Mon Feb 03, 2020 12:40 pm

symonray wrote:
Mon Feb 03, 2020 12:15 pm
Like any ICE unit, I would think the Rex engine would occasionally need to run an extended period of time to fully warm up. After coding my car to activate the hold SOC function, I use the Rex on my work commute in the afternoons (an hour commute covering 30 miles of distance) 3 or 4 times a month to allow the engine to run the entire time. I imagine you could run the Rex to give it a chance to fully warm up without activating the hold SOC function, but it would require running on a low SOC, which may not be the best practice for the high voltage battery.
I was curious about this aspect of US REX. Running the hold state of change is a benefit to avoid battery degradation vs. waiting for a low start of charge to occur. Would be interesting to learn if any folks have info. on this scenario.

jadebox
Posts: 8
Joined: Fri Apr 19, 2019 12:29 pm

Re: Hold state of charge vs. normal US REX operation

Sat Feb 08, 2020 6:40 am

I doubt that using HSOC is either beneficial or harmful to the REX or batteries. Neither is doing anything different than when the REX runs at a different State of Charge.

What is beneficial is that it gives you more control over when the REX runs which can be helpful at times. For example, during one part of the trip I make occasionally, I run the REX using HSOC during the middle part of the route when I am on a highway. That way the REX runs when I can't hear it due to the road noise. It saves the battery charge for the end of the trip, so I don't have to hear the REX when going slower in town.

I can also time the usage of gas so that it runs low when I get near an area with plenty of gas stations rather than somewhere with few gas stations (and no chargers!). :-)

jadnashuanh
Posts: 5196
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Hold state of charge vs. normal US REX operation

Tue Feb 11, 2020 8:07 pm

Not currently true in the USA, but in a lot of markets, there are places that charge a fee for using an ICE in say city centers...preserving the main battery can save a huge amount of money if turned on earlier so you still have sufficient battery power to traverse those areas entirely on battery power.

If your power use is significant when it turns on automatically at about 6%, you could run the risk of it shutting down certain functions if the REx can't keep up and the SOC continues to drop. Running fast, uphill, or with the HVAC cranked, all add to the power drain, and you can exceed the capacity of the REx to keep up. Having a bigger buffer can help.

Keep in mind, it was designed as a city car where your top speeds and distances aren't large, so exceeding the REx's capacity wouldn't be an issue when used as designed.
Jim DeBruycker
2014 i3 BEV, 2021 X5 45e
(The i3 will be sold soon, <17K-miles, interested?)

panamamike
Posts: 121
Joined: Sat Jan 25, 2020 10:21 am

Re: Hold state of charge vs. normal US REX operation

Fri Mar 13, 2020 7:08 pm

jadebox wrote:
Sat Feb 08, 2020 6:40 am
I doubt that using HSOC is either beneficial or harmful to the REX or batteries. Neither is doing anything different than when the REX runs at a different State of Charge.

What is beneficial is that it gives you more control over when the REX runs which can be helpful at times. For example, during one part of the trip I make occasionally, I run the REX using HSOC during the middle part of the route when I am on a highway. That way the REX runs when I can't hear it due to the road noise. It saves the battery charge for the end of the trip, so I don't have to hear the REX when going slower in town.

I can also time the usage of gas so that it runs low when I get near an area with plenty of gas stations rather than somewhere with few gas stations (and no chargers!). :-)
This is a good point.

Does REX end up running at it's maximum speed if used to hold state of charge on the highway? Wondering if it may run at a lower speed given there is more room to play with than at 6.5%

jadnashuanh
Posts: 5196
Joined: Thu May 22, 2014 2:07 pm
Location: Nashua, NH USA

Re: Hold state of charge vs. normal US REX operation

Sat Mar 14, 2020 7:07 pm

The computer won't run the REx at max RPM until it has warmed up some, and it will continue to run later on until it fully warms up to avoid moisture issues. Once warmed up, it will adjust the RPM based on the current load and where the SOC is relative to when you turned it on (it will try to raise the SOC to the point where it was activated, if the load isn't too large). Once warmed up, it could also shut down when it realizes it doesn't need to run to maintain.
Jim DeBruycker
2014 i3 BEV, 2021 X5 45e
(The i3 will be sold soon, <17K-miles, interested?)

JohnWasser
Posts: 83
Joined: Sat Mar 07, 2020 6:15 pm

Re: Hold state of charge vs. normal US REX operation

Sun Mar 15, 2020 8:30 am

One risk of NOT using HSOC at a value higher than 6.5%: If the REx should fail you only have about 6-8 miles to get to a charger before your battery can't move the car. I think I would set it to 20% around here and maybe higher in areas where chargers are more scarce.

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