# regenerative braking and bms



## Tyn245GL (Oct 12, 2009)

The Elithion Lithiumate (Pro) can do it.


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## tomofreno (Mar 3, 2009)

MemphisPapa said:


> I've been looking into BMS products to make the most of my Lithium batteries, but looking into the specs and how they work I began to wonder about if this would limit the effectiveness of my planned usage of regenerative braking of my AC system.
> 
> I like the function and display of the Ligoo BMS. Are there BMS products that are designed for AC systems and regenerative braking?


 What do you mean? Are you concerned with over-charging with regen? Most AC controllers permit you to set a max battery voltage so the controller limits regen current to keep battery current below this. That is how I protect my cells. I have a minibms which has HVC and LVC but the HVC only works during charging, and only LVC during driving, so it won't protect against regen overcharging cells. Been in operation about 2 1/2 years this way with no problem from regen overcharging.


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## Ziggythewiz (May 16, 2010)

tomofreno said:


> What do you mean? Are you concerned with over-charging with regen?


Is this really even an issue for anyone? Unless over 50% of your driving up to some point in your trip is downhill, I can't see it being an issue. Are many people king of the hill?


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## MemphisPapa (Feb 16, 2012)

Well it's good to know that this isn't an issue. No, I don't live on a hill and no, I have not converted many vehicles. Like a good newbie, I started reading up and I had noticed that the slave modules were relatively low current. I suppose the current generated from my AC motor would be distributed over all cells. I also understand that the controller has parameters to determine the amount of breaking and generated current and probably not likely to generate as much power as it consumes. I haven't seen any BMS systems specifically mention compatibility with AC systems. I wondered if there were potential issues or if there were specific management systems to be sure to stay clear if I intended to use regeneration breaking.

Thanks for helping.


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## steven4601 (Nov 11, 2010)

For those who are lucky enough to live in the alps or something similar, charge the pack less than absolute full. This would allow to recapture energy while regenerating x-meters / miles downhill. The amount of energy that is regenerated depends mostly on your vehicle mass.


joules = m.g.h
watt-hours = joules/3600

g = 9.8m/s^2

example: mass = 1300kg, hill height 2000meters ( 1.25miles)
joules = 1300 * 9.8 * 2000 = 25 mega joule
or 7kwh!

So if the battery pack is 20kwh, charge upto a maximum of 13kwh energy contents*1. That way the energy you put in driving up hill will be recaptured.... Perhaps GPS altitude information might be usefull for the BMS 

**1 See my next reply for re-evaluated values *


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## major (Apr 4, 2008)

steven4601 said:


> For those who are lucky enough to live in the alps or something similar, charge the pack less than absolute full. This would allow to recapture energy while regenerating x-meters / miles downhill. The amount of energy that is regenerated depends mostly on your vehicle mass.
> 
> 
> joules = m.g.h
> ...


Hi steven,

Your calculation might hold in a frictionless vacuum, but in the real world the road load always uses energy. So if it was a 10% grade, 12.5 miles would be traveled. At 500 Wh/mi, friction and aero losses would be 6.25 kWh. So that the net energy at the bottom of the decline would be 0.75 kWh gained. 

This still doesn't include drivetrain efficiency, but we could consider those losses part of the 500 Wh/mi. With a little headroom, he could charge the 20 kWh battery to 19. The real killer is getting back up the mountain. There you have the change in P.E. plus the road load or 13.25 kWh needed. So he would have only about 6 kWh to use down in the valley.

Regards,

major


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## steven4601 (Nov 11, 2010)

Of course, losses have to be accounted for.

Considering air resistance, faster the decline, lesser the amount of energy can be recaptured. Considering friction losses, I think those are rather static, either going fast or slow over a certain distance. 

Estimated / rough measurements of efficiency of a BMW Z3 RWD drivetrain with manual trannny is about 77% @ 50km/h. 
- 50kmh requires about 4.5 to 5kW of battery power, or 3.85kW wheel power on level grounds. 
- Declining 2000meters @ 50km/h would give about 7kwh of potential energy back. 
- @ 50km/h. 7kwh potential energy * 0.77 drive train efficiency is about 5.4kwh battery energy that can be recaptured.

With a decline of 10%, or distance of 20kilometers @ 100watt/hour per kilometer:
energy required for the distance = 20km * .1kwh/km = 2kwh
potential energy regained for height = 5.4kwh 
total gain = 3.4kwh.

Hope I didn't miss anything.

How does it help to throw in a random value like 500 watt-hour mile? Going up hill with a school bus ? 


edit: energy available in the valley after the decline:
Assuming the 20kwh pack is full @ 20kwh at the bottom of the decline, about 10kwh energy is available for driving in the 2km deep valley. The other 10kwh is required to go back up @ 50km/h.


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## major (Apr 4, 2008)

steven4601 said:


> Hope I didn't miss anything.
> 
> How does it help to throw in a random value like 500 watt-hour mile? Going up hill with a school bus ?


It was just an example. The vehicle and speed were not specified. A random value was just as good as a random BMW


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## tomofreno (Mar 3, 2009)

MemphisPapa said:


> Well it's good to know that this isn't an issue. No, I don't live on a hill and no, I have not converted many vehicles. Like a good newbie, I started reading up and I had noticed that the slave modules were relatively low current. I suppose the current generated from my AC motor would be distributed over all cells. I also understand that the controller has parameters to determine the amount of breaking and generated current and probably not likely to generate as much power as it consumes. I haven't seen any BMS systems specifically mention compatibility with AC systems. I wondered if there were potential issues or if there were specific management systems to be sure to stay clear if I intended to use regeneration breaking.
> 
> Thanks for helping.


 I suppose you are thinking if the bypass voltage on the bms is exceeded you would drive some of the regen current through the shunts? This could happen, but shunt current is determined by the ratio of cell terminal voltage to shunt resistance, so even if a cell is driven to 4V, this is still less than about 1.5 A for typical shunt resistances. But it is not something you should do.

I live on a hill, with about 150 ft or so drop. If I charge the pack full, the controller cuts back regen so much that I coast down the hill freely, which feels like I am applying the accelerator compared to a normal descent with small amount of regen to regulate speed, and I have to apply mechanical brakes repeatedly to stay within the 25 mph speed limit. If the pack is more than 8Ah or so below full, full regen is applied including about 70A when I come to a stop at the bottom (25 mph speed limit), and the pack voltage as indicated on my meter doesn't exceed the max I have set. This is what I normally do, which reduces my total range by about 5 miles out of 75. My pack is top balanced so no cells are driven above about 3.45V at this max voltage setting (cell rest voltage after full charge is about 3.4V). If I decelerate suddenly while traveling at 50 mph, resulting in higher regen currents on the order of 150A while the pack is 8Ah or so below full, regen will be limited as the higher current drives the pack voltage up.


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## tomofreno (Mar 3, 2009)

major said:


> It was just an example. The vehicle and speed were not specified. A random value was just as good as a random BMW


 Here is some data: My car requires about 6.5kW to maintain 40 mph on level terrain, or about 160 Wh/mile. Down a 6.3 mile 4.8% average grade it generated 5.5Ah, about 655Wh (119V pack V) of regen into the pack. Took 25.6Ah to go up at about the same speed, so about 21% regen.

And some more, and some accounting for losses here:

http://www.diyelectriccar.com/forums/showpost.php?p=280832&postcount=16


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