Soapbox on regenerative braking

Technical discussion on converting internal combustion to electric
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Soapbox on regenerative braking

Post by Electrocycle »

put the braking resistor in a tank of water and use it for cabin heating :)
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Post by Goombi »

That makes sense --- the cream is getting thicker lol
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Post by weber »

zeva wrote: PS: Coulomb, I reckon a big shunt for dissipating regen energy is useful even when the batteries are happily accepting a charge.

ThunderSkys etc can only take about 1C charge current which doesn't represent a huge amount of regen braking force. But it's still better to slow a vehicle down with a motor than with friction brakes since there's nothing to wear out in an (induction / PMSM) motor. So you might as well maximise regen and have a big shunt resistor to bleed off excess power as heat? My 2 cents..


It seems that way until you actually try to buy or design such a resistor. Dissipating 20 to 50 kW as heat, without starting any fires is difficult, and so is simply finding the space to fit this very large incandescent object. Think how much space a 500 W floodlight occupies. You'd need 40 of them just for 20 kW.

Of course it doesn't have to be able to do 20 kW continuous, but then it needs a lot of thermal mass, which means a lot of actual mass, and temperature monitoring.

After a few attempts like this, friction brakes start to look like a really good way of dissipating this power, since you have to have them anyway.

The problem with not having a braking resistor, as Tritium_James mentioned to me re the AC Porsche 911, is that it's bad not knowing whether you will get braking or not, when you lift your foot off the accellerator (depending on the state of charge of the battery).
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Post by acmotor »

Well not quite that bad re braking resistor size.

Braking resistors need only be 10% duty cycle and can be pushed 10:1 in power by industrial design. e.g. A 1kW braking resistor can handle 10kW for two minutes by manufacturer's design. More than enough for most braking exercises (after all it will only get used when the pack is full or braking is over the pack recharge current).

e.g. from Danfoss handbook... for 10% duty
VLT   seconds on   motor kW   resistor ohm    resistor power   part #
5042        120        30,0               15               4.8kW             175U1852

All VFDs with braking output are told the braking resistor power and resistance and calculate/control internally so as not to overheat the resisitor. i.e. It has all been thought of before.

12A regen on a 1200kg vehicle is plenty (from experience) so no issue with battery pack capacity. Don't be put off regen on the basis of recharge current. Read the red suzi thread on this forum for some live data.


I would note that one of the big differences between AC and DC regen is the RPM at which regen can occur. In DC the RPM must be such that the voltage of the 'generator' must be sufficiently greater than the battery voltage otherwise no regen can occur (obviously). This is hard when the armature voltage may be less than 1/3 the battery voltage as is often the case in an EV application. Some DC regen attempts use boost switching to get this voltage up to pack voltage. The result is that there is little regen available at lower revs and eventually none as the 'generator' voltage drops. Some regen systems hide this by actually driving the armature to zero RPM using power.

Those of you who are old enough to remember generators on ICEs compared with alternators will recall the lack of output at lower revs or idle.

Alternators behave very much like the regen in an 3PIM except the 3PIM can go to nearly zero RPM since the stator is actively driven, not passive (just diodes). An alternator is also fairly lossy as a generating device ... man it come off an ICE afterall. Efficiency was not a design criteria !


Now..
ACIM regen is inherently able to supply full regen torque down to the nominal slip speed of the emotor e.g. 60RPM (1.2kmph) on my 11kW 4 pole.

The fact is that the VFD is a box of smarts and will feed a reverse frequency and continue the regen torque to zero RPM. (or further if you program it too i.e. go into reverse). The VFD can also supply its torque boost (used to be called voodoo until the data was logged) and now we know it is just increasing v/f ratio. Thus full overload regen is available to zero RPM. Locks the back wheels... from experience there ! Image

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Post by Goombi »

More cream? ACmotor i am still waiting for your answer.. to my e-mail re AC power
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Post by zeva »

weber wrote: It seems that way until you actually try to buy or design such a resistor. Dissipating 20 to 50 kW as heat, without starting any fires is difficult, and so is simply finding the space to fit this very large incandescent object. Think how much space a 500 W floodlight occupies. You'd need 40 of them just for 20 kW.

All true, but most road-going vehicles already have a system designed to dissipate huge amounts of heat - the old water cooling for the engine. The radiator etc is designed to dissipate several times more heat than all the energy used to actually propel the vehicle (based on 25% efficiency of ICE) * so can surely handle all the braking energy you throw at it. Just need a big water heater in the loop and an electric water pump (possibly powered by regen energy itself?)

I used to think the lack of any liquid cooling system in EVs was a big advantage, and sure the simplicity is nice, but I'm starting to think a water cooling circuit not such a bad idea. Water cooled electric motors can be both much more power dense and weatherproof than air-cooled motors. Water cooling controllers is a great way to support high continuous power, as is sometimes necessary if the car is climbing a long hill. And it'd also be a handy place to sink regen energy.

* Edit: On second thoughts this is not correct, much (most?) of the engine's wasted energy would be leaving the car as hot exhaust. But the radiator can still dissipate a lot of power..
Last edited by zeva on Mon, 20 Jul 2009, 16:07, edited 1 time in total.
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Post by acmotor »

BTW, rest assured you will get regen even without a braking resistor if you have just had your foot on the accelerator ! i.e. battery pack will not be full.

The regen may reduce to ~20% if the battery pack becomes full. (the VFD can lose some power deliberately internally in this case) Most VFDs can absorb some regen energy.
I am speaking industrials here.
Note... a feature for custom made EV units. They are quite capable of 'wasting' some power and getting warm under software control.   Image
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Post by Goombi »

If one drives a EV just like one drivee ICE and if you are happy with 75 Km/h and 50-70 Km distance then you do not need any cooling.
We are all sliding into the impossible performances travelling 10 Km and braking the sound barrier. The cars that some of you visualise will never be build and are not practical. Why can't just everyone settle for the most practical, cheapest and best performing metro car.The 8K EV is a true reality not a dream and achievable for almost everyone's budget with little enthusiasm.
What other option is there for us mortals,, we can keep discussing and debate all possibilties untill we grow beards
Did you know that making 8k EV is easily selable for 12-13K Test the market ACMOTOR put your EV on ebay-- see how its going to go make sure you place a reasonable "reserve"
and after you sell yours you can finally get stuck into a decent DC well performing and economical hearse.[[[[[

Thats enough cream for one day



Last edited by Goombi on Mon, 20 Jul 2009, 16:40, edited 1 time in total.
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Post by acmotor »

Difference is, I'm still learning and developing from my EV. Most DC EVs finished their development 100 years ago. Well not quite. The mosfet came along 20 or so years ago.

Hey Goombi, it is not a contest, DC just suits some people.
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Post by zeva »

acmotor wrote: Note... a feature for custom made EV units. They are quite capable of 'wasting' some power and getting warm under software control.   Image

My thought on this subject would be to have an independent current shunt device in parallel with the battery pack which "clamps down hard" at a specified voltage threshold to avoid overcharging the pack. I realise this could be built into the controller (or BMS) but it seems like a pretty trivial bit of logic, coupled with a big PWM switch and resistor/heater element, so the controller wouldn't really need to know about it. The controller should still restrict its regen voltage to slightly above the current shunt's threshold just in case the shunt fails etc.

BMS cell modules supporting very high shunt currents would be another route, though this seems like a bigger engineering challenge since you wouldn't want all the heat to be near the batteries etc. Easier to rely on fairly low shunt currents to maintain balance on the pack and a single large shunt for mass power dissipation, I think..
Goombi wrote: If one drives a EV just like one drivee ICE and if you are happy with 75 Km/h and 50-70 Km distance then you do not need any cooling.
We are all sliding into the impossible performances travelling 10 Km and braking the sound barrier. The cars that some of you visualise will never be build and are not practical. Why can't just everyone settle for the most practical, cheapest and best performing metro car.The 8K EV is a true reality not a dream and achievable for almost everyone's budget with little enthusiasm.

This is true, what we are discussing is well beyond the realms of the average backyard converter and I am certainly in favour of anything which gets more EVs on the road NOW, but one day EVs will need to do everything an ICE vehicle can if the general public is to embrace them. This includes towing trailers uphill at freeway speeds etc! So while some may see it as "pie in the sky" stuff, I do think it is valuable to discuss ways to surpass existing performance barriers in current EVs for future designs.
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Post by Goombi »

Just one point.re Overcharging batteries via Regen.
If it is established that normal driving with regen will in reality generate no more the 10-12% of power. Why worry about overcharge
Every 100m of travel under power every 100 m under regen will only pump 10-12% to batteries. If under normal city driving you travel 1 Km you will gain extra 100m free run.. Such are the probabilities. But never OVERCHARGE
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Post by acmotor »

Ah, it warms my heart Hoops !
zeva wrote: My thought on this subject would be to have an independent current shunt device in parallel with the battery pack which "clamps down hard" at a specified voltage threshold to avoid overcharging the pack. I realise this could be built into the controller (or BMS) but it seems like a pretty trivial bit of logic, coupled with a big PWM switch and resistor/heater element, so the controller wouldn't really need to know about it. The controller should still restrict its regen voltage to slightly above the current shunt's threshold just in case the shunt fails etc.


Ian, that is exactly what is built into industrial VFDs (they also monitor the braking resistor and the IGBT driving it for faults) and can terminate regen in the case of a fault. Industrial VFDs are big and expensive but rather smart.

The EV adatption to this would be to link the BMS O/V signal to this shunt control as well for obvious reasons of limiting any one cell voltage. Hang on, there is an input already on the front panel that can be programmed from regen control - connect it to the BMS. You beauty !


On that old topic...
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Post by acmotor »

Goombi wrote: Just one point.re Overcharging batteries via Regen.
If it is established that normal driving with regen will in reality generate no more the 10-12% of power. Why worry about overcharge
Every 100m of travel under power every 100 m under regen will only pump 10-12% to batteries. If under normal city driving you travel 1 Km you will gain extra 100m free run.. Such are the probabilities. But never OVERCHARGE


I had to read it a few times but yes, that is part of the regen advantage... recovery, so extra range. It is probably worth as much as a few extra batteries, or the result of maybe 200kg less weight, or some serious aerodynamic mods.... and it is free with AC !

But yes, your point. all the overvoltage talk is only when you have full charge. (doesn't happen often !!) Image
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Post by Goombi »

Hey and its free with DC !
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Post by zeva »

I agree about overcharging being unlikely under normal circumstances, but can think of two instances where a big power shunt would be valuable:

1) People living at the top of hills, so spend significant time going downhill first thing. (Not common but I know someone this does apply to.)

2) Braking harder than the batteries could accept the energy. I still think electromagnetic (motor) braking is better than friction braking, even if the energy is just wasted. Admittedly this is due to a limitation of the batteries which may be eliminated soon (e.g AltairNano's tech).

I actually plan to test the "huge current shunt" idea on UWA's electric Formula SAE race car project as part of my masters. We'll be running ThunderSky batteries and plan to trial the car without any mechanical rear brakes - just using the motors to brake rear wheels - so will have a lot of extra braking energy to dump somewhere!
Last edited by zeva on Mon, 20 Jul 2009, 17:42, edited 1 time in total.
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Post by weber »

zeva wrote:All true, but most road-going vehicles already have a system designed to dissipate huge amounts of heat - the old water cooling for the engine. ... Just need a big water heater in the loop and an electric water pump (possibly powered by regen energy itself?)

Zeva, that's a darn good idea. Pity we already sold ours.
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Post by Tritium_James »

zeva wrote:All true, but most road-going vehicles already have a system designed to dissipate huge amounts of heat - the old water cooling for the engine. ... Just need a big water heater in the loop and an electric water pump (possibly powered by regen energy itself?)
You don't even need a very large radiator, you just need a large-ish reservoir. Water can adsorb a very large amount of energy per amount of temperature change. I guess it's back to the 10:1 duty cycle thing that acmotor mentioned, but you can optimise it for living on top of a big hill with the ratio of reservoir to radiator sizes. No need to have that big ICE radiator (with associated aero losses) if you don't need it!
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Post by Johny »

zeva wrote:My thought on this subject would be to have an independent current shunt device in parallel with the battery pack which "clamps down hard" at a specified voltage threshold ...
I have to add a brake resister controller when my vehicle is up and running and I have thought about and discussed this a bit. If you go for a straight switch at a certain voltage (assuming a little hysteresis) then you are using the pack as the time constant for the brake resister switch on-off rate.
I'm not sure why but it seems to me that a PWM system would be gentler on the pack. Have PWM ramp over about a 3% range with maximum pulse width at the highest pack voltage you decide.

The other aspect is that monitoring systems will not be as "fooled" with a faster pulse width (easily filtered) as they would with a relatively slow switch on-off that I would imagine was happening at a single digit Hz rate.
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Post by Hemonster »

Turn on the heater before starting your trip .... that should get the batteries down a bit Image

I found with my headway cell testing that at 2C charge rate on a medium flat pack, the voltage on the cell very quickly rises to beyond 3.65V - is it then common to allow the cells to go up to say 4.0V during regen?
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Post by coulomb »

Hemonster wrote: ... is it then common to allow the cells to go up to say 4.0V during regen?

Interesting question. There may be a recommended charge voltage, say 3.65, and an absolute maximum voltage, which is where you could allow regen to approach, or even reach.

What Weber and I have to decide soon is what to do with Sky Energy SE cells, which specify 3.60 V as the maximum. Does that mean we should balance at 3.55 V and stop regen at 3.6 V?

I suspect it will be fine to balance at 3.60 V and regen up to some higher figure, perhaps 3.8 V or 4.0 V. But will EV Components honour the warranty if the cells occasionally see 3.8 V?

Hopefully some experiments will provide some clues.
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Post by Goombi »

The consumption is much greater then the regen input you will never overcharge your batteries besides most batteries are smart they will NOT let you.
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Post by Tritium_James »

Goombi with an AC drivetrain and lithium batteries the round-trip regen efficiency is about 70%, not 10%, so it's certainly possible to exceed maximum voltage on the pack during regen.
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Post by Goombi »

Tritium_James,
One has to ask a question what is causing all the heat that could melt the wiring??
Once a electric motor gets hot it does not use anymore energy because of the motors heat it uses is the same. If the motor is oveloaded then you will create heat if its oveloaded for short time then it will get hot -but still operate-- if you run the motor on overlead mode constantly then you will burn the motor. Cooling a electric motor is not the solution (has anyone seen or heard of electric motor being cooled by water of refrigerant gas etc? The suggestion is quite ludicrous. Any motor that gets very hot should be replaced by bigger capacity unit. Or reduce the demand on the motor.
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Post by Goombi »

Tritium_James,
Wherer did you get these figures? 70%?
The only way you could possibly ( but not likely) overcharge your batteries if you were to live on Mt Fuji and work in the gully below is this the case?
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Post by Electrocycle »

there are many water cooled electric motors, especially in "production" EVs.

For continuous operation (industrial), water cooling doesn't have much of an advantage over air cooling - but in EV use water cooling allows the motor to be overdriven by a much larger margin for short periods.


Mal lives at the top of a hill and his battery pack will go over voltage if he uses too much regen immediately after leaving home with a full charge.
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