Regen braking - how do batteries like it?

[Tiberius]

Hi all,

Just been doing some calcs on regen braking, and there is potentially quite a lot of energy that has to be handled. The short answer (and fairly intuitive really) is that you have to put back into the battery at the same sort of peak current as the battery will handle going out. The difference is that it is only for a few seconds.

How do the different battery chemistries like 30 or 40 A going into them?

Lead acid seems to take it ok as that's what automobiles do to them after starting the engine.

What about NiCd and NiMH? Is it just a reverse of the chemical process that generates forward current? What are the losses?

But with Lithium batteries, what does the BMS do when it sees a huge charging current?

Nick
(Apologies if this all been dealt with before)

 

[fechter]

I think heating is the main issue. Most batteries are rated for a higher discharge rate than charge rate. Hybrid cars that use Nimh seem to handle it fine as long as the maximum allowable voltage is not exceeded. Same for lead-acid and NiCd as far as I know.

For a breif braking period, I think they can handle a lot, but on a long hill you could possibly generate significant heat. Some kind of over voltage cutout would be a good idea.

Lithium is still kind of new. A123's can take a high charge rate, but they're spec'd at a lower rate than you can safely discharge them. The BMS on a duct tape or Ping type pack will definitely be unhappy with high charge rates, as the shunts will not be able to handle much current and there is a fairly wimpy FET that controls the charge current.

 

[Lessss]

I don't like regen braking becasue if you exceed the amperage the batteries can handle or even the fuse, your controller/bms cuts out the brakes altogether and your back to friction breaks. This can happen unexpectedly and if you are not prepared for it you will suddenly be moving forward faster then you thought until you can engage the alternative brakes.

Now if instead of cutting out it put power into a electric motor powered hydraulic break or lights or something to shed the excess power then it would be ok. As it is I don't trust regen braking.

 

[Tiberius]

Thanks, guys,

I'm beginning to wonder if its one of those things that are absolutely wonderful in theory, but founder on the practical details.

In my calcs, lowering myself down the hill near my house gave about the same total energy as stopping suddenly from speed. But one happens over a period and one in a few seconds. So its the stopping from speed that gives the current peaks.

Nick

 

[Lessss]

Good regen braking needs to have the option to use it or not. In other words it is only regen when you select regen if you are planning on slowly stopping as you approach a light or to slow down a bit on a hill. As the primary means of braking, until there is a way to shed excess power spikes, my opinion is = not good to have.

 

[drewjet]

I can only offer my real life experience with my motorcycle with lithium batteries. My maximum amperage on acceleration is 150 amps. The highest amperage input on regen is from top speed and is 30 amps and lasts only a few seconds. The amps drop as the speed bleeds off.
This is used in normal stopping, I live in Florida and have no long hills to comment on.

 

[fechter]

You got the regen working with the Kelly controller?

 

[deardancer3]

Hi all,

....
How do the different battery chemistries like 30 or 40 A going into them?
......
What about NiCd and NiMH? Is it just a reverse of the chemical process that generates forward current? What are the losses?

But with Lithium batteries, what does the BMS do when it sees a huge charging current?

Nick
(Apologies if this all been dealt with before)

From a user level, The bionx system handles regen to NIMH and Lith quite well. But I dont think bionx has a lot of current going in or out of their battery packs.

ON NIMH, i notice if I have a fresh well charged pack, the regen meter shows nothing going into the pack on downhill regen braking until the pack has used up some charge.

dick

 

[Lessss]

WEll the world of 150 Amps on a motor cycles is a far cry from 20 amps of a bicycle. My comments were directed to electice bicycles. P.A.B.'s.

 

[Toorbough ULL-Zeveigh]

NiMH are the worst for accepting charge while hot, check any RC forum for the cautions about letting your pack cool completely b4 charging & not running it more than twice in a day or else risk drastically shortening its life.
Also look at a typical NiMH charger 2-4 amps is the usual recommended rate without shortenting their life, with 6 amps the max if you're willing to trade life for shorter charge time because of the extra heat generated.
A small NiMH pack like on a ebike will get hot on an extended run of the motor & then you hit it with a high current charging spike with the ebrake, it's not a good combination.
A larger pack like in a car perhaps can handle it better that it doesn't heat up as much, idk.

But I'm fairly certain that regen shortened the life of my NiMH pack by half, considering how I otherwise babied my battery.
In particular the symptoms of how my ebrake acted up in the week prior & the pack finally giving up the ghost during braking while going rather slow on level ground in cool weather leaves me with no other conclusion.
The damage appears limited to a single cell that will no longer take a charge but there are about a half dozen others that have a very high self discharge & go totally flat to zero volts within a couple of days indicating a punctured separator consistent with overheating.
The remaining cells will at least hold a voltage even if left sitting for over a month so I'm presuming they will be ok for a time but I'll be replacing the rest while I'm at it.
To hopefully keep it from re-occuring I'm going to switch to either a plug brake or a small sacrificial parallel NiCd regen pack.

 

[drewjet]

You got the regen working with the Kelly controller?

Yes I did. It would appear the first controller was faulty. I thought I had updated my thread on that info.

WEll the world of 150 Amps on a motor cycles is a far cry from 20 amps of a bicycle. My comments were directed to electice bicycles. P.A.B.'s.

I realize that, however the ratio of 150 amps power to 30 amps regen or a 5:1 ratio is what is applicable.

 

[fechter]

Another thing to consider is due to losses in the system, the amount current you can charge with during regen will be substantially less than the current drawn during an acceleration of the same force. If the regen current gets too high things tend to break in the drivetrain too, so a modest regen limit will be desirable anyway.

 

[Tiberius]

Just done some further calcs and,

At 20 mph, the Kinetic Energy of a bike and rider is of the order of 1 Watt-hour (Wh) so that's the most you could hope to recover.
To travel at 20 mph that 1 Wh would last you for about 10 secs.

So you would be better off closing the throttle a few secs early and coasting up to the stop. The only time regen would gain you anything would be in an emergency stop. But in an emergency stop,
a) the current generated might be too high and you would need to dump it rather than put it in the battery,
b) you would want a mechanical backup anyway, and you would have operated that at the same time.

The other case is running down a hill dragging the brake to keep the speed down. That's much more suitable for regen, and I reckon you could get about 2 Wh for every 100 ft descent.

So, YMMV of course, but on a typical journey I make there is scope for regen to recover less than 10 Wh, out of an input of about 150 Wh.

Nick