Regenerative braking, MP3 with internal controller

[ingesa]

Because I have hydraulic disk brakes, I didn't connect the brake handles with regen switches. But now I just connected a separate switch for it. This works fine, the motor brakes well. But looking at my cycle analyst I see that the current is only going into the battery for about 1 second, then the amp goes to zero, but the motor continues to brake. Anyone else experienced this? I had only used about 1 Ah of the battery (GM 48V 10Ah), so maybe the battery BMS kicked in. I will test it again tomorrow with lower state of charge. But why does the motor brake even though no current is going into the battery? Where does the energy go, into heat in the phase wires and controller?

[Kris]

I'm getting decent regen currents with the internal controller SP.

Most battery pack BMS have a separate charge input so the BMS can't switch off motor regen currents coming in on the discharge circuit. They will isolate the discharge path on low voltage or over current but the over voltage function seems limited to the charge path.

[ingesa]

Thank you for answering. Yes, the battery has a separate charge input, so the BMS should not be causing this. Is it possible that the controller only direct the current in a loop through the phase wires, and not to the battery while braking? If this is the case, the motor should be rather hot after braking down a hill..

Inge

[Kris]

Some early controllers simply shorted the motor output apparently, so called dynamic breaking? There is a lot of iron in the SP so I don't expect to see much of a temperature rise with regen since the energies involved are not that great. You will only ever recover 10% - 15% max of your battery pack capacity through regen.

[ingesa]

The controller is supposed to have regenerative braking, I think. I bought it last summer, so it's not that old. It also works for about one second, I see -300 to -700W before it goes to zero. But I don't have a lot of trust in these controllers, I had to change it two times. The others just didn't work. Nothing burnt or anything wrong to see on them..

I agree that it's usually not much to gain using regen braking. But I use my bike in a quite hilly area of Norway, can be 300 m or more up in altitude, and 300 m down again. In these long downhills I think it should be possible to regenerate quite a lot, given that the generator efficiency is about the same as the motor efficiency. After a trip up a mountain (500 m up in altitude), I did a check on how much energy was used compared to the theoretical minimum, the energy necessary to lift the bike + me (about 100 kg) 500 m up. I was suppriced that even though I moved 6 km with an average speed of 25 km/h (with the friction this represent), the energy used was less than twice the theoretical min. (I did pedal a little bit, but I think that was negligible).

Energy used: 5 Ah * 48 V = 240 Wh = 864 kJ
Theoretical min: 100 kg *9,81 m/s2 * 500 m = 490 kJ

Say that if moving slowly downward (minimize air resistance) it is realistic to regenerate 60% of the potential energy. This would be 294 kJ, or 34% of the energy used on the way up. Then I think regenerative braking would be worth it.

[Morgen 3Eman]

Hi Kris,


Can you tell us what sensor 2 is measuring?  The graph has no labels to tell us what is being shown.  I assume one axis is time, but what are the units, and where is the measurement being made? 

TTFN,
Dennis

[Kris]

Those figures make sense Ingesa. If you use  roughly twice the theoretical limit going up then it is reasonable to assume you would loose half the 490 kJ of stored potential energy coming back down. Thus you might expect to regen 245kJ (68Wh)?

Apologies Morgan that graph is battery current, so the -6A at 7:59pm is regneration current going down heart break hill & the positive spike just after is motoring back up again.