18 July, 2015

DIY: Turn your bike into e-bike for yourself - Step 5/5: Final details: Active Braking, charge control and legality

This wounded wire is my brake;)
After a few months of using battery-powered bike, I am going to comment how it feels. On the one hand it does very well with the uphills, but the downhills are another thing. In my neighborhood we have slopes with between 8 and 10% drop, and the bike does not stop fast enough with his brakes (disk brake only in the back). I have found that its weight (about 30 kg) is a major handicap, and although putting many banks in each cell increases the li-ion battery life and autonomy, it makes very difficult to go upstairs with it (impossible to carry on daily to an apartment), or braking, the brakes suffer a lot (especially at low speeds when the engine brake don't work).
Although we could use engine braking to stop and derive the energy to the battery, but you can not raise too much the regeneration (engine braking); above 15 amperes (1,5C) batteries suffer much from excessive current (even without reaching 4.2V per cell), and they degrade faster, besides the Golden Motor control is proportional to speed (recovers more amperes the higher the speed), so we could be sending a damaging 40A to the batteries if the slope is steep and we go fast.
Another option is to put LiFePo4 batteries, much more tolerant, but they have less energy capacity per kg, in addition to its high prices. The A123 for example accept for each 26650 cell type up to 10A continuous recharge. For this moment I can' afford them (although they come equally profitable having more recharging cycles).
After investigating, found a interesting solution  about using a resistance as a powerful engine braking (what  a great well of wisdom is Endless Sphere about electric behicles), ¡and they have proved that it works wonderfully!:

Example of resistance brake on a bike from a friend of Endless

Using a motor phase as active brake

If we derive by a relay a motor phase of the engine power to a resistance induction, we get the cheapest and more effective braking system of all (with consequent savings). 
We only need the following materials:

Relay, high temperature tape and thread copper 0.75 to 1.5 mm. Indispensable!
And of course welder, tin, 2.5mm cable, etc.
There are about 5 meters to wind  wire about 1 mm. in diameter (too thin and we could melt it); the length depends on how strong we want to have it, and with patience, taut it so that it is well in contact with the metal to transmit heat (I have left spaced by 2 millimeters to better distribute the heat for the picture, but not but more because the magnetic fields could be too far apart), and I think that works induced (impedance) between different laps fields are locked together at high frequencies (what would work best if the sheath is made ​​of steel), leading the electricity as heat (heat only the cables appear where it turns), but do not take me very seriously, I could be wrong and it could be that most of the heat is caused by a simple short-circuit:

And so it is finished; I used high temperature tape of polyamide to hold it in place and to assure that it will not melt with the heat (this tape holds up to 500 °C):

Then I connected to the active side of the relay, I used one of solid state, that must be for AC (alternate current) like mine (current phase is changing everytime in an engine of this type):

I welded it to motor phase (green and yellow) peeling some cables and soldering them

You can see the detail of copper wire soldered; the red and black wires can withstand high temperatures (later I realized that with a normal 2.5 mm would be more than enough):

I've connected the input relay only to the rear brake, taking current from the last cell of the battery (it only need 3 to 32V to activate), consumption is ridiculous therefore not significantly discharge cell (activated with a few mA).

This way, half of the energy is sent back to batteries and the other half converted into heat; thus the front brake is to activate regeneration battery, and we can raise the percentage of regeneration control 70% as when braking with both brakes, and there is no danger of overloading batteries (we have to activate both brakes whenever we need powerfull brake).
The best thing about this system is that braking is proportional to the speed; higher intensity with more speed braking (motor absorbs more energy); the bike is more manageable and secure.
The only drawback of using only one phase is the "brrrrrrr" sound is makes as we activate only half of he coils, especially audible if we go slowly, it seems that something in the engine will break, but nothing further from reality. This is smoothed by using both brakes and from 10 kms per hour and more.
Another option is to connect each phase to resistance, with two independent relays, activating each brake with each of them, thus there is no energy recovery but braking capacity would be spectacular.

The connections terminated (just left isolating some joints with heat tape and properly secure the relay)
The relay is not necessary to join to a heat dissipating sheet unless we plan to use much engine braking; I tried to go down a 500 mts long slop with 50 mts drop and the resistance had reached 40ÂșC, but the relay 100A 380V barely warmed. If you use one of less amperage would need to attach some metal to dissipate temperature.

It is cooled very quickly when in contact with aluminum sheath

Viewing battery charge (per cell)

On the other hand it is important not to overload or download too much battery (always between a healthy 3,1V and 4,1V); I found inexpensive viewers 10 cell voltage, so I put a small monitor up to 8 cells, leaving monitor the first two, which were better than the last two:

Voltage monitor  cell; important to know the status of each pack
I found other more economical monitors for 8 cells but they are usefull only as configurable low voltage alarm:

What Jacopo did was to take advantage of the port monitor alarm 8S to cut the flow of current, protecting the battery.

However, after months of having exposed the Cell-Log 8M to ther weather, the thin plastic that covers the screen and polarizes light to make visible the data was being damaged quickly.

With rain and sun plastic polarizer was damaging to disappear
So finally the data could not be seen more. I searched for a fix. I looked between broken devices with other liquid crystal display (and found an old broken Wattmeter), from which I extracted this part, 

and I tried to invert tones; I placed it in the position that interested me, with data in black on white.

Once trimmed and fixed with a few drops of epoxy glue in each corner:

The display comes to life:

Bike legality

In Europe we have to comply with these strict legal conditions:
  1. Real output Power 250W maximum per bike (very low in my opinion if you live in a mountainous area).
  2. Activated only by pedaling (I wonder what danger would be to have only 250W...).
  3. The engine should only help to reach 25 kms per hour (reasonable).
  4. The engine must be deactivated if we brake (logical).

Power limitation seems unfair; if you have to overcome a deep slope it is very low; This should be limited depending on the slope; in plain 250W and 500W in uphills (as I have), it is simple to do with an electronic gyroscope.
And engine powering only by pedaling... this is silly, it's almost more dangerous that powering only with handle on the flat, 'cause of the danger of losing your balance by moving the legs.
As for the rest ... it is reasonable, but must also amend the law to include maximum braking distance. You can not pretend to stop a bike and its occupant from 25 km / hour to 0 with simple brakes as I've seen some ...

More info and sources:
Different configurations of engine braking in an electric bike 
To make the bike legal
Changing the battery A123 26650 A123 Systems 20Ah LiFePO4 battery Discharging Test

No comments :

Post a Comment