e-bike: battery build, part 2 of 3

Moving on to the BMS (Battery Management System), here's the top-side of the BMS that came with my kit.

And the wiring diagram that EV Assemble ( http://myworld.ebay.ca/evassemble ) emailed me when the kit shipped from China.

(Diagram is copyright EV Assemble and is shown here purely for instructional purposes.)

And these are the connectors that came with the kit, and that were attached to the charger.

I didn't like them.  So I decided to modify all of the connectors that came with the kit, starting with the seventeen sensor wires that would have to be connected to the battery pack.

The sensor wires pick up the characteristic voltage drop that occours when a lithium chemistry battery reaches it's full capacity charge.  When it senses that one of the batteries in the pack has reached capacity, the BMS will drain off some of the current that the charger is pumping into that battery in order to avoid over charging.

(At least that's my understanding of how it works.. please leave a comment to correct me if you know better.)

I picked up a pack of quick connectors at Sayal, drilled a small hole through the metal tang, and then trimmed off the plastic connector where a wire would normally be attached.  I did this because the wires for the sensors are far too small to work with the usual way of attaching these quick connectors.

Then I soldered one of these modified quick connectors to each of the sensor wires.

The next step was building the wiring harness that would connect the BMS to both the charging unit and to the load.. the e-bike.  After a bit of thought I decided to re-use the connectors from the bike's original SLA battery pack for the load part of the circuit, and have a separate connector for the charging part.

Since the load side socket is a standard 3 prong AC connector, I decided to cannibalize a monitor power cable to build the charge side connector.  I didn't want to have two 3 prong AC sockets on the battery pack's casing, as that might get confusing as to which socket was for charging and which was for the load.

Here's what my circuit diagram ended up looking like.

[ image removed ]

EDIT:  I've made an important update to the above diagram!  I had the polarity of the plug going into the e-bike backwards.

EDIT EDIT:  Okay... looks like I may have had the polarity right in the first place.  I've deleted the diagram and will be uploading a corrected version soon, once I've gone over everything and fixed it.

I picked up some Y shaped quick connectors and bolts to use for hooking it all together.  Here's what it looked like with the first wire put together and bolted to the BMS.

Unfortunately I didn't notice until too late that there is a bus wire between the two boards of the BMS which, due to where it's soldered to the pad, gets in the way of the nut used to attach one the wire to the "Out-" pad.  I took apart the two boards to get a look at the problem.. just had to undo the four screws and pull the boards apart, like so.  (I've put a circle around the problem area.)

Fortunately I was able to snip off a bit of the bus' wire and give the nut just enough space to fit.

Having the boards apart made it easier to bolt on all of the wires.

And with the sensor wires plugged in, here's my finished BMS, ready to be connected to the battery and tested.

First step in connecting the BMS to battery is to attach the sensor wires.  First I unplugged the sensor wires from the BMS again.  If they were left plugged in then the BMS would begin sensing the status of the batteries as I went along, attaching them, which would cause it to begin trying to balance the charges of the batteries.  If it tried to do this while only having a couple of batteries connected it could quickly burn out the BMS as it tries to balance charged batteries against batteries that it has no readings for at all.. the ones that have not yet been hooked up.

If you have the sensor wires plugged in to the BMS and you start hooking things up, you will see an LED on the BMS come on, some time around when you're hooking up the third sensor wire, and the BMS will begin to heat up.  If that happens remove the sensor wire you just connected, unplug the wires from the BMS, and carry on.

Once I had the sensor wires unplugged, I connected the "0" sensor wire to the negative end of the battery pack.

Then I flipped the pack over and connected the "1" wire to the negative end of the next cell in the pack.

Then flipped it back over again and connected the "2" wire to the negative end of the next cell.

And carried on along the cell like this, flipping it over and connecting the negative end of the next cell to the next sensor wire.  Be very careful that you don't accidentally cause a short while you are working with the bridging bars.

There we go, all the sensor wires hooked up.  Notice that I've got the battery pack sitting in an enamelled "butcher's tray".  I thought this was a reasonable safety precaution, just in case something shorted out and one of the batteries leaked or even caught fire.

At this point it's safe to plug the sensor wires into the BMS, so I did that, grabbed the power supply and took everything outside onto my concrete patio in order to do a test charge.  I didn't want to try the first charge indoors, just in case of catastrophic failure, leaking acid, flames, etc.

But it worked fine.  Here's the charger, with the Power LED and Charging LED both lit up.

And here's the battery pack and BMS, with an LED or three lit inside of the BMS, indicating that it's balancing the charging between the batteries.

So that's the rough build of the battery pack completed.  Since there were no flames or magic smoke escaping, I brought everything back inside and finished charging the pack.. which only took a few minutes since it seems they were shipped from China with a full charge.

Next step will be to do a load test to make sure the batteries work with the bike and to test their capacity.  So I'll have to partly re-assemble the bike, which is currently in various boxes in my work room.