Tuesday, 30 June 2015

Battery charging circuitry part 1

Because the battery voltages in the e-bike are going to be around the 44-50V range, buying a charger/power supply to charge this thing is likely to be expensive.* So after consulting my semi-partner in crime Jared at work, we came up with an idea to upon connection, automagically "break" the battery pack in half for charging. This is explained below...

On the left hand side is the normal configuration of the battery pack when riding the bike. Each one of the battery symbols represents a "pack" of 18 batteries, in 6S3P configuration (a series string of 6 packs, in parallel 3 times). These two packs are connected in series, combining for a total voltage of ~50V. The right hand side shows the charging configuration. Pack one and pack two are now in parallel, effectively halving the required charge voltage to ~25V.

So how to achieve this?

Shown above is my switching scheme, using two single pole, double throw relays (SW1 and SW2). The diagram shows the relays in their "normal" mode, connecting the two packs in series. When energised, the relays will swing to their alternate position, connecting the packs in parallel.

I went ahead and bought a couple of relays from element14.com. I chose Durakool automotive relays (DG85B-8011-76-1012-DR), because can handle the currents and voltages I expect to pump into the motor.  After they arrived, I connected them up to a power supply and a scope, and tested the design.

I simulated two battery packs using a dual-channel benchtop power supply at work. This way I could set the current limits really low, just in case I stuffed something up. As can be seen below, this likely happened multiple times.

The relays connected up with alligator clips.
Sure enough, the relay circuit appeared to work in principle. The packs were reconfiguring from series to parallel on energising the relays, and reverting back again upon disconnecting the power.

Jared warned me about current spikes across the batteries when disconnecting the relays. These come as a result of the coils in the relays wanting to discharge upon the rapid change in current during disconnection. These spikes could potentially damage the battery packs, so I needed to quantify the effect, in order to determine the scale of protection necessary for mitigation.

I connected a scope across the battery packs, and activated and deactivated the relays. As can be seen, a spike of ~4V is present during the switching, though not for long. This will most likely be dealt with by fitting 25V zener diodes across the packs. More on that, along with the rest of the switching circuit, later.

*This statement is not based on any research/facts. Look, I just wanna play around with relays alright?

Sunday, 14 June 2015

Bike frame

Behold, the Leichtsport "Senori". This will provide the skeleton for the build. One of the main requirements for the bike build was not having to need to change clothes to ride the thing. Hence, I made sure the bike had a step-through frame, and a chain guard allowing Jenna to be able to ride in a dress. It also doesn't look too dorky (unless I ride it), which was another requirement.

One thing I'm a little concerned about with the bike is the tyre size. Printed on the tyres is:
28x1-3/8x1-5/8. According to http://biketiresize.com/ this is the same as 700 x 35C - but I'm not going to believe it till some new tyres are fitted.

The frame is a bit rustier than what it looks like in the picture (and the picture I saw on Trademe) - so I may end up completely stripping the frame and re-painting it.  However all this stuff will happen after I've installed the motor and gotten things running.

Tuesday, 9 June 2015

Battery testing

Here are the 42 battery cells I'm re-homing from the scrap pile at work. They're 4,400 mAh, 3.7V, Li-Ion batteries, with a protection circuit built in. They're not the fanciest battery in the world, but at the price I got them at (free), they're pretty unbeatable!

The first thing to do was to find out the maximum current they could put out. The protection circuit in these units effectively limits the amount of current the battery can discharge. The reason being, if these things let out too much current, they will heat up, and become quite dangerous. The downside is the current limiting circuit chokes the maximum power limit of the bike.

According to the battery datasheet, the maximum discharge current is 2.4~4.5A. I decided I better test this out, and see how much juice I could pull out of these suckers. I bought some high current load resistors from Trademe for a few bucks, connected them to a few batteries, and started measuring the current with a multi-meter, and an oscilloscope.

High current resistors (3 and 5 ohm)

Turns out the current limit kicks in at 3.6A. Now I understand the current limit of these things, I can go ahead and tailor the battery pack for the power output I want. I could potentially disassemble the battery pack, and remove the protection circuit, but as the pack will be sitting quite close to my girlfriend's butt, I figure I'll leave it connected for now, until I understand the system better.

Battery specs:

Manufacturer: Golden Cel Battery Co. Ltd.
Model: CELALB18650-PCM
Max discharge current output: 3.6A
Nominal voltage: 3.7V
Capacity: 4400mAh

Saturday, 6 June 2015

Jenna's bike

It's been awhile since I've posted anything on here. Been too darn busy releasing records with my pals in Proton Beast and Anthonie Tonnon and the Successors. Somewhere in there I also built a TR-808 drum machine, but since it was just from a kit I didn't think it was blog-worthy.

Anyways in other news, I've decided my next project is putting together an E-bike for my girlfriend Jenna. It's worthwhile me doing it, as I can get some of the more expensive bits (i.e. the battery) for free from work, and liberally plunder the senior dudes for advice.

In terms of requirements, the bike has to:

  • be really easy to ride;
  • not look dorky;
  • not require a change of clothes;
  • be breakdown-proof;
  • be able to carry a reasonable amount of cargo;
  • be powerful enough to deal with Auckland hills; and
  • have a range long enough for a return trip to work plus some.
Anyways, I'll try to post my progress up here, as the project moves along.