Tuesday, January 15, 2013

Distributed Charger and First Pack Charge

Jan 15, 2013 - Its been 8 months since my last post. My job at JPMorgan has kept me pretty busy the last 6 months leaving me drained when I got home. I was able to get a good bit accomplished over Christmas vacation though.

One of the major accomplishments was getting the batteries wired up to the charging socket. Soldering twenty 16gauge wires onto an area about a square inch was fairly challenging and using a lighted magnifying glass was extremely helpful.

Another challenge was the actual placement of the socket. I had initially considered placing it on the left front fender well but felt uncomfortable placing it there with the twenty wires exposed even in some sort of box. Since the gas tank and filler were in the front trunk area and to fill the tank, one had to actually open it up, I figured why not open it up to charge her up as well. So here's the socket attached to one of the crosswall supports:

Here's another shot:

Another one with a three-quarter perspective view:

And yet another one even further out, so you can get a better idea of how its mounted where:

Here's another one with the pigtail:

And yet another one with the pigtail mated to the socket:

An explanation of the concept of the distributed charger is probably warranted here. As I had originally intended to use lead-acid deep-cycle golf-cart batteries, I had always intended on using some form of distributed charging, meaning each pair of 6V batteries would have their own 12V charger. The concept of individual chargers would keep the batteries better balanced. But as Lithium-Ion batteries were coming into the mainstream, I decided to pop for them and forgo the weight of the lead acids. Mark Hazen of EVHelp.com LLC was, at the time, manufacturing his own patent-pending controllers and distributed chargers.

Mark used TK-Lambda SWS300-15 single output general purpose power supplies to power his charger. The SWS300-15 is about $142/supply from digikey and he incorporated 10 of them into my charger with room for 2 more if I need to add them. Here's a shot of the charger mounted on the wall near the car and the breaker panel. Notice the electrical outlet on the left dedicated to the charger. Each pair of outlets is wired to its own 30A breaker as each bank of 5 chargers will initially draw about 2000W.

Here's a shot of the charger with the panel door open exposing the 10 individual power supplies:

Here's a closeup shot of the 5th charger with the current sensing circuit attached:

Now the current sensing circuit is needed for the Lithium batteries as the manufacturer recommends cutting off the charger once the current in the constant voltage phase of the charging profile reaches 0.05C of the 200AH batteries, which is 10A.

I charged the batteries for the first time Sunday nite for about 6 hours before I shut it off and went to bed. Not that I don't trust the charger, I'd just rather err on the side of caution. I adjusted the initial charge voltage to 14.6V (3.65 * 4) while the batteries were charging. I also e-mailed Mark and asked how the charger knew when to switch from CC to CV. Each battery is comprised of 4 lithuium cells.

The next day I received an e-mail from Mark instructing me to adjust the charging voltage with the batteries disconnected. When I turned the charger on, I noticed that the no-load voltages were around 15.1V or 3.8V/cell well under Thundersky's recommended 4.25V/cell charging voltage. I then adjusted the no-load voltage to around 14.6V on each power supply or about 3.65V/cell. I then turned the charger off, connected it to the vehicle, and turned it back on. Within 5 minutes, the charger then automatically shut off as it had reached the 10A constant voltage current. Apparently, the batteries were only at about 50% depth of discharge and only needed 4-6 hours of charging to get them back to full charge. This was their first charging since I acquired them almost two years ago. Here's an even closer shot of the current sensing circuit. I would post the the actual circuit diagram but Mark requested that I didn't as it his proprietary design.

The charger also comes equppied with a built-in voltmeter/ammeter and a rotary selector switch with 12 positions. The voltmeter is wired directly to one of the AC plugs so I can plug in the connector and use the voltmeter without powering on the charger.

All in all a very cool setup. Mark did an outstanding job on the design and construction of the charger. The cutoff circuit was my idea but his design.

In my next post, I'll show some of the progress I've made on my battery monitoring system. Until then, caio.