Turning a perfectly good sailboat into an engineering project
[scroll down for pictures]
Hi everyone, it’s Joe. I’m overdue for an update on how things are going at Saleae with regards to the new product and all that. However before I do that I thought it would be fun to bring people up to speed on the boat.
The boat is a 30-foot, 1983 Catalina 30 named Kathleen. We got a good survey and learned all about the boat and created a very long list of things to fix or bring up to code, etc. Most of the items are now complete. There are still a few significant ones that will get done later this year, such as replacing the lifelines and lightning protection.
Old Atomic 4 Gas Engine
A few weeks ago we took our perfectly functioning gasoline engine out of the boat and sold it for a song on craigslist. So this is a bit reckless, or foolhardy or in any event risky – now we have a non-useable project boat sitting in the marina. Why ever would someone do this?
Do not fear we are replacing that ancient, corroding, pollution spewing dinosaur with a nice quiet and clean electric system. A few things of note: this carbureted engine (Atomic 4) has no pollution controls whatsoever. The engine sucks up seawater, mixes it with the exhaust, and spews this into the bay. Any oil or other muck that comes off the engine winds up in the bilge and gets pumped overboard – although to be fair the oil-absorbing pads you put down there do help somewhat. You are supposed to carry all sorts of spare parts because at any time the engine could break down. To top it off, gasoline fumes are heavier than air and you can actually blow up the boat if you have a gas leak and start up the boat without sniffing first. Apparently, fiberglass is actually a fuel at high temperatures and if ignited the entire boat can burn to the waterline in under a minute. Honestly I’m not really THAT against gas motors in boats, but this particular one had to go.
New Electric Motor
The new electric motor is the ME0201014201 from Motenergy. [From the manufacturer] “This is a 3-phase, Y-connected Permanent Magnet Synchronous Motor with an axial air gap.” I’m not all that up to speed on how AC brushless motors work but apparently they are pretty cool. It weights 22lbs.
I’m not sure how heavy the old one was, but something on the order of 400 or 500 lbs. Removing it was quite the undertaking as you might imagine – involved removing several parts of the boat, and rigging up the boom as a crane, and lots of pulleys for mechanical advantage. Mark, James and I were able to get it without harming it or the boat, but it wasn’t easy or without risk. I have a feeling if we had managed to drop the motor from say 10 feet up or so it would have punched a hole in the boat and it would have sunk right there in the slip, haha.
CAD & Motor Mount
Mark and I did our best to CAD up the inside of the motor compartment so that we could design up a new motor mount. Unfortunately the geometry down there is all curved and irregular so there’s really no getting much better than an inch or two of precision. So when I designed the motor mount I decided to laser cut it out to fit check first – which my friend Jason kindly did for me.
Just last week I sent in the final (for now) version which was water jetted out of ¼” aluminum plate by http://www.bigbluesaw.com/.
**Motor Controller **
We also got a Sevcon Gen4 motor controller for the motor. This is a very professional controller. The downside is that it’s complicated as hell. Every last thing can be configured over CAN – it’s more of a business-to-business product. Nevertheless the resellers will program up the basic profile for your motor and you can control it with an analog input and two digital inputs. Mark and I really wanted to control it strictly over CAN but it’s looking like that will take more effort than it’s worth at this point. We did get a USB-to-CAN (VScom USB-CAN) dongle and got it working – although the existing SDKs, etc. were pretty useless, at least for C# development so we ended up rolling our own interface layers for it. In fact according to mark that USB can controller just uses an off-the-shelf CAN IC from Phillips.
For cost per kWH at first it looked like lead acid was really the only contender. But it’s pretty terrible – the internal resistance just kills the performance and you have to seriously downgrade the amp hour rating to accommodate this. Lithium ion of course does not have this issue. There are several companies that make lithium ion batteries that are literally drop in replacement for lead acid – really awesome stuff, like http://www.mastervolt.us/marine/products/li-ion/mli-24-160/ This battery is 4.3kHh – about what I’m looking for – and is super easy to use – but tragically costs well over $5,000. This is many times the cost of lead acid. However, I hated the idea of going lead acid so I kept looking around and started finding some extremely good deals on lithium ion, in particular ones from Sinopoly. What’s really cool about these things is that no matter what size you get, you get the same cost per Wh. That means you can start with a small pack, and then grow the pack over time by adding more cells in parallel. So I got x15 40Ah cells at 3.2V nominal – 48V and about 1.9kWh – for about $700. http://evolveelectrics.com/Sinopoly.html — these guys were quite helpful.
Now, the down side of these cells is that you’re totally on your own managing them. You need a charger and a battery management system (BMS). And everything out there in this category is pretty mom and pop – not a lot of stuff that I was excited about. I did end up getting a massive charger from Elcon (the 1500) which I’m not exactly thrilled about (I really want a way to set up the charge profiles myself because I’ll be changing the battery capacity in the future). In addition I got what seems to be a pretty cool solar charger from Genasun called GV-Boostlithium. Haven’t tried that out either. In fact, what I have done is individually charged each of the batteries with one of those awesome hobby chargers that Sparkfun sells — https://www.sparkfun.com/products/10473. If only there was a charger/balancer that good that could charge a 15-cell system at 40A…
The other thing you need besides a charger is a battery management system. This does a couple things – if anything goes haywire, it disconnects stuff so you don’t blow anything up. This includes master control over whether or not the charger is connected, and if any loads are connected. It also keeps tabs on the “state of charge” – monitoring the energy in and out of the pack so you have some idea of how much juice is left. Perhaps most importantly it balances the pack. Here’s how that works:
Battery balancing 101
The cheap way to charge lots of batteries at the same time – assuming those batteries are in series – is to just put a big voltage across the entire set and start pumping in coulombs. There’s two basic steps: first control the voltage such that you get a specific current (current control) – typically .3C (30% of the amh-hour rating per hour). For a 40Ah that means 12A. You do this until the voltage rises to some threshold, such as 3.6V per cell, but it depends on the battery chemistry. My 48V nominal pack would be at a voltage of 54V for instance. Then the charger goes two mode two: Maintain exactly that voltage per cell – reduce the current slowly so that the voltage does not rise. Eventually the current reaches zero (or near enough) and charging is done. This is why the first part of charging is much faster than the last 20% or so.
Here’s the problem: the cheap charger can’t actually control the voltage across each cell. It can only control the voltage across the entire pack. That’s where the “balancer” comes in. (Typically the balancer is a function of the BMS).
Each cell has a slightly different impedance, and over time will become more or less charged than other cells in the string. This is a problem because more charged cells will have higher voltages when being charged – so the charger may think all cells are getting exactly 3.6V, but in reality some cells are lower than that, and some cells are higher; the more unbalanced the pack becomes, the more likely it is that the highest voltage cells will be over-charged and ultimately blow up.
The balancer typically discharges over-charged cells. Literally, it switches in a power resistor and bleeds off power. There are a number of strategies to do this, and generally speaking it’s all fairly inelegant and inefficient. What I have in mind is to dynamically control the impedance of individual cells using a mosfet and power resistor – I think this could work fairly seamlessly without needing to stop charging to do the balancing (this would assume the pack was already fairly balanced). With unlimited money, the best way would be to have a dedicated, electrically isolated charger for each cell. Honestly, the cheapest way is to undercharge the pack (to be safe), then manually go around with a volt meter, power resistor, and kitchen timer and discharge cells that were getting overcharged. Once the pack was nice and balanced, you could charge it all the way up. The pack only slowly becomes unbalanced, so I think you technically could get away with this.
Of course I just had to roll my own BMS. Which explains why it is not done yet. I did design one up and got it built but it’s just a proof of concept at the moment, and I’ll need to spin at least one more board.
**2400W AC Inverter **
The boat will also have AC power of course, enough AC power in fact to run a microwave, or pressure washer, or on-demand hot water heater, or whatever else you might think up. To provide this I got a really inexpensive 2400W 48V DC to AC Inverter on Amazon for ~$210. The one I really would have trusted was this unit from Tripp Lite but as it costs a hefty $690 it seemed worth it to give the cheaper one a shot. I opened it up, and actually I’m impressed with it. They got all the cost out of it and yet it seems pretty efficient and the quiescent draw is very low. We’ll see how long it is until it blows up. It certainly won’t stand the marine environment, but that’s another blog post.
The boat naturally also needs a computer, and I just so happened to have an incredibly slow and all-around terrible netbook which was donated to me from a friend. I maxed out the ram to a whopping 2GB, and removed every last background application and it actually works rather well – assuming you only open one thing at once. It’ll run C# nicely.
When all this is done, we’re talking autonomous waypoint navigation, and drive-by-Wii-nun-chunk, among much else. Stay tuned.