My Bespoke Battery Bank

This is literally how the universe reacted when my plans were formalized.

Reality Intrudes

Things started spinning out of control with the delivery of the thirty-two cells that we’d ordered from a local supplier in Australia. We’d decided against ordering directly from China, since we weren’t thrilled with the idea of wiring thousands of dollars off sight unseen to a business there, then taking the delivery and importation on ourselves. So we went with a local vendor, Lithium Power. I link to them because I think that Peter, who runs the company, did a fine job and don’t blame him for the delays.

But the delays happened. There was confusion about where the batteries where, what ship they were one. What routes they were taking to get from China to Australia. But was important, at the end of the day, is that poor Will beat the batteries here by several weeks. So much for fun in the sun and two months of French food.

The exact date of the battery delivery remained fuzzy. We were trying to pin it down to within a week, so we could let the marina know when to expect us. We didn’t want to be late and have the batteries show up without us. But we didn’t want to show up too early, because the Slip Fairy wasn’t going to be leaving any free nights in the marina under our pillows.

We ended up spending a weekend without batteries in the marina, but I took that time to remove the old batteries, start poring over the plans and manuals, and figure out what was going to work and how it was going to go in.

Ordering Up a Storm

Preparatory to the project, I’d had to design the system, which included deciding how much power I wanted (how many cells, what size), how I was going to charge it (we needed more capacity), how we were going to secure the batteries to the boat, and how I was going to keep myself from accidently destroying the batteries once I finished the installation (pick out a Battery Management System).

Don’t worry, I’ll make a nice little table of all this stuff at the end.

“Batteries” and “Cells”

Back to the truly “Bespoke” angle of this project, I had to configure the batteries. When you buy a twelve volt battery for your car, it is in fact a group of six “cells”, each having a “nominal voltage” around 2 volts. Stick six together, and you get a 12V “battery” which is really nothing more than a bunch of cells organized to get a target voltage for your application. Don’t believe me? Go look under the hood of your car…pop open the water fill caps on your battery. There are six….one for each individual cell.

LiFePO4 cells have a Nominal Voltage of around 3 Volts. So instead of sticking six in a group to make 12V, you stick four. In our case, you eight in series to get to 24V. Then have to decide on the capcity – how much power do you want, measured in Amp-Hours.

Be forewarned – I will frequently and sloppily, without regards to your state of confusion, use the terms “batteries” and “cells” interchangeably. Don’t worry…all the math works out the same.

Parallel vs. Serial

Batteries are connected in two acceptable ways that don’t make sparks and smoke. Parallel – connecting the positive and negative poles to each other, or Series, where the positive pole of one battery is connected to the negative of the next and so on, daisy chaining the batteries from positive to negative poles.

The main difference is that connecting them in Parallel increases the capacity, but does not change the voltage. Batteries in series add their voltage, but capacity does not change. Two 200 Amp-hour (Ah – one unit of capacity for battery storage) 12V batteries connected in parallel makes one big 400 Amp-hour battery. If you take those same 200 Ah 12V batteries and wire them in Serial, you will end up with one 24V battery with 200 Ah of capacity.

So when I say I have to “configure” my cells into batteries, what I am talking about is how they will be wired in order that I get the right capcity AND the right voltage.  You end up with this:

Ignore all those other wires, doohickeys and doo-dads you see for now. I’ll get back to them. You will see this picture again.

What you are seeing there is groups of four cells wired in parallel You can see the bars running between each positive and negative post in the group. Each individual cell has 180Ah of capacity, so each little group is one 3V cell with 720Ah of power.

Then, all eight groups are run in series to bring the voltage up to 24V. If you look at the first group (lower right corner) you will see a fat wire running from the positive terminal of cell 1-4 to the negative terminal of cell 2-1. Then you will see a wire from 2-4 Postive running to 3-1 Negative. And so on – this makes all of these connecting into what whacking great 24V battery.

Once I’d sorted out how many cells I needed, all I had to do then was order them!

The Safety Net

I’ve described the general functions of the Battery Management System in the last post; I won’t belabor it here. Suffice it to say, it is a means to idiot proof to the batteries, in order to keep the idiot in the equation (me) from over charging or over draining the batteries and destroying them. We settled on the Emus BMS, made by Elektromotus, an Italian company.

But you need more than the BMS, the BMS is just the brain, and the eyes and ears monitoring the batteries. It needs hands to turn the batteries and chargers on and off, as well as switches, power supplies, wiring, and so on. Using a series of Relays (electromechanical switches that can be controlled by another circuit) and Contactors (pretty much like relays, but with a fancy name because they can handle much more power without melting), the BMS can enable it’s safety features, like turning the boat batteries off in the middle of a movie because it lose the signal from a cell monitor…

So all this, in my view, is part of the BMS even if it isn’t sourced from Elektromotus. They just provide the brain, and some of the monitors and sensors.

Strapping it Down

Vibration and motion is a battery killer. So LFP cells have to be secured so there is as little of each as possible. In addition, since we’re on a boat, there is a non-zero chance that we might roll over. Having all the batteries fall out of the boat onto the ceiling in the event of a knockdown really doesn’t increase your odds of recovering and finishing the passage. So you need something tough.

We ordered some battery frames from the provider of the BMS. They weren’t what we expected and caused some troubles and delays, but I won’t get into that now.

Charging it Up

One of the big drivers in doing this project was cutting down out charging and generstor run time. One way to do this is to increase your charging capacity.

The other consideration for chargers is that LFP batteries are fickle beasts. They don’t match all the charging rules for Lead-Acid batteries and need to be handled differently. Our old chargers were not going to get it done.

Going to a new high-tech battery with a smart BMS, getting chargers that the BMS could control directly seemed a sensible option. I may come to regret that (more later on that one, too), but the idea that the system responsible for protecting the batteries from over charging could actually control the battery chargers directly seems pretty sound. So we ended up special ording some smart chargers from the states that we couldn’t buy in Australia.

Bits, parts, tools, and wires

Above were the major and easily identifiable components. But when the time came to install all this stuff, there would be need for a constant supply wiring, new tools, a bench power source, and many other items. I’m not sure it bears going into, with a few exceptions (TOOLZILLA!!!). But there have been almost daily trips to various vendors for nuts, bolts, screws, wires, terminators, tools, fuse blocks, switches, and so on.

There were also a few other semi-major components to get. Once I removed the old batteries we would lose all 24V power on the boat until the new batteries were ready to go. Not such a great idea, as our refrigeration, house lights, computers and so on work off of battery power. In a slip we plug into shore power, but that still just runs a battery charger for most applications. The boat runs on the batteries for it’s core systems. So I needed to replace that with a power supply. When the batteries arrived, I needed to “balance” the cells with a precise power supply. Another tool…hopefully one we can sell to the next guy trying this project.

Putting it together

There were a lot of things to order.