Category: Electric propulsion

Some of you who follow this blog are interested in the technicalities of setting up an electric boat. This post is for you – it has details about testing the Lithium-Ion batteries to be sure how much capacity they have.

First some background – some pecularities of these LFP batteries mean they require different management than regular lead-acid batteries. As this is my primary energy source for the motor, and you could consider it as a fuel tank, I need to know how much fuel is in the tank. I need to know how full is the tank, and how much fuel does it hold.

These batteries are 200 Ah capacity and I want to run a test to prove that they do in fact have this much capacity.  That is, how much fuel does the tank hold.  I had run into a problem earlier, when a low voltage alarm warned me that the batteries are flat – that is, the tank is empty – after I’d used only 30 amphours.  But at that time I couldn’t be sure that the tank was full to begin with – or even close.

So for this test, the first step was to make sure the tank is full. You would think this is pretty simple, but this highlights another pecularity of LFP cells. Individual cells can have different charge/discharge rates – I’m not entirely sure why, but I expect its due to differing internal resistence.

Each cell has its own regulator – and here I’ll call them cell modules – and this cell module will protect the cell from over charging. When the cell reaches 3.84 volts, the cell module begins to bypass power – its just burns it off. When that happens a red LED light comes on. Here’s a photo of the cells in the workshop after fitting of the cell modules and wiring them together.

Anyway, to be sure that the tank is full, I need to see red on every cell.  So I charged the pack and gradually the cell modules began to light up red. It got to 13 of the 16 cells were red, and the other three remained stubbornly green. The cells are in a difficult location under the cockpit and its not easy to reach to each cell with the probes of a multimeter to measure the voltage.  So I ran wires to each cell and then back to a common point so I could easily measure the voltage of individual cells, and I could individually charge each cell.  Running these wires was quite a project – it took many days and I still have four to do.

I could now charge the individual cells and was able to do so on the three reluctant cells until each showed red.  So I had all the cells to red and was ready for a discharge test by early this week.

Luckily I scored a jetty on Monday – and being tied to a jetty is a good place to run the test.  So I ran the motor at 5 to 20 amps depending on the tidal flow at the time.  And I monitored the Link 10 battery monitor.  It shows cumulative amphours drained from the batteries.  I wanted to drain them to 80% discharge (160 amphours) to begin with, and then go lower while closely monitoring the voltage of individual cells.

Over two days I used up power and at 144 amphours the low voltage alarm went off. Bugger!

This is worrying.  I should easily have this much power. This highlights another pecularity of these cells. The voltage hardly drops at all for most of the disharge. It gradually creeps down – and then it falls off very suddenly to low voltage. So its difficult to monitor the available powet by watching voltage alone.

I checked the voltage of individual cells. All the cells that were attached to the common board – the easy ones to measure – were reading 3.2 volts – which means they are ok.  I had to crawl down under the cockpit to measure the other four – and I could see that on one of them, the light was off, meaning low voltage.  I measured it at 2.64 volts.  The alarm went off at 2.5 volts, and then after turning the motor off the voltage recovered to 2.64 in the 5 minutes it took me to measure the voltages.

What now? I’m consulting the supplier for advice. Perhaps because the batteries are new there’s some cycling or such that they need to ensure full capacity.  I hope so.  In the next few days I’ll be busy with other things and in that time the sun will begin to charge the batteries back up again.

But it looks now that the tank holds not even three quarters of its rated capacity – and that due to just one cell being low.

The Blue Mountains are delightful at this time of year and I was planning to spend another day there with Con and Di when I got a message to call Deb urgently…  And her message was call Miki, urgently – something’s happened to the boat but its ok.

There’s a flood in the Caboolture River, and Current Sunshine has broken off her mooring. Talking to Miki I can hear the water flowing through Monty’s boatyard. Every boat has dragged its anchor.

I’ll catch the next train train to Sydney – leaving in 20 minutes.  So Con takes me to the station and in another three hours I’m on a Qantas flight to Brisbane. Qantas let me fly on my cheap non-changeable ticket that was booked for the next day.

Back at the boatyard the level of the river has dropped and current is faster than usual but not racing. Steve shows us some film of the earlier pandemonium, and explains what happened.

Hoodwinked was the first to drag and she crashed right into the middle bow of Current Sunshine. Her bowsprit went right through the netting on the safety lines of Hoodwinked and locked on. The impact of the collision was enough to fracture the stainless steel swivel on the top of the mooring chain, and together they were swept downstream in the flood.

Eagleloosenberg, a big steel schooner, was next to go and she dragged her mooring until she collided with the steel ketch Annalise and Tony’s landing barge. Phil on board Dragon Song was swinging off the back of the barge, and the growing collection of boats in this group. He is upstream of Current Sunshine and saw her break off her mooring – he tried to call me but at 2 am my phone was off.

Here’s a photo of Annaliese and Current Sunshine on a sunny day. Having her collide with Current Sunshine doesn’t bear thinking about.

Next thing Phil noticed that his flotilla was beginning to drag. They dragged right on down to where Current Sunshine had been moored not long before.  By now Current Sunshine and Hoodwinked had gone out of sight around the bend.

Phil spent a ragged night aboard Dragon Song. At 5am Miki was leaving for work and, not expecting to be wading to the car, suspected something was up! She checked on Current Sunshine but in her place saw the barge and the collection of big steel boats. She raced back…  “Steve Steve! Wake Up! Current Sunshine is gone.

Steve counted the masts in the group of boats collected together and couldn’t find Current Sunshine. Holy shit! He looked again, further afield and could make out Current Sunshine’s mast sticking up above the mangroves downstream somewhere. He and Pete borrowed a dinghy and set off to find her – and Miki clambered her way through flooded fields to follow. Around the first bend they found her parked nicely with her stern shoved into the mangroves and with Hookwinked cleverly anchored again.

They boarded and hatched a plan of rescue. She was for the moment safe enough but the eddy was turbulent and could break her loose again. They first removed Hoodwinked from the bowsprit, and then dropped Current Sunshine’s anchor, and let out plenty of warp. Steve gunned the motor and she pushed out into the current. But in that Eddy she spun around her anchor doing doughies until they could tie a line off to the mangroves. The motor performed well, and Steve gave it just enough power to push her against the current, and manouvre her to get her tied up and to set another stern anchor.

With three lines set, she settled down and being in the backwater she was out of the main current.

With hindsight, its clear that her breaking off the mooring was the best thing that could have happened. Had she stayed on her mooring, the barge and the other steel boats would have come down on her and done untold damage.  As it is she has some scratches and gouges from Hoodwinked and from backing into the mangroves. And the bowsprit is bent.  Relatively minor really.

Current Sunshine is now back at Monty’s – I need somewhere to keep her while I go to Vietnam for a couple of weeks. Steve and Miki will move aboard to keep her company while I’m away. Luckily Noggy has a mooring available here for the next month that I can use. Isn’t it great how the universe provides for me…

After leaving Myora anchorage at Straddie, I went to Bribie Island – I was too late to catch the tide for entering the Caboolture river. And yesterday we had a great sail across Deception Bay. The wind being really light – even calm – the crew decided to swim.  And then 15 minutes later we had a shark cruise past us, and then we spotted another very white one just as at the entrance to Caboolture River.

Before leaving Myora I did manage to get a picture of one of the elusive turtles,

 

and one of a sea eagle that was playing in the wind eddies around the boat…