Elver Auxiliary Propulsion System

Introduction

This section describes the development of a better power source for the Elver's electric auxiliary motor. It consists of a LiFePO4 battery bank with electronics to control and monitor its operation. It is charged by a solar array. It powers a trolling motor. This page covers the inception, development and testing of this system.

Motivation

April 2015

In April 2015 I took the Elver to Apalachicola for the Apalachicola Classic Boat and Car Show. The day before the show we put in for a sail at a boat ramp at Scipio Creek Marina. The wind was blowing directly up Scipio Creek and we had about 1 mile to go to the bay. I motored out of the basin and pointed into the wind - or tried to. This sailboat like most others likes to point downwind under power. So it requires some authority to get it pointed the right way. But it was proving much more difficult than expected. I looked around and soon noticed that the battery voltage was 20 volts instead of 24. I was a bit mystified at this since I thought I had properly charged the batteries before hand. I struggled to get the boat down the creek to Apalachicola bay. After about 20 minutes we finally succeeded. We sailed for a while and then headed back. It was much easier to head down wind so we enjoyed a leisurely ride up the creek and observed the scenery as we went by.

After returning home from the boat show I attempted to charge the batteries. I soon realized that both batteries had one dead cell each. I was a bit frustrated because these batteries were only a year old.

What to do?

I started really thinking about other types of batteries such as lithium ion. After a lot of pondering I settled on LiFePO4. These have several advantages over flooded lead acid. In short they are:

• About twice the energy density for a given weight.
• Are happy with a state of charge from 10% to 90% and left that way for extended periods. This is in sharp contrast to lead acid chemistry which has a useable capacity between 50% and 100% and likes to be charged back up immediately.
• Completely dry like AGM; there are no fumes.
• They will supply high current out without much voltage sag.
• They have a low Peukert factor.
• Almost unlimted cycle life
• Much greater longevity while being used or just sitting in storage.
• LiFePO4 is much less likely to burst into flames than other Lithium ion chemistries, everything else being equal.

There are disadvantages though.

• They cannot tolerate overcharging.
• They have a much higher initial cost.

Also I wanted to use solar panels as a charging source.

At this point the stage was set for new development. The following sections detail this development of this system.