Rewired: Part 3

Vero Beach
No LL

Yesterday I re-re-wired [sic] the Link 10 monitor. I decided that the signal wires that I used were too fragile and would prove to be too easy to damage and too unreliable. The wire came from a spool of wire that I bought at a Radio Shack store. I regret buying it. I also regret the marketing practice of packaging goods in clamshell plastic packages that make it impossible to really inspect the goods before buying.

Anyhow, I rode the bus to the hardware store and bought some heavier guage wire. Then I returned to the boat, ripped out the old wires and installed the new. Then I ran the generator long enough to get the battery fully charged. Now the Link 10 is installed and calibrated and we can start using it seriously.

So what is a battery monitor and why would we want one? Simply stated, it is a device that accurately measures only three things -- battery voltage, battery current and time. From those three things one can calculate a number of derived quantities. Most significant is amp-hours, the measure of battery consumption. Also, one can calculate kilowatt-hours, and estimate time remaining before the battery goes dead. It can also keep historical records on the number and depth of charge-discharge cycles that allow you to spot trends. An optional serial interface (which I don't have) allows you to download log files of the measurements into an Excel spreadsheet so that you can analyze and plot data to your heart's content.

The value of a battery monitor is to prevent you from overcharging your batteries. Boats use deep discharge batteries that are unlike the batteries that you use in your car. Deep discharge batteries are designed for fewer but deeper lifetime charge-discharge cycles. Without a battery monitor, you have only battery voltage to judge the charge state of the batteries. Voltage is a poor indicator, especially when the current draw is variable.

Just guessing as to the battery charge state invariably leads to overcharging. It is better to be safe and overcharge rather than to undercharge and wake up to a dead battery. Overcharging costs money. It costs fuel, engine wear and tear and reduces battery life. In rough numbers, I think that I need to charge our batteries about 1 hour per day, but in the past 4 years I have averaged 2 hours per day, just to be safe. What is that worth?

• One hour per day excess charging * 250 days/year at anchor or under sail. (When we run the motor all day on the ICW, charging is "free.")
  
• Engine replacement ($15000/10000 hours)= $1.50/hour
• Fuel ($4*0.3 gallons/hour) = $1.20/hour
• Battery Life extended from 1.5 years to 3 years $.18/hour

Tht total is $575/year wasted costs. WOW! Compare that to $200 for a battery monitor. The investment can pay for itself in a very short time.

My monitor includes an optional fourth measurement -- battery temperature. The temperature is used to refine the estimate of time remaining on the battery according to Peukert's Equation (see below). The capacity is further corrected by 0.5% per degree C battery temperature.












I would have loved to have been the engineer who designed, programmed and documented the Link 10. It would have been a juicy snack for an analytical engineer; especially for an engineer who made a lifetime career out of understanding E=I*R (Ohm's Law.) Most of my friends also made careers out of E=I*R. Too bad finance and economic theories are not as simple as that.