Getting Juiced at the Dock

LUX has one of the original Sentry FR1240/3X (12v, 40A, 3 bank) chargers made by Dometic. When we rescued LUX from the Nanny Cay boat yard, the house bank was at or just below 10 volts. That's the level that's a fully discharged battery. We put the charger on and it slowly came up to 12 volts over three days. The charger just didn't seem to be performing the way a 40A charger should. We knew that something was amiss when we couldn't get a good charge in marinas and the engines would start off with high charge loads. We were back home, so it was easy to remove it, examine the guts, and decide what action to take.

The case was somewhat rusty. It looked like someone spilled salt water on the unit. Some parts of the case are aluminum and some are steel - both had corrosion. Upon removing the case, we found that the main capacitor leads were also corroded and charred (the pictures are after we repaired the wire). The capacitor is highlighted with a red circle in the first picture.

The capacitor is the same as a motor run capacitor, available at any HVAC shop. I had one handy and connected it to the charger. The charger worked! (Drat; now we have to put off buying the Victron Inverter/Charger.) We fixed up the case and reinstalled it on LUX.

The Sentry charger is a ferro-resonant charger and therefore acts much like a 2-stage charger. It is basically a constant voltage charger, with current limit. As the battery voltage increases, the current decreases. It has a cut-off and cut-in circuit that stops charging when the battery voltage reaches the cut-off point and will restart charging when the voltage drops to the cut-in voltage. The manual recommends 12.8v for the cut-in voltage and 13.9v for the cut-out voltage. We have AGM batteries, which can take higher voltage.

There's a nice table on the BD Batteries web site (http://www.bdbatteries.com/mcharging_procedures.php) about Lifeline batteries that says a 2-stage charger should charge at 14.2-14.4 volts for the bulk phase and 13.2-13.3 for the float phase. So increasing the set points on the Sentry to 13.0v on the lower set point and 14.2 v for the upper set point would emulate the 2-stage charger. The West Marine advisor page "Selecting a Battery Charger", shows 14.6v and 13.4v, respectively for accept and float phases (http://www.westmarine.com/WestAdvisor/Selecting-a-Battery-Charger).

The end result is that we now have a good charger. Perhaps someday we'll get the Victron so that we have a really good inverter/charger.