Kristin and I have done quite a bit of cruising on the Great Lakes, much of which involves lots of anchoring. Without the ability to safely anchor for the night, we’d miss out on most of the beauty of Lake Huron’s North Channel, Lake Superior’s Isle Royale, and even Lake Michigan’s South Manitou Island. We usually prefer the privacy of anchoring over staying in a marina near a lot of Lake Michigan towns, too, and anchoring is easier than docking when I’m singlehanding.
However, all this time away from a dock means I have to keep Priorities pretty self sufficient from an electrical standpoint. Electrical power is needed for a bunch of stuff including lighting, electronics, and especially the refrigeration systems. Since I’m not going to leave the diesel engine or generator running all the time (that would be obnoxious!), we depend heavily on battery power when not connected to shore power.
Since my house batteries are central to the electrical system, keeping track of how charged they are is very important.
One of the most valuable electrical improvements I’ve made to either of the boats I’ve owned, including the current Priorities, was installing a battery monitor shortly after buying her. A battery monitor is a small instrument in the cabin that helps me keep track of how charged my house batteries are, and therefore when to recharge them with the engine or generator.
Similar to how a smartphone has a readout showing the percentage of battery remaining, a battery monitor can display the current state of charge (SOC) of the house batteries on my boat as a percentage. It displays other useful information, too, like voltage, amps in or out, and amp hours consumed. By keeping an eye on the percent SOC, and how quickly it’s decreasing, I can avoid costly deep discharges.
While a lead acid battery (flooded or AGM) can be discharged to near zero, doing so puts a significant amount of wear on it and greatly shortens its life. My house battery bank is expensive ($750), and industry data shows if I avoid discharges below 50% SOC they should last many more seasons. Since my battery monitor only cost me around $250, I found it a worthwhile expense.
My Xantrex LinkLITE battery monitor derives the percentage by continuously measuring amps in and out of my house battery bank with a sensor called a shunt. When time is factored into the equation, the number of amp-hours consumed can be derived (and displayed). Programming in the battery bank’s size (in amp hours) allows it to display battery state as a percentage. While my battery monitor is subject to error, it’s way better than guessing.
Why not just use voltage to determine state of charge? Theoretically, a 12V battery that is 50% discharged should have a voltage of around 12.1V when not under any loads. Using this method is difficult on a cruising boat… when the fridge cycles on and puts a load on the battery, voltage immediately drops about 0.1 or 0.2 volts. When the fridge cycles off, the battery voltage recovers, but very slowly. Compound this with a continuously changing load, like the autopilot drive working the rudder with each passing wave, and I have to guess a lot. My solar panels are also constantly changing output as shadows of the rig and clouds pass over parts of the panels, making me guess more.
Once the batteries are charging, I can use the battery monitor to check how effective the charging system is performing. As I bring the engine RPMs just above idle, I can immediately see my alternator’s output increase as it hits its optimum speed around 1500 RPM. Amps flowing into the battery should be realistic (Charging System Output – House Loads = Charge Current into Batteries)… several years ago I noticed my old stock alternator struggling to charge my batteries, and I ended up replacing it.
Any unusual values warrant attention. However, finding problems when the batteries are less than half discharged (50% SOC)… long before the batteries are dead… means I have time to troubleshoot and hopefully solve the problem before much damage to the batteries occurs. I had a minor alternator issue while anchored in the North Channel last year, and wrote about it in my Boat Repair in an Exotic Location post. Having a battery monitor allowed me to troubleshoot and then confirm my repair worked long before we would have been left with a dead house battery.
Before I had solar panels, all my battery charging was from diesel power (the propulsion engine or generator). Since “topping off” the batteries to 100% takes a VERY long time, I’d wait until the charge current into the batteries slowed to around 40A or so (about 85% SOC), then shut the engine off for some peace and quiet. This made me feel confident in my batteries’ health more than the “charter boat” method of running the engine for an hour in the morning and an hour in the evening. I also have found on cooler days… when the fridge doesn’t run as much… I can run the motor far less and still sufficiently charge the batteries.
Installation
There are several models of battery monitors on the market. Victron makes one that constantly gets high marks in the marine industry. I chose the Xantrex LinkLITE since it provided all the features I needed at a slightly lower price. If I were living aboard for years I’d probably get the Victron version. The Balmar Smart Gauge came on the market after my install, and it’s supposed to be really good, too (it’s a slightly different system, though).
I installed my battery monitor as part of a larger electrical project that included adding a separate engine start battery. The most difficult part was working with large battery cables.
Current flow in and out of the house battery bank is measured with a shunt that is wired between the negative terminal of the house battery and the rest of the DC negative system of the boat. NOTHING can bypass the shunt for the system to work correctly, so it could be a complicated project on some boats. On my boat the battery cables are huge (2/00 AWG), requiring large swage-type terminals done with special tools. (For more information on working with battery cables, I highly recommend RC Collins’ Marine How To Website) Since almost all DC current flows through these cables, they must be done right!
Due to space constraints of the battery compartment on my Catalina 400, I mounted the shunt in a normally dry section of the bilge a few feet away from the battery compartment. At the time of the install, I planned on keeping Priorities in fresh water forever, so corrosion in the bilge wasn’t much of a concern. Salt water boats may need to install the shunt in a more protected location.
Since the battery monitor is always on during the season, power wires to the LinkLITE are only fuse protected but not switched. I remove the fuses during winter storage to eliminate the tiny drain on the battery from the monitor. The wires to the shunt are a pair of twisted wires (sold as “instrument wire”) and routed away from the large battery cables to minimize interference.
Programming
After the wiring was completed, the LinkLITE needed programming. Most was pretty straightforward, though I needed to contact my battery manufacturer for the Peukert component. The Peukert component has to do with how much a high current flow affects the rate of charge and discharge… for my Trojan T125s, it’s 1.23. If this number is way off, the %SOC value could end up being inaccurate.
I’ve also needed to tweak the “Auto Synchronize” settings a few times, especially since solar power output fluctuates so much. Occasionally the monitor has “Auto Synchronized,” or reset the %SOC to 100%, way before the batteries were fully charged. For example, when a cloud passes overhead, the solar panels’ output current drops. Sometimes it drops to a current equal the house load, resulting in no net current into the batteries. If the voltage is around my programmed “sync voltage,” the monitor can mistake this for a fully charged battery. I’ve found it best to set the “sync voltage” parameter to just over the charger’s programmed float voltage and the “sync current” parameter to only 1% of battery capacity. Both values need to be met for it to “auto sync” the %SOC to 100%. My solar charging system is new, and I’m still tweaking it. If it becomes an issue again, I could set the “auto sync” voltage to a very high number that never gets fulfilled, thus never “auto syncing.” I would then need to always manually sync the monitor every few days when cruising.
The LinkLITE doesn’t interface with my shore power or solar chargers, so any inaccuracies with the charge status won’t negatively affect their output.
Valuable Addition
Overall, I remain very happy with my battery monitor. When cruising I check it many times a day, giving me peace of mind that my batteries will continue to do their job for many more seasons and not leave me in the dark. It’s one of the most valuable improvements I’ve made to Priorities.