A: The other problem might be the type of wiring used to measure the voltage across the shunt. Since the voltage differential is so tiny, it’s important to have a twisted shielded cable between and battery monitor. Furthermore, make sure none of the twisted shielded cable is running along AC lines or large DC lines as well. You could test this by running a good shielded twisted pair not permanently run and see if you still have the problem. Otherwise, you might be looking at a bad battery monitor.
Assuming you have an engine and house battery bank or two equally sized batteries, you basically have two choices: either a 20-amp or 40-amp multiple output smart charger. If your house bank is 200 Ah or more I’d suggest going with a 40-amp model.
As a rule of thumb when sizing chargers, choose the charge rate based on the battery size. For flooded batteries a conservative charge-rate should be between 10 to 20 percent of the battery capacity. For AGM and gel batteries, you can go between 10 to 35 percent. Over the last year, we have had great success with ProMariner Charger, which will charge both flooded and AGM batteries.
A: Without knowing the battery size we cannot answer this question definitively, but I seriously doubt that an 80-watt solar panel could fulfill the bulk charging rate for any reasonable battery bank. Therefore the problem lies with the alternator and regulator, not the solar panels charging at the same time.
It is ideal to have one maximum power point tracking monitor (MPPT) per panel or at least an MPPT that can handle the total wattage of two panels put in parallel. Unless both panels always experience the exact shading, I’d suggest getting an MPPT per panel, that way you'll get the most efficient charging all the time.
A: The following advice applies to lead acid batteries, such as flooded, AGM, and gel. While cruising it is totally fine to stop charging before 100 percent. Remember that lead acid batteries follow the bulk, absorption, and float charge curve. In order to maximize the efficiency of your charging while off the docks I advise boaters to stay within the bulk charge. Once the absorption stage is reached the charge rate drops. Therefore don't bother to charge past 80 to 85 percent unless you have AC shorepower or are underway and going somewhere.
As a reminder, flooded batteries should rarely be discharged below 50 percent of capacity and AGM/gel batteries no more the 30 percent of capacity.
About the author: Jeff Cote is the owner of Pacific Yacht Systems, a full service shop delivering marine electrical and navigation solutions for recreational boats. Visit their website and blog for info and articles on marine electrical systems, projects and more: www.pysystems.ca.