Keep Charged - Charging Scenarios Using Battery Combiners and Battery Isolators

A 12VDC marine electrical system typically relies on one battery to start the engine and another battery to power the “house” loads, such as refrigeration, lights or an inverter.  In an ideal electrical world, engine and house battery banks would always be separate and have their own charging source.  However, many boats only have one charging source, such as an alternator or inverter/charger, so marine electrical designers are forced to share one charging source to recharge more then one battery bank. 

In this month's Tech Talk we are going to look at some charging scenarios using battery combiners and battery isolators to automate and simplify the charging process of multiple battery banks with only one charging source. Battery combiners and battery isolators serve similar purposes but achieve sharing a charge differently.  Let’s look at what they do and how they are different?

Battery combiners, also called Automatic Charging Relays (ACRs) or Voltage Sensitive Relays (VSRs) are designed to put two battery banks in parallel (i.e. connected together) automatically when there is a charging voltage and separately when there is no charging voltage. Battery combiners will put two battery banks in parallel when there is charging voltage but leave the batteries isolated during discharge.  Battery combiners operate based on a specific voltage so the batteries are parallel when charging (i.e. parallel at 13.3 VDC) but isolated when discharging (i.e. disconnect when voltage gets to 12.8 VDC).    The challenge with battery combiners is uneven battery banks, specifically when one battery bank is discharged heavily and another battery bank is full.  When two battery banks are in parallel (joined together), they will instinctively want to even themselves out and the higher voltage battery will want to lift the lower battery bank’s voltage.  This inrush of current from one full battery bank to an empty bank can easily exceed the maximum current rating of the battery combiner. Furthermore, the fuses installed to protect the battery combiner will trip/blow. What makes this all challenging for a marine electrical system is how do you know when the fuse has been tripped or blown?? Especially if you don’t know what to look for. This is why we choose to install battery combiners for battery banks that are generally similar in size and also won’t be uneven or unbalanced to avoid nuisance tripping of the fuses protecting the battery combiners. Popular models include the Blue sea Automatic Combiner, BEP Voltage Sense Relay, and the Xantrex Digital Echo Charge.

Battery Isolators also allow one charging source, such as an alternator, to charge multiple battery banks. Think of them like a gate valve, allowing the current to flow from the alternator to two or three battery banks without any of the battery banks “seeing” one another.  By using diodes or Field-Effect Transistors (FETs) to only allow current to flow in one direction, a fully charged battery cannot pass current to a partially charged battery.  For instance, most battery chargers have isolators. This is how a battery charger can recharge two or three battery banks simultaneously. The challenge with a diode battery isolator is that the diode causes a voltage drop of .6 to .7 VDC. Modern FET battery isolators offer all the benefits of diode combiner but without any significant loss. This high efficiency makes FET battery isolators very popular and a common tool in PYS electrical designs. A popular FET battery isolators is the Victron ArgoFET.

Battery Combiner Scenarios. A battery combiner is perfect for a boat with an outboard or two small, similarly sized battery banks (one starter, one house). In this scenario, the alternator is not that big and neither are the battery banks, which makes a battery combiner well suited for the application

Battery Isolator Scenarios. If you have a single engine (one alternator), a starter battery, a house battery and a thruster battery then a battery isolator is perfect device.  Each individual battery will never “see” another battery and therefore the maximum current going into the battery is limited to the size of the alternator. No one battery bank will share its voltage with another battery bank, thereby avoiding  nuisance tripping of the fuses.

If you have two engines (two alternators), two starter batteries and a house battery bank then you could put a battery isolator on each alternator’s positive output.  The alternator, via a battery isolator, would then charge its respective starter battery and the house bank.  All batteries get a charge at the same time, yet no battery bank is in parallel with another.

If you have a red 1-2-Off manual switch and constantly forget to switch it back and forth when you are running or if you have a charger with multiple outputs and only one alternator then you can use a battery combiner or battery isolator to manage the charge voltage sharing for you.

As with most marine solutions, you have to determine what works best for your individual needs.  If you have multiple battery banks onboard and require redundancy then a battery isolator will ensure that no single battery drains the other batteries.  If you have similar sized battery banks and only one charging source with no excessive draws then a battery combiner is the way to go.

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: