How do I charge two battery banks on my boat and still keep them separate?  There are a number of ways of doing this:

This question arises for boats of all sizes.  Nobody wants to be stuck out on the water with a dead battery and so all but the smallest boats end up keeping one battery in reserve for starting.  How to charge both batteries and still keep them separate is one of the questions we most often get asked at PKYS.  This article was updated in May 2023 to add in the category of Battery to Battery chargers which weren't an option when the article was first written in 2019

  • The simplest way, if you can remember to do it, is to turn your battery switch to "BOTH" when charging, and turn it back to 1 or 2 when you are finished.  This solution is unsatisfactory for those who sometimes forget and then end up with a flat battery from time to time. The Blue Sea 9001e is an example of a 1-2-Both battery switch. If you want to automate the process and make it idiot proof, then you need to pick one of the other options. This 1-2-Both switch is the way most boats come as standard, and the least satisfactory method.
  • The best way, if you can manage it, is to have two alternators.  This allows independent charging of each bank, and by fitting a combiner switch you get the added feature of a backup alternator for either bank in the event of a failure.  This is a technically challenging project which is usually not an option for most installations.
  • A hugely popular and successful way is to use a battery combiner such as a Blue Sea ACR  ACR stands for Automatic Charge Relay. This is a solenoid which connects the batteries together when they are being charged and separates them when they are being discharged.  Blue Sea designed a special Add-A-Battery kit with a special dual bank battery switch and an ACR to totally automate the process.
  • The newest way to do this is the one that gets used most often for Lithium Battery installations is the Battery to Battery Charger.  This allows a totally different charge profile for each battery type.  Take your alternator to whichever battery you decide, and then use the Battery to Battery charger to take a different charge profile to the second battery.
  • The old fashioned way was to use a battery isolator, which is a set of diodes which allow the charge to go to each battery but doesn't allow the batteries to connect to each other.  One problem is that there is a voltage drop across the isolator which means heat is generated and your precious energy is wasted.  It also means that you can only use an externally regulated alternator so that it can sense the voltage downstream of the isolator. Isolators have had a new lease of life lately with the availability of FET Isolators.  These do the same job with lower losses, so reducing heat build-up and improving reliability.  At PKYS we now stock a range of these Victron ArgoFet Isolators
If you have Lithium Ion batteries this whole question becomes more challenging.  Lithium batteries have very constant voltage throughout their state of charge.  The voltage only drops off right at the end before they are empty, and rises only near the full point.  This constant voltage doesn't give the signal necessary for a standard ACR battery combiner.  The topic is discussed further in my blog post about adding an alternator to a Lithium Battery setup.


Liang Wang

Date 11/17/2020

Peter Kennedy

Date 5/25/2023


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Stuart Scheinberg

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Josef Farrugia

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Peter Kennedy

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