Battery Isolators, Separators, and Combiners.

When you want to expand your battery capacity.

Off grid ham radio has an addictive quality about it. You tell yourself, “I’ll get a modest battery and solar panel and just fool around a little bit. I’m not too serious about this,” thinking that’s as far as you’ll ever go. Next thing you know, you’re adding more and more until you have what looks like the progeny of a NASA satellite and doomsday prepper outpost. That’s what happened to me.

At some point in your obsession, you’ll likely want to expand battery capacity. But how can you do that without altering or messing up the battery system you already have? Battery isolators, separators, and combiners are a few options. None of them are perfect, but one of these should get you close.

The two-switch solution.

The cheapest and easiest method of incorporating various batteries is to simply place a single pole, double throw (SPDT) switch between the batteries and the load, and another SPDT switch between the controller and the batteries. After that, you can manually select which set of batteries you want to use. Battery Isolators, Separators, and Combiners.

This method allows the user to select which battery will charge and which battery will power the load. It only works if the batteries in each bank are the same type (AGM, lithium, flooded, etc.). Because each battery type has different charge voltages, the controller must be set internally for the battery type you plan to use. Unless your controller has an easy way to change the battery type (and you always remember to change the setting when you switch banks), you can’t mix different battery chemistries using this method. Furthermore, only one bank can be charged at a time. Battery Isolators, Separators, and Combiners.

Battery isolators, separators, and combiners



  • Super simple and inexpensive.
  • Requires no extra electronics or hardware other than the physical switches.


  • No automatic switching between battery sets; load must be transferred manually.
  • Cannot mix different types of batteries without reconfiguring charge controller every time the load is transferred.
  • Only one battery bank can be charged at a time.

Two charge controllers?

Wouldn’t it be great if there was a charge controller with two separate outputs? Preferably, these outputs could be user-configured to different battery types.

I looked all over the internet and could not find any controller that fit this “unicorn” criteria. I did find one controller with dual output, designed for marine applications. Unfortunately, one of the outputs is only a 1 amp float charge.

For all practical purposes, the only way to have a system where different battery banks are always connected to a controller is to have more than one controller. Each controller can be set up for a particular type of battery. You will have the added expense of the extra controller(s) and you still cannot combine the output of the batteries to power the same load.

Installation is straightforward. Simply set up each controller and battery bank as an independent system. All they share is the solar panels. Once again a SPDT switch lets you choose which set of batteries will power the load.


  • Allows charging of different battery types at the same time.


  • Expensive —need to buy two controllers.
  • Battery banks are not complementary. You can still use only one bank at a time to power your load.

Battery Isolators.

A battery isolator allows two batteries to charge off the same source without the risk of one battery pulling the other down. This could be a good alternative to running two solar chargers.

Using different battery types with an isolator will probably not work. If there is a way to set separate charge voltages for each side of the isolator (highly unlikely), then you’re in business! Isolators are most often found in commercial automotive/transportation applications where one battery starts the engine and another powers the lights & accessories. They are a must-have for hams who run mobile radio with a separate battery for the radio. The West Mountain Radio ISOpwr+ is a very popular choice.


  • A good substitute for dual controllers.
  • Helps prevent discharging batteries too far.


  • The batteries cannot be combined to power one load.

Battery separator.

Unlike a battery isolator, a battery separator will allow two batteries to assist each other when the load requires it. A solenoid switch will open or close as needed to supply extra power. On most units voltage connect/disconnect set points are either fixed or somewhat variable with DIP switches. One of the risks of battery separators is that a small parasitic load can deplete both batteries over time.  Battery separators are very hard to find in anything other than a 12 volt system voltage.


  • Inexpensive.
  • Each set of batteries can power the same loads.


  • Very difficult to find units that will work on 24 & 48 volt systems.
  • Potential for long term parasitic loads to drain both batteries.

The Magnum Energy ME-SBC Smart Battery Combiner.

I’ve been using the Magnum Energy ME-SBC for about eight years now and can attest that it’s a great product if you understand what it is, and isn’t. It’s a very unique device and I haven’t found anything similar made by another manufacturer.

The Magnum combiner is meant to allow two battery banks to act as one. This is not the same as switching a load between batteries or independent batteries that power separate loads while connected to the same charge source.

A battery combiner will connect and disconnect two or more batteries when they reach certain voltage thresholds. This allows different sets of batteries to act as one power source as long as they are both within the set voltage window.

The ME-SBC works on 12 or 24 volt systems and will handle 25 amps. It does have a built-in option to activate a relay to handle loads greater than 25 amps. You can combine dissimilar batteries as long as the voltage settings are appropriate for the type of batteries. A great feature is the continuously adjustable voltage settings, as opposed to fixed DIP switches.

Also, note that the secondary battery does not directly power a load. It only supports the primary battery.


You can arrange the main and auxiliary batteries in strings. The Magnum inverter and remote monitor is for illustration only and is not required. GRAPHIC COURTESY OF SENSATA TECHNOLOGIES.


  • Allows dissimilar batteries to work together.
  • Continuously variable voltage settings
  • Front panel status LEDs


  • More expensive than the other options.
  • Secondary battery does not directly power a load.


Here is an Off Grid Ham article from way back in 2015 that reviews the Magnum Energy MB-SBC.

Here is a really good explanation of the difference between battery isolators and separators.


12 thoughts on “Battery Isolators, Separators, and Combiners.

  1. randall krippner

    Thanks for pointing out that ME-SBC unit. I’ve been haunting a few of the solar power forums and I’ve run into a few people who could definitely use it and I’ll pass the information along to them.

    Since I got involved in this stuff I’ve seen people do some very silly and even some very, very scary things to try to expand the capacity of their systems, especially when it comes to trying to combine batteries together. There are a lot of people out there who don’t understand basic electrical systems so it’s a good thing there are sources like Off Grid Ham to help people learn how to work with this stuff.

    Sidenote: The whole house solar system is still working well. It has been anticlimactic really. No muss, no fuss, no drama. I just flip a couple of breakers and the whole house goes off grid and everything just keeps working normally. The only bottleneck is my lack of solar power. I just don’t have space at the moment to add more solar panels. Expanding that is going to have to wait until we get the roof on the garage replaced in the next two year or two. The lack of solar limits how much I can run the system. If the budget permits I might be able to squeeze some higher capacity panels into the same space as a temporary measure. A couple of weeks ago we had a power failure that lasted three or four hours and we ran the whole house off just the batteries and still had about 80% battery capacity left when power came back on. Now that I know everything works as it should I need to clean up the wiring, run some of the cables through conduit to meet code and some other minor stuff.


    1. Chris Warren Post author

      There are a lot of safety issues not addressed in this article. I’ve seen many dangerous setups. Off grid power is real power; always take it seriously!

  2. JR Hill

    As Randy pointed out batteries are often misunderstood. About all I can say is this can be a dangerous subject.

    “A man has to know his limitations” Dirty Harry.

  3. Randall Krippner

    “Next thing you know, you’re adding more and more until you have what looks like the progeny of a NASA satellite and doomsday prepper outpost.”

    Ha! I love that line! Yep, that’s me. That I can get free power from nothing but sunlight gets my brain coming up with all kinds of weird ideas. Some of them actually amateur radio related. I’m thinking of putting the AC200Max and some folding solar panels in the back of the Buick to power everything, along with my TS-2000 or FT-450, a laptop, a coffee maker (essential for me), a couple of lights and turning it into a self contained mobile POTA station. Come to think of it, might be fun to take a setup like that around to the local county fairs and community picnics to promote amateur radio in general.

    1. Chris Warren Post author

      I have a portable system that I use for public demonstrations and it’s quite popular. I’ve presented to Boy Scouts, at hamfests, and a kids summer day camp. Covid and my already overbooked personal schedule kind of ended my one-man outreach, but it was fun and I’d do it again if I could.

    2. Eric G

      After my LG Chem/SolarEdge battery was indefinitely backordered I picked up a minimal critical loads backup battery and generator from Ecoflow (Delta Max and SmartFlow generator gasoline only). Last weekend during Field Day was it’s first big test. After charging it up on my house’s solar system (during the day) and running with a 100 Watt panel it ran for about 7 hours before it called for the generator to start. We had already made more than the required number of solar QSOs for the bonus points so it did its job. I have it programmed to start the generator when the battery hits 30%, and it recharged the battery to full in about a half an hour. The radios and lights in the tent pulled around 1200 Watts on TX and ~60 W on RX. I heated up chili on an induction hotplate that pulled about 1 KW for several minutes too. After the recharge the battery ran the radio well into the wee hours, kicking on again sometime before sunrise. It ran out of fuel at 60% recharge (the gauge was at 30% of the 1 gallon tank when Field day began) and I decided to not bother refilling because it wouldn’t be necessary before FD was over. As it turned out our CW tent’s generator quit running so we shut down the digital tent and took the battery over. If there were any noise issues I didn’t get any complaints about them. And speaking of noise, because the generator only ran for about 20 minutes at a time the digital tent was acoustically extremely quiet. Very pleasant place to work without the constant drone of the generator, just the cycling of the fans on the radio and battery.

      1. Randall Krippner

        That Ecoflow with the matching generator is a really nice system. I considered that one before I bought the Bluetti but I have a little Yamaha 2KW inverter that’s whisper quiet that I could use to recharge if necessary so I spent the extra money on an expansion battery instead.

        I had to drag out my old 7.5 KW Generac gas generator a few weeks ago to test an EG4 Chargeverter 5000. I’d forgotten how loud that thing is. Even inside the house with the generator about 20 feet away the noise was irritating. I think that was the first time that generator ever had to work hard for an extended period of time. When that chargeverter maxes out it draws more than 5 KW at 240V. The Generac is bad enough when it’s just idling. Under that kind of load you’d want to wear hearing protection if you had to work near it. The chargeverter was impressive, BTW. I ran it up to the full 5KW output for a while to make sure it worked then dialed it down to about 4KW output and just let it go, charging my house’s battery bank during the test. Ran it for about 3 hours straight. It’s going to end up permanently wired into the battery bank as a backup charging system.


        1. Chris Warren Post author

          Getting rid of my 3600 RPM “screamer” and replacing it with an inverter generator is high on my wish list. I hope to get a Honda, and relegate the conventional generator to running my welder.

      2. Chris Warren Post author

        I’m glad it worked out for you, Eric. Your approach illustrates a great point…power management! You don’t necessarily need to run a generator all the time. Run it only long enough to charge the batteries and you can stretch a tank of gas for a really long time.

  4. Eric G

    Most of this stuff has been figured out by the telecom industry decades ago. The simplest answer is to use the same batteries across all strings, and keep the same voltage across all devices. If you have to change voltage do it outside of the battery “plant,” after your inverter. You loose some efficiency but you also keep it simple.

    1. Chris Warren Post author

      You are correct. Telephone companies use a standard 48 volts (it’s actually 52-53 volts). Anything outside of that is done separately.

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