Cross-Bank Battery Charging—Splitters and Relays

Reading Time: 9 minutes
I'm thinking I will show all my diagrams with separate load and charge busses, since with the rapid rise of lithium batteries that's the best option for anyone installing a new system, even lead-acid—might as well be lithium ready. That said, if I had a lead-acid system that was working I would not rip it all out just to go over to separate busses.

In the last chapter, we looked at best practices for battery bank separation, now let's dig into the actual hardware, the stuff all us nerds fixate on.

I'm assuming liveaboard offshore cruising as I analyze these options. If you are primarily coastal day sailing and often in marinas, much of this won't apply, but reading it will still be useful.

Let's take a look at the options:

  1. Why Most New-To-Us Boat Electrical Systems Must Be Rebuilt
  2. One Simple Law That Makes Electrical Systems Easy to Understand
  3. How Batteries Charge (Multiple Charging Sources Too)
  4. 5 Safety Tips For Working on Boat DC Electrical Systems
  5. 7 Checks To Stop Our DC Electrical System From Burning Our Boat
  6. Cruising Boat Electrical System Design, Part 1—Loads and Conservation
  7. Cruising Boat Electrical System Design, Part 2—Thinking About Systems
  8. Cruising Boat Electrical System Design, Part 3—Specifying Optimal Battery Bank Size
  9. Balancing Battery Bank and Solar Array Size
  10. The Danger of Voltage Drops From High Current (Amp) Loads
  11. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 1
  12. Should Your Boat’s DC Electrical System Be 12 or 24 Volt?—Part 2
  13. Battery Bank Separation and Cross-Charging Best Practices
  14. Choosing & Installing Battery Switches
  15. Cross-Bank Battery Charging—Splitters and Relays
  16. Cross-Bank Battery Charging—DC/DC Chargers
  17. 10 Tips To Install An Alternator
  18. Stupid Alternator Regulators Get Smarter…Finally
  19. WakeSpeed WS500—Best Alternator Regulator for Lead Acid¹ and Lithium Batteries
  20. Smart Chargers Are Not That Smart
  21. Replacing Diesel-Generated Electricity With Renewables—Loads and Options
  22. Efficient Generator-Based Electrical Systems For Yachts
  23. Battery Bank Size and Generator Run Time, A Case Study
  24. Battery Options, Part 1—Lithium
  25. Battery Options, Part 2—Lead Acid
  26. A Simple Way to Decide Between Lithium or Lead-Acid Batteries for a Cruising Boat
  27. Eight Steps to Get Ready For Lithium Batteries
  28. Why Lithium Battery Load Dumps Matter
  29. 8 Tips To Prevent Lithium Battery Load Dumps
  30. Building a Seamanlike Lithium Battery System
  31. 11 Steps To Better Lead Acid Battery Life
  32. How Hard Can We Charge Our Lead-Acid Batteries?
  33. How Lead Acid Batteries Get Wrecked and What To Do About It
  34. Equalizing Batteries, The Reality
  35. Renewable Power
  36. Wind Generators
  37. Solar Power
  38. Watt & Sea Hydrogenerator Buyer’s Guide—Cost Performance
  39. Battery Monitors, Part 1—Which Type Is Right For You?
  40. Battery Monitors, Part 2—Recommended Unit
  41. Battery Monitors, Part 3—Calibration and Use
  42. Battery Containment—Part 1
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Matt Marsh

Re. diode isolators.
We use a cheap diode isolator on the RV to allow its house battery to be charged by the truck, without back-feeding the truck or the 12V side of the 3-way fridge when the alternator is not spinning. They are effective for this purpose, but I would absolutely not recommend or install one for any application with a charging current of more than about 30 A.

Re. contactor vs. relay. Yes, I get the pedantry here. That said, in current industrial controls practice, “contactor” refers specifically to a normally-open relay that makes or breaks a connection between input and output terminals without a common terminal, is rated for high current and high voltage, and is mechanically guaranteed to open all terminals simultaneously and to suppress any internal arc – i.e. it cannot seize in the “on” position. The distinction is important when you’re wiring up 480V 3ph motors; you’ll use relays to switch and multiply the control signals, but the point where the control signals switch the three 480V phases will have a motor contactor.

Re. fuses. If in doubt, always fuse it. And yes, FETs of any kind can (and do) fail shorted — when they overheat, the dopants in the silicon diffuse through the bulk, rendering the entire chunk of silicon conductive, and therefore break down the p-n / n-p barriers that give the device its switchable one-way properties.
I believe there’s an exception in the standard for short high-current wiring runs that connect an engine starting battery directly to an engine starter. Just about anything else should be fused at the battery. Anything that can keep producing dangerous levels of current after the battery fuse has blown – solar panel regulators, for example – might call for a fuse at the source as well.

Vesa Ikonen

I´ve had a Mastervolt FET isolator (”Battery Mate”) for 10+ years. It is cheap, simple, has worked flawlessly, and the voltage drop is negligible.
Would not bother even considering a charging relay instead of it.

Vesa Ikonen

One thing to note, though. I just recently learned the hard way that Mastervolt really does everything they can to keep you from servicing their products yourself:
a mastervolt battery charger suddenly died on me. I opened it, and found a blown fuse.
Did the manual mention there is an internal fuse?
Not a word of it, not even the fuse specs to be found anywhere.

The only mention about servicing in the manual was ”please contact an authorised service partner…”

To replace a 0,25€ fuse??

John Deakin

I totally agree with you John regards the misuse of terms to describe something.
Me. Is this circlip made from 316 Stainless?
Salesperson. ‘Oh yes Sir, it’s stainless…..what’s 316?’
Marketing and sales staff don’t you love them 😱🤬
But l agree with Matt’s explanation of a contactor vs relay. There’s a big difference.
Great article as always John, thank you.
Ps hope l’m still on your Christmas list 🤣

William Murdoch

I am a retired engineer with 34 years experience working in chemical plants in North Carolina and Tennessee. My use of the words relay and contactor matches Matt’s although I use the term motor starter for the contactors that start electric motors because they may have overcurrent protection and a reverse function (and perhaps other things). And, “a bloody great relay” would have done fine during the five years I worked in the UK.

Andrew Reddon

I have been using a Victron FET based isolator for 4 years with my LFP house bank and and an AGM starting battery. My charge control set up gives the manufacturer’s recommended profile from my alternator and on shore power (14.4 volts constant voltage until charging current falls to 5% of bank capacity then shuts off all charging). My AGM starting battery specification says that it can be charged at an absorption voltage of 14.4 volts (spec is 14.3 +/- 0.1 volts) which seems perfectly compatible with my Lithium charging profile. The absorption phase is certainly much longer that the AGM likes or needs and it gets no float but I see no evidence that the start battery has suffered from overcharging in any way from 4 years with a lot of exposure to the lithium profile. That said, I do not motor much when coastal cruising and I run my alternator 1-2 hours a day on passage for my electrical needs. Seems very simple and understandable to me and I have had no problems to date.

Michael Albert

How about a Balmar digital Duo Charge? I admit I don’t know which category it fits into but it charges my AGM start battery very well. It appears to be able to handle different battery chemistries.

William Murdoch

I was going to ask about the Xantrex Echo Charge that I use to recharge my engine starting AGM battery from my lead acid flooded battery(s) house bank that is itself charged by the engine alternator and a solar panel. With a 14.4V limited output and a maximum 15A output it seems to me to be a good solution. I’m waiting to see what I missed.

Matt Marsh

Xantrex’s Echo Charge is also a DC-to-DC charger, similar to Balmar’s Duo Charge.

Cory Hall

I wonder if a FET splitter could be used in conjunction with a DC-DC charger? Could you charge a house lithium bank and an AGM starter at the same time and then when the lithium is full and they drop out the AGM could buffer and let down the alternator? (not sure what the FET would do when the BMS shut them) If you were concerned with the lithium bank needing a higher voltage you could use the DC-DC to top it up from the starter bank/alternator. I think if this because I use 4- 30A dc-dc chargers from my large 170Ah starter/alternator battery to charge the house lithium bank so that when they close they wont fry the alternator via the field.

Terence Thatcher

Thanks for directing me here. As usual. my questions are answered. Brilliant.