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:
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.
Hi Matt,
Didn’t know that about contactors, thanks for the correction, although it will probably always be “a bloody great relay” to this old tech!
One point on not fusing starter motor conductors, you are right that there is an exception in the standards, but it my view they should always be fused: https://www.morganscloud.com/2022/03/19/8-checks-to-stop-our-dc-electrical-system-from-burning-our-boat/
Rod Collins has a great article advising the same: https://marinehowto.com/battery-banks-over-current-protection/
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.
Hi Vesa,
Thanks for the endorsement of the Mastervolt. I have never used their kit, but hear pretty consistent good things.
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??
Hi Vesa,
Thanks for the warning. Seems the way of the world, sadly.
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?’
Doh!
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 🤣
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.
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.
Hi Andrew,
Sounds good, and thanks for the real world report on the Victron FET. As I say in the article your engine battery should be fine as long as you don’t have a lot of solar or motor a heck of a lot, and even then 4 years would be a conservative life. Like most everything, this one is profile dependant.
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.
https://balmar.net/products/digital-duo-charge/
Hi Michael,
Just took a look at it, and it’s a DC/DC charger, which we will cover in the next chapter in a week or so.
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.
Xantrex’s Echo Charge is also a DC-to-DC charger, similar to Balmar’s Duo Charge.
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.
Hi Cory,
I guess maybe something like that might work, but, at least to me, so doing is ignoring the core design problem here which is that the alternator regulator should be told by the lithium BMS to stop charging prior to any sort of shutdown. Also, if you are set up that way, then all of the charging sources can feed a single charge buss on the house side and the engine start battery be charged by a single DC-Dc charger, which is by far the simplest and most elegant way to design a lithium based systems.
That said, if you already have lithium batteries that don’t communicate, I would leave things as they are with the alternator charging the AGM and stick with the DC/DC, since you have already paid for them, and not add the complications of a FET as well. Or, if you want to get more out of the alternator change over to an FET, but that will mean your AGM will not be ideally charged.
Bottom line, I can’t see any elegant way to get the best of both the DC/DC and FET splitter.
Another key point to keep in mind is that none of this deals with the fundamental dangers of load dumps (blackouts):
https://www.morganscloud.com/2022/04/25/why-lithium-battery-load-dumps-matter/
So, particularly if you do have batteries that don’t communicate, I would recommend adding a vital loads backup as detailed here: https://www.morganscloud.com/2022/07/03/building-a-seamanlike-lithium-battery-system/
Thanks for directing me here. As usual. my questions are answered. Brilliant.