Simplifying COMPLEX CHARGING SYSTEMS on Boats 🔧 | Ask The Expert with NIGEL CALDER

Added: 2026-05-28

[00:00:00]It is a bit of a convoluted way of doing things and you've got the voltage losses through the the battery isolation diodes.

[00:00:08]Uh so, the whole system probably could do with a review.

[00:00:13]Hello and welcome to another one of our Ask the Expert series here at Boat How To. We're Nigel and Jan and we answer your questions about boat systems.

[00:00:23]Today, we have a longer question where it comes to charging different battery banks. Our alternator has its positive conductor wired to the input on a battery isolator that has three output terminals. One output is connected to a DC-DC charger that supplies the lithium ion phosphate house battery bank.

[00:00:42]The other output terminals supply AGM batteries for the engine and generator starters and the bow thruster.

[00:00:50]There isn't a Balmar APM, an alternator protection module, wired to the alternator.

[00:00:57]Will this setup prevent a voltage spike cascading through the electronic systems should the lithium ion battery BMS disconnect the house batteries?

[00:01:06]Are there any issues you see with this configuration?

[00:01:10]Okay, it's a long question and it seems to me like a little bit overly complicated the system.

[00:01:16]>> It is a bit but but the the answer to the first bit, if the lithium ion disconnects, will there be a voltage spike cascading through the system and will have effectively will the alternator blow out his diodes? The answer is no, because there's still two batteries connected to the system, the AGM batteries. There's not going to be an open circuit on the alternator. Uh he he will, of course, or she will lose everything that's connected to the lithium ion battery because it's just disconnected. So, presumably that's most of the house systems.

[00:01:49]It is a bit of a convoluted way of doing things and you've got the voltage losses through the the battery isolation diodes.

[00:01:57]Uh so, the whole system probably could do with a review, but there is one one good feature about it, which is the alternator is not directly connected to the lithium-ion batteries because it probably doesn't have a suitable charging algorithm for those batteries.

[00:02:13]So, the DC-to-DC charger that's in the circuit is actually providing that algorithm, I'm sure, for the lithium-ion batteries to protect them.

[00:02:22]But, I'm sure there would be much simpler ways of reconfiguring that system that would improve the performance as well.

[00:02:29]For example, just connecting the alternator to the starter battery and then have two DC-to-DC chargers that go from there to the lithium-ion battery and to the bow battery. Add one more piece, but remove a lot of different older systems like the battery isolator, which is yeah, not really state of the art anymore. And actually thinking about it, Jan, as as you say that, we're going from the alternator to the isolator to the bow thruster's batteries. So, there's no current limiting process built in there.

[00:02:59]So, the the conductors up to the bow thruster battery have to be sized to handle the maximum output of the alternator, which is probably quite a powerful alternator. Whereas, if you took the isolator out of there and and put in place, as you suggested, DC-to-DC converter, you can put current limiting into the circuit and then you can have much smaller conductors going up to the bow thruster batteries. And then when the bow thruster is operating, it'll suck the batteries down a little bit and then that DC-to-DC converter will catch up afterwards. So, yes, there are much better ways to to wire this than than we're looking at here. Yeah.

[00:03:33]And actually, if you want to learn how to properly do this and learn about various options, we do have a whole module on charging systems where we also talk about these charging yeah, setups with multiple battery banks, multiple charging sources in our boat electrics 101 course. So, go check that out at boathowto.com and see you soon.