Fusing an alternator on a house bank

[s/v Jedi]

Originally Posted by Jetx

Interesting idea… one that I haven’t seen mentioned before. Can you give an example of what a suitable diode would be?

In normal operation (in the absence of a high-voltage spike), would the diode pass any current from the house battery to the start battery, or would the batteries effectively be isolated unless/until a spike occurs?

I’m pretty sure that a battery diode isolator is what is meant. There’s diodes connecting the alternator to start battery plus other diodes connecting the alternator to the house battery. Here is one: https://www.victronenergy.com/batter...tery-isolators

The FET based isolators are an improved version with minimal losses. Here is an example of that: https://www.victronenergy.com/batter...tery-isolators

The problem with these is that the alternator regulator’s charging profile applies to every connected battery, so unless they are all the same chemistry, only one chemistry is charged correctly and the other suffers.

The dc-dc charger setups fix the issue.

[TxCoastSailor]

Here is the data sheet for the diodes I used.

My setup is as follows:

All primary charging systems (alternator, solar, wind, AC battery charger) are connected to the LFP house batteries. There is a Victron DC/DC Smart Charger between the LFP house batteries and lead acid start battery. It keeps the start battery charged. The start battery is floated at 13.6V

The maximum charging voltage on the LFP batteries is 14.4V. When the charging voltage gets above about 14.2 the diodes will start to trickle a bit of current into the start battery and its voltage will rise to about 13.8. The absorption time on the LFP batteries is limited to 1/2 hour and they pretty much stop accepting charge above 14.4V. So most of the time the diodes are off and the LFP batteries are isolated from the start battery.

Unclamped load dump pulses are about 100V for about 1/2 second with the current being limited to the output of the alternator. In my case that is about 100A. Since the diodes are rated for 200A each they should be able to handle the pulse. The load dump should be clamped at about 15V, 14V across the lead acid battery and 1V across the diodes. All of our electronics are rated for at least 16V max so they should survive. I have never experienced a load dump so I don’t know for sure.

Cheers,
TCS

[Jetx]

TxCoastSailor… thanks! That’s a really interesting approach, and one I haven’t heard of before.

Playing devil’s advocate, what prevents the LFP house bank from continually feeding the start battery if the B2B charger fails and the start battery is severely depleted… or, to throw out the doomsday scenario that gets tossed about periodically (and I think a little too hysterically) the start battery suffers an internal short?

[TxCoastSailor]

If the start battery charger fails the diodes will charge it and keep it at 0.6V below the LFP house battery voltage. I guess you could see that as a plus.

If the start battery really shorts it will probably blow the diodes, i.e. they will act like a fuse. You could put a fuse in that line as well or on the negative terminal of the start battery.

We have three LFP batteries in parallel and they all have ‘isolation’ fuses on the negative terminals so that if any of them short those fuses will prevent the other batteries from dumping their energy into the short.

I don’t have an isolation fuse on the start battery. I will probably add that at some point.

[MistyCat]

Originally Posted by BjarneK

I was thinking about how to fuse an alternator in the scenario where it is used to charge a house bank and an echo-charge or similar device is used to keep the starting battery charged....

In my rewired and repowered Niagara 35, we placed a 120 Amp thermal fuse on the feed from the 80 amp alternator to the house battery bank, close to the battery. We have a Xantrex Echo Charge so that both house and engine start batteries can be charged from the alternator. This would seem to address the issue you raised. The man who designed the rewiring project for me - who is well acknowledged in our region as an expert - insisted that the engine start battery would not need to be charged from our solar panels or shore charger, given that the diesel (a new Beta 30) requires no more than about 1 Ah to start it, maybe a bit more if I use the pre-heater. We just finished our second season with this set up and it works like a charm. We put about 100 hours a summer on the engine, averaging about 60 days out (we are in charter service so we are departing and returning to a dock at least once a day, and that's when the engine runs).

Attached here are a photo of the fuse and a PDF with a written explanation and diagram of the wiring plan.

[Jetx]

Originally Posted by TxCoastSailor

If the start battery charger fails the diodes will charge it and keep it at 0.6V below the LFP house battery voltage. I guess you could see that as a plus.

If the start battery really shorts it will probably blow the diodes, i.e. they will act like a fuse. You could put a fuse in that line as well or on the negative terminal of the start battery.

We have three LFP batteries in parallel and they all have ‘isolation’ fuses on the negative terminals so that if any of them short those fuses will prevent the other batteries from dumping their energy into the short.

I don’t have an isolation fuse on the start battery. I will probably add that at some point.

Thanks TxCoastSailor... makes sense, and is very helpful. Seems like a pretty clever solution.

[Sonosailor]

Simple question: Can I use a Blue Sea terminal fuse block and fuse to protect the alternator wire at the positive bus bar? Is a 100 A fuse appropriate for an 80 A alternator with a small bank that can never receive 80 amps anyway?

[Wotname]

Simple answer - Yes.

Expanded answer - Yes, no problem.

[hjohnson]

My alternator directly charges my LFP house battery. I use a Victron bus bar for my high side/charging bus, which in turn is connected to my alternator via 2 awg wire. You had better believe it’s fused.

My Alternator is rated for 85A, I put a 160A megafuse on it.

The fuse is there to protect the wiring. LFP batteries can supply essentially infinite current.

Fuse it. But at the battery end.

[hjohnson]

Originally Posted by Sonosailor

Simple question: Can I use a Blue Sea terminal fuse block and fuse to protect the alternator wire at the positive bus bar? Is a 100 A fuse appropriate for an 80 A alternator with a small bank that can never receive 80 amps anyway?

The fuse is largely there to protect your wiring, not the alternator. The maximum size of fuse is going to depend on the size of wire you use.

[mitiempo]

I would fuse the alternator at 150% of its rating. Mainesail states the same.