Battery Charging With Two Alternators

At the moment, Jazzcat has two inadequate batteries that power the domestic systems and start the port engine. I want to upgrade and can see a way to fit three ~100AH leisure domestic batteries and use the existing 60AH engine battery to start both engines, but with the ability to start from the domestic bank if necessary,

Plan A is to connect the port engine alternator to the start battery isolator, so no mods needed, and the starboard one to the domestic battery isolator, but that will mean the starboard alternator will have no connection to the start battery. One disadvantage is that one alternator would be working hard to keep the lights on and the fridge running, while the other will basically spend ten minutes recharging the starter battery then go back to sleep.

Can anyone see any problems? Is there a simple, inexpensive way to use that spare capacity on the domestics without both alternators getting hurt or confused? They're standard Beta 20 alternators from 25 years ago, so nothing clever.

Thanks in advance.

How about a Victron Argofet, two engine input, 2 battery bank outputs ?

How many hours of the year will the batteries take more than one alternator can comfortably deliver?
How big is the real gain from changing anything?

Standard alternators from >20 years ago might only be giving 14.0 or 14.2 volts.
If you want to get more into the batteries, an upgrade might be a better way t o spend money?

Generally, two controllers trying to regulate the same function is at best, bad practice.
If you want to buy power supplies which share a load when paralleled, it costs extra.

If you're intending to keep the existing batteries and add an extra new one, you could run the domestics as two banks, where maybe one bank does the fridge only or something, and one engine charge that bank, the other deals with everything else.

Adding a LiFePO4 battery and a DC/DC charger is another possibility..

The 'best way forwards' might depend on exactly how you feel the existing setup is not meeting your needs, and how you want to use the boat.

Personally I'm not rushing to get into Lithium, but neither would I be keen to spend £300 on lead acid without serious thought.
How tired are the existing batteries?

B27 said:

How many hours of the year will the batteries take more than one alternator can comfortably deliver?
How big is the real gain from changing anything?

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Why have one alternator doing nothing, when it can be helping to charge the batteries ?

B27 said:

Standard alternators from >20 years ago might only be giving 14.0 or 14.2 volts.
If you want to get more into the batteries, an upgrade might be a better way t o spend money?

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This makes no sense at all. You just said the batteries won't take more than one alternator can output, but now you suggest replacing the alternators. ?????

B27 said:

Generally, two controllers trying to regulate the same function is at best, bad practice.
If you want to buy power supplies which share a load when paralleled, it costs extra.

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This also makes no sense at all, where did power supplies come from ? As for having two "controllers", that's nonsense, thousands of boats have more than one charging source, multiple solar controllers, mains charger, alternators, etc etc.

I have two alternators, each one charges the engine batteries and the domestic bank, exactly what the OP wants. No problems at all and been doing so for the past 35 years, with LA batteries.

B27 said:

If you're intending to keep the existing batteries and add an extra new one, you could run the domestics as two banks, where maybe one bank does the fridge only or something, and one engine charge that bank, the other deals with everything else.

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But splitting the domestics into two banks will further complicate charging and will also further reduce charge acceptance rates.

B27 said:

Adding a LiFePO4 battery and a DC/DC charger is another possibility..

The 'best way forwards' might depend on exactly how you feel the existing setup is not meeting your needs, and how you want to use the boat.

Personally I'm not rushing to get into Lithium, but neither would I be keen to spend £300 on lead acid without serious thought.
How tired are the existing batteries?

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The OP didn't mention Lithium.

PaulRainbow said:

How about a Victron Argofet, two engine input, 2 battery bank outputs ?

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I see Argofets with 2 or 3 outputs but only one input. Would you connect both alternators to the single input or could you use a Victron Diode Battery Combiner to combine the outputs from the two alternators and feed this into an Argofet.
There would be a voltage loss through the battery combiner but small as it uses Schottky diodes

Alternatively use two low loss splitters, such as Victron Argodiode battery isolators , one for each alternator ( Or even 2 Argofets ) each connected to the two battery banks.

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I think the electrics should be set up just like any single engined boat with alternator to engine start battery then diode or VSR to charge domestic batteries.
Then add the second alternator in parallel with first. Thhis means tha ton start up the first alternator will be charging the start battery while cranking the second engine. This is the typical arrangement for twin engined light aircraft with auto type alternators. The thing is they also have a load balancing regulator to even out the charge and load current of the 2 alternators. Desirable as one alternator/regulator could over power the other when simply paralleled. (but may not be a concern in practice. But still allow one alternator to power the plane or boat if necessary.
This may be of interest Twin Alternator Voltage Balancing

Just thinking out in text ... my thoughts are that care is needed not to cut off charge from any alternator while running. Second that two alternators banging into same circuits surely cannot be good without some form of control. Third - I think better to have each alternator charging its respective bank before charging to other ....

It may sound a bit outside realm - but wouldn't suitable diodes achieve possibility ? I know when we have two power sources in various other gear I have - its common to use diodes to prevent one power source battling the other ...

Just commenting ...

Thanks for the responses

PaulRainbow said:

The OP didn't mention Lithium.

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I'd love to go lithium, but I haven't got a spare £1000 to do it, nor have I the knowledge to DIY from scratch.

VicS said:

I see Argofets with 2 or 3 outputs but only one input.

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That's what I saw, too. How would that work with the two alternators?

Stemar said:

Thanks for the responses

I'd love to go lithium, but I haven't got a spare £1000 to do it, nor have I the knowledge to DIY from scratch.


That's what I saw, too. How would that work with the two alternators?

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I dont know if the two alternators could simply be connected in parallel to the single input. William H seems to suggest they could be paralleled. Otherwise combine their outputs using a Victron battery combiner as I suggested. (It's like a diode splitter but with the diodes the opposite way round to combine two sources into one as opposed to splitting one into two.) There are two different current ratings available.

My first thoughts were to use two diode splitters , one connected to each alternator, but ordinary diode splitters will drop the volts too much. Hence my suggestion in #5 to use two low loss splitters such as the Victron Argodiode. This is basically what Refueler is suggesting.

Perhaps PR will return to expand on his suggestion to use an Argofet

Thank you VicS ... I was just commenting - trying to think of a way to beat the problem of two alternators banging away at each other.

Refueler said:

Thank you VicS ... I was just commenting - trying to think of a way to beat the problem of two alternators banging away at each other.

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I dont think they will be "banging away at each other" but unless they have been carefully matched there is bound to be a small difference in their voltage outputs. This probably wont matter while there is a good demand on then from partially discharged batteries but when they become close to fully charged and the regulators start to take effect the alternator with the higher voltage output will take the lions share or perhaps all, of the load. Maybe even this wont matter.

I think the same will apply even with the suggested solutions using the "Blue box" equipment.
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I have 2 alternators driven by the same engine to charge domestic battery and the engine start battery. These batteries are also charged by solar panels at the same time.

If and when I combine both my domestic and engine start batteries this means that 2 alternators charge both battries at the same time. as its DC there is no phase issue so there should be no issue.

If the cutout voltage is different on the 2 different alternators the lower alternator will just cut off first but the second alternator will keep charging until it reaches cut out voltage

It will act just like 2 batteries in parallel

I'll re-phrase my first question:
How often will the OP see any gain from any of this complication?

VicS said:

I see Argofets with 2 or 3 outputs but only one input. Would you connect both alternators to the single input or could you use a Victron Diode Battery Combiner to combine the outputs from the two alternators and feed this into an Argofet.
There would be a voltage loss through the battery combiner but small as it uses Schottky diodes

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For some reason i had it in my head there was a two input Argofet, but you are correct, these is only single input. Vetus make one, but it's about double the cost of two Argofets. You could connect both alternators to the input, but it's likely that you'd only get charge from one, as one would sense the voltage from the other and "think" the batteries were charged.

VicS said:

Alternatively use two low loss splitters, such as Victron Argodiode battery isolators , one for each alternator ( Or even 2 Argofets ) each connected to the two battery banks.

.

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Now my brain is in gear, i would use two Argofets.

Our experience with 2 engines was that if you used both you doubled fuel consumption for maybe a 20% increase in speed. 2 engines would be useful if you have a tide window to catch or punching into seas - but otherwise a waste of money. We would thus run only one engine. Both engines, MD2020, charged a common battery, LA 400 amp, bank. We had one engine start battery, 60 amp, also charged by the engines. We had a 1,2, both off switch but almost never, maybe 2 times, used it - allowing the house bank for engine start. The house bank ran everything, including windlass. But we would have the engines running if using the windlass - simply for manouverability. As mentioned, start engine, leave to run for a few seconds, when you check water flow etc - then start the second engine (lower stress on the battery).

So, unless you are flush, or in a hurry - most of the time when motoring - you will only use one engine.

I don't recall the means to manage charging - but it was installed 25 years ago and there might be improvements since then.

We tried to balance the engine hours, if we were motoring we would used one engine for 2 hours and then the other. If motoring into a light head wind we would use the leeward engine. If the batteries were full - we would bake bread , turn the fridge to colder or 'make' water.

In tight locations, you will already know, 2 engines give you superb control, one engine is very difficult

Jonathan

PaulRainbow said:

For some reason i had it in my head there was a two input Argofet, but you are correct, these is only single input. Vetus make one, but it's about double the cost of two Argofets. You could connect both alternators to the input, but it's likely that you'd only get charge from one, as one would sense the voltage from the other and "think" the batteries were charged.

Now my brain is in gear, i would use two Argofets.

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Paul, I don't want to confuse the OP here, but, I am being a little thick on this one. Looking at the original post I first thought "Just connect the alternators in parallel, but make sure they both sense from the same source, through a VSR or what ever"
You wouldn't want one sensing "start" and the other the "house" as one would in that case still charge both batteries.
If as you suggest above and use one Argofet would not the sensing be for what ever battery required charging? Does the Argofet take care of switch the charge for each battery as required? Admittedly if the regulators in each alternator are slightly different. i.e. one set at 14.2 and one at 14.5 then one would possibly "cut out" before the other, or possibly "cut in" first.
I don't know, not being familiar with Argofets etc. As the resident sage on these things. What do you think?

Using one engine unless in a confined space, which is something I've heard of but had forgotten, suggests that I should stick with one alternator, one battery, but fit a VSR linking the banks. I know Victron do one that works in both directions, I don't know if they all work that way.

Alex_Blackwood said:

Paul, I don't want to confuse the OP here, but, I am being a little thick on this one. Looking at the original post I first thought "Just connect the alternators in parallel, but make sure they both sense from the same source, through a VSR or what ever"
You wouldn't want one sensing "start" and the other the "house" as one would in that case still charge both batteries.
If as you suggest above and use one Argofet would not the sensing be for what ever battery required charging? Does the Argofet take care of switch the charge for each battery as required? Admittedly if the regulators in each alternator are slightly different. i.e. one set at 14.2 and one at 14.5 then one would possibly "cut out" before the other, or possibly "cut in" first.
I don't know, not being familiar with Argofets etc. As the resident sage on these things. What do you think?

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I originally said one Argofet because i mistakenly thought they did a two input version. I'd be wary of connecting two alternators to one Argofet (in parallel) for the reasons i said above. It might, of course, not have a major effect, that will depend on the alternators.

The Argofet acts in the same way as the Olde diode splitters, without the heavy voltage drop. It simply keeps the battery banks isolated from one another, it does not individually regulate charging to the different banks.

Stemar said:

Using one engine unless in a confined space, which is something I've heard of but had forgotten, suggests that I should stick with one alternator, one battery, but fit a VSR linking the banks. I know Victron do one that works in both directions, I don't know if they all work that way.

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Running on one engine can save some fuel, but Neeves calculations are off. Running on one engine does not cut the fuel bill in half. If you shut one engine down you either lose speed, or have to increase RPM of the running engine.

Actually, we might have been over thinking this ! You could leave one alternator connected to the "engine" battery and the other connected to the leisure bank and fit a Victron Cyrix 120 between the two banks. The Cyrix is dual sensing, so all batteries will get charged no matter what charging source is active on either bank.

If you still want to use a single battery to start both engines you can, connect the port starter to the engine isolator but disconnect the charging cable from the alternator and connect it to the domestic bank.

I don’t really see the difference between having two alternators connected in parallel to one battery, versus having two alternators connected to two separate batteries connected together via a Cyrix relay.