Linking alternators - bad idea?

[Piers]

What if I charged one of my battery banks from both engines' alternators at the same time?

Why am i thinking of doing this? The kit now being driven from the Aux battery bank demands almost the same current as one of the alterators can provide. Hence the thought of paralleling both alternators.

Any foreseable problems?

 

[Andy Cox]

What if I charged one of my battery banks from both engines' alternators at the same time?

Why am i thinking of doing this? The kit now being driven from the Aux battery bank demands almost the same current as one of the alterators can provide. Hence the thought of paralleling both alternators.

Any foreseable problems?

I would have though the best option would be to fit 'smart' multi stage remote regulators to get the most out of the alternators and then install a good quality charge splitter which can divert the necessary current to the batteries that need it, i.e. your house bank.

It's not a cheap exercise, but but depending on your existing alternator and charging set up it can dramatically increase the amount of power going into the system and to where it's needed.

Andy

 

[kashurst]

I replaced my battery charger back in February and discovered that both my alternators and the charger are all connected together. Battery charging is controlled by one of these:
https://www.mastervolt.com/products/battery-mates-electronic-battery-isolator/battery-mate-1603-ig/

seems to work really well.

 

[halcyon]

What if I charged one of my battery banks from both engines' alternators at the same time?

Why am i thinking of doing this? The kit now being driven from the Aux battery bank demands almost the same current as one of the alterators can provide. Hence the thought of paralleling both alternators.

Any foreseable problems?

Been doing doing this since the early 80's for twin engine boats with three battery banks, both alternators charged the service bank, never a problem.

If you have twin engines and two battery banks, fit a Durite or similar VSR between banks, don't gain by expensive units. You get two gains, due to house load the battery voltage may be low due to limited supply from alternator, a second alternator gives you more amperage available. This gives more amperage to battery and better charge voltage, thus better charging.

Brian

 

[petem]

I thought this was the standard config for all twin engined boats to prevent a battery discharge in the event of an alternator failure.

 

[BartW]

Fit a Victron Cyrix between the 2 pos poles of the 2 battery bank

 

[5teve]

I set mine up this way last year. 3 battery banks 2x 1 way vsr's on start up cranks charged first then as they hit voltage the vsr opens up to the house bank.. once both open then both alternators are linked, though in practice you won't get double current as one invariably runs slightly higher voltage than the other and sends the lower voltage one into idle (in a
simplistic description) no issues so far and cranks the house bank 600ah back up quickly...

The only issue I had was my 3 bank charger, so I have the vsr operation disabled when mains is present via a small PSU powering a relay which breaks the negative to the vsr's

Steve

 

[Piers]

What if I charged one of my battery banks from both engines' alternators at the same time?

Why am i thinking of doing this? The kit now being driven from the Aux battery bank demands almost the same current as one of the alterators can provide. Hence the thought of paralleling both alternators.

Any foreseable problems?

Thanks for all the feedback, but I'm sure I'm any the wiser - maybe because I only have O level Physics! What i'm hearing is that allowing two separate alternators (port and stbd) to feed one battery bank is OK.

Am I missing something?

 

[halcyon]

Thanks for all the feedback, but I'm sure I'm any the wiser - maybe because I only have O level Physics! What i'm hearing is that allowing two separate alternators (port and stbd) to feed one battery bank is OK.

Am I missing something?

Not sure of your point or problem, as said we have been doing it with twin alternators charging two or three battery banks since 1984 without a problem.


Brian

 

[MapisM]

Fit a Victron Cyrix between the 2 pos poles of the 2 battery bank

B, I'd be interested to understand more about that. Good topic to be debated in the cockpit with a beer or something in a few days! :encouragement:

@Piers: fwiw, the OEM setup in my DP (which they used in all their boats up to 80', allegedly with no problems) is rather unusual, but addresses also your needs, sort of.
TBH, I wasn't so keen on it from a theoretical standpoint, when I realized how it works.
But it's a 2004 boat, and she didn't explode till now, so hopefully she won't also in the future, touch wood!
There are two battery banks, one for engines start (same bank for both engines) and one for domestic.
One alternator is connected to the first, and the other to the latter.
BUT, there is also a relay that automatically keeps connected in parallel the starting and the domestic banks, whenever the engines are running.
So, in practice, both alternators are always keeping both banks recharged, in parallel.

 

[Zing]

It’s a question I am faced with too. I have a single engine and a genset, but two sets of starter batteries, one for each. The engine batteries failed, so I replaced them and now, few months later the genset batteries have failed, so I thought why bother with the second set? I could simply connect the genset leads to the engine starter battery also.

I have separate charging for the house batteries, which can serve as a back up starter battery too, so redundancy is dealt with.

My house batteries are charged with paralleled AC driven chargers and that works fine, so it seems that paralleled starter battery alternators should also be fine.

Is my thinking OK? If it is, I think Piers’ question will also be addressed.

 

[James L]

There is one issue you may have with two alternators charging the same bank of batteries.
I have this and it works fine, but when I changed one alternator to an aftermarket one I now sometimes get interference in the rev counters.
Every so often as I go over about 1200 revs one or both rev guages will drop to 0, once I get back over about 3000 revs they'll pick up again and work fine.

Someone more knowledgeable than me explained that they were interfering with each other and messing up the signal to the rev counter, nothing serious, but was a bit worrying the first few times.

 

[BartW]

B, I'd be interested to understand more about that. Good topic to be debated in the cockpit with a beer or something in a few days! :encouragement:


...
One alternator is connected to the first, and the other to the latter.
BUT, there is also a relay that automatically keeps connected in parallel the starting and the domestic banks, whenever the engines are running.
So, in practice, both alternators are always keeping both banks recharged, in parallel.

that is exactly what the cyrix does,
it has a small controller,
detecting if there is a "charger" on at least one of the battery's ( >12,5V DC), and linking the two batt. banks in that condition.

if one of the two banks voltage is below <11,5, it won't engage. (for safety)
and it has a contact for a switch to force the relay to engage (for emergency engine start, ...)

these voltage settings are programmeable.

I have this setup in our 28ft Karnic, succesfully installed since 2008.

 

[halcyon]

There is one issue you may have with two alternators charging the same bank of batteries.
I have this and it works fine, but when I changed one alternator to an aftermarket one I now sometimes get interference in the rev counters.
Every so often as I go over about 1200 revs one or both rev guages will drop to 0, once I get back over about 3000 revs they'll pick up again and work fine.

Someone more knowledgeable than me explained that they were interfering with each other and messing up the signal to the rev counter, nothing serious, but was a bit worrying the first few times.

The alternator driving the rev counter has a lower regulator setting and is cutting back, this reduces the pulses to the rev counter ths low RPM.

As said any ties I have been supplying systems for two alternators since 84 without a problem, all you get is one alternator cutting out if the load is low and thus voltage high, load supplied by higher voltage unit.

Ref post #13, 12.5 volt is way to low, a new fully charged battery can be upto 13.2 volt no load, a old one should e 12.7 volt, so relay remains engaged. That was why we set the original VSR's at 13.6 volt, minimum alternator regulation. The 12.9 volt drop out was related to flat battery on charge voltage, nominally 13.0 volt, so below 12.9 you are not charging, no charge source or faulty alternator.

Brian

 

[MapisM]

that is exactly what the cyrix does....

Interesting to hear all that, B.
Also in my DP, the parallel relais can be forced on through a dashboard switch, for emergency start.
I don't think there's any "intelligent" bit in my setup, though: the relay just puts the two banks in parallel as soon as the engines are running, without any voltage pre-check.
Otoh, I can perfectly see why Victron thought to add such feature.
In fact, I might even consider adding it, coming to think of it...

 

[Piers]

YES! Just had an email from Delco Remy tech Support stating: "Yes you are able to parallel the alternators."

Phew.

 

[Piers]

What if I charged one of my battery banks from both engines' alternators at the same time?

Why am i thinking of doing this? The kit now being driven from the Aux battery bank demands almost the same current as one of the alterators can provide. Hence the thought of paralleling both alternators.

Any foreseable problems?

OK, the objective was twofold:

1) To be able to charge the accessories battery bank from both alternators to double the potential charge whilst en route.
2) To prevent the engine start battery bank form becoming discharged.

hence, I have fitted a Blue Sea Automatic Charging Relay https://www.bluesea.com/products/7611/BatteryLink_Automatic_Charging_Relay_-_12V_24V_DC_120A cabled as follows.

The relay is wired on the heavy duty side by a large cable from each battery positive to the main terminals of the relay.

Normally the relay senses the battery from a common negative and a battery positive of the battery to be preserved, i.e. not go flat. In this case, the batteries stay connected until the threshold voltage is reached on the protected battery and then disconnected automatically.

I have interrupted the negative sensing with a switch so I keep control of when the relay operates, defeating the Automatic Paralleling and maintaining the batteries separate. By making the switch the unit is able to sense and providing the sensed battery is not already too discharged (i.e., below the threshold) it will re-connect the batteries for parallel charging.

I have an LED to indicate that the unit is paralleled.

Having made two channel crossings, I can confirm the system works brilliantly.

 

[BartW]

OK, the objective was twofold:

1) To be able to charge the accessories battery bank from both alternators to double the potential charge whilst en route.
2) To prevent the engine start battery bank form becoming discharged.

hence, I have fitted a Blue Sea Automatic Charging Relay https://www.bluesea.com/products/7611/BatteryLink_Automatic_Charging_Relay_-_12V_24V_DC_120A cabled as follows.

The relay is wired on the heavy duty side by a large cable from each battery positive to the main terminals of the relay.

Normally the relay senses the battery from a common negative and a battery positive of the battery to be preserved, i.e. not go flat. In this case, the batteries stay connected until the threshold voltage is reached on the protected battery and then disconnected automatically.

I have interrupted the negative sensing with a switch so I keep control of when the relay operates, defeating the Automatic Paralleling and maintaining the batteries separate. By making the switch the unit is able to sense and providing the sensed battery is not already too discharged (i.e., below the threshold) it will re-connect the batteries for parallel charging.

I have an LED to indicate that the unit is paralleled.

Having made two channel crossings, I can confirm the system works brilliantly.

for those who are interested and don't want to buy in US,
this is exactly the same as what a Victron Cyrix does.
https://www.victronenergy.nl/battery-isolators-and-combiners/cyrix-battery-combiners

 

[MapisM]

this is exactly the same as what a Victron Cyrix does

Darn, I forgot to ask you to show it to me the last time we met.
Oh, well. That's what happens when you are more focused on eating/drinking/going at sea, as opposed to fixing stuff.
I can't complain, all considered!