Size dc to dc charges

[B J]

I'm replacing my 3GM30 with 3YM30 but new motors only have a single alternator I'm told that you charge starter battery from house batteries using a dc to dc charger but what size charger do I need

[MerMike]

What type of batteries do you have, how many batteries do you have, what is their capacity, what is the manufacturer's recommended charge rate, and what is your budget?

[Orin]

Because I don’t have enough information to answer this directly this will be a bit long winded. Please bear with me.
Bottom line, total your loads and make sure that your charger output - constant loads is greater than the minimum recommended charging rating… while not allowing charger output to exceed the batteries maximum charging rating.
To get these for a flooded lead acid an approximation would be 25-30% of rated capacity and would be 30-40% of rated capacity (20 hour Ah rate often just listed as Ah on batteries) for AGM batteries. This also varies by manufacturer, technology employed, and chemistry.

It depends on a great many factors. It’s possible to do as you’re indicating charging the house batteries only with alternated power and then using a dc-dc converter to charge the start battery. This option is especially attractive if you want your house and start batteries to be a different voltage, chemistries or for any number of reasons. In this case I would assume that the only loads on the start batteries only loads are the engine starter and engine instruments. This is not the only option and does present some challenges.
If you have a battery disconnect that has a 1-2-both selector for combining the start then you may want an auxiliary contact on that turns off the dc-dc converter when the batteries are combined. This is because if the batteries are combined the dc-dc charger will be trying to charge both banks and will deplete both banks slowly for no gain.

Anyway if you have a 100Ah AGM battery for starting and a 3A instrument load being the only continuous load I would ideally throw a 40A charger at it.…. But only a 30A if you’re using flooded lead acid batteries. If you get it too low or high your batteries won’t last as long. How much their lifespan suffers depends on how far off you are there.

If the batteries in the house bank and starting bank are of similar type you can always just install a battery isolator (diode block) that simply splits the power to both banks but prevents the battery banks from back feeding or equalizing.

Apologies if there are grammatical errors here. Typing this on my tiny phone.
Best Regards!
Orin

[B J]

Start battery is 75ah house battery 2 x 200ah all lead acid alternator rated 90 amp what's the best option to charge both
Thanks

[Jp Nomad]

Victron Argofet isolators
can do 1, 2 or 3 banks
Are good option if all same type of batteries
And are a lot cheaper than dc todc chargers

The 200 amp will be better for your system that the 100 amp

[Orin]

Best is an awfully subjective thing. Simplest with the least fuss? Put a battery isolator in. That’s a pretty big discrepancy in battery size, but in normal use your start battery won’t be as deeply discharged as your house batteries.

https://www.fisheriessupply.com/newm...tery-isolators

If you’re optimizing for battery health a dc-dc converter would work well. 20A is what I would recommend there. If you don’t have an aux contact on your battery disconnect you could always do something like run the “external switch” input of the DC-DC converter to “Accessories” or “run” position of your ignition switch. That way it would only charge as your engines running or if you intentionally put the key in that position to charge the start battery.

18A is the closest standard size victron has for this.
https://www.fisheriessupply.com/vict...arger-isolated

The DC-DC converter option is a little more complicated but in terms of cost they’ll be similar when you’re done as you’ll have more battery cable and lugs with the battery isolator.

Most systems are set-up with just an isolator. Both would work just fine for you I would think.

[Jp Nomad]

Also if you want more control over charge settings, alternator control & protection, engine load & ramp up etc.

A wakespeed ws500 advanced alternator regulator and

Victron argofet 200-2

Might be worth considering

[Orin]

Originally Posted by Jp Nomad

Also if you want more control over charge settings, alternator control & protection, engine load & ramp up etc.

A wakespeed ws500 advanced alternator regulator and

Victron argofet 200-2

Might be worth considering

That victron unit looks good. The guys at Wakespeed are great. I’ve got an externally regulated alternator from Balmor before their brainpower all went to start wakespeed. A bit pricy, but agreed worth considering. I would say regardless of the system it’s a good add. Being able to use an actual charging profile on your batteries is always a plus.

It would then also be technically possible to limit the output current in the event that the only battery that needed charging was the start battery.

[B J]

Looks like the victron argofet will be the easiest
Next question , this is in a catamaran I've got one 90 amp alternator and was only going to charge house battery from the one motor (mainly relying on solar) do I bite the bullet and get another another alternator, will I then need a balancer
Thanks

[mvweebles]

OP has full lead-acid batteries for house and start. Why bother with a DC-DC charger at all? With a 200AH house battery, old-school was to charge start batteries, then a combiner (ACR) to the house bank. Worked well for 40-years or so.

That said, if budget allows, using the traditional alternator to charge house and adding an externally regulated alternator such as a Balmar to charge house bank would be a decent option too.

[Orin]

Originally Posted by B J

Looks like the victron argofet will be the easiest
Next question , this is in a catamaran I've got one 90 amp alternator and was only going to charge house battery from the one motor (mainly relying on solar) do I bite the bullet and get another another alternator, will I then need a balancer
Thanks

I can't tell you what's best for you. Perhaps I am not the best person to speak to yacht electrical design as I do this for fast ferries primarily. That said, what we do:

2 hulls each with alternators and their own start battery, run a cross-over cable to combine start batteries if needed. You can then eliminate the function of the battery switch to combine house and start batteries as you'll already have redundancy there.

Alternatively you can buy battery bank isolators that allow for 2 alternators and 2 or 3 banks.

Alternators do fail periodically, and the risk analysis on what happens if that fails is up to you. Whether you'd want to carry a spare, have a redundant system, or rely on solar in that event is up to you.

[Maine Sail]

Originally Posted by B J

Start battery is 75ah house battery 2 x 200ah all lead acid alternator rated 90 amp what's the best option to charge both
Thanks

A simple ACR will work just fine..


https://marinehowto.com/automatic-charging-relays/





.

[Orin]

Originally Posted by Maine Sail

A simple ACR will work just fine..

https://marinehowto.com/automatic-charging-relays/


.

This operation can be cheaper as well by just using your battery switch. Start with the battery switch in 1 then click over to both while running and 2(house) when at anchor. The logic for when to combine the banks can allow for rapid discharge from the start bank to the house bank. This can be bad for battery health.

Each way you approach this has its ups and downs.