DC to DC charger size

MFD · Apr 21, 2025

I have the original Hitachi 80 amp on my yanmar.
Start battery is still AGM, house bank is new 2x460AH LifePO4.
I bought a Victron OrionXS 50 amp DC=>DC charger.
I currently have it set to limit itself to 40 amps on the input side thinking this would be a reasonable starting point for some testing.
I only have a few hours on the engine so far and it has seemed okay.

When I do some sustained motoring distance later this year and its warmer outside, I plan to watch data on the Orion and check the alternator temperature, etc. and see if I can adjust it up to the full 50 - or (hopefully not) need to lower it.

These Hitachi's are notoriously horrible alternators for sailboats, and getting even 50% (40a) out of them is probably asking too much once they warm up. Maine Sail has a pretty good article on them. One day I might replace it, like when it finally fails.

Johann · Apr 21, 2025

ice breaker said:
I have a question for all of you that have switched to lithium house banks but kept the AGM start batery. What size DC to DC charger did you install? I have an 80amp alternator on my Yanmar. I would like to go as large as possible in order to facilitate faster house charging, but I do not want to kill the alternator or the start batery. Your comments would be appreciated.

Thanks

I have been using the old Victron Orion Smart Tr 30A DC charger from an 80A Hitachi for about 5 years and no problems. The old Orions are pretty inefficient so the alternator was outputting about 40A with no overheating. But my engine room is fairly well ventilated. You could use the new efficient Orion XS 50A. You can adjust the output and input current setting in the Bluetooth app. There shouldn’t be any worries about the starter battery, as there is an ignition activation for the Orion, as well as input voltage lockout/engine running detection.

Stu Jackson · Apr 21, 2025

MFD said:
Maine Sail has a pretty good article on them. One day I might replace it, like when it finally fails.

Here is the article, in full:

Hitachi/Yanmar Alternators: (by Maine Sail)

Some alternators though, such as those made by Hitachi and found on Yanmar diesels, are dumber than a pound of beetle poop. Actually, to the alternator, they are pretty smart but to your batteries and the speed of charging they are flat out stupid. Why?

Hitachi alts with dumb regulators, and some others, limit voltage but also reduce voltage based on alternator temperature. This is a self protective feature installed in the internal dumb regulator to prevent the alternator from cooking itself. Remember voltage is the pressure that allows more current to flow. So, if we reduce the absorption voltage, then we also reduce the current the alternator is supplying.. Any battery at any state of charge simply will not accept the same current at 13.4V that it did at 14.4V and as a result the alternator will run cooler. What do you suppose this does to your batteries over time.......?

The problem is that when cold you will get 14.3V to 14.4V out of the Hitachi but as the alternator heats up the dumb regulator begins to reduce the CV/voltage limit based on the alternators internal temperature. It is not uncommon to find a Hitachi alternator at 13.4V when hot. This is REALLY, REALLY DUMB....

If you have a dumb regulator, and notice the voltage dropping, it is likely a temp compensated dumb regulator. Get rid of it or plan to buy new batteries more often.

If you have a temp compensated alternator or a Hitachi alternator on a Yanmar you really are in dire need of external regulation if deep cycling a larger battery bank.

This is from:
http://forums.sbo.sailboatowners.com/showthread.php?t=125392
and these, too:
Hitachi Alternator and Smart Regulator Instal Question - Cruisers & Sailing Forums
Most practical way to upgrade Yanmar alternator? - Cruisers & Sailing Forums

Johann · Apr 21, 2025

MFD said:
I have the original Hitachi 80 amp on my yanmar.
Start battery is still AGM, house bank is new 2x460AH LifePO4.
I bought a Victron OrionXS 50 amp DC=>DC charger.
I currently have it set to limit itself to 40 amps on the input side thinking this would be a reasonable starting point for some testing.
I only have a few hours on the engine so far and it has seemed okay.

When I do some sustained motoring distance later this year and its warmer outside, I plan to watch data on the Orion and check the alternator temperature, etc. and see if I can adjust it up to the full 50 - or (hopefully not) need to lower it.

These Hitachi's are notoriously horrible alternators for sailboats, and getting even 50% (40a) out of them is probably asking too much once they warm up. Maine Sail has a pretty good article on them. One day I might replace it, like when it finally fails.

A couple of summers ago we motored about 20 hours (two 10 hour days) with the 80A Hitachis at 38-40A output the whole time. It was Florida hot and we ran the AC the whole trip so the Orions stayed in bulk at 30-32A. I had just added cooling fans to the Orions so I was keeping track of their output and efficiency. As long as you have some ventilation I think you will be fine at 40A from the XS.

Stu Jackson · Apr 21, 2025

ANY and ALL alternators WILL WORK more cooly when they are current limited.

Maine Sail · Apr 21, 2025

ice breaker said:
I have a question for all of you that have switched to lithium house banks but kept the AGM start batery. What size DC to DC charger did you install? I have an 80amp alternator on my Yanmar. I would like to go as large as possible in order to facilitate faster house charging, but I do not want to kill the alternator or the start batery. Your comments would be appreciated.

Thanks

Use a Victron Orion XS50 and set it at 40A…

marcham · Apr 21, 2025

ice breaker said:
I have a question for all of you that have switched to lithium house banks but kept the AGM start batery. What size DC to DC charger did you install? I have an 80amp alternator on my Yanmar. I would like to go as large as possible in order to facilitate faster house charging, but I do not want to kill the alternator or the start batery. Your comments would be appreciated.

Thanks

Do you have an external regulator with a temperature sensor? (ie: WakeSpeed, Electromaax)

With a regulator that monitors temp, you can get the max output till max temp is reached. I've set mine to 80 deg C, which is pretty conservative and I rarely see it go over 75 deg C.

Are you planning to connect the alternator to the house battery? You're not getting the full benefit of lithium without making this important change.... then all you need is a small dc-dc converter (10-15 A) to charge the start battery from the house (wired to only be enabled when the ignition is ON).

The hitachi 80A are well built.... but you will only be getting a small amount of amps out of it while it's connected to the starter battery, especially if you don't have an external regulator.

With a 100 Amp alternator, my 630 Ah bank is usually fully recharged daily with 1-2 hrs of motoring (enough time to start, raise anchor, motor out of the bay raise sails, drop sails, motor in, drop anchor...).

MFD · Apr 21, 2025

@ice breaker said they have the original 80-amp alternator that came with their Yanmar.
One reason people do this is to avoid the costs and work of updating the engine based charging system when installing a LifePO4 house bank. At least that was my reason and I am setup the same way.
Basically I just left the alternator and start battery alone, took out the old combiner charging system and put in the DC=>DC start=>house.
One day I may go back and upgrade the alternator and do an external regulator and reverse the DC=>DC charger, but that is not on the roadmap anytime soon. Very little of my charging throughout the year comes from the engine anyway so it is just not a high priority for me.

I only have a few hours on the engine since the change, but notes show that on both runs, the alternator (plus start battery) settled in around 13.7 volts and 40 amps (I have input set to limit at 40) to the DC=>DC charger and stayed that way. I purposefully had the LifePO4 house bank somewhat discharged as a simple test scenario.

So in summary, the alternator is giving the LifePO4 house bank everything it is allowed to give via the configuration of the DC=>DC charger.

@ice breaker - it looks like you have a Hunter 41AC, I have a 41DS. You probably already know this - you will want to look at the breakers at the top of your main battery panel. There are pair of 150 amp breakers there, one for each battery bank. They connect to a solenoid between them that is activated by the engine ignition key switch to combine the two banks. To keep the charging systems isolated you want those two breakers off. Otherwise you will be putting wrong voltage Hitachi 80 amp alternator juice into your nice new LifePO4 house bank. Unsure how the DC=>DC charger interacts if the two banks are combined during an emergency like the start battery died. I doubt it would be a problem, it probably does little/nothing, and could be shutoff remotely anyway.