DC/DC Charger for start battery

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   One of my small projects for our haulout early next year is to fit a DC/DC charger for the engine start battery and dedicate the engine alternator charging for the house bank. I gather someone would have done this already. What size DC/DC charger should I fit? 
Thanks in advance for the input.

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Wayne's World Members Profile Send Private Message Find Members Posts Add to Buddy List Admiral Joined: 18 July 2012 Location: Cruising Status: Offline Points: 1434	Post Options Post Reply Quote Wayne's World Report Post Thanks(0) Quote Reply Topic: DC/DC Charger for start battery Posted: 24 October 2025 at 02:22
	One of my small projects for our haulout early next year is to fit a DC/DC charger for the engine start battery and dedicate the engine alternator charging for the house bank. I gather someone would have done this already. What size DC/DC charger should I fit? Thanks in advance for the input.
	Wayne W Cruising, currently in the Pacific until the end of 2026.

marsella Members Profile Send Private Message Find Members Posts Add to Buddy List Rear Admiral Joined: 21 June 2022 Location: Antigua Jolly H Status: Offline Points: 592	Post Options Post Reply Quote marsella Report Post Thanks(0) Quote Reply Posted: 24 October 2025 at 03:24
	Some discussion on this topic was here: https://www.myhanse.com/lithium-and-agm-batteries-a-warning_topic15238.html I was initially excited by this simple modification but decided to keep the original setup as it keeps engine bat charged no matter what.

Lyn Members Profile Send Private Message Find Members Posts Add to Buddy List Captain Joined: 04 February 2022 Location: Hampton, VA Status: Offline Points: 190	Post Options Post Reply Quote Lyn Report Post Thanks(0) Quote Reply Posted: 24 October 2025 at 14:11
	The first owner of the boat added a second alternator: Mastervolt Alpha 12/130 Multi-groove alternator to the Volvo D3-150. There is also an Alpha Pro Charge Regulator that you can't see here. That charges the house bank as a back-up if the generator has an issue AND I have a shorepower problem, which did actually happen this past summer. I may have this wrong ... but I think the original engine alternator is still on, depicted just behind the white Alpha alternator, driven by the belt. For other non-relevant reasons, I added a DC-DC charger for the engine start battery: Mastervolt Magic, the same as used to maintain the bow & stern thruster batteries. Compact & very inexpensive by boat standards. Edited by Lyn - 24 October 2025 at 14:12
	Jon S/V Lyn 2017 Hanse 588 \| Hull 19 \| Deep draft \| 150hp \| 220v & 110v systems \| Lithium House Bank

Black Diamond Members Profile Send Private Message Find Members Posts Add to Buddy List Admiral Joined: 24 October 2015 Location: Newport, RI, US Status: Offline Points: 1275	Post Options Post Reply Quote Black Diamond Report Post Thanks(0) Quote Reply Posted: 25 October 2025 at 00:36
	I guess I'm curious as to what problem you are trying to solve. Dedicating the engine alternator might top off the house bank faster, but is that really a problem? I would think with solar or an upgraded alternator the existing setup could speed up enough to meet your expectations.
	Rick S/V Black Diamond Hanse 575 Build #192, Hull# 161 Newport, RI

AnnSea Members Profile Send Private Message Find Members Posts Add to Buddy List Captain Joined: 03 March 2008 Location: Australia Status: Offline Points: 159	Post Options Post Reply Quote AnnSea Report Post Thanks(0) Quote Reply Posted: 25 October 2025 at 01:21
	Hi Wayne, I have a Victron DC-DC 20amp charger. Initially I had the 30 amp. 20 amp is way more than enough. Keep in mind that it will run your house bank down unless you have a way of topping it up (solar etc). Yes, it will mostly be in float mode, but it is always on - a consideration for when you are away from the boat for awhile. This setup works even better if your house bank is lithium - they accept as much power as you can throw at them, and are usually at a slightly higher voltage than the start battery. I'm four years into this setup and so far it's perfect, regards, Tim
	Tim AnnSea 370e #418

Wayne's World Members Profile Send Private Message Find Members Posts Add to Buddy List Admiral Joined: 18 July 2012 Location: Cruising Status: Offline Points: 1434	Post Options Post Reply Quote Wayne's World Report Post Thanks(0) Quote Reply Posted: 25 October 2025 at 05:42
	Rick, The issue I am looking to fix is that - a guru electrician in Sint Maarten noticed our generator was starting off our lithium house bank and he suggested that was not a good idea so I changed the generator to start off the engine start AGM. There are cables to jump (the generator or engine) from the house bank if necessary. For some reason the alternator on the generator does not charge the start battery. Originally the alternator was shot and I replaced it but the new one doesn't charge either. This is generally OK but over a period of no use of the engine and starting the generator the start battery starts to get low. If I run the engine or motor sail for a while all is good but if we spend too much time at anchor without running the engine but using the generator the start battery will gradually go down. I have been occasionally charging the start battery with a 10 amp Victron charger. But I thought if I had a DC/DC charger in place it would be constantly topped off. We recently added some portable solar panels (2 x 220w) to our system which now gives us just under 1500w. This seems to mainly meet our needs unless we have some cloudy days in a row.
	Wayne W Cruising, currently in the Pacific until the end of 2026.

Pzucchel Members Profile Send Private Message Find Members Posts Add to Buddy List Rear Admiral Joined: 13 March 2020 Location: floating Status: Offline Points: 659	Post Options Post Reply Quote Pzucchel Report Post Thanks(0) Quote Reply Posted: 25 October 2025 at 06:39
	I may step in with a useless philosophical consideration, but I like to share it anyway because it's the conclusion I got exactly on the same subject. I have 6x lead batteries , thrusters generator and main engine , and two large lithium batteries from mastervolt, the last one doing 90% of the job. even for starting the engine , as I discovered this summer... I have solar panels too, and two mppts. as Leon pointed out various times , and I completely agree , there is no perfect design. however, ALL the issues I had in the last 5 years are ALL related to lead batteries. I had to replace all of them at least once. despite the lead technology being culturally accepted without critical analysis because it is there since a large number of years , it has two fundamental problems: 1) the intrinsic fragility of the chemistry 2) the absence of onboard BMS that can handle device -specific individual issues: unit temperature, charging /discharging currents control, float, cell normalization. can we invent outside solutions and devices to take care of these two problems ?yes , like we have palliatives for terminal diseases...they help , but they don't fix the fundamental issue. so my strategy is : I don't invest into fundamental modifications of my system , but over time I am going to replace all 6x lead based batteries with lithium (or sodium) batteries... times change. we have moved from the art of starting a cold engine , to fuel-less cars that don't care if the engine is cold or warm ...sometimes you can't fix the past issues without a step change. Edited by Pzucchel - 25 October 2025 at 06:47
	Hanse588#55

Black Diamond Members Profile Send Private Message Find Members Posts Add to Buddy List Admiral Joined: 24 October 2015 Location: Newport, RI, US Status: Offline Points: 1275	Post Options Post Reply Quote Black Diamond Report Post Thanks(0) Quote Reply Posted: 25 October 2025 at 09:54
	Wayne, it sounds like something else is going on. On my boat the gen set will throw (say) 110 amps into the batteries (all of them) thru my mastervolt charger. If I turn off the charger, the gen set alternator alone will throw 45-70amps into the batteries (all of them, start, thrusters, house, etc). Not sure why your gen set is not charging the start battery. Why not simply add another small battery for generator start? Why mess with the engine alternator charging setup? FWIW - Edited by Black Diamond - 25 October 2025 at 10:16
	Rick S/V Black Diamond Hanse 575 Build #192, Hull# 161 Newport, RI

Mark Pullen Members Profile Send Private Message Find Members Posts Add to Buddy List Commander Joined: 20 July 2022 Location: London, UK Status: Offline Points: 90	Post Options Post Reply Quote Mark Pullen Report Post Thanks(0) Quote Reply Posted: 25 October 2025 at 10:00
	Hi Pzucchel, have you watched Clark's Adventure on YouTube? He has designed a BMS that specifically allows the combination of Lead and Lithium. It might be worth a look as you transition. https://www.youtube.com/watch?v=VZSr6o5fLHE&list=PLsT7_jPsZM5ogT6or244F49-Gy7pYFV7V&index=11
	Mark

Pzucchel Members Profile Send Private Message Find Members Posts Add to Buddy List Rear Admiral Joined: 13 March 2020 Location: floating Status: Offline Points: 659	Post Options Post Reply Quote Pzucchel Report Post Thanks(0) Quote Reply Posted: 25 October 2025 at 12:53
	interesting...but as i said i took the decision of putting my money into lithium batteries, not trying to modify my setup to have lead coexisting! they last more, they have their own bms, they are lighter, they can be fully discharged and it's not a drama...and sodium batteries will come with their own bms, they will be safer, even lasting longer, and they can will produced for 20USD/ kWh...so i look forward!
	Hanse588#55



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