DC/DC Charger for start battery

Hi Wayne, if you already have lithium and lots of solar then DC-DC is perfect. I worked with Lead Acid for 10 years and while I'm no expert I do have good hard won knowledge. Lead Acid is fine (for the price) *IF* you keep it charged up. Don't let it get below 70-80% full and you will get good life out of good quality batteries. My previous set of AGM's lasted 11 years. My current ones are 5-6 years old. Let your lithium do the cycling up and down (in charge) and your lead acid do the short sharp heavy loads then recharge them. And keep in mind that lead acid is pretty foolproof - no fancy electronics involved - they make a great emergency backup in case of BMS failures etc.

The Bank Manager referred to below also seems an excellent product - if I was starting out again I would use it,

Tim

Rick, the electrician in Sint Maarten, Adam, fixed a couple of issues introduced when the Victron dealer in Malta oversaw the fitting of our lithium house bank and the additional 3000w/120amp charger in 2019. We had problems with the original charger/inverter and Adam showed me the Malta install had used smaller size cable to connect the new inverter which was also longer than the original cable for the first unit. This all made sense to me.  Whilst doing this work Adam noticed our generator was starting off the house bank (originally AGM but now lithium). His suggestion was to change this to either the engine start battery or add a separate generator start battery. Because Adam was the guru and we had worked with him for a few seasons whilst in the Caribbean I made the change. The easiest change was to start from the engine start battery. This suited me because generally I'm the "don't add extra complexity, cost or tech to a simple situation" type of guy.   

I thought adding a DC/DC charger to charge the engine start battery would be an easy fix to the problem of the new alternator on the generator not charging. We are now in Fiji and will not return to Aust until Oct 2026 I think the best solution would be to put up with the issue for that period and find a good electrician in Aust to sort out the charging. From what I have seen of the standard of technicians since being in Fiji I don't think I will get it fixed here. Particularly in the more remote areas we will be in.   

Currently when running our inverter/chargers on the generator we get about 200amps into the batteries, when running the Volvo about 100amp, solar (1465w) puts in upto 80amps and when sailing the Watt & Sea puts in about 20amps. When we fitted the original solar (1025w) I worked out we used about 250ah a day. But since then (2017) we have added an additional freezer, Starlink, extra appliances ( bread maker, air fryer etc etc) added the water heater and washing machine to the inverter circuit I think we can now use up to 450ah a day. So we increased our solar with two portable panels so we now have 1465w which on normal days works fine. But on the bad solar days we still need to run the generator. If we are running the watermaker (now 240V) we also need to run the generator. If we are filling scuba tanks we need to run the generator. So I will need to get the generator alternator charging sorted long term I can deal with in for now. 

Great to hear the thruster parts were of use. Your low tech mod to the combiner looks like my type of option. Our house bank of lithiums get to 100% charged normally 5 out of 7 days from our solar so this option would probably work well for me. What brand of combiner did you modify and how hard is it to modify? 

Our tunnel bow thruster is working a treat and no issues with it since fitting it in early 2023.