2x Dc2Dc in opposite direction, other ideas?

The house bank was just updated with Lithiums (800ah). The lithiums are connected to only the Inverter for supplying 120v to power the fridge, MW, charging devices, floor freezer, etc. Lets call it house bank #1. The daily draw is around 400AH a day on this bank.

The previous house batts were 3x 8D 260ah AGM fullrivers which I moved to power the 12V dc panel (as well as the davits crane and windlass), and to start the two 3208TA. Not sure of the draw but possibly around 200ah/day but I am not totally sure. I am installing a shunt to get a more accurate value.

750W solar panels are connected to the lithiums. Solars provide around 280-320 AH/ day.

A dc2dc was installed to pull 20/40 amps from the starting batteries and send it to the lithiums, when underway.

Overall, house bank #1 is charged by solars+/- alternator when underway.

The starting batteries/house bank #2 are only charged from the alternator when underway.

In an effort to send some charge to house bank #2 for the days on Anchor, I am thinking of installing a Victron dc2dc to take power from the lithiums (house bank #1) and send it to house bank #2 (starting batts & one mode fullriver). In effect sending some of the solar power to house bank #2. I would have to find a way to limit the duration of the power transfer from the lithiums, not sure how to do that right now but working on it.

So I would have 2 dc2dc connected between 2 house banks but going in opposite directions.

Does this make sense or Am I missing something?

Another option is finding a way to split the solar charge between house bank 1. & 2. Possibly with a manual selector. I would leave the Lithium charging profile to the AMG & lithiums. So at 2pm for example , I would manually switch the destination charging from lithium to AGM. I could find a way to get this done automatically yet.

I would appreciate any comments or advice.

Thanks

Raylee

Sounds more complicated than it needs to be, and you describe making it more complicated! What stops the existing DC2DC from draining the start bank at anchor? Is there a relay to turn them off when engine shutsdown or something?

What I do (900AH lithium house bank) is have the alternators charge the lithiums. Balmar external regulator, programmed for lithium. I have DC2DC units (2 x 30A) charge the start's from the lithiums. You could do the same, use your existing DC2DC to use lithium as source and the 8dD's as destination, running fulltime. The start's wont draw much after engine shutsdown. Solar to house bank only. No d for switches, timers or other gubbins.

To me it sounds to complicated. But I am not the sharpest tool in the shed.

It sounds like back#2 has more life in the batteries and you don't want to remove them. I believe in keeping it simple as can be. I would just add another 200 to 400ah to bank1 and just use bank2 for starting. Less wiring, less components, no manual or auto switching, small starting bank is less wight more useable power from bank2.

I might be missing something by not seeing the setup.

Good luck!

A diagram would sure help, but I think I understand. It does indeed sound like you're trying to salvage the old 8Ds rather than abandon them. Noble but results in a complicated sub-optimal system. Charging your 800ah LFP house bank via a 40a charger isn't enough.

Is there a reason not to go a tried-and-true layout? Move all your 12v loads to the LFP House Bank. 8Ds become starter batteries (one per engine)? Hi-output alternator goes to LFP House Bank with a DC-DC charger to your starter batteries plus whatever parallel switch you want. Simple and efficient.

Thoughts?

Peter

Why have two house banks? You end up with a complicated mess with loads and power sources on each bank. Keep in mind DC to DC converters going in opposite directions will eventually just run down both banks. Turning the entirety of the storage energy into waste heat. You can avoid this with monitoring and relays to ensure only one DC converter is on at a time but more stuff to break.

I would do 1 starter bank and 1 house bank.

I am following because I only have 1 bank=starting and house. Eventually I'd like to separate the start and house sides.

Thank you all for the comments/ideas. I have setup the 2 banks for right now for a few reasons. The cabling for the 12v breaker panel is 8 feet away from the cabling for the inverter/charger. I would need to run connecting cables between them, which I am trying to avoid. Yes, the fullrivers have still a lot left in them and utilizing them would extend help reduce generator time. And finally, until I get to know the BMS of the lithiums, I wanted to run the high-amp demanding functions such as the windlass & davits on the trusted AGMs. My marina neighbor lost his Victron-lithiums (BMS cutting out) 2 miles from the marina. He subsequently added AGMs for windlass & thrusters operations.

My Wesmar thrusters are on a dedicated 48V bank of Lifeline AGMs.

Due to space constraints, I am unable to add more lithium batteries. As mentioned earlier, the Li bank is being charged with the Solars and dc2dc when underway. I installed the highest amp alternator that fits the 3208s but its not a smart unit.

Am I wrong in saying that not too long ago, we had situations with various combinations of dedicated batteries for starting, genset, thrusters, windlass & house. We only had shore or genset power( maybe a small solar), to charge the combinations. Now we have lithiums, big solars, dc2dc, ACR, Buck boost, charge controllers & smart alternators with centerfielders, ext BMS, smart shunts, etc to utilize but aren’t we still dealing with different banks to manage? I do not see that things got less complicated.

I will do a dry run and see how it goes and refine.

Thank you all again for the input.

Raylee

Hooking up two DC-DC chargers between two battery banks sounds like an energy loop to me that will accelerate depletion. But I'm not sure. Make sure you check the batteries for elevated temperatures at rest, a sure sign of an energy loop.

According to the OP, you use 400AH/day. With 3.7kw of solar, depending on your latitude, you might see 280AH/day, and that's on a good day. So you will be charging via a DC-DC from your #2 bank which is charged by an alternator. And you have a 48V system for your thrusters but no indication of how these are charged.

I posted a thread a couple months ago postulating that it's starting to make sense to go to a dedicated LFP bank for inverter (but using server-rack batteries and inverter) with a 12V sub-system for necessary electronics (certainly not three 8Ds), so I'm not averse to your general premise. You may want to make a schematic of your entire DC system including all charge sources.

Good luck -

Peter

I've contemplated having bi-directional charging between banks. There are cases where it makes sense, and this may be one, IMO.

I haven't done it, but once you have shunts there are tools that can turn the chargers on/off based on SoC between banks. I think that Victron Cerbo can do this. Or if it is for exceptional use a dedicated on/off switch for the charger can easily be configured, with voltage limits to avoid fully discharging the source battery in case you leave it on inadvertently.

Thank you Peter & Jeff. I did an 6-hr trial yesterday. Left the dock with fully charged batteries, then cruised for 3hrs. Bank 1 was running inverter loads and consuming 12-16A/hr, Bank 2 was running all DC loads including electronics, AP, etc and was consuming 10-14A/hr.

The alternators charged Bank 2 (which stayed fully charged) and dc2dc sent a charge to Bank 1 (Solars were also charging Bank 1.. Bank 1 remained fully charged). So for the 3 hrs underway… Both banks remained fully charged and I suspect a good deal of Solar & alternator energy was not used & lost so to speak.

I dropped the anchor after cruising for 3hrs and the alternator dc2dc shut down when engines are not running. I stayed at anchor for just over 3hrs with only Solars as the energy source charging bank 1. I generally measure actual Solar gains to be between 3.4-4.2KW. With a median of around 3.65KW/day.

For the 3hrs at anchor, I ran the MW, used the induction cooktop (I always use the LP stove & oven at anchor but decided to use the induction for testing), watched TV for a bit, worked on the laptop and charged devices. The solars kept charging Bank 1 lithiums and the second dc2dc was charging the Bank 2 AGMs between 6-15AH (I am using a 30A Victron Orion turned down to 50%).

At the end of 3hrs at anchor, Bank 1 lithiums were at 96%, Bank 2 AGMs were at 100%. From what I learned, I probably need a lower powered dc2dc to charge the AGMs, or possibly following Jeff F’s idea of using a Cerbo or such to control the 30A Orion.

As background, I have 735AH usable in the lithiums bank 1 and 360AH realistically usable in the fullrivers bank 2. I will do an overnight dry run this weekend and see. I anticipate running the genset every so often to run mainly the watermaker, dishwasher, trash compactor, etc. Thrusters are 48V and have their dedicated charger (shore or genset) and a large dedicated Lifeline AGM bank that only needs topping off every month if that. I was trained on a boat with no thrusters, so don't use them much.

Raylee

Raylee said:

At the end of 3hrs at anchor, Bank 1 lithiums were at 96%, Bank 2 AGMs were at 100%. From what I learned, I probably need a lower powered dc2dc to charge the AGMs, or possibly following Jeff F’s idea of using a Cerbo or such to control the 30A Orion.

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I think if it were me I'd be happy to use the LI bank to keep the AGMs at 100% when at anchor. Better to cycle the larger LI bank than the AGMs. I don't see any issues with a 30a charger.

I think Peter is right to be concerned about an energy loop. You only want this charger on when the other is off, I think. Maybe switched on ignition, where ON activates the AGM -> LI and OFF activates LI -> AGM. Or just turn on the Orion via Bluetooth when at anchor.