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doublebubble asked

Lithium bank with AGM backup for critical systems.

I'll try to keep this understandable, basicly at the moment on our Catamaran we have 2 x battery banks one for house and one for engine start (2 seperate engines 2 sperate start batt's), I'm in the middle of changing over to lithium for the house supply and leave the starter batteries for starting only, 2 x Orion DCDC chargers for the alt charging from AGM to Lithium. There is installed already a battery switch to combine the two banks. My thought is to change the switch to a 3 way switch to be either Batt 1 or batt 2 or off, and not to be able to join the two banks together, the thought is that if for some reason the Lithium bank shut down it would leave the boat black so by switching to Batt 2 would bring back online the critical systems e.g. lighting, GPS, Autopilot etc until the issue is repaired. The lithium bank will be connected to a Victron Mutiplus 3kw which wold leave us with no AC but that's fine for the interim as we have a 6kva genset, would this work or have I got this all wrong.

Lithium Battery
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Alistair Warburton avatar image
Alistair Warburton answered ·

A line diagram would help but none the less...

Check that the two AGM batteries are not being charged by one source because if they are, and you separate them, one will not charge at all. I am assuming they are separate hence the two ISOLATED DC-DC chargers to charge the Lithium pack from either/both.

Connecting two batteries together, when the SOC of either is not known is a very bad plan, in fact the only thing worse than that would be directly connecting the Lithium to the AGM do not do that, ever.

I am assuming 24V here but 12V would work, just bigger bits... How much DC power do you need?

You could... Connect all the negatives/gourds, they are probably common anyway but wiring would need to be big enough.

Add a single switch to each battery positive, all three, add a high current Schottky diode, or several in parallel on a heat sink depending on the current required, after each switch.

This will allow you to tick any DC source, or any combination of them, without the risk of batteries charging batteries directly. Current will flow, with a 0,5, is volt drop, from which ever connected battery has the highest terminal vorlage at any point in time, naturally without any control system being required. You could also add a voltage stabilizer at the common point after the diodes to boost/Buck the 24/12,ish, volts to a stable 24/12V supply.

With big enough cables and diodes the inverter could run from the common point but you would have to get creative with the switching to bypass the diode in the Lithium positive when it was the only battery connected.

A more reasonable plan is to split your AC circuits, low draw critical stuff and everything else.

Now add a second, much smaller inverter, as a standby for critical circuits, and a changeover on the AC side to move them to the available AC. Motorized switches that do this are readily available so when you turn on the standby inverter you critical AC loads will use it automatically whatever battery is feeding it. that would be switch No4

Hope that helps, give me a shout if you want to talk detail and pictures.


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doublebubble avatar image doublebubble commented ·
Thanks for the reply, 12v system with 720ah lithium bank, there will be 2 AGM batteries, one for each engine with a combiner switch between the 2 AGM start batteries in case one start battery is flat for some unknown reason, DCDC on each alternator to be able to charge the Lithium bank via the two alternators, the AGM's will never be able to be connected to the Lithium (the only common positive connection between the Lithium & the AGM would be the 2 DCDC chargers), two 12v power source positive cables one from the lithium & one from the AGM's to a 3 way switch, batt 1 batt 2 and off, the lithium bank will be our main house bank feeding the 12v positive house power and the Multiplus , if for some reason the BMS shuts down the lithium bank and I loose all 12v house power to the boat I'd be able to select Batt 2 from the AGM's to get 12v power back to the positive load switch panel for lights, navigation equipment and so on and to be able to start the engines, I was hopeing it would be as easy as that, there are no AC critical loads only our 240v freezer so we'll just have to deal with that as it happens until we can get the main Lithium bank back on line.
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