LiFePO4 reference diagram, 12V version

[fxykty]

Originally Posted by krid2000

fxykty, what software did you use to draw this nice diagram?

Thanks!
Dirk

MS Visio

[fxykty]

Originally Posted by s/v Jedi

Wow, you have your diagram down to the last lightbulb, impressive

What were you thinking when you saw people calling my diagrams “over the top” and “complex”?

The only thing I would like to see different is the house bank voltage. My preference would be 48V but I settled for 24V due to availability of parts I needed.

Also, keep an eye out for redundancy. How do you recover from a component failure for supplying power to electronics etc. Not sure if I saw a battery dedicated for that etc.

Edit: I went back to the diagram to check and see you don’t have that. I highly recommend to add this. Did you see my DC Main panel? In the lower left corner there is the power for electronics. I have a small AGM battery and the dc-dc converter is isolated like yours but it is set as a charger for the agm. Also, there are two of them. The two double pole breakers are in the outputs of those converters.

Thanks!

When we did our change from AGM to LFP three years ago I unfortunately did not consider changing to 24V. To make the change now I think only the charger/inverter and windlass motor would need to be changed, but that’s really expensive and I’ve already done the sunk cost of running the big wires for 12V, sigh.

As a cat I’ve got two AGM start batteries. I can parallel one or both to the load and/or charge bus. For the very unlikely situation of the LFP battery going offline (now the even more unlikely chance of both going offline), I consider the 30 seconds of no power at all as a reasonable compromise. Frankly, I don’t even have room anywhere reasonable for another battery in our salon - we’re a small boat inside.

Regarding “What were you thinking when you saw people calling my diagrams “over the top” and “complex”.” I was thinking, “this is awesome, thanks so much”. Complex, maybe, but fit for purpose. As others have written in this thread and elsewhere, the greater current capabilities of LFP batteries require a systems approach to installation, not just a simple swap of LFP battery for a lead acid battery. YMMV

[fxykty]

Originally Posted by s/v Jedi

I modified the diagram for 12V house batteries. Asyou can see, much is the same. Changes:



1. House batteries from 4x 700Ah Winston cells, 9kWh each

2. BMS and Battery switch 12V versions

3. dc-dc converters are 12-12 versions

4. Inverter/charger is a 12V version

Revisiting our proposed design for two LFP batteries in parallel per your diagram. We currently have separate charge and load bus - you show a combined bus that has both charge sources and loads on it.

A single bus certainly simplifies disconnects of each battery, but it does mean that a LVC event for one battery means that it cannot be easily charged as it is disconnected.

Is there any benefit to maintain separate charge and load bus?

[s/v Jedi]

Originally Posted by fxykty

Revisiting our proposed design for two LFP batteries in parallel per your diagram. We currently have separate charge and load bus - you show a combined bus that has both charge sources and loads on it.

A single bus certainly simplifies disconnects of each battery, but it does mean that a LVC event for one battery means that it cannot be easily charged as it is disconnected.

Is there any benefit to maintain separate charge and load bus?

It is nearly impossible to get one battery to LVC while the other is still online, even when batteries are at drastically different capacities and SOC. The reason is that the near empty battery will not take part in load sharing anymore as soon as it’s voltage starts dropping into the lower knee of the SOC graph. It can linger there, while the other battery runs the show and equalizes to the same SOC levels.

I don’t think you should worry about LVC at all. When this happens, simply start charging and when the BMS detects charge voltage it will reconnect, or some primitive ones need some nudging to get back online but that is documented too.

Also, I don’t think the combined busses are better. I just think that for a sailboat it’s good and complex enough