Feedback on DC plan for LFP/LA system

[Ryban]

This is the latest connection diagram I have drawn up for our DC power distribution and charge wiring and am looking for some feedback.

Key components and design considerations:

• LA start battery, and LFP house bank
• Standard alternator (existing)
• Ability to start the engine from the house bank if needed
• Ability to charge the LFP bank from the altnerator (if needed/desired)
• Ability to charge the LA battery from the solar/dockside power

Our LFP batteries wil be located in the cabin nearish to our DC panel, and our start battery is going to be lcoated close to the engine. Thus the start selector switch will also be close to the engine to keep cable runs short, and ideally we aren't messing with this very much. The other two switches will be co-located in a cabinet with our batteries, or possibly next to our DC distribution panel for more visibility.

Most of the components I already have on hand, except for the busses which I've been waiting until I had the design nailed down.

[User 09]

May I ask....what happens if your lead acid start bank goes bad? Does the alternator have a voltage regulator so it can be changed to charge lithium?

How do you throttle down the voltage if you have to motor for a long while with a bad lead acid battery that had to be disconnected, and now the alternator has to send all it output to the lithium bank. Wont this fry your alternator?

It appears the alternator output passes through the start battery, then through the Orions, can the alternator go through the Orions without a lead acid start battery connected?

Curious, i'm not familiar with how they work.

[Ryban]

That's a good question.

If the LA battery goes bad, it can probably, at least in most cases, be kept in the circuit as a load for the alternator, but just won't hold much of a charge, at least that's normally how lead acid batteries die.

If the problem is bad enough that it has to be removed from the circuit, then I think I might just accept a dead alternator, given we have redundant and sufficient solar sources. Not shown, but there will be a total of 4 discrete MPPT chargers, and our panels are located in different places. Even if a complete knock down strips the boat of solar, we will a 200W deployable stowed below.

[User 09]

Originally Posted by Ryban

That's a good question.

If the LA battery goes bad, it can probably, at least in most cases, be kept in the circuit as a load for the alternator, but just won't hold much of a charge, at least that's normally how lead acid batteries die.

If the problem is bad enough that it has to be removed from the circuit, then I think I might just accept a dead alternator, given we have redundant and sufficient solar sources. Not shown, but there will be a total of 4 discrete MPPT chargers, and our panels are located in different places. Even if a complete knock down strips the boat of solar, we will a 200W deployable stowed below.

What if...you put the alternator output wire, battery pos+ cable and the DC-DC charger on a small two or three stud pos+ busbar? If connected this way and the DC-DC charger was always turned on when the engine was running, i wonder if it would blow the alternator? Or, if it became necessary to disconnect the FLA battery, if done with the engine turned off and then the DC-DC charger turned on before the engine was started, if this would allow alternator voltage to pass through the DC-DC charger?

[newhaul]

To many what ifs in this .
Your diagram looks good . I'm retired us navy damage control specialist and that's even more redundancy than I would install.
Now the what iffs. 1) Fla start battery .what are the chances of a new when you install this system battery failing in 10 years? It will show signs it's getting weaker long before it fails. Replace it every x years to avoid the death possibility.
Personally I would not have the system to start from the house bank wired like you have. I would just go with a starter relay with a momentary switch to energize it if ever needed .

[Downeaster]

Very well thought out and nice schematic, by the way. I guess if you will be doing serious, long range cruising all of the redundancy is low cost insurance. But there is a price... when wired up, those 5 switches will take up a lot of room. And making sure you (or the crew) dial the right 'combination' in a panic situation is another consideration.

@newhaul - very interesting idea with the starter relay. I may consider that for my design so thanks.

[donradcliffe]

Should work, if the LFP BMS have enough current capacity to start the engine.

The other way LA batteries fail is a shorted cell. Impossible to predict, but the good news is that you can get a LA replacement in Timbuktu.

[Ryban]

The starter relay is an interesting option too. I'll look into that.

There are technically 5 switches, but 2 of them are built into the batteries themselves, and only designed to be used to take a battery offline for whatever reason (see photo below).

Of the other 3 switches, one will only ever be used if the start battery dies. The bank selector should only ever have to be used if the LFP batteries fail or are for whatever reason being consumed much faster than we can replenish them. The final switch is probably the one that might get used the most often, as we may want to switch around which way the DC-DC chargers are directing current.

The BMSes I'm using can handle 200A continuous each (so 400A combined if evenly loaded), so should be able to start the engine.

[Ryban]

Oops, forgot to attach the image.