What start battery to pair with LifePO4 house bank?

[c.spots]

Originally Posted by SpaceRnglr

I just can't find the answer to this question for my upcoming electrical upgrade. I want to install a LiFePO4 house bank but a cheap FLA for the start battery seems to make the most sense. What engine start battery do people normally pair to a LiFePO4 house bank, and how do they charge it?

The boat lives on a swing mooring currently, but is being prepared for extended cruising. I have no generator or AC chargers. My electrical setup will be very close to this link and solar will meet 95+% of my power needs with the rest from the alternator: https://marinehowto.com/wp-content/u...1-1024x539.jpg

Option A, set all charge sources to LifePO4 friendly values (I would have to externally regulate my current alternator), connect them to the house bank, and use a simple Blue Sea ACR that I already own to charge the start battery. The ACR closes at 13.0V, so I suppose it would always be closed as the LiFePO4 would almost always be above that voltage. Maybe this doesn't work at all...

Option b is to connect the 14.4V charging alternator to the start battery and use a battery to battery charger to charge the house bank. This would limit the alternator current to 60A (I have a 120A alternator) and is more expensive than option A. B2B charges seem to only work in one direction so I don't think the solar on my house bank would keep my start battery charged. That could be a problem if I am away from the boat for a few weeks (although I could start from the house bank).

Option C is to connect the alternator to the start battery and just rely on the battery parallel switch for those few occasions when I need a boost from the alternator. This means I don't have to buy any new equipment.

Option D, forget the start battery as LiFePO4s still have sufficient start voltage even at 0 SOC. Honestly, I would rather have a start battery...

I feel like I am over thinking this, but haven't cracked it yet. Any experience from others?

I like your option A.
This assumes your major charge source is not the alternator, but solar. Set your charge voltages very low, so it shuts down long before the BMS does. It also lets the solar do most of the work when it can, taking wear off the alternator. But make sure you have an alternator temp sensor.
1. It gives you an emergency start battery in case of battery failure. When the voltages reaches 12.7-12.8, the circuit closes, separating the batteries: leaving reserve power in your start battery.
2. Since the circuit on the VSR is almost always open, your start battery becomes part of your house bank. I haven’t seen anybody mention this yet. Could it decrease the battery life by a few cycles? It could also increase battery life since the larger battery bank doesn’t need to drain as low.

Thoughts please.

[DeeBee]

I'm considering the following, other than being a little pricey it keeps things separated pretty well.

This is for a sailboat that will not be plugged in as it will live on a mooring can all season so shore power is not available.

12v Lead Starter with the Renogy 100w 12v Premium kit which includes a MPPT

24v LiFePO4 100ah House battery with the Renogy 200w 12v (hooked in series to make 100watt 24v) with MPPT. (I realize it's under sized, but I'm not cruising and my power consumption, for now, is way less than available power.)

This is the part I'm not sure about...
I would connect the engine alternator (unsure the size as it's a new-to-me boat) to a switch then to both MPPTs. The switch would allow me to select between the start and the house bank when needed. The normal position would be to the starter.

Questions:
Will the 12v alternator work to charge the 24v house bank? The MPPT specs says it auto-detects voltages, but do the input and output voltages need to match?
https://www.renogy.com/rover-li-20-a...ge-controller/

If it's charging the LFP batteries and they are full, will the MPPT protect the alternator when it stops charging? Or would it be safer to put a B2B between the alternator and the MPPT?

[SpaceRnglr]

Originally Posted by DeeBee

Will the 12v alternator work to charge the 24v house bank? The MPPT specs says it auto-detects voltages, but do the input and output voltages need to match?
https://www.renogy.com/rover-li-20-a...ge-controller/

If it's charging the LFP batteries and they are full, will the MPPT protect the alternator when it stops charging? Or would it be safer to put a B2B between the alternator and the MPPT?

You wouldn't run the alternator through any device including MPPT charge controllers. Alternators are connected directly to the battery possibly via a combined charging bus. Most alternators are internally regulated, but some are more sophisticated and use external regulation so the charging parameters can be modified. Your 12v alternator cannot charge the 24v system--do not connect them or you will damage your equipment!

I'm not saying running through MPPT can't possibly work, but why do it? Here is a link to Victron discussing this topic https://community.victronenergy.com/...c-charger.html
I've owned my boat 3 years now and you can find some pretty silly threads in my history of me trying to take shortcuts. So far, very few have worked.

Under your plan you will have to leave the alternator connected to the 12v start battery. You could run a battery to battery charger between the two batteries and you can find them that will convert voltage (e.g. 12v to 24v). That seems a reasonable option in your case as you would otherwise have to replace the alternator with a 24v version.

Right now my alternator is only charging my start battery. This is a real waste of my 120a alternator, but I have 1,000w of solar so I don't really need the power. I have a plan to eventually install a Balmar external regulator on my existing alternator and run it to the house bank, but that is somewhere down the line. If I do that I will need some way to charge my start battery, or perhaps I just get rid of it... Either way that is a non-urgent question somewhere pretty far down the list for me.

FYI, I have attached the schematic of my DC charging system. I hope it is helpful!