Wiring 2 Battery System, treat at 1 battery or use the marine switch?

[bostonbuzz]

I'm planning a day/small cruising sailboat battery install with 2x 100ah 12v LIFEPO4 batteries. I believe that I don't need a lead-acid battery whatsoever since the LIFEPO4 can handle the cranking and it simplifies things. My issue is that if I use one battery for "house" and one for "starting" as is usual in boats so that you don't accidentally drain both while asleep or listening to the radio, what happens when I switch back to "ALL" on my marine battery switch? The batteries will be at a different SOC and then suddenly be connected in parallel. Is this bad?

Why wouldn't I just treat them as 1x 200ah 12v battery by wiring them in parallel before the switch? The only problem is I have to prevent the batteries from going below X% so that I can still start the engine - but I can just monitor them and I have a small generator on board - so no big problem there.

The other main disadvantage is if 1 battery goes bad, then I will have a little problem and potentially a dangerous one??? If I simply wire them in parallel, should I at least put a fuse on the positive between the two batteries or is this not needed since I'll have a 200A fuse on the main + anyway?

Thanks for the help.

Edit: not sure how my Victron 75/10 solar charger or ac chargers will be charging both batteries if they are not connected to one another in parallel either. Do I need two of each??

 

[jasonhc73]

This is an interesting topic watching you debate your self.

I think you answered yourself when you said you need one as a backup. You shouldn't change how you operate just because you change the chemistry. They run down just like wet batteries do, they are just much longer-lasting, in real-world application basically double what the same AH of a wet battery is rated at.

Would you ever parallel any charged battery with a dead battery? Sure, to jump-start an engine.

It's still just a battery.

They are for all intents designed to be drop-in replacements and used just like wet batteries. The cost is quite steep, however, you get the full capacity, not simply 1/2. Many are guaranteed to work 10 full years, and even then you still have 80% of usable capacity. It's a very tall hurdle to convince someone to spend $1000 on a car battery that will easily last 15 years when you can do it for $80 every other year.

 

[SolarRat]

I'd use both as one big house battery, but would keep the LA starter battery as a separate system. If you want to use only the lithium house battery you'll need to use some sort of DC to DC charger between the alternator and the battery bank, otherwise you may greatly shorten the life of your alternator...they aren't designed for lithium and can't handle a constant high amp charge the lithium battery will draw.

 

[DanHitch]

A different SOC will cause one to flow and equalize your SOC on both. 200aH is large for a medium sailer unless you are using large AmpHrs somewhere.. Best to learn what you daily use and get. Inverters usually cut off ahead of the newer Lithiums cut offs and allow for your gen / charge start.

 

[bostonbuzz]

Thanks for the input folks.

I plan on using the isolation manager for alternator safety since it seems like the cheapest and easiest way to do itHere.

I have 200ah because I though that was a good amount and my bms for each battery can only handle 100A so I need both batteries to start the engine. I don’t even have an inverter. Using Victron mppt and ac chargers programmed to LIFEPO4.

I will post a wiring diagram.
Thanks

 

[bostonbuzz]

I still don’t understand a few things.
How can my solar charger charge the batteries when they are not combined? It’s a victron 75/15 sized just to keep the batteries topped up and a mooring light on. I don’t want to have the switch to “both” all the time.
when one is worn down and I connect them in parallel, wont that overdo the C rating of the lifepo4 cells? (2c).
I’m using a balmar 614 regulator, which the folks at balmar said I can convert my “dumb” regulator to be controlled by.

 

[danphillips]

I agree with SolarRat, keep a LA for starting, do not subject your LifePo4 batteries to engine starting loads unless you are stranded. Utilize a battery to battery charger to keep the starting battery charged at the correct voltage. Nothing worse on a boat than not having enough voltage to start your engine. Always - safety first.

 

[bostonbuzz]

Thanks. I’m going to have to think hard about that. Adding another dc-dc charger and a LA battery and the wiring to me equals more complexity, all while the lifepo4 batteries are totally capable (batteries rated at 3c discharge ~600amps, and BMS to 180amps). They also have a lot more juice to start than LA.

 

[kernel]

I'd use both as one big house battery, but would keep the LA starter battery as a separate system. If you want to use only the lithium house battery you'll need to use some sort of DC to DC charger between the alternator and the battery bank, otherwise you may greatly shorten the life of your alternator...they aren't designed for lithium and can't handle a constant high amp charge the lithium battery will draw.

Can tailor charge current, or purchase just enough

 

[kernel]

Blue Sea 7611 DC BatteryLink Automatic Charging Relay 120 Amp

Blue Sea 7611 DC BatteryLink Automatic Charging Relay 120 Amp Aux Battery. Automatically combines batteries during charging and isolates batteries when discharging. Ideal for emergency, specialty vehicles, RV and marine applications.

powerwerx.com

These might work. Not sure how many amps you need to start the engine. This is 120A automatic.

 

[kernel]

This is bigger. Not sure that you wouldnt want a manual batt1/batt2 switch. Battery combining can be overridden.

Blue Sea 7622 Automatic Battery Separator for Auxiliary Batteries 12V/500 Amps

Battery separator acts as a smart switch to connect independent battery banks only when charging voltage is present. Otherwise, batteries are isolated and discharge between banks is prevented. Ideal for adding an auxiliary battery to your main battery.

powerwerx.com

 

[Danny]

I use a cheap lead acid starting battery as a reserve you could shore charge it and use a switch to select it in emergency. Maybe Parallel your lifepo4 house and install a Victron SBP

. Not sure how you will charge your lifepo4. See my channel for my project details.