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Old 30-05-2019, 03:05   #1
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What start battery to pair with LifePO4 house bank?

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?
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Old 30-05-2019, 04:41   #2
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Re: What start battery to pair with LifePO4 house bank?

May I kindly suggest that you spend a few hours doing research and reading the existing threads about LiFePO4 house banks?
This topic is a basic design issue with almost all LiFePO4 marine installations, and several solutions exist to address it. They have all been covered in this forum, even in another thread very recently.
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Old 30-05-2019, 04:55   #3
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Re: What start battery to pair with LifePO4 house bank?

Use the b2b to charge the start from the house, not the other way around.
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Old 30-05-2019, 05:23   #4
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Re: What start battery to pair with LifePO4 house bank?

Starter could just get a dedicated charge source, no need for it to ever be combined with House.

Or, if your Starter batt is cheap enough, just put a low-amp VSR / combiner to charge it off the LFP bank.

Replacing it every 4 years instead of 6 is cheaper & simpler than investing in a DC-DC.

All major sources should go to House directly and settings be tweaked for LFP.
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Old 30-05-2019, 05:43   #5
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Re: What start battery to pair with LifePO4 house bank?

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Use the b2b to charge the start from the house, not the other way around.
Exactly what I do
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Old 30-05-2019, 07:17   #6
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Re: What start battery to pair with LifePO4 house bank?

Another alternative is to treat your starting battery like a starting battery instead of a second house battery (shocking, no?)

On my boat I have a 24V house system, and a 12V system that does nothing but start the main engine and genset. The main engine has 2 alternators, a very small 12V for the start battery and a larger 24V alternator for the house system. The genset has the cutest little 10AMP 12V alternator.

The 12V alternators are both internally regulated. So my start battery is treated like every start battery in every internal combustion vehicle on the road. Short, shallow discharge, immediate recharge. How long did the last battery you put in your car last???

If the start battery ever dies, it is a piece of cake to use jumper cables to a single battery in the house bank to get the engines running.

Advantages: The start system is completely independent of the house systems. The start battery can be whatever size and chemistry you like, independent of the house bank. No expensive electronics (b2b charger). Redundant charging systems (engine & genset) for the start battery.

Disadvantages: More moving parts (an extra alternator), Buying and mounting extra alternator.
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Old 30-05-2019, 16:17   #7
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Re: What start battery to pair with LifePO4 house bank?

That image you had link to is not for LFP.

I'd be tempted to charge LA with Alternator, LFP with Solar. May be B2B charger that has flying leads so you can charge either way.


If your swinging on mooring and your absent, do you intend having HV protection in case solar gets excessive and if you do How would you protect from consequent low voltage.
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Old 30-05-2019, 18:51   #8
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Re: What start battery to pair with LifePO4 house bank?

I did what you want to do. I have Rellion lifepo 100 amp hour batteries. They do not have a way to shut down the the alternator if the management system decides to shut the batteries down to protect them. The sudden loss of the batteries from the alternator while running could end up frying the Alternator. The Lifepo with two 100 amp batteries limits my 180 amp alternator to 100 amps, so using the regulator I derated the alternator. My starter battery is an odyssey starter rated battery. My solution was to charge primarily the starter battery, the solar does a good job, 600 watts, of keeping the Lifepo's charged. Initially I had my 30 amp victron ac battery charger set to parameters that worked for the lifepo, and were close to what the odyssey should want. I noticed battery acid on the outside of that battery and contacted Odyssey. It turns out odyssey batteries need higher then 30 amps. so I took the starter off the solar and the ac charger. It now only charges after starting the engine, like a car. I have an ACR switch so I can charge the lifepo if needed. (or jump the starter battery) The starter battery acts as a continuous charge receiver for the alternator if the lifepo shuts itself off. I am thinking of adding a third Lifepo, but will likely leave the alternator alone.
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Old 30-05-2019, 19:21   #9
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Re: What start battery to pair with LifePO4 house bank?

It looks like there are two votes:

1) leave the alternator alone and let it charge the start battery. The house bank will be charge by solar only.
2) externally regulate the alternator, connect it to the house bank, and use a B2B charger to charge the start battery.

Does anyone know of a cheap low amp b2b charger just to charge the start battery? Sterling would do the trick but is $$$ and 2A would work.

There was a good question about over charging from Oceanrider--the solar controllers are 100% programmable so this shouldn't be an issue although i will check again. Overcharging from alternator is a risk, hence this post. As for under voltage cutoff frankly the solar is so oversized I don't see how this is a risk. I suppose both MPPT controlers could burn out when I am away and the fridge runs a week or two with no chargers, but that seems pretty remote. I will give this a bit more thought.
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Old 30-05-2019, 20:34   #10
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Re: What start battery to pair with LifePO4 house bank?

Quote:
Originally Posted by SpaceRnglr View Post
It looks like there are two votes:

1) leave the alternator alone and let it charge the start battery. The house bank will be charge by solar only.
2) externally regulate the alternator, connect it to the house bank, and use a B2B charger to charge the start battery.

Does anyone know of a cheap low amp b2b charger just to charge the start battery? Sterling would do the trick but is $$$ and 2A would work.

There was a good question about over charging from Oceanrider--the solar controllers are 100% programmable so this shouldn't be an issue although i will check again. Overcharging from alternator is a risk, hence this post. As for under voltage cutoff frankly the solar is so oversized I don't see how this is a risk. I suppose both MPPT controlers could burn out when I am away and the fridge runs a week or two with no chargers, but that seems pretty remote. I will give this a bit more thought.
Yes the common answer I got when I was looking into this was to go the option 1 route. It is a perfectly valid way to do it Im sure.

I went the B2B route, which as mentioned isnt the cheap option, because I wanted to maximise my alternator charging capacity.

My thinking is that the Start battery needs relatively little charging. So why 'spend' an Alternator on it. The main/ house bank is what really needs the Amps put back.

So I have a couple of 110A 24V alternators feeding my main bank. Then a B2B trickling off to keep the start batt topped off.

But this just the way I have chosen. Obviously not the only way, possibly not the best?
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Old 31-05-2019, 02:05   #11
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Re: What start battery to pair with LifePO4 house bank?

I checked with Victron MPPT supplier, Yes can reduce float voltage to 13.5V thinks that would be safe. LV protect is easier. Heres a cheaper DC2DC charger. https://www.youtube.com/watch?reload=9&v=EcVjPap9dkY


Of course Victron agent says cell management of battery is required. I'm not so sure, think cell monitoring is enough and is economical. My insurance to get me home would be a larger start battery than normal, by swinging Mppt feed to Starter and change mode if necessary.


Tell me if my approach is reasonable or not.
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Old 31-05-2019, 06:44   #12
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Re: What start battery to pair with LifePO4 house bank?

13.5V is higher than I usually use for charging LFP for longevity, especially at low current rates like solar.

13.2V or lower is what I would set Float to, but at least be able to isolate / stop charging manually once you see it's Full.

Get a good FLA for Starter and then any solution you like for getting it topped up will do in practice.

inexpensive ACR don't worry about voltage

Renogy 20A DCDC

even switches to direct charge source output yourself.
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Old 31-05-2019, 09:50   #13
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Re: What start battery to pair with LifePO4 house bank?

I had seen that Renogy B2B charger. It needs an outside 12v signal to turn itself on, like from your engine ignition circuit. I suppose it will work well if I leave the alternator connected to the start battery and wire it to turn on whenever the engine is running. Even the 40a version would limit charging to the LifePO4 house bank with my 120a alternator and I still wouldn't have anything maintaining my start battery if I am away for long periods...

I found this B2B TecMate / OptiMate TM-500 trickle charger that is a lot cheaper and smaller than any other option I have found: https://www.amazon.com/gp/product/B0...k_ql_qh_dp_hza
Instruction manual here: https://www.manualslib.com/manual/15...?page=7#manual

If I decide to install an external regulator and connect the alternator to the house bank I think it would work well to keep the start battery topped up (via any charge source). I know the charge capacity is only 2a, but that should be plenty for replenishing the start battery. The downside I see is that I have no idea how much power it uses to maintain the start battery. If if uses the full 2.7amps all the time it would use almost as much power as my refrigerator over the course of 24 hours. That said, I think it would use a lot less than 2.7a once the start battery is charged. If I did this I would also install a low voltage disconnect on the LiFePO4, which may be a good idea anyway (see earlier post in this thread).

Has anyone used the small TM-500 B2B trickle chargers? Any obvious flaw in this logic?
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Old 31-05-2019, 10:07   #14
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Re: What start battery to pair with LifePO4 house bank?

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I had seen that Renogy B2B charger. It needs an outside 12v signal to turn itself on, like from your engine ignition circuit. I suppose it will work well if I leave the alternator connected to the start battery and wire it to turn on whenever the engine is running
Yes exactly.

But again, why worry so much about a Starter batt, just replace it as needed?

> Even the 40a version would limit charging to the LifePO4 house bank with my 120a alternator

No, do NOT use a charge source on your LFP that does not feature user-custom setpoints!

Then put the alt output to Starter, and go with a Sterling DCDC to condition the charging properly, better for the bank and protects the alt/VR from overcurrent.

Or the external VR upgrade, will give those benefits wrt the alt source only, but (probably) give higher amps.

> I still wouldn't have anything maintaining my start battery if I am away for long periods

Completely separate issue.

> I found this B2B TecMate / OptiMate TM-500 trickle charger

Actually as long as the 2A max is OK, those are **fantastic** for their intended use, fitted quite a few over the years. Note their cutoff to protect the source battery is 10V, way too low afaic, I'd put a higher-setpoint LVC on the input side.

I guess one way to look at it is, if it stops being effective, means something else is wrong with your setup 8-)

> The downside I see is that I have no idea how much power it uses to maintain the start battery.

There are cheap Ah counters you can place wherever you like, but IMO you're overthinking again.

Just put a single high quality BM on the LFP bank.

LVCs should be adjustable per circuit or even per load device.

The master bank-protective one is last-ditch, part of BMS functionality.
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Old 31-05-2019, 10:30   #15
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Re: What start battery to pair with LifePO4 house bank?

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> I found this B2B TecMate / OptiMate TM-500 trickle charger

Actually as long as the 2A max is OK, those are **fantastic** for their intended use, fitted quite a few over the years. Note their cutoff to protect the source battery is 10V, way too low afaic, I'd put a higher-setpoint LVC on the input side.

I guess one way to look at it is, if it stops being effective, means something else is wrong with your setup 8-)

> The downside I see is that I have no idea how much power it uses to maintain the start battery.

There are cheap Ah counters you can place wherever you like, but IMO you're ove

Just put a single high quality BM on the LFP bank.

Iíve already got a Balmar SG200 so once the charger is installed I can surely measure the amps. What I canít find in the manuals is how much power it uses in maintain mode. If it is a power hog I might look for other options since it will always be on. I guess power use during maintain should be quite low so this is sounding like a good option.
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