Solar + MPPT to LifePO4 First, or to AGM First?

DIY
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Good day!

So I’m trying to add more electric options to my boat to be more off grid capable.

I am a beginner to electrical stuff :slight_smile:

Sorry for the wall of text, I will try to explain this as best I can with my limited knowledge.

I have a couple of same AGM batteries, one for engine starter, and one for ‘consumption’. There is a lever to turn on only the engine start battery, a lever to turn on only the ‘consumption’ battery, and a lever to combine the two. The engine start battery charges higher and holds more power for longer than the consumption battery which after full charge falls almost immediately to 12.4V, and 12.2 not long after. I have been running my fridge and lights on the combined setting, and it only lasts a couple of days before the fridge stops working.

Somehow I feel like combining the two is actually dragging down the better starter battery, and they seem like they are wearing out quickly. As I understand AGM is better for starter, and LifePO4 is better for consumption.

So I’ve invested in a new system which includes:

A 180 watt solar panel.
A Victron MPPT 75 | 15 to charge the LifePO4.
A 100 Ah LifePO4 battery to connect to the fridge (which consumes the most).
A Victron Orion 12 | 12 – 18 isolated DC-DC charger to charge the LifePo4 when the engine runs.

This will take most of the load off of the AGM batteries, and they should last longer, and because engine start is a primary safety concern.

But there is a problem: the AGM batteries do not get topped up and kept healthy by the solar. They only charge from shore power or engine. And I wonder how to fix this. I’ve seen ‘two way’ DC-DC chargers but they are out of my budget range for now ($460- 3000!) yikes that could be a number of more batteries.

And I’m considering doing the opposite with my setup IE:

Connect the Victron MPPT 75 | 15 to charge the AGMs instead of the LifePO4s, and use the DC-DC charger to both charge the LifePO4 when the engine runs, and when the AGMs are full, to charge the LifePO4s from solar (will this work?)

It seems most people do the opposite setup, IE MPPT connected to LifePO4, but then the AGMs can sit for long times not fully charged, and as I understand they will wear out even faster and they are very expensive.

Any advice to make this work, without spending a fortune much appreciated :slight_smile:

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The simplest and many people here have done this is to add solar to the AGM battery as you are considering. On the DC to DC charger you then set up engine shutdown detection with a start voltage of say 13.2V (you could set this lower if required, trial and error once installed) and a shutdown voltage of 12.8V. Once the solar increases the AGM voltage to 13.2V the DC to DC charger will start charging the lithium battery and the important thing is if there is not a lot of solar then the DC to DC charger will limit its output current to keep the AGM at 12.8V so that the AGM battery does not discharge, 12.8V being a 100% full AGM battery. Once the lithium battery is full, then the solar will then raise the AGM battery voltage to make sure it is completely full. This is slightly less efficient because the solar charge goes through 2 controllers to get to the lithium battery, both with their own losses.

Another option is to fit the solar to the lithium battery and install a trickle charger from the lithium battery to the AGM such as one of the ones linked below, I use the Ablemail version as it is very configurable but very expensive. There are others depending on your location. This is more expensive, a bit more wiring but it is more efficient.

Votronic trickle charger

Battery Master trickle charger

Ablemail trickle charger

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Thanks for the quick and detailed response! I’m still trying to digest all this…
So option one sounds simpler and uses the hardware I already have, which sounds good. But then since one of my AGM batteries is getting old it may take a lot of solar every day to keep topped up. I guess maybe I’ll just test and see.

Another concern is what happens when the solar is generating power, and the engine is running or shore power is on, and the AGM batteries are being charged by two sources at the same time. Will this cause problems?

Separately with option 2, can I use the DC to DC charger to charge the LifePO4 battery from shore power, and when the engine is running, and at the same time have the trickle charger set to move power the other direction to the AGM? Will these two conflict?
Thanks again :slight_smile:

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Also it seems the main thing I’m missing from option 2 is the ability to charge the LifePO4 from shore power or engine alternator. Is this correct?

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Option 2 is all your current hardware, DC to DC charges lithium from AGM and solar charges lithium so you then have a trickle charger to go from lithium to AGM to keep the AGM happy when you only have solar charging.

2 charging sources is not a problem, you have them all set up to have a target charging voltage and when that is reached they will all reduce charge, you can not put extra charge in. What you can do for option 1 is set the solar charger absorption voltage 0.1V higher than the alternator, then if there is lots of sun the alternator will stop charging before solar, maximising use of solar. On Option 2 you would set the solar voltages 0.1V higher than the DC to DC charge voltages. On my battery bank I have direct alternator charging, solar charging and shorepower charging, I can run all 3 at once if required but as the batteries fill up shorepower stops, then alternator and finally solar.

In Option 2, if the alternator or shorepower are charging the AGM then the trickle charger will not work, it just stops charging because the AGM battery voltage would be too high, it does this automatically.

Finally, if one of your AGM batteries is tired then it may well let some current pass through it even when full. Perhaps you should remove the tired AGM and just run the engine from the one good AGM.

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Well that was a whole lot of information, very well stated. Thank you very much!
So I’m leaning toward option 2, it seems the better choice overall, though a little more complicated, worth the effort. Would you agree?

Also it seems that since I have a voltmeter on the way, I should start researching how to properly test that AGM. Any particular criteria for when you would keep it as a secondary starter battery vs throw it out? It’s space could be used as well…

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Not sure how best to test the AGM with basic equipment.

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OK, well thanks for all the info :slight_smile: