Options to charge/maintain 12v AGMs from 48v Lithium

[americano]

I need a solution to charge and maintain a 12 volt AGM house bank from my 48 volt lithium battery bank. There are a lot DC-DC charging solutions, but I'm not finding 12V DC-DC chargers that take a 48V input.

Since I'm in the process of setting up a new boat I appreciate all constructive feedback and I'm especially grateful for corrections to flaws in my reasoning.

The boat is a new sailing catamaran that comes from the factory with a 12v AGM house bank (420Ah) and a 60A 110vac to 12vdc charger.

I'm adding solar panels and a 600Ah 48v lithium bank, with two Victron Inverter/chargers (one for 240vac loads and a separate unit for 120vac).

Here is a schematic diagram of the system:


I'm concerned about maintaining and charging the 12v AGM house bank. It powers a lot of critical systems and the life of AGMs is dependent on proper charging.

I'm confident that the 60A factory charger will be able keep up. But, since its powered by the inverter, if either the charger or the inverter should fail, I'll be unable to operate lights, pumps and electronics. I'm not comfortable with two single points of failure on such a critical system.

I think the best solution would be to place a DC-DC charger. But there's the rub. I'm not finding chargers that take a 48 volt input. There are numerous DC-DC converters and I've found one MPPT controller whose manufacturer (Sterling Power) recommends it for this application.

So I guess I have three questions:
1) Can someone recommend a 48V DC charger that can output a 12V AGM charging profile up to 50 or 60 Amps?
2) Could the Sterling Power Products 50A MPPT controller do the job?
3) Would a DC-DC converter be an adequate or even better solution?

I think the first option could be the best because it purpose built for this precise job. It should keep the AGM bank in an optimum state of charge using a specific AGM charging profile.

The Sterling MPPT50 should effectively work like a charger with an AGM charging profile. I was originally looking at a Victron MPPT until I read that they specifically recommend against this use case. I read something saying their algorithms can run into problems in this scenario. Further reading neither confirmed nor dispelled the issues... it just lead to some people say it works, others say it wont. However, Sterling supports it. They put out this video:

video

In any case, the 50A Sterling MPPT is cheap enough that I'm probably going to buy one anyway.

Finally, the DC-DC converter also makes sense. If I understood the working principle correctly, the converter will provide variable current at a constant voltage. It will output a fixed DC voltage. As long as the DC load is within the converter's capacity, the current should flow directly from the converter without drawing from the batteries. If the current draw increases above the converter's capacity, current will flow from the batteries. When the battery voltage drops below the voltage supplied by the converter, excess current will begin to flow to the batteries maintaining their state of charge.

This option raises more questions: Where would I set the output voltage? Should it be at a float stage voltage, or slightly higher? What should I take into consideration to spec the capacity of the converter?

Finally, the most important question: ¿which is the "best" charging solution for this scenario?

[americano]

Someone pointed me to this charger. It could be just what I need.

Mastervolt Mac Plus 48/12-50 https://www.mastervolt.com/products/...plus-48-12-50/

Is anyone familiar with this charger? Anyone know where I can buy it in the US?

[HeinSdL]

Victron Energy Orion-Tr 48/12-30A (360W) - isolated

[Texasvlad]

All solar controllers (MPPT or PWM) does exactly that thing: to convert higher voltage DC from solar panel ( some 25V some 75V) to 12V

[hlev00]

Re: MPPT you may find this useful or at least another pointer for more research.
https://community.victronenergy.com/...y-charger.html

[HeinSdL]

Originally Posted by HeinSdL

Victron Energy Orion-Tr 48/12-30A (360W) - isolated

Oh sorry, too small

[Statistical]

A DC to DC converter simply outputs at a constant voltage. It can power loads and can be used to keep the battery topped off. A DC to DC charger is likely programmable and has 3 stage output (bulk, absorb, float).

So a lot depends on how you normally use it. If you normally have the banks disconnected that is the converter/charger is off and you let the house bank cycle up and down then you will want a charger with 3 stage output.

On the other hand you could use a converter capable of powering your house loads it remains on all the time and powers the loads. Technically you wouldn't even need a 12V bank at all. However keeping the 12V bank allows you to have backup/emergency power source in case the 48V system or converter fails giving you 12V power for critical loads for some period of time.

[Statistical]

Originally Posted by HeinSdL

Oh sorry, too small

On option would be to use two and now you have redundancy. Are you sure that 360W is too low? That is 8.64 kWh (720 Ah) per day. How much are you running on the house DC loads?

[Stu Jackson]

Originally Posted by americano

Can someone recommend a 48V DC charger that can output a 12V AGM charging profile up to 50 or 60 Amps?

Why do you need this size chagre rate for just the AGMs? From your diagram, they are just start batteries which are rarely depleted much if at all. nd unless you have serious access issues for those two batteries, AGM simply is a poor choice for boat batteries.




Given what else is on your diagram, why 48V? You have two diesel engines. Are their alternators 48V?

[Mal Reynolds]

Originally Posted by americano

Someone pointed me to this charger. It could be just what I need.

Mastervolt Mac Plus 48/12-50 https://www.mastervolt.com/products/...plus-48-12-50/

Is anyone familiar with this charger? Anyone know where I can buy it in the US?

I don't think they have been shipped to the US yet. Annoying as this was announced 6 months ago. Ordered two from www.invertersupply.com two weeks ago and they said it would be shipped in two weeks. I reached out to them and they said another week. We'll see.

I'm sure Victon is working on a competitive product, but we've no idea when they might release it. https://panbo.com/victrons-new-orion...-developments/

Please let us know if you find a reliable US source.

[CaptainRivet]

Originally Posted by americano

I need a solution to charge and maintain a 12 volt AGM house bank from my 48 volt lithium battery bank. There are a lot DC-DC charging solutions, but I'm not finding 12V DC-DC chargers that take a 48V input.

Since I'm in the process of setting up a new boat I appreciate all constructive feedback and I'm especially grateful for corrections to flaws in my reasoning.

The boat is a new sailing catamaran that comes from the factory with a 12v AGM house bank (420Ah) and a 60A 110vac to 12vdc charger.

I'm adding solar panels and a 600Ah 48v lithium bank, with two Victron Inverter/chargers (one for 240vac loads and a separate unit for 120vac).

Here is a schematic diagram of the system:


I'm concerned about maintaining and charging the 12v AGM house bank. It powers a lot of critical systems and the life of AGMs is dependent on proper charging.

I'm confident that the 60A factory charger will be able keep up. But, since its powered by the inverter, if either the charger or the inverter should fail, I'll be unable to operate lights, pumps and electronics. I'm not comfortable with two single points of failure on such a critical system.

I think the best solution would be to place a DC-DC charger. But there's the rub. I'm not finding chargers that take a 48 volt input. There are numerous DC-DC converters and I've found one MPPT controller whose manufacturer (Sterling Power) recommends it for this application.

So I guess I have three questions:
1) Can someone recommend a 48V DC charger that can output a 12V AGM charging profile up to 50 or 60 Amps?
2) Could the Sterling Power Products 50A MPPT controller do the job?
3) Would a DC-DC converter be an adequate or even better solution?

I think the first option could be the best because it purpose built for this precise job. It should keep the AGM bank in an optimum state of charge using a specific AGM charging profile.

The Sterling MPPT50 should effectively work like a charger with an AGM charging profile. I was originally looking at a Victron MPPT until I read that they specifically recommend against this use case. I read something saying their algorithms can run into problems in this scenario. Further reading neither confirmed nor dispelled the issues... it just lead to some people say it works, others say it wont. However, Sterling supports it. They put out this video:

In any case, the 50A Sterling MPPT is cheap enough that I'm probably going to buy one anyway.

Finally, the DC-DC converter also makes sense. If I understood the working principle correctly, the converter will provide variable current at a constant voltage. It will output a fixed DC voltage. As long as the DC load is within the converter's capacity, the current should flow directly from the converter without drawing from the batteries. If the current draw increases above the converter's capacity, current will flow from the batteries. When the battery voltage drops below the voltage supplied by the converter, excess current will begin to flow to the batteries maintaining their state of charge.

This option raises more questions: Where would I set the output voltage? Should it be at a float stage voltage, or slightly higher? What should I take into consideration to spec the capacity of the converter?

Finally, the most important question: ¿which is the "best" charging solution for this scenario?

Interesting regarding the MPPT of Sterling.
I would simply replace the AGM bank with a bank from 280 or 304AH their new 316AH EVE lithium with a conactor BMS or Electrodacus BMS and call it a day. The you can charge with a DC2DC converter. Much simpler. No need for lead in that system at all.
AGM are new sell them.

[wefleenor]

I just installed a 48V 600A bank with 3 12V AGM batteries for two engine starts and one gen start. I have the Victron Energy Orion-Tr 48/12-30A (360W) - isolated installed.

The system is all Victron and blessed by 2 distributors and Victron.

[HeinSdL]

Originally Posted by Statistical

On option would be to use two and now you have redundancy. Are you sure that 360W is too low? That is 8.64 kWh (720 Ah) per day. How much are you running on the house DC loads?

Having recommended the Victron 30A dcdc charger I then realized the OP wanted 50 to 60. So I commented on my own post with the 'too small' response. However, in the meantime I came to the realization that 30A -continuous- should not be too small, it's a lot of energy being delivered which surely should cover all but the most lavish setups.

[s/v Jedi]

Of course you can use two Victron Orions in parallel, but they do need to be the smart ones that can charge a battery correctly rather than just output a fixed voltage.

I didn’t see a Victron model for this last time I looked.

[HeinSdL]

Actually that is right, the Victron model is not a charger, it's a converter. Not for battery charging according to the manual. So probably not suitable. I do wonder though, could the output voltage not be set to the battery's float level and leave it there?