ningaloo avatar image
ningaloo asked

Charging 12V AGM from 48V lithium

I am commissioning a new boat which will have 3 x 160Ah AGM batteries for the 12V house bank. I have a 310W solar panel and propose using a SmartSolar MPPT 100/30 for this. I also have a "spare" 1200Wh 48V lithium battery and I'm wondering if I could use this as an alternative source for topping up the 12V system through the same solar regulator (having a switch to ensure only one source at a time). I would only plan to use this when sailing at night when my solar contributes nothing and I'm using a fair bit of power with fridge, instruments, lights and autopilot.

For the sake of this scenario I am not worried about getting the 48V battery recharged - that is another issue.

Any drawbacks in this arrangement?

battery chargingMPPT SmartSolar
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4 Answers
dazey77 avatar image
dazey77 answered ·

Why would you not use an Orion DC-DC converter for this? e.g. Orion-Tr 48/12-9

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ningaloo avatar image ningaloo commented ·

I have looked at these however I thought that these were "just" voltage converters. Do they also take the SOC of the 12V into account and have multi- levels of charge based in this?

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ningaloo avatar image
ningaloo answered ·

So I have done some more reading up on the potential issues using the mppt regulator for dc-dc charging. I understand that there is a possibility of the regular shorting the input(?) to protect the battery from overcharging. This may be ok when using PV but not a battery as input.

I cannot find any 48-12 chargers but there are 48-12 Orion converters which seem to be recommended for my scenario.

So my follow up question is how this might be configured. If I got the 9A converter this would meet my needs. Does the device limit the current to 9A from the 48V input? I'm thinking I would set the charging voltage to be 13.8(?) for the AGM and manually switch off when/if the SOC reaches a predetermined level.

Does this sound right?

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Kevin Young avatar image Kevin Young commented ·


I have the same requirement - what solution did you use in the end?

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tsawind avatar image tsawind Kevin Young commented ·

I am looking for an elegant solution to this problem as well. I have a camper with 50amp service, 48v lithium bank, and 2 Quattros. The factory converter is a WFCO (WF-9855) power converter and a 12v lead-acid battery. (This converter takes 120vac+- input to charge the lead battery and keeps the whole 12vdc system working) There is no logical reason that Victron does not have a 48vdc to 12vdc smart charger/converter 55amps or greater. I really don't want to use 2 or 3 Orion-Tr 48/12-30, for well over $400, to just have a dumb conversion happening, keeping my 12v battery partially charged. This is a common issues with these systems and we all want a solid solution.

I'm am almost positive that I could use a MPPT to do what we want to do. PV input from the 48v batteries. Output to the factory 12v battery. Fully programable charging output, adjusts based on temperatures, very low parasitic draw, talks to the cerbo gx ect ect. There is a lot of misinformation on the web, and a lot of people are saying this is "unapproved usage" ect ect.

Please clarify a good way, and/or the right way, to replace these factory converters for 48v systems, even if it isn't Victron branded, at least give us some recommendations go from 48v to 12v on these factory setups.

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Justin Cook avatar image Justin Cook ♦♦ tsawind commented ·

For the record, yes, it is an unsupported use of an MPPT to connect a battery to its PV inputs. That could easily damage the MPPT, and thus have no warranty - it is a solar charger, not a battery-to-battery charger. Some have done this with success, it is true, while others have tried and failed, causing damage to the MPPT and then being upset when the damage is not covered by the MPPT's warranty.

I'm not sure why there's such consternation over finding 48-12 DC-DC chargers, a simple Google search will reveal multiple options.

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tsawind avatar image tsawind Justin Cook ♦♦ commented ·
Which product, or products, would you suggest for the 48-12 DC-DC charger, to replace our factory unit? Keep in mind, my Google search results are not the same as yours. I am in the U.S.A currently.
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ronski avatar image ronski commented ·
The 48-12 Orion converters have a remote control terminal, use this in conjunction with the relay on the Cerbo, then you can have some simple Node Red logic to turn the Orion converter on and off as required.

I suspect the the Orion may try and supply more than 9A, thus over loading it.

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sebcbien avatar image
sebcbien answered ·

Hello, I have this setup and I use an Orion Orion-Tr 48/12-9.

Be aware that no power can be present when you switch it on (via remote)

See here: Orion-TR 12/12-9A as charger output doesnt goes up until batterie ist disconnected (several hundred pieces) - Victron Community (

To overcome this, I've used two big diodes in parallel so there is no voltage.

This setup is working perfectly since one year, I'm outputting something like 13,5V becaus I ave an Euro 6 and want to give spare to the alternator to charge when braking

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dtemp avatar image
dtemp answered ·

We just did an install with a 48V lithium bank and a 12V AGM back. We used 2 of the Orion TR 48|12 30 DC-DC converters. They are "dumb" devices where 12V voltage is set with a potentiometer. The behavior we see is when they are set to 13.2V, as the 12V bank gets to about 12.9 volts they will start supplying amperage. They hold the 12V bank at about 12.6V. Here is the catch, they do not charge the 12V bank, they basically cover all the loads on the DC bus, and you see their output go up and down with the loads. If the loads ever exceed the output of the DC-DC converter, it draws that power from the 12V bank. Once the loads drop below the capability of the DC-DC output, the DC-DC converter will hold the 12V bank at that SOC. So they work very well for keeping a 12V bank from falling down, but our experience is they do not charge (unlike the smaller DC-DC Chargers that can actually charge from one bank to another). That has been our experience, and we are quite happy with it as that is what we wanted, the 48V bank is the fail safe to the 12V bank falling flat and we key generator starts off the SOC of the 48V bank.

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