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Charge 24V system with 48V

MaikaiLife

MaikaiLife

Solar Enthusiast
Joined
Nov 16, 2022
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Location
North Las Vegas
I am seeking assistance in identifying the right hardware solution to enable a 24V system to recharge from a 48V system during emergencies.

Here is the scenario: a home has two separate solar systems. The primary system is substantial, featuring 60kW of 48V batteries connected to a Sol-Ark 15K, which powers the entire home except for one critical load circuit. The secondary system is a smaller 24V Victron setup that runs a single critical circuit. This smaller system ensures that when maintenance is performed on the large system, essential services like Starlink and the POE network remain operational.

I am looking for a Victron device that when the SOC drops to around 20%, I need it to draw power from the 48V system to recharge the 24V batteries.

Any recommendations or guidance on the appropriate Victron device to achieve this would be greatly appreciated. (y)
 
MaikaiLife said:
I am seeking assistance in identifying the right hardware solution to enable a 24V system to recharge from a 48V system during emergencies.

Here is the scenario: a home has two separate solar systems. The primary system is substantial, featuring 60kW of 48V batteries connected to a Sol-Ark 15K, which powers the entire home except for one critical load circuit. The secondary system is a smaller 24V Victron setup that runs a single critical circuit. This smaller system ensures that when maintenance is performed on the large system, essential services like Starlink and the POE network remain operational.

I am looking for a Victron device that when the SOC drops to around 20%, I need it to draw power from the 48V system to recharge the 24V batteries.

Any recommendations or guidance on the appropriate Victron device to achieve this would be greatly appreciated. (y)
Click to expand...

I don't think Victron makes one of their "smart" programmable DC to DC chargers for 48v down to 24v.

You can use their dumber one, and use it as a "supply" instead of a "charger". Just set it's output voltage down to whatever you deem an acceptable voltage for taking over your loads from your 24v battery system.

They are expensive for the given output though, so hopefully you don't need much amperage.

1719081778172.jpeg
 
Here's one idea. Not saying it's a good idea but it's a start.

Victron 48V Phoenix 1.2kVA feeding Victron IP67 Blue Smart Battery Charger (s).​

 
cs1234 said:
Even the small Victron 75/10 MPPT would possibly work depending on his loads.

View attachment 223889
Click to expand...
MPPT's are are not typically designed for the amount of amps they would be faced to deal with when hooked up to a battery instead of a solar array. Not saying they won't work but they certainly aren't intended for this sort of application. You'd want to see if Victron would sign off on it before you did it.
 
OzSolar said:
MPPT are are not typically designed for the amount of amps they would be faced to deal with when hooked up to a battery instead of a solar array. Not saying they won't work but they certainly aren't intended for this sort of application. You'd want to see if Victron would sign off on it before you did it.
Click to expand...
They work pretty well as dc/dc stepdowns, I've done so with my 100/30 for 24->12, but Victron cautions against: some error states cause the MPPT controller to short/crowbar the input, harmless on a solar panel but disastrous when connected to a battery. Using a fast-blow fuse on input might partially mitigate the risk.
 
twrlp said:
They work pretty well as dc/dc stepdowns, I've done so with my 100/30 for 24->12, but Victron cautions against: some error states cause the MPPT controller to short/crowbar the input, harmless on a solar panel but disastrous when connected to a battery. Using a fast-blow fuse on input might partially mitigate the risk.
Click to expand...
I've done it dozens of times without issue with pretty much everyone's MPPT. My comment was only meant to caution that should something go wrong you're most definitely on your own.
 
Excellent suggestions, and I appreciate everyone's input! 👏

I have a question: how can I configure either of the suggested devices (DC-to-DC converter or MPPT) to only draw energy from the 48V system when the state of charge (SOC) of the 24V system drops to a specified level, say 20%?

Here are some additional details about the Victron system: it's a self-contained setup mounted on a board, including a Phoenix Inverter, shunt, charge controller, battery balancer, and Cerbo GX. Is there a way to control when the suggestions engage based on the SOC?

The power usage on the 24V system is minimal, running a total of about 300W AC, so the maximum current draw can be quite low, around 5-7 amps.

Thank you for your assistance!
 
Play with the volt .
So that it only go on if the volt are about that use
Really easy .
My charger its on 13.2 volt so it only charge if the battery is on 60%
Use see attach file
You see the 12volt file x2 for 24 volt and x4 for 48 volt systeem
 

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Last edited:
MaikaiLife said:
Excellent suggestions, and I appreciate everyone's input! 👏

I have a question: how can I configure either of the suggested devices (DC-to-DC converter or MPPT) to only draw energy from the 48V system when the state of charge (SOC) of the 24V system drops to a specified level, say 20%?

Here are some additional details about the Victron system: it's a self-contained setup mounted on a board, including a Phoenix Inverter, shunt, charge controller, battery balancer, and Cerbo GX. Is there a way to control when the suggestions engage based on the SOC?

The power usage on the 24V system is minimal, running a total of about 300W AC, so the maximum current draw can be quite low, around 5-7 amps.

Thank you for your assistance!
Click to expand...

The 75/10 MPPT has a bluetooth configuration method using your phone. You can set the voltage in that to be whatever voltage you want it to start charging at. It will only do 10 amps at whatever voltage you set it to charge at though, i.e. 25 volts @ 10amps = 250watts / 25.5 volts @ 10amps = 255 watts. They do have larger versions of these MPPT controllers of course, pick whatever size you want to supply your desired wattage.

The dumb version, the 48v to 24v converter, it does 16 amps at whatever voltage you set it to output. It uses a little screw to set the voltage output I believe. But that would be enough to cover your loads possibly, but it is 2-3 times more expensive than the 75/10 MPPT. It's also "approved" to hook up to a battery, unlike the 75/10 MPPT which is "designed" for solar panels as the input source, not batteries.


EDIT..

You can't technically tell either of these devices to only turn on and do something when the SOC of the 24v battery is at 20%. But, they will use such a tiny amount of power when the 24v batteries are above whatever voltage you tell them to start outputting voltage at, that it is almost like they are completely turned off.
 
What about this idea... the 48-24V Orion converter has a relay that can trigger it. What if I use the Generator Start stop feature on Relay 1, to trigger the Orion device to start/stop at various charge levels defined in the Gen Start/Stop feature?

1719090100167.png
 
MaikaiLife said:
What about this idea... the 48-24V Orion converter has a relay that can trigger it. What if I use the Generator Start stop feature on Relay 1, to trigger the Orion device to start/stop at various charge levels defined in the Gen Start/Stop feature?
Click to expand...

That sounds complicated, in true Victron fashion. I'm sure it probably works. However, if you just use the dumb voltage method.. you are probably talking idle usage of like 1 watt or something.
 
cs1234 said:
That sounds complicated, in true Victron fashion. I'm sure it probably works. However, if you just use the dumb voltage method.. you are probably talking idle usage of like 1 watt or something.
Click to expand...
So you're suggesting, just leave it on all the time and it will just help keep the batteries topped off with minimal continual draw from the 48V 1200amp battery bank? But wouldn't this essentially always be feeding from the 48V system rather than only utilizing it when there isn't enough solar to bring the 24V system out of the end zone around the 80% DOD?
 
MaikaiLife said:
So you're suggesting, just leave it on all the time and it will just help keep the batteries topped off with minimal continual draw from the 48V 1200amp battery bank? But wouldn't this essentially always be feeding from the 48V system rather than only utilizing it when there isn't enough solar to bring the 24V system out of the end zone around the 80% DOD?
Click to expand...

Current only flows from the 48v-24v device when the battery bank is a lower voltage than whatever voltage you have it configured to output.

MPPT/Charger set for 24.5 volts
Battery bank is at 24.6 volts
NO CURRENT FLOWS - only wasted power is from the very small idle draw of the Victron MPPT/Charger doing the 48v-24v conversion

MPPT/Charger set for 24.5 volts
Battery bank drops to 24.4 volts
CURRENT FLOWS from MPPT/Charger until bank hits 24.5 volts.. then it stops until battery bank dips again.. well not technically stops.. they both supply power to loads equally at 24.5 volts. It's basically like a second battery that only kicks in once you hit 24.5 volts. It just doesn't really "turn off" again until the battery hits 24.51 volts. Once something else charges that battery up to 24.51 volts or higher, it is essentially off.. in idle.

While the battery bank is at 24.5 volts or lower, the LOADS will be being supplied by the MPPT/Charger at 24.5 volts, until the loads go over whatever wattage it is capable of outputing. At that point, the 24v battery will add more power into the mix and then get charged back up to 24.5 volts again when the loads drop below the maximum wattage the MPPT/Charge is capable of outputting.
 
cs1234 said:
Current only flows from the 48v-24v device when the battery bank is a lower voltage than whatever voltage you have it configured to output.

MPPT/Charger set for 24.5 volts
Battery bank is at 24.6 volts
NO CURRENT FLOWS - only wasted power is from the very small idle draw of the Victron MPPT/Charger doing the 48v-24v conversion

MPPT/Charger set for 24.5 volts
Battery bank drops to 24.4 volts
CURRENT FLOWS from MPPT/Charger until bank hits 24.5 volts.. then it stops until battery bank dips again.. well not technically stops.. they both supply power to loads equally at 24.5 volts. It's basically like a second battery that only kicks in once you hit 24.5 volts. It just doesn't really "turn off" again until the battery hits 24.51 volts. Once something else charges that battery up to 24.51 volts or higher, it is essentially off.. in idle.

While the battery bank is at 24.5 volts or lower, the LOADS will be being supplied by the MPPT/Charger at 24.5 volts, until the loads go over whatever wattage it is capable of outputing. At that point, the 24v battery will add more power into the mix and then get charged back up to 24.5 volts again when the loads drop below the maximum wattage the MPPT/Charge is capable of outputting.
Click to expand...
Okay, so if the loads on the 24V system exceed the 120W output of the Orion device, the batteries will supplement the difference to meet the demand. When the load decreases, the Orion device continues to supply energy until it reaches the configured output voltage level, then shuts off.

My only concern is that unless the load consistently exceeds 120W, the 24V system will essentially act as a pass-through, continually drawing from the Orion device. While I'm open to this approach, I also want the 24V system batteries to cycle occasionally down to 20% SOC to allow them to rebalance and maintain their health.

It seems that both methods would achieve the desired outcome, but the key difference is:

1. **Dumb Method**: Always pulls energy from the 48V system unless idle, ensuring continuous power but potentially limiting the 24V batteries' opportunity to cycle and rebalance.

2. **Conditional Method**: Only engages when the 24V system reaches a configured lower SOC threshold, likely remaining inactive on sunny days but allowing the 24V batteries to cycle and maintain health.

I'm interested in both approaches but leaning towards the conditional method to give me more granular control over the on/off status of the Orion unit.

Thank you very much for your constructive feedback, this discussion has been super helpful and I'm already getting multiple application ideas for these Orion devices.
 
MaikaiLife said:
Okay, so if the loads on the 24V system exceed the 120W output of the Orion device, the batteries will supplement the difference to meet the demand. When the load decreases, the Orion device continues to supply energy until it reaches the configured output voltage level, then shuts off.
Click to expand...
Correct.. though it doesn't shut off.. it's just idle and not doing anything as it is at the configured voltage so it's equal with the battery.

MaikaiLife said:
My only concern is that unless the load consistently exceeds 120W, the 24V system will essentially act as a pass-through, continually drawing from the Orion device. While I'm open to this approach, I also want the 24V system batteries to cycle occasionally down to 20% SOC to allow them to rebalance and maintain their health.
Click to expand...

You just configure the Orion/MPPT (whichever) to output a voltage that is the equivalent of 20% SOC on your 24v battery bank. 25.8v on this chart.. you might have to experiment to find the actual voltage of your bank.
LiFePO4 Battery Voltage Charts (12V, 24V & 48V).jpg

MaikaiLife said:
1. **Dumb Method**: Always pulls energy from the 48V system unless idle, ensuring continuous power but potentially limiting the 24V batteries' opportunity to cycle and rebalance.
Click to expand...
It doesn't really limit anything.. you configure it at whatever level you are willing to have the battery drop down to. It will then "float" it at that voltage.. but no higher. Your normal battery charger (solar I'm guessing) will charge it up the rest of the way when the sun comes out. The "float" that the Orion is doing doesn't do anything when a higher voltage is coming in from somewhere else.. it's like it's not even there as far as the battery / other charging source are concerned.
MaikaiLife said:
2. **Conditional Method**: Only engages when the 24V system reaches a configured lower SOC threshold, likely remaining inactive on sunny days but allowing the 24V batteries to cycle and maintain health.
Click to expand...
Yes, you would have more control using the more advanced features in the Cerbo/Multiplus. You get to do it based on actual state of charge of the bank that way. Personally, I probably wouldn't bother.. but I understand the desire to do it a cooler way than simple voltage level control.
 
cs1234 said:
Correct.. though it doesn't shut off.. it's just idle and not doing anything as it is at the configured voltage so it's equal with the battery.



You just configure the Orion/MPPT (whichever) to output a voltage that is the equivalent of 20% SOC on your 24v battery bank. 25.8v on this chart.. you might have to experiment to find the actual voltage of your bank.
View attachment 223936


It doesn't really limit anything.. you configure it at whatever level you are willing to have the battery drop down to. It will then "float" it at that voltage.. but no higher. Your normal battery charger (solar I'm guessing) will charge it up the rest of the way when the sun comes out. The "float" that the Orion is doing doesn't do anything when a higher voltage is coming in from somewhere else.. it's like it's not even there as far as the battery / other charging source are concerned.

Yes, you would have more control using the more advanced features in the Cerbo/Multiplus. You get to do it based on actual state of charge of the bank that way. Personally, I probably wouldn't bother.. but I understand the desire to do it a cooler way than simple voltage level control.
Click to expand...
Excellent points, and I might have another application where I can use both methods. This has been excellent, thank you so much for your feedback and explanations.
 
cs1234 said:
The 75/10 MPPT has a bluetooth configuration method using your phone. You can set the voltage in that to be whatever voltage you want it to start charging at. It will only do 10 amps at whatever voltage you set it to charge at though, i.e. 25 volts @ 10amps = 250watts / 25.5 volts @ 10amps = 255 watts. They do have larger versions of these MPPT controllers of course, pick whatever size you want to supply your desired wattage.

The dumb version, the 48v to 24v converter, it does 16 amps at whatever voltage you set it to output. It uses a little screw to set the voltage output I believe. But that would be enough to cover your loads possibly, but it is 2-3 times more expensive than the 75/10 MPPT. It's also "approved" to hook up to a battery, unlike the 75/10 MPPT which is "designed" for solar panels as the input source, not batteries.


EDIT..

You can't technically tell either of these devices to only turn on and do something when the SOC of the 24v battery is at 20%. But, they will use such a tiny amount of power when the 24v batteries are above whatever voltage you tell them to start outputting voltage at, that it is almost like they are completely turned off.
Click to expand...
Thare 2 model of the 75/15 serie
Bluesolar (no bluetooth)
Smartsolar (have bluetooth)
 
Posting my solution for everyone else.

I was able to get the Orion hooked up to the generator relay on my Cerbo S GX and trigger it based on SOC to enable and slowly feed 5A max from my 48V system into the 24V system and charge up the batteries.

Here are the videos to show my setup and see the solution in action.


 
HarryN said:
Victron is just a brand, not a legal document that must be followed.

The easy way to charge a 24 volt battery from a 48 volt / 120 vac power system is to just plug in a 24 volt battery charger. IOTA is an example brand.



www.solar-electric.com

Iota DLS-27-40: 24 Volt 40 Amp Regulated Battery Charger

www.solar-electric.com www.solar-electric.com
Click to expand...

Its a lot of over head and power lost .
From 48 volt to 120 volt have a lost of 10%
Than from 120 volt go back to 24 volt again a lost of max of 10 % .

Really a good way to do it.
A good think to use dc to dc charger with almost 0 lost
 
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