12V battery box charged from my 24V battery bank

Mof

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Jul 7, 2021
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Hello, first time poster, please have patience.

I am planning to build a power box with my son as a learning opportunity (for both of us). It will use 4S 60AH Lifepo4 cells with a BMS. We'll add USBs, 12V cigarette lighter and probably use a 500W inverter. Separately, I have an off grid solar array with a 24V 1300AH SLA battery bank and Midnite Solar Classic 150 charge controller. I would like to charge the power box from the 24V bank (via relay) using the "waste not" feature of the Midnite CC.

My question is this. If I feed an MPPT charge controller in the power box from my 24V bank, how do I limit the current? I am afraid the battery box will pull too much current from my large battery bank and will overwhelm the batteries, CC, BMS. How should I limit current coming out of my battery bank? I am thinking about this situation properly?

Hope this question makes sense...

Thx.
 

HRTKD

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A DC-DC charger may be a more appropriate device for charging the 12v battery from the 24v battery. This type of charger is good for limiting the number of amps pulled from the source.
 

Mof

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Thanks. Any recommendations for a DC-DC charger? From what I can find, they are quite pricy.

Perhaps I find a 120V current limiting DC power supply and plug my nose to the inefficiency of the whole process.

I.E. Panels -> CC -> Battery Bank -> Inverter -> Power Supply -> powerbox CC -> powerbox batteries -> powerbox inverter -> 120 appliance (which might have it's own power supply)

What do you think my % efficiency will be? LOL. I have to remind myself that a big part of the reason for this is education.
 

HRTKD

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In my system, it would be a Victron DC-DC charger. The charger could hang directly off of the 24v battery bank. Or, like you suggested in your first post, off of the Midnite Solar charge controller. Many of the Victron devices are programmable so you can set a voltage threshold to begin/end charging.
 

Johnjayjrv

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I have a victron 100/15 that I connect a 48v battery box to the PV input to charge my Trailer 12v battery. The Victron will only pull from the 48v what the max PV input is for the 12v battery. I'm assuming any other MPPT would function the same.
 

Mof

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I have a victron 100/15 that I connect a 48v battery box to the PV input to charge my Trailer 12v battery. The Victron will only pull from the 48v what the max PV input is for the 12v battery. I'm assuming any other MPPT would function the same.
Ahhhhhh... so in your case, the 48v battery box act as 'panels'.

In my situation, the 1300Ah 24v SLA battery bank would act as the 'panels'. So I could use a small mppt (say 20A) in the 60Ah Lifepo4 powerbox and it will pull at most 20A from the 24V bank. So 20A is actually a limit that the MPPT imposes? Or is it a current rating that is not to be exceeded?
 

Wibla

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An MPPT is just a DC-DC converter with extra steps. I'd still prefer to use a regular DC-DC charger for this kind of "duty" though.
 

Mof

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Why? I'm not contesting you, just want to understand your preference for using a DC-DC charger.
 

Wibla

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Simply because a non-isolated Orion 24/12-20 is way cheaper than a Victron 20A MPPT and does the job(y)
 

Johnjayjrv

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If you already have an MPPT that u use with panels it's just good to know u don't have to buy one more device.
 

Johnjayjrv

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Ahhhhhh... so in your case, the 48v battery box act as 'panels'.

In my situation, the 1300Ah 24v SLA battery bank would act as the 'panels'. So I could use a small mppt (say 20A) in the 60Ah Lifepo4 powerbox and it will pull at most 20A from the 24V bank. So 20A is actually a limit that the MPPT imposes? Or is it a current rating that is not to be exceeded?
Yes, u don't want to exceed the current. I, although, don't see me building a 100v battery bank. Lol
 

HRTKD

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Ahhhhhh... so in your case, the 48v battery box act as 'panels'.

In my situation, the 1300Ah 24v SLA battery bank would act as the 'panels'. So I could use a small mppt (say 20A) in the 60Ah Lifepo4 powerbox and it will pull at most 20A from the 24V bank. So 20A is actually a limit that the MPPT imposes? Or is it a current rating that is not to be exceeded?

Maybe. Your SLA battery bank would likely have a nice high voltage to start with and good wiring to limit voltage drop. If the voltage drop is significant then the number of amps pulled would certainly exceed 20. A 20 amp MPPT is going to output a maximum of 20 amps. The input amps/volts will fluctuate.
 

Wibla

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You'll never see >20A on the input of a 24 -> 12V DC-DC charger / buck converter / MPPT that tops out at 20A output.
 

gelmjw

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I also use a spare 15A MPPT controller to charge a 12V from a 24V battery.

With MPPT, it is Watts in = Watts out.

24V * 7.5A = 12V * 15A.
 

Yurtdweller

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I'm in a similar situation. I hope you don't mind if I pop my question in here. I live in a yurt. Fully off-grid. I run a 12v system which is fully built, including a 2kw inverter, a 650w inverter, lots of 12v devices, and a 12v 100ah lfp. I am about to buy 8x280(3.2v)prismatics. I could wire them as two 12v batteries, but I could, of course, wire them as 24v. My cc can handle 24v charging. My plan is to build the 24v pack, and then run a dc to dc charger to maintain the 12v battery on my existing system, so that I do not need to buy new inverters and buck converters. At this time, I simply do not need more inverter power, just more storage capacity. It seems that a dc to dc charger is less cost than a huge buck converter or a new 24v inverter. Also, I have the advantage(?) of only one bms for the 24v pack, instead of two 12v bmss. Is there any obvious flaw in this plan that I am overlooking? Other than my whole system relying on the dc to dc charger? If that device goes down, I can still reconfigure back to my existing setup.
 
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