Charging one battery bank from another

I am drawing a blank, brain lock, and either there is a simple method I am missing or a more complicated answer.
After building out a large LiFePO4 battery bank/inverter/solar system for my RV, I was left with enough spare parts and ideas to build a few DIY Milk Crate and Ammo Box style solar generators and power banks. I have a small ammo box with a 12Ah battery with a powerful LED spot light, and USB accessory charging ports. I have an SAE connector in which to plug a LiFePO4 charger connected to AC power. This unit is very handy and useful, and very portable. I also built a couple of milk crate and tool box solar generators, complete with solar charge controllers and inverters, plus accessories. One with a 280Ah 4-cell battery and the other with a 160Ah 4-cell battery. Both LiFePO4. These are great back up for the house and the RV, but are of course heavy and not so handy. All of these are 12v DC systems.

Is there a way to connect my small portable ammo box 12Ah system to one of the larger systems so that the larger system will charge the smaller system? I cannot think of how to do this, or if it is possible. It would however be very useful---to charge the bigger systems with solar, and in turn use those systems to charge the smaller hand-held portable. Is there a simple connection, or do I need an intermediary device to manage the connection and power?

Does either one of them have AC output via an inverter? You could have the bigger one hooked up to an inverter, then use the AC charger to charge the smaller one.

Any name you wish said:

Does either one of them have AC output via an inverter? You could have the bigger one hooked up to an inverter, then use the AC charger to charge the smaller one.

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Yes, both of the larger systems have pure sine wave inverters incorporated... so yes, could easily plug a charger into an inverter and use that to charge the smaller portable, but that just seems so inefficient to go from DC to AC back to DC. Was hoping there was a simple and more direct way staying in DC 12v.

constantspeed said:

I have an SAE connector in which to plug a LiFePO4 charger connected to AC power.

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48v -> 12v buck converter -> pigtail? Make a small charging station in a cabinet or somewhere you can just wire in a cord?

Looks like one will dump its charge into another pretty quickly like too much so.

Rednecktek said:

48v -> 12v buck converter -> pigtail? Make a small charging station in a cabinet or somewhere you can just wire in a cord?

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Thank you for trying... Everything is already 12v. The bigger systems are 12v, but with 280Ah or 160Ah batteries, and I want to use them to charge smaller 12v batteries, such as a 12Ah battery system. I cannot just connect them. I think I need a device in the middle, some form of DC to DC device to regulate, but something small. I don't need a buck to step down the voltage. Just want to take from the big and give to the small.

constantspeed said:

Thank you for trying... Everything is already 12v. The bigger systems are 12v, but with 280Ah or 160Ah batteries, and I want to use them to charge smaller 12v batteries, such as a 12Ah battery system. I cannot just connect them. I think I need a device in the middle, some form of DC to DC device to regulate, but something small. I don't need a buck to step down the voltage. Just want to take from the big and give to the small.

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Oh, if everything is already 12v then you can just "parallel" the battery banks via pigtail. The 12Ah will top off long before the larger system even notices the dip.

32 volt boater said:

Looks like one will dump its charge into another pretty quickly like too much so.

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Interesting... just glanced at it. But batteries I am talking about are very dissimilar in capacity.

Rednecktek said:

Oh, if everything is already 12v then you can just "parallel" the battery banks via pigtail. The 12Ah will top off long before the larger system even notices the dip.

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Really? That simple? I could just connect these two in parallel, and nothing melts down? (the larger raw cells have a BMS, this is just to illustrate)

No you cannot just take a full one and dump it into a empty one unless you like fried BMS. You can wait until your main packs are almost empty and the voltages are very close. Then parallel them and let the PV charge everything together. Any other method is going to require voltage conversion and some loses.

OK, I sit corrected. I'm so used to lead acids I figured it would be about the same. The more you know!

A DC to DC charger will do it. While you are charging the larger bank. But a good one is expensive.

Actually, a battery Isolator would be a lot cheaper.

Since I do HVAC and have plenty of old used electric kits (or resistors) sitting around. I just run one of those in series to restrict the current flow to something manageable. A typical 280 amp hour 16S full battery going into an empty will flow slightly over 300A with my .20 milliohm cells. With my heat kit installed it drops down to 32A. I have two breakers I can turn on for 15 or 32A. I can add more heaters as I'm using a 6ga wire, and I switched breakers to Square D or Schneider QOU. I don't leave this unattended and it always sets outside. No fan is needed, because the heaters are only dissipating about 5% of what they normally dissipate in your indoor air handlers. The coil temperatures get hotter - 180-220F, than you want to lay your hand on with 48V. 24V setups run about 100F.

RV10flyer said:

Since I do HVAC and have plenty of old used electric kits (or resistors) sitting around. I just run one of those in series to restrict the current flow to something manageable. A typical 280 amp hour 16S full battery going into an empty will flow slightly over 300A with my .20 milliohm cells. With my heat kit installed it drops down to 32A. I have two breakers I can turn on for 15 or 32A. I can add more heaters as I'm using a 6ga wire, and I switched breakers to Square D or Schneider QOU. I don't leave this unattended and it always sets outside. No fan is needed, because the heaters are only dissipating about 5% of what they normally dissipate in your indoor air handlers. The coil temperatures get hotter - 180-220F, than you want to lay your hand on with 48V. 24V setups run about 100F.

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@RV10flyer, FYI, I am a Bonanza flyer. So I am trying to find a smaller solution, something I can build into my milk crate that in turn, my small unit can plug into to charge. Ideal would be 12v (14.5v) at 6amps to charge the 12Ah battery (max charge with BMS). Since all systems I have built are 12v (14.6v LiFePO4), I do not need step-up or step-down from lower or higher voltage, as in a boost. Not sure if something like this would work... https://www.amazon.com/PEMENOL-Auto...9Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=

robby said:

No you cannot just take a full one and dump it into a empty one unless you like fried BMS. You can wait until your main packs are almost empty and the voltages are very close. Then parallel them and let the PV charge everything together. Any other method is going to require voltage conversion and some loses. The only bill I

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Would something like this work? This is where I get lost in the weeds.

constantspeed said:

Would something like this work? This is where I get lost in the weeds.

https://www.amazon.com/PEMENOL-Automatic-Converter-Adjustable-Regulator/dp/B0B84TZZV3/ref=sr_1_38_sspa?crid=2ZJFGVSBCIQCJ&keywords=14.4v+dc+to+dc+converter&qid=1663348635&sprefix=14.4v+dc+to+dc+converter%2Caps%2C90&sr=8-38-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEzU1NLV09POTI1SEdVJmVuY3J5cHRlZElkPUEwNjg3NDkwMjU3S1pRUkc0QlJVUiZlbmNyeXB0ZWRBZElkPUEwNDg4NTIzMUkxTkUwRVA2NTU2QSZ3aWRnZXROYW1lPXNwX210ZiZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=

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It's Limited to 5 Amps only.

robby said:

It's Limited to 5 Amps only.

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Yes, but that is close. The 12Ah battery has a max charge current of 6 amps. There doesn't seem to be a good solution for what I have in mind, in terms of charging from the big battery to the small. So I am going to try a different direction, and see if I can make the small one more independent, like the larger ones, and have it also charge from solar. Will try a small MPPT controller to fit in the ammo box. I think I can still stuff something like this in, and use an external SAE connector for it. Connect it to a 50w or 100w portable flex panel, and this should work. I can use an external LiFePO4 charger plugged into AC power to charge any of them, but then again, that is not the point of being off grid, and sort of a last resort solution. This might be the best solution to help the little ammo box stay independent.

constantspeed said:

@RV10flyer, FYI, I am a Bonanza flyer. So I am trying to find a smaller solution, something I can build into my milk crate that in turn, my small unit can plug into to charge. Ideal would be 12v (14.5v) at 6amps to charge the 12Ah battery (max charge with BMS). Since all systems I have built are 12v (14.6v LiFePO4), I do not need step-up or step-down from lower or higher voltage, as in a boost. Not sure if something like this would work... https://www.amazon.com/PEMENOL-Automatic-Converter-Adjustable-Regulator/dp/B0B84TZZV3/ref=sr_1_38_sspa?crid=2ZJFGVSBCIQCJ&keywords=14.4v+dc+to+dc+converter&qid=1663348635&sprefix=14.4v+dc+to+dc+converter,aps,90&sr=8-38-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEzU1NLV09POTI1SEdVJmVuY3J5cHRlZElkPUEwNjg3NDkwMjU3S1pRUkc0QlJVUiZlbmNyeXB0ZWRBZElkPUEwNDg4NTIzMUkxTkUwRVA2NTU2QSZ3aWRnZXROYW1lPXNwX210ZiZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=

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I figured you were named after Hartzell ?.

I would do some "experimenting" as you know the category I like. I'm not an EE, I wish. Do at your own risk. I'd purchase a 0-100A DC Clamp-On Amp meter, $45-175 on Amazon. Mine for work is 600A rated and $400, but not necessary for solar at our level. It will always come in handy. Here is a resistor to try...