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DC-DC assistance

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Hello all,

I have some electrical background (Navy) but am definitely rusty. I could use some assistance in trying to figure out what I thought would be a simple one. Is there a way to have a DC-DC Bridge? Bi directional? I know with AC a transformer can go both directions. But with DC, I am finding products where I am stuck going one direction (Step up or Step down).

My issue: I have a 12v Battery bank (2 6v) and I want to use a Solar panel and charge controller designed for 24v. I would like to make this as simple as possible with the fewest items/devices or points of failure. On the 12v side is the standard load of an average RV. On the 24v side I would have a few POE powered devices. (Cameras, Wifi Access Point)

I would like to use this unifi device as the charge controller as I like the built in interface:

I have been kicking around a couple of ideas that if what I'm after does not exist I can work my way around the limitations.

1. Add in both a step up and step down DC converter: using items like the two links below.
Step Down:
Step up:,aps,168&sr=8-8

**Would the above solution even work with the ins/outs tied together? Or am I creating some type of loop here that would require some diode or limiter.

2. In place of the step up/down converters, I could add in another very small 12v battery in series to bump voltage up. And in doing this since my cells will now be different sizes than my 2 6V golf cart batteries I would have to add in some balancers or charge equalizers so I don't overcharge the tiny 12v. I would add two of these: One wired between the 2 12v sources (2x6v series batteries and 1 small 12v) and another between just the 2 6v's so they charge equally as well. I realize the second is not as critical as the first.

I realize there may be a much more efficient way to accomplish what I am after and I am all ears to suggestions.

Thanks in advance to any and all who offer advice or assistance.
There are two arrangements that you could use.

The first one is to have a step down for 24 V to 12 V, and a step up for 12 V to 24 V. To stop them both running all the time and power getting lost, you could turn off the 24 V to 12 V converter if the 24 V dropped below maybe 25 V. So when the solar panel is running, it will exceed 25 V, and that will be converted to 12 V to charge the batteries. Also, you could set the 12 V to 24 V converter to output maybe 23 V, and it will simply stop when the voltage is above 23 V. That assumes that your camera and POE devices are OK on 23 to 25 V, which they probably are.

The second method is to have a 24 V to 12 V step down connecting the solar panel to the batteries. It would run whenever there is any voltage more than the battery voltage from the solar panel. You then have a 12 V to 24 V step-up converter that creates a separate 24 V supply for the camera and POE devices. That would mean that when the sun is shining, the camera and POE are powered via a 24 V to 12 V step down and then a 12 V to 24 V step up, which isn't all that efficient but it probably doesn't make all that much difference in the overall scheme.

Either of those has the batteries being charged by a 24 to 12 V step down converter, and the 24 V comes from a 24 V charge controller. There are at least two problems there. Firstly, you need about 14 V to fully charge a 12 V lead-acid battery, so you need to regulate to 14 V.

The second, and more difficult, problem is that the 24 V to 12 V converter is probably arranged to take as much power as it wants from the 24 V supply. In other words it's expecting a battery on the input, and a load on the output that takes less power than the supply can provide, which isn't the case. The 24 V solar charge converter is also going to be expecting a battery, and it is probably going to try to maximise the power output of the solar panel by running the solar panel at it's maximum power point (MPPT - Maximum Power Point Tracking) but it can only do that if there is something like a battery to put the current into. I can't see a 24 V to 12 V converter working nicely with an MPPT.

An MPPT converter can only work by running the solar panel at a different voltage to the batteries that are being charged. The MPPT converter is a step-down (or possibly step up, or even up or down) converter, but one that is run to a different set of rules than a fixed voltage converter. As a result of the voltages being different, it may be possible to find a MPPT converter that will accept 30 ish V input and will work with 12 V batteries. That would be a far better arrangement than having a 24 V charge controller followed by a 24 V to 12 V converter.

Running the 24 V loads from the batteries via a 12 V to 24 V converter is fine. Every component is doing what it is supposed to do.
Thank you for your insight. on that. It sounds like the step up/down's are not the clean way to accomplish what I want. So what are your thoughts on using a small 12v battery in series with the RV's 12v (2 6V's also in series) and battery equalizers so that the RV's 12v system sees 12v and the solar/POE side sees 24v? Do you think that is the cleaner more efficient way to go? That way the 24v solar and 24v charge controller would all see what they are expecting. Maybe?
Bi directional? I know with AC a transformer can go both directions. But with DC, I am finding products where I am stuck going one direction (Step up or Step down).
In AC, as I understand you, power can go from primary to secondary, or secondary to primary, like in the power substations. That is what I´d call bi-directional.
Step up/down is another matter, but there sure are devices that can do that, say convert 10-30V to 24V DC.

Regarding your circumstances, a solar charger aimed at a 24V system tied to a 24/12V converter that is charging your battery will either simply not work because the load it sees is very erratic, or it will be very inefficient and you won´t gain as much energy from the sun as you could.

I just made electrical installation for a friend in a 20ft off-grid container, so basically an RV without wheels, and I can only suggest that you add another battery and make your system completely 24V, especially since you don´t already have any high power loads that require the 12V system.

From my experience, there is no problem getting 24V LED light bulbs, kitchen counter lights, a small 24V water boiler and water pump, 50L chest-style fridge with an actual compressor (instead of horribly inefficient peltier), 24VDC to 230VAC inverter to charge your notbook etc. Also, all the fuses, wire sizes and required current demands in a 24V system are half that of 12V system, saving you some hassle when installing the wiring.
I agree with Kubeek that going to 24 V is an improvement.

If you want to keep the main battery and load at 12 V but with an additional battery in series, the additional battery will rapidly get fully charged. You therefore need an equalisation circuit. The equalisation circuit will have to be able to take the full current from the solar charger.

The equalisation could just be dumping the current to a big resistor when the voltage of the series battery gets to 14 V, but that is lost power. More efficient would be an isolated DC - DC converter, converting the 14 V on the series battery to charge the 2 x 6 V set, so the 2 x 6 V set would receive all the current that the solar charger is giving out, plus whatever the DC-DC converter is producing.

I don't know how large your series battery would have to be. I would aim for at least a battery that would take 1 hour to charge from the solar panels. Also, I don't know if there are DC-DC converters that would do what you want without modification. You basically want one that will run at full power at 14 V input and turn off at 13 V input. Finally, the whole lot may be not worth it as two more 6 V batteries and a big 24 V - 12 V converter would solve all your issues with off-the shelf products.
I think the main issue here is having completely the wrong equipment, and trying to 'bodge' a solution with unsuitable parts.

I would suggest the 'best' solution would be to rewire the solar panels for a 12V system (which would probably be simple to do?) and replace the 24V controller with a 12V one - and sell the 24V one to recoup some of the cost.

Failing that, use the 24V panels and controller with a 24V battery pack - no series rubbish with the existing 12V battery - an entirely separate 24V battery system. Then use a DC-DC converter to charge the existing 12V battery, as and when required from the 24V battery.
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