Lithium Batteries and Upgrading the 12V DC Power System

Last fall, I wrote about installing a PC power supply to power the remaining 12V DC loads in my RV, but that’s getting replaced with a DC-DC converter.  The power supply did the job it was supposed to, without fail.  But there are a few drawbacks with it:

  • The power supply converts AC power from the inverter to 12V DC.  That means the inverter is on, and there’s an extra conversion that wastes energy.
  • Under load, the 12V output from the power supply is lower than I’d like.  At the source, it drops to about 11V, and my rig’s 12V wiring is mediocre at best.  Vent fans, water pumps, etc. all run a little slow.
  • The power supply doesn’t respond well to sudden load increases.  This is most evident with the stereo system.

Installing a DC-DC Converter

First things first.  I bought one of these:

It’s rated for either 25 or 30A of 12V DC output, which will take care of everything I have running off of the 12V distribution panels.  Some quick testing showed it’s better than 90% efficient, and with no fans, it’s totally quiet.

Where does the DC-DC Converter Go?

The output of the converter needs to connect to the 12V distribution panel, which is just to the left of the wall separating the bathroom and rear hallway.  It needs a 48V input, which is available down at the inverter and charger just on the right side of that same wall.  Sometimes things just work out fairly easy.

The converter could go anywhere between those two locations, but it’s cheaper and easier to run the smaller wire needed for the 48V supply side, and we’ll waste less by keeping the 12V side as short as possible.  That means putting the converter in the cabinet with the distribution center.

Running the wires took all of about five minutes, and I elected to put connectors on both ends of the converter so it could be unplugged easily. The connectors also different from each other, to make sure that we prevent plugging them in backwards or reversing polarity.

Wire Sizing

On the 48V side of things, a 100% efficient converter would need 7.5A.  If we allow for 10A, and a 3% loss on the round-trip run, we could use as small as 16AWG wire.  If we expected to spend a lot of time at full load, sizing up might make sense to reduce losses.  But experience so far shows that about 95% of the time, the load is less than 10A at 12V, or about 2.5A at 48V plus conversion losses.  16AWG is fine and will net <1% in ohmic losses.

On the low voltage side of the converter, we have a very short run–about 2 feet round-trip.  But considering the overall run lengths, we don’t want to add another 1-3% in losses.  16AWG would be perfectly fine, but we’ll run 10AWG, which more in line with what the terminals on the distribution panel are designed for.

How does it work?

Lots better. Voltage drop under load is significantly reduced, it’s a lot smaller, and silent.  It gets a little warm, but not bad at all.  The output voltage is a true 12V.  And the price is hard to beat.

Now that the power supply in the bay under the RV is not needed, I expect to use that for storage.  For a full-timer, that’s a big win.

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