Recommendations for powering small 12V loads from inverter?

Plowman · September 2014

My small off-grid solar electric system (see signature) is set up about 120' from my RV, which I have parked in the shade so I don't have to run the AC in summer. Too far for 12V, so I run a line from the inverter into the RV.

My problem is that I need to power three 12V loads: A propane refrigerator (~0.3 amps for the controller) and two water pumps (1.7 amps + 4.4 amps).

I've been using an old worn-out Sprinter 150 ah 12V AGM battery and a Xantrex 5 amp charger (I know, mismatched, but the Sprinter acts more like a 50 ah batt and since it's on its last legs I don't care if I kill it). I don't have a good sense for exactly how much power the charger draws over time. I just plugged it into a Kill-A-Watt and hope to gather a week's worth of data to see exactly how much it's drawing. My guess is an average of ~160 Wh/day.

Seems like there's a better way to do this. I need a 24/7 source of 12V power to run the fridge, maximum draw of 6.4 amps @ 12V, usually <0.5 amps. Suggestions?

I was looking at some small 12V converters, but they don't appear to be very efficient. This Schumacher says input is 120VAC and 1.5 amps, while output is 12VDC and 6 amps. That's 40% efficiency, which seems rather low. If my math is right, this little converter would draw considerably more power than my current set up.

zoneblue · September 2014

Re: Recommendations for powering small 12V loads from inverter?

What you are looking for is an energy star rated 12 plug pack or swtich mode power suuply unit. Theres are made for computers and the like up to 200W or so. Have a rummage through these:http://www.ebay.com/bhp/12v-dc-power-supply

You can also make such things by talking on old ATX computer case, and removing the PSU. Some of those are rated even higher into the 600w range. Efficiency is normally pretty good into the 90s.

ramloui · September 2014

Re: Recommendations for powering small 12V loads from inverter?

Probably not the most efficient, but I use my Iota charger to power my 12V Shurflo pumps. I use it because I had it on hand and did not need it for another purpose at the time.

Plowman · September 2014

Re: Recommendations for powering small 12V loads from inverter?

ramloui wrote: »

Probably not the most efficient, but I use my Iota charger to power my 12V Shurflo pumps. I use it because I had it on hand and did not need it for another purpose at the time.

I tested my Iota 45 a while back, as a converter it drew 8.5W with no load, about the same as my RV converter. That's 17 Ah/day @ 12VDC. I think my separate battery and 5 A charger draws less, but still need to measure this over the course of a week or two.

One of the benefits of using my Iota or RV converter is that I could use 12V LED lights, which draw a lot less power than my current 120V lamp. Might balance the extra power the converter uses.

Plowman · September 2014

Re: Recommendations for powering small 12V loads from inverter?

zoneblue wrote: »

What you are looking for is an energy star rated 12 plug pack or swtich mode power suuply unit. Theres are made for computers and the like up to 200W or so. Have a rummage through these:http://www.ebay.com/bhp/12v-dc-power-supply

You can also make such things by talking on old ATX computer case, and removing the PSU. Some of those are rated even higher into the 600w range. Efficiency is normally pretty good into the 90s.

It's hard to tell how much power these little units draw. Here's one 6 A converter, they say 72W, which is just the DC output. Doesn't say what the AC input is aside from 120V (no amps listed). I'm assuming the 72W draw is at maximum output. Any idea what it would be if I was only drawing 0.5 A?

If it draws 72W all the time, that would be far higher than I could deal with. 72W * 24 hours = 144 Ah @ 12VDC, which is 64% of my battery capacity. Yikes.

Recommendations for powering small 12V loads from inverter?

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