Boondocking electrical system: $50

[tomhawk]

I have recently finished much of my trailer and am waiting for somewhat warmer weather to go camping.

So far, what lighting I have is all AA battery powered. Since I am moving up from tent camping this is still pretty luxurious to me. I want more. More light and the possibility of powering a fan and charging some electronics is attractive. While I am deciding just how much is enough I have come up with a stop gap.

I recently acquired an electric mower. It is powered by two 40V Lithium battery packs with 4 and 2 Ah capacity. Their combined capacity is over 200 Wh. That should be plenty for my short term goals.

Several power tool battery pack companies sell adaptors for these batteries. The one for my battery system has one that provides a 12V cigarette lighter type port, some USB ports and an inverter to generate 120V AC. I did not find attractive that the inverter was unswitched and thus, always on, generating noise and phantom battery draining current. I did not need this feature. Also the circuit did not have an on/off switch. The transformer would be another phantom current user.

Here is my solution. It was just as expensive as the commercial product but does not have the feature I did not want and I could add an on/off switch. Also it probably has a better set of USB ports.



The key component was a step-down transformer that converts the 40V battery output to 12V. The pieces of sheet metal used for the battery contacts required some careful adjusting but none of the construction took much effort.

Here is finished installed battery.


Here is the result powering a 6 watt string of LEDs.
At that rate it should last over twenty hours.

Plenty for a few hours a day on most trips I anticipate. This seems like a big improvement over AA
batteries.

I saw that California has passed a law encouraging (requiring?) electric yard tools. I am a fan. The reduced noise alone makes me far more willing to mow my lawn. If it keeps me from hiring someone to do it, clearly it is cost effective. More than that, the noise and fumes I am not sending to my neighbors is worth a lot to me. I like that I can use the battery in my trailer too.

A question I am not able to answer is how much "phantom current" am I losing if I leave the battery connected to the step down transformer even though no devices are receiving 12V output. I am thinking that I need to turn off the power to the transformer when not using the electricity in order to save battery charge. How significant is that?

Another possibility if this is a significant power drain would be to have a timer somehow to disconnect the battery when not in use. If that is a good idea, does anyone have a suggestion how I might do that?

Finally I ran into a product called a "switched transformer" ( or something like that) that I believe somehow circumvents the "phantom current" issue altogether with some electronic shut off circuit.
Am I confabulating this or is this an item that might be useful to solve this problem?

Thanks,
Tom

[Pmullen503]

You are probably reading about a "switched" power supply. They are more efficient than "linear" power supplies.

If you are worried about phantom loads, put the switch between the battery and the transformer.

[tomhawk]

I did put the switch between the battery and the transformer. I can prevent "phantom current" by switching it off there.

I would like to put the battery in the galley and use a cord to power my light source in the cabin. However, even after I disconnect the light, I will have go to the galley to turn off the battery at night.

I may have to store the battery in the cabin. It is that inconvenience I was trying to avoid. I am not sure how much power loss leaving it connected all night would cost.

Thanks,

Tom

[Tom&Shelly]

Here is my solution. It was just as expensive as the commercial product but does not have the feature I did not want and I could add an on/off switch. Also it probably has a better set of USB ports.

I saw that California has passed a law encouraging (requiring?) electric yard tools. I am a fan. The reduced noise alone makes me far more willing to mow my lawn. If it keeps me from hiring someone to do it, clearly it is cost effective. More than that, the noise and fumes I am not sending to my neighbors is worth a lot to me. I like that I can use the battery in my trailer too.

A question I am not able to answer is how much "phantom current" am I losing if I leave the battery connected to the step down transformer even though no devices are receiving 12V output. I am thinking that I need to turn off the power to the transformer when not using the electricity in order to save battery charge. How significant is that?

Thanks,
Tom

Looks nice!

I mowed my folks lawn on a riding mower with gas engine from the age of 4 on (my Dad would be in jail for the stuff he let us do, nowadays ), and mowed my own lawns with gas push mowers for years. Finally learned I didn't need a grass lawn. That's a better solution, IMO, than grass in places with little water, including California. Saves time and energy.

Can you use the ammeter function on a multimeter to measure the phantom current? You'll have to break a connection and connect the meter in series. Another solution would be a clamp-on ammeter, but you'll need one that works on DC. unless you can borrow one from somewhere.

Tom

[Pmullen503]

You could try to measure the current drain. It'll be small and hard to measure directly. You could look at voltage drop in say 24hrs. with nothing running from a full charge and compare that to a known draw from your lights or other load. That should give you some idea if it's worth worrying about the parasitic draw.

You should have some type of low voltage cutoff to prevent your battery from draining completely.

[tomhawk]

Finally learned I didn't need a grass lawn. That's a better solution, IMO, than grass in places with little water, including California.

In places where it does rain, keeping growth at bay is a constant battle. In the midwest, the default land form is prairie. Prairie is 8' tall.

You could try to measure the current drain. It'll be small and hard to measure directly.

As you suggested, I put a ammeter across the 40V feed to the transformer. I get 20mA.

Recollecting my meager physics (W = A x V), 40V x 20mA = 0.8W.

That means my battery will lose around 20 Wh of its 200 Wh capacity each 24 hr period I leave it switched on.

Assuming these results are true, maybe I won't worry too much about it. Just keep it switched off if convenient.

Thanks,

Tom

[tomhawk]

We just returned from an overnight at a rural county park in eastern Iowa. We arrived Sunday afternoon and were surprised to see we were the only campers there. It was right next to a nice river with lots of geese and other forest birds. Off in the distance you could hear cattle and horses.

We have been hoping to do another trial run of the new trailer to test out various additions made over the winter. I have camped as early as the first week of April in other years but this year there were highs only in the 40s or raining most days we could go. But Sunday promised over night lows in the 50s and no rain.

The weather is forecast to reach near 90F each day for the next few days. It looks like this year we are going directly from Winter to Summer. No spring!

All went well. We got to use the lawn mower battery set up to run some lights for the cabin and the galley. Here is a view of the galley after sundown.


For the lights, I used some 12V LED strips my son was no longer using for his room at school. I soldered some 2.1 x 5 mm connectors to the ends of segments and attached them to a piece of 0.5 inch aluminum angle. I covered them with some strips of plastic milk jug material using clear plastic packing tape to make a light diffuser.


These strips draw about 1 watt/ foot. I used a 2 ft strip in the galley and a 4 ft strip in cabin.

As planned, I left the 'ON' switch on all night. The battery ran down a bit overnight due to the 'phantom' current from the step down transformer. My calculations must be close. At the rate of current draw I should be able to have battery powered lights for over 4 nights on a single charge.

Since the LED lights and wire were items I had already and I am only borrowing the battery from my lawn mower, the outlay for the system is under $50.

Tom

[tomhawk]

We now have 3 weeks of camping with this battery. It worked as well as I could have hoped. At one point, we spent 5 straight days in a non-electric camp site without recharging. We had plenty of light in the cabin to read before dozing off in the evening even though it was late in the summer with shorter daylight hours. Having plenty of light to cook dinner was nice. We even charged our phones a few times.

I am quite happy with the weight of these components, 3 lb. Our trailer fully loaded is under 600 lb. Having a lead acid battery would seem to defeat our quest for a light trailer.

A problem with this system is that it seems there is no way to recharge from our 12V automobile system. The charger for the 40V battery requires more watts than our AC converter can supply. I have not been able to identify a simple system to work around this. Further I have not seen any easy way to use a solar charger.

12V batteries seem to be the standard for all of these applications.

It turned out we were fine not charging our 40V batteries until we got home but I think it would be nice to have the option to do that on the road or with solar.

Does anyone have a suggestion for a way to charge non-12V batteries when no 120V AC outlet is available?

Thanks,

Tom

[tomhawk]

Here is an idea for a very low power entertainment system that plays well with our limited battery storage.

I do enjoy watching a movie from time to time. On camping trips, sometimes it is just too cold and wet to enjoy a campfire. Sometimes it is nice to just be entertained.

Here is our set up. It uses a 4 watt video projector that has an internal USB rechargeable power source with a capacity of about 10 watt-hr. This is sufficient to show a 2 hr movie. We have a battery we normally use for charging cell phones with a 50 watt-hr capacity. With this we could supply power to the projector for 10 more hours.

The projector is 3" x 3" x 0.8". It has a screw tripod mount so we could attach it to the front wall of the cabin. We hung a fragment of an old white bed sheet from the rear ceiling for a projector screen.

I made a "foamie" box, to protect the projector when it is stored, out of some scrap foam insulation and canvas that probably could withstand a hit from a loose dutch oven.

Video system parts (total weight < 1lb):

1. Video projector
2. Foamie projector storage box
3. USB thumbdrive (<=16 gBytes)
4. USB battery, 10,000 mAmpHr
5. USB/micro USB cable
6. Mini tripod and wall hanger

Here is the projector mounted on the front wall of the trailer cabin. An old bed sheet hung from the rear ceiling serves as a screen. The projected picture width is over 3 feet wide.


The audio provided by the internal video projector speaker is pretty low volume and quality. In the confined space of the trailer cabin it is surprisingly sufficient. There is an audio-out jack that we can use to power some old USB computer speakers if we want better sound quality.

The projector has an HDMI video-in port as well. It can thereby stream video from a phone or laptop. This would run down the batteries on those sources however. The most efficient source is from a USB thumbdrive or microSD card.

[Pugwinkle]

Where do you purchase that projector? What kind is it?

Here is an idea for a very low power entertainment system that plays well with our limited battery storage.

I do enjoy watching a movie from time to time. On camping trips, sometimes it is just too cold and wet to enjoy a campfire. Sometimes it is nice to just be entertained.

Here is our set up. It uses a 4 watt video projector that has an internal USB rechargeable power source with a capacity of about 10 watt-hr. This is sufficient to show a 2 hr movie. We have a battery we normally use for charging cell phones with a 50 watt-hr capacity. With this we could supply power to the projector for 10 more hours.

The projector is 3" x 3" x 0.8". It has a screw tripod mount so we could attach it to the front wall of the cabin. We hung a fragment of an old white bed sheet from the rear ceiling for a projector screen.

I made a "foamie" box, to protect the projector when it is stored, out of some scrap foam insulation and canvas that probably could withstand a hit from a loose dutch oven.

Video system parts (total weight < 1lb):

1. Video projector
2. Foamie projector storage box
3. USB thumbdrive (<=16 gBytes)
4. USB battery, 10,000 mAmpHr
5. USB/micro USB cable
6. Mini tripod and wall hanger

Here is the projector mounted on the front wall of the trailer cabin. An old bed sheet hung from the rear ceiling serves as a screen. The projected picture width is over 3 feet wide.


The audio provided by the internal video projector speaker is pretty low volume and quality. In the confined space of the trailer cabin it is surprisingly sufficient. There is an audio-out jack that we can use to power some old USB computer speakers if we want better sound quality.

The projector has an HDMI video-in port as well. It can thereby stream video from a phone or laptop. This would run down the batteries on those sources however. The most efficient source is from a USB thumbdrive or microSD card.

[tomhawk]

Where do you purchase that projector? What kind is it?

Kodak Luma 75 purchased from the "Big Online Store"

We have been using a water resistant external speaker connected by an auxillary cord to improve the audio. In a small trailer space it now has impressively good audio.

A complaint I have about this projector is the minimal documentation. It is not documented, but it does not recognize USB sticks that are larger than 16 gBytes. It does play several video formats, but is poorly documented which ones.

The battery life is not long enough for a longer video, but it will play while charging from a small USB power supply that I show in the image. I think some other projectors I have seen will not play while charging.

One important caveat is to avoid upgrading the firmware update they advise on their support page. It fried my projector. They provided me with a replacement, but I would rather not go through that again. They did not offer any explanation why the update did that.

I am happy with this product so far.

Tom

[tomhawk]

Here is an update on this battery system. We have now used this to provide power for 50 nights. Everything seems to be working fine. When not in the trailer, we use it to mow the lawn. No complaints.

I had made the parts that coupled the battery to the circuitry out of some glued together 1/4 inch plywood (see previous for photo). It worked ok but it was a bit tricky to align everything. My daughter recently started doing some 3D plastic printing as a hobby. There are many digital designs for useful objects available for free or nominal charge on several web sites. There is some open source software that allows one to create their own designs as well. I found a digital plan for an adaptor for my battery type. I downloaded the file and sent it to her. She printed it using $1 worth of materials in under 20 minutes. Here is the result replacing the plywood adaptor.



It is a definite improvement.
The battery slides in much easier and it actually now snaps into place as the battery
was designed to do. Plus it looks as if it should works better.

I don't currently have ideas for using this technology for other items but this worked so well I would think it has got to be useful for more.

Tom

[bdosborn]

That's a great use for a tool battery. I made a similar rig to power a Starlink Mini dish using Milwaukee batteries I already had.

The low battery disconnect for tool batteries is typically built in the tool and not the battery. I picked the Milwaukee battery connector I'm using because it has a low voltage disconnect built into it. You might look at a battery meter so you can track your energy usage and not run your battery too low. That's what the meter is doing in the photo; I wanted to see how long the Starlink Mini would run from a 5 a-hr battery (3 hours). Finally, to charge the battery I would try using a small inverter plugged into a cigarette lighter in your car to power your the 40v battery charger. I checked my Milwaukee charger and it pulls 115VA when charging a battery.
Bruce

[bobhenry]

gallery/image.php?album_id=1521&image_id=37621

Used battery wired to dc breaker then fed to an automotive style fuse box and fed to the dc components all the grounds return to the grounding lug bar and finally to the battery. Don't you just love my UL approved electrical enclosure