Solar assisted UPS to be installed in an RV - Page 1
Forum section: electronics / projects

author: jakeson
author_rank: Newbie
timestamp: May 26, 2014, 05:41:17 pm
content: Hello,I need to install in inverter in my camper. Its a small camper, so I don't need much, it will only ever run laptops and small electronics. When I went to buy an inverter I could not believe the cost, and with this thing in my hand it occurred to me, how is this any different than a UPS if you break it down. I already have a solar system on my camper (150w 24v system)If I take an old APC 1000 ups I have (runs two 12vDC in series = 24vDC):-take the battery leads from the solar charge controller (GP-PWM-25) and attach that to the battery in the UPS-remove the buzzer (cause that would get old fast)-tie the 110v power cord to the power coming from from the utility power, so when its plugged in I still charge the UPS battery-Take the 110v out from the UPS and make all the 110v outlets in the RV plug in to the UPS.(Did I mention I am in north america? so all 110vAC, none of that crazy 220vAC)Talk me down here, am I nuts? cause I have all of that, none of this 700$ crap on an inverter when I seem to have everything laying around. Do I need a diode to protect the solar charger? should I have a relay to cut off the solar panel when plugged in to utility? Or am I just missing something obvious here?Thanks in advance, and I did look for any similar projects, but didn't find any relevant.-Jim

author: Lightages
author_rank: Unknown
timestamp: May 26, 2014, 06:15:32 pm
content: Most inexpensive UPS's are square wave or modified square wave output, not sine wave. So the first issue is if this will be a problem for your equipment. Computers all use switching power supplies so they don't care at all what the quality of the power is. Things like motors, radios, some TVs, refrigerators, etc, do care. Motors might burn out or operate very poorly with non-sine wave power. Radios might pick up the harmonics from the square wave and make them unusable.Then there is the design of a cheap UPS. It has a limited output capacity and also they are rated for a use time of their battery capacity. It might not survive continuous use at or near rated output.It really is worth the money to get a proper sine wave inverter. This is one place where trying to go cheap is false economy. It will fail, and maybe cause other problems making it money in the garbage. There usually is a reason why things are done the way they are. If they could be done cheaper and easier, they usually are already being done that way.Edit: I suppose if you have the parts, it costs nothing to try. You will need to ensure that you don't have the UPS and the utility connected at the same time. Without knowing more about your solar system it is hard to say what else you need to do.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 26, 2014, 06:37:10 pm
content: You don't need pure sine for electronics unless you're using critical instrumentation. In fact, the optimum waveform is more like a square wave (with the peak equal to that of the sine wave, e.g. 170V peak for 120V mains) with the edges controlled to avoid EMI. An easy way to get that is to modify a cheap inverter (probably not so cheap for the less common 24V input) to output a square wave (usually just involves hacking the TL494 controlling the output side) and then add some series inductance (passive PFC choke from an old PC PSU works nicely) for controlling the edges.That said, modifying a UPS is probably the cheapest way to get a 24V inverter. Just derate it and add some fans.

author: johansen
author_rank: Super Contributor
timestamp: May 26, 2014, 06:41:38 pm
content: you should have no problems provided you don't connect the battery to the 120vac output somehow. make sure the two circuits are isolated.I believe the battery ground is connected to the steel chassis of the apc1000, which is ground. however both 120vac outputs are floating if i'm not mistaken.however, don't connect either one of them to "ground" and you won't have any supprises.I have heard from another that placing an inductor on the 24v side of the transformer will reduce no load losses significantly, i haven't made measurements on my apc1000 yet to see if its significant. as you might be aware, the transformer's leakage inductance is used to filter the sine wave pwm, and they have a 10 or 20 uF capacitor on the 120vac side to null the HF out. however, the iron core is quite lossy at 8Khz, so a separate inductor will help quite a bit.you can easily add a couple more mosfets in parallel to the existing 8, that will reduce losses significantly.Also, you can solder two additional 10awg wires in parallel with the primary leads to remove about 1mOhm of resistance from the system. those 10awg wires will get warm during heavy load btw.overload capacity is pretty good, you shouldn't have a problem starting a refrigerator.yes, it is a sine wave inverter.

author: jakeson
author_rank: Newbie
timestamp: May 26, 2014, 08:04:20 pm
content: Great info!! Thanks!Interesting about the inductive loads. Didn’t think about the fridge, If I recall its 12v, been a while since I had to do anything with it, usually with buddies with a bigger fridge. Heater runs 12v should be ok there. I am putting a deck in, but again 12v. I don’t think anything running on the 110v should be an issue.If I hook the 110vAC up to the 12vDC side I deserve the resulting fire lol. But the warning is definitely appreciated!I had planned to under rate the whole thing, IE 1000 running a laptop and that’s about it. Possibly Gameboy charger for the kids or something. Not a huge fan of pushing things to the max.As for doubling up the fets, not a bad idea, I will do that. I have some 8awg laying around; I will use that for this. I had planned the run to the batteries to be 10awg to prevent loss.I will do some digging to find out if this is a full or half sign.Lightages: I had thought about putting a relay in so that if utility power came on a relay would disconnect the solar power from the batteries. I think perhaps I will just go with that option. Thanks for the heads up.Also just a funny thought about you comment: "There usually is a reason why things are done the way they are."Thats why they call us makers and hackers.. As Doc once said from fraggle rock "What do you mean "Why am I doing it"? ? ?, I don’t even know HOW I am doing it yet!"Thanks guys this is great to have some sanity check before proceeding.-Jim

author: jakeson
author_rank: Newbie
timestamp: May 26, 2014, 08:10:03 pm
content: Oh, just for interest, here is the solar controller manual. I am NOT looking for you to do the leg work for me, just figured if your interested, its a good one. I am very pleased with it:http://gpelectric.com/files/gpelectric/Docs/Manuals/Go_Power_MAN_GP_%20PWM-25_vE.pdf

author: Legit-Design
author_rank: Frequent Contributor
timestamp: May 26, 2014, 08:18:30 pm
content: Get car chargers for those devices? You have laptop, you can probably get a car charger for it. Same goes for gameboy and possibly a phone charger, just get good chargers and not the cheapest chinese crap. Car charger takes in 12/24VDC and puts out what your laptop desires, around 12-19VDC. If you have inverter for that laptop first the inverter makes 110VAC then your laptop mains charger makes that into 12-19VDC. You skip two steps on inefficiencies and replace them with one. Also 12/24VDC is much safer than 110VAC.

author: jakeson
author_rank: Newbie
timestamp: May 26, 2014, 08:31:52 pm
content: Yeah thank you for the advice. Where I can I go with a 12 V adapter. It's more for the situations that I can't plan for. it's amazing how much power is often wasted in the process.Thank you for the suggestion

author: johansen
author_rank: Super Contributor
timestamp: May 26, 2014, 09:18:25 pm
content: Use 8 awg for the battery cables and if you intend to keep the loads below 500 watts don't waste your time doubling up the fets in the H bridge, its just not worth the risk for a few watts saved.starting a refridgerator should max out the 5 LED load bar graph and light up the red unbalanced load LED but it should still start properly.if you really want to make it easier to start, you might experiment with a capacitor in series with the start winding of the fridge.

author: Skimask
author_rank: Super Contributor
timestamp: May 26, 2014, 09:53:05 pm
content: How big is this solar panel anyways?

author: jakeson
author_rank: Newbie
timestamp: May 26, 2014, 10:19:02 pm
content: 1x150 watt I will be adding a second panel eventually

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 26, 2014, 10:50:26 pm
content: You don't need to disconnect the solar panels. The controller will do that for you.If small loads are all you're running, 12V would make more sense.

author: jakeson
author_rank: Newbie
timestamp: May 27, 2014, 05:32:22 am
content: Ya I completely agree, 12v would be fantastic, but both the APC 1000 and the APC 2200 that I have are 24v and don't have anything else to use. Not totally sure I can even go 24v because the panel I have is 12v, I will have to find a second panel to make it work. Unless someone knows a secret magic trick to use a 12v panel charge 24v batteries?Seriously though, I am going to a local ecycler in the AM to see if they have a APC750 because most of them are 12v.By the way, I checked out a APC1000 APC1500 and APC2200 today on a scope and all of them were full sign wave output when running on battery. That is encouraging. I also replaced the Pizio electric speaker with an LED so I can get beep codes just in case I have some issues later on down the road. Worked perfect. Now to try and track down a 12vUPS.I plan to post this build on my site and when its done a coals notes here. So far this is the most information I have been able to find on the intergooglewebs about this topic in one place. Thank you eevblog.-Jim

author: sleemanj
author_rank: Super Contributor
timestamp: May 27, 2014, 05:54:37 am
content: Quote from: jakeson on May 27, 2014, 05:32:22 amBoost converter.

author: jakeson
author_rank: Newbie
timestamp: May 27, 2014, 06:19:58 am
content: Magic!

author: Harvs
author_rank: Super Contributor
timestamp: May 27, 2014, 06:40:02 am
content: Quote from: sleemanj on May 27, 2014, 05:54:37 amOn a 150W STC rated panel, which probably has an NOCT rating around 100-110W, it's likely not going to be worth it though. Probably be better off getting a second panel to series connect. You obviously also gain power spending the money on a second panel compared to a boost converter.