DC -> AC inverter, UPS questions/discussion

There is merit in your suggestion if there is a UPS system already available. When my last UPS system 'gave up the ghost', I didn't bother with replacing the batteries but put it by the curb.

Bob Wilson

 

Bob, In hooking up a generator from my RV to house, I simply plugged into a outlet in my garage. This gave available power at any outlet THRUOUT the hose. Naturally, I shut everything down first and pulled the main power. I then applied loads gently while monitoring. As a final test I only applied the TV, Refrigerator and computer, one light. This was about 20 years ago and it worked fine. It eliminated the need for a long heavy extension.

 

I have a Xantrex ProWatt 1750 mod sine wave for sale. Mint never used. I need a pure sw.
$50 plus shipping to USA 48 only. Paypal only. PM me.

 

I am interested, why are you selling it Ed?

 

I want a pure SW.

 

Hi Alsteckprius:
Sorry, I was offline this weekend. I've heard that it's not a good idea to put batteries in parallel if they are not of the same type (power, capacity, etc), because the variations between the two creates something of a feedback between them that causes the batteries to drain on their own. That's one of the selling points to a battery isolator. I'm not sure that's a relevant concern when the batteries are actually being fed power by an external source (the DC-DC converter), so it might well be overkill. Someone familiar with marine electronics (not me) would be a better source of information.

 

Hi Bob:

Great info. My solution was intended as an emergency only solution-- not a permanently install-in-the-car solution like the one you built. The second battery would be kept in the garage along with my emergency kit.

I'm not sure if you're suggesting this (feel free to correct me if I'm wrong) but I'd be reluctant to replace the OEM battery with something like a starter battery. Priuses obviously don't have the same starter demands as a regular car, so high crank power isn't essential. Also, most inverters can only handle 2x nomimal wattage for short durations before they blow out their fuses, so high crank power might be overkill.

I like the circuit breaker idea. Are you thinking that it'll trip faster than the (expensive) DC-DC Converter fuse? I haven't really researched this, but does anyone know how fast that fuse (or fusible link) in the Prius will blow and under what kind of load?

 

Wow, this sounds brilliant, but would it really work? I'm just not familiar enough with UPS's. Specifically, does a UPS pass through line voltage exclusively unless the power fails? If so, then what would trigger it to "assist" that line voltage when there is a sudden high load (in this case, "line voltage" would be coming from the Prius-inverter system)? I thought these things were kinda "dumb" and just had a relied on a relay that flipped one way or the other (i.e. AC or battery-inverter) based on line current. If I'm wrong (and this idea would work), I'm going to go out and buy the biggest darn UPS I can find.

 

The caveat is I think a UPS would be unhappy with a modified sine wave inverter as its mains. I would use only a pure sine wave Inverter in front of a UPS. Especially if its a cheap UPS which actually puts crap back on the ac line in the form of low amplitude square waves which your Inverter will not enjoy.

Pure SW's are expensive.

And there's no magic bullet here. A UPS will be another additional load on the Inverter and will have its own inrush when a big load is presented to it. Maybe even bigger than if it was not there as now your charging a sagging battery too.
There is no free power.
UPS's are hungry.

 

Yeah, unfortunately, I'm coming to the same conclusion that there is no one perfect solution. My advice to anyone beginning down this confusing path is:

1. A 1kw Pure Sine Wave inverter plus a Kill-A-Watt is a fine solution in a pinch if you're an energy miser. First be sure to think about what you want to run in an emergency and test the entire load (before the emergency) using the Kill-A-Watt and a power strip to see if it can handle it at peak.
2. If you can't get by with 1kw or have high peak loads (e.g. an old refrigerator or air conditioner), then accept the fact that your solutions are going to be expensive. One is to hook into the Prius traction battery (e.g. a Converdant solution). The second is to research marine solutions and be prepared to buy a (quiet) generator, an expensive pure SW charger-inverter, and bank of at least 4 deep cycle batteries hooked up in parallel in your garage.

 

I picked up a 1KW Inverter today. Tested it out, biggest load today was my garage door, it used about 300 Watts for that. Still havn't done my whole house test. Ie: principle loads such as Fridge, TV, Laptop and a couple of lights. Today my bench simulated the house, had night light on .14 Watts, solder gun, florescent light and garage door. Engine never did come on!

 

Consider the portable "jumpstarter packs" available at any hardware store. Most have a ~18AH AGM battery and a ~400W MSW inverter, I'm sure you know the type. Strap one of these into the back, and add the appropriate wiring (and isolator?) to feed it from the Prius's 12v bus.

Pros:

• It's removable to save weight when you don't anticipate needing it.
• It's portable to carry elsewhere, if it's needed elsewhere.
• It can run standalone, assuming the internal battery is adequate.

Cons:

• When removed, the battery should be connected to a maintainer.
• Commercially-available versions won't tuck under the cargo lid.
• Inverters in such things are small, not anticipating an external 12v source.

And those last two cons are easily handled by building one's own. I think pairing the inverter with a secondary battery, but making the whole thing removable, is the most sensible architecture. Thoughts?

 

It works! This thread, and the others on the same subject have been fascinating reading, even for an electrical neophyte like myself. But I finally decided to take the plunge after the double whammy of Irene and Sandy. I set up a new 775 CCA Die Hard AGM (with marine type terminal adapters, about $180 at Sears) in parallel with my 2010 Prius' 12V using 4 gauge cables. Then another set of 4 gauge to a Power Bright 1100W inverter (about $90 from Home Depot on-line). From the inverter, two 15A 50 foot extensions into the house. I ran my older, small (about 16cf) refrigerator, my TV, phone, cable box, modem, router, receiver, laptop and a couple of lamps for 7 hours, maintaining zero degrees in the freezer and doing no apparent damage to the car, or any of the components.

The inverter is nice, because it has a temp. controlled fan, so it doesn't run the batteries down continuously. It also has a digital readout for input V. and output watts. I never saw more than 350W. on the display, although I'm sure I was never looking when the fridge started up. I don't know how much fuel I used because the gauge remained on the same segment throughout the experiment. The cables from the Prius' battery stay, safely insulated, in the storage bin over the spare tire. The battery (with a float charger) and the inverter now go onto a shelf in the garage, waiting for the next disaster. I can hook everything up in about 5 minutes using the two large wing nuts that came with the marine terminals.

Thanks to all for their advice.

 

Bob, how much loss was there with the Anderson connectors? What current rating were the ones you tried?

And did you find a good 80 A, 12 V circuit breaker?

 

I'm doing this from memory but I remember it was in the same order of magnitude as the 120A circuit breaker . . . ~40-80 W.

I remember I could sustain ~930 W (resistive load) holding the battery voltage right at 12.8-13.0. It looked like permanent cables (permanent inverter mounting!) would deliver my target 1 kW. But I have been thinking about a semi-permanent installation.

A permanent installation gives me 110 VAC inverter power while driving down the road. But I don't do that many 'trips'. So another alternative is to keep the inverter in the storage area; ground wire stored with the battery, and; B+ wire and wrench on the inverter in the storage area (aka., non-conductive nylon cord holding them together. Enabling the inverter is a little more involved:

• Take out of storage bin and lay on top of rear floor board.
• Remove battery cover and terminal 'cover'.
• Bolt the B+ to the inverter.
• Connect the inverter ground to previously installed ground cable.
• "Protect" the terminals (aka., make sure nothing bad can fall into a bad place.)

This can be done under a LED flashlight. Start the car, turn on the inverter, run the contractor AC cable into the house.

The advantage of this approach is I don't have to permanently mount the inverter in the car. The disadvantage is using it 'on the road' is not so easy. But I already have light-duty, cigarette lighter inverter and on the road we're only looking at laptop and USB based charging. I don't need a 1 kW load traveling down the road . . . or at least my wife hasn't asked for it . . . yet.

The only reason for having higher power when traveling would be if we had one of those pod-based, espresso machines . . . distracted driving in the extreme!

Bob Wilson

 

+1 on all of that. I remember reading a torture test that Computer Shopper did back in the early 90's (back when they were a serious computer tech mag) on computer power supplies. They hit them with wild voltage swings, wild frequency swings, etc. and didn't have any problems with the output stability. Modern international standards ensure that most electronics' power supplies are even more robust today.

 

Almost right. Batteries in parallel need to start out paralleled while new (you can't add one 3 years later), of the same size and amps. They also need to be tapped correctly, e.g. two batteries in parallel should have the + tapped from one battery and the - from the other. The reason for all of these rules for even load sharing while charging and discharging. Different ages, different sizes, incorrect battery cables, etc. all result in slight differences in internal resistance, and even a slight difference results in an imbalance. One battery will end up discharging faster and recharging faster leaving you with a dead battery in 3 years when it should have lasted 5+. And since you can't parallel a new battery with the older one that is left (for the same reasons), you end up having to scrap both of them and getting two new ones.

Lead acid batteries do have a self-discharge rate, usually 1 to 1 1/2% per day for flooded lead acid in warm weather. Gell cells like you find in most UPSs are about half that. Cut all those numbers in half again during cold weather (if the batteries are outside). Which is why if they aren't on a float charger they should be fully recharged once a month in warm weather and every 2-3 months over the winter. But there is no such thing as a "feedback".

 

Check out my signature for a fairly cheap solution which will give you 3000 watts continuous, 4000 for an extended period, and well above that for surge.