Help with basic inverter construction

tnine

Hi all,
I'm doing a long road trip in my SUV around some national parks. I want some portable power (without running the car) to charge my headlamp and GPS batteries, run my computer etc. I picked up a decent secondhand 700W DC->AC inverter. Using that and a marine battery, a marine battery case I now have a functioning 700 watt inverter.

I'm having trouble deciding how to charge it when on the road. Our SUV is a 2014 Subaru Forester. It has a 120W 12V power port in the rear that is unpowered when the vehicle is not running, so I don't have to worry about draining the Subaru's battery when using this port to charge the marine battery. Ideally, I'd like to take my DC power plug adapter and connect it directly to the battery. This comes with a 10 amp fuse. I purchased these at ACE this morning.

http://batterytender.com/products/accessories/cigarette-lighter-adaptor.html

http://batterytender.com/products/accessories/ring-terminal-harness.html

Can I use these to plug directly into the battery from the power port, or do I need some sort of charge controller? I don't want to change any wiring in my car, since it will void the warranty.

Thanks,
Todd

Cariboocoot

Re: Help with basic inverter construction

Welcome to the forum Todd.

There should not be a problem recharging your Marine/RV battery from the alternator as that is basically what they are designed to be recharged from. There's no big issue with bringing them up to a high Absorb Voltage and holding it for an hour.

Having the receptacle turn off with the ignition does solve the problem of drawing current from the vehicle battery.

You should not need a charge controller on the output, as the alternator's output is already regulated. it will detect the second battery as a load and increase output to the system until it balance (the battery charges).

Some issues you may have:

1). Insufficient time from the engine to recharge the battery. This will depend on how deeply you discharge the battery and how much you drive to recharge it. You will want some reliable system to measure its SOC before you get in trouble here. Voltage is not really a good method of judging this. Battery monitors cost a lot of money. A hydrometer is fairly accurate, but requires you to actually check the SG and that means dipping into the cells (acid is nasty stuff). No perfect answer here.

2). The current limitation of the accessory port. A discharged battery may demand more current than this is designed to handle, leading to frequent fuses popping and not getting the battery recharged. The only way around this is to be able to charge from the vehicle battery directly if the current demand is going to be high. Find out how much current the outlet can actually handle. With any luck you can install a small circuit breaker between it and the battery that will trip before the fuse blows.

3). Voltage drop. Depending on the wire size and length from the alternator, the Voltage even at low current levels may be a tad low to bring the battery in the back all the way up. Again this is one of those "monitor it and give it more charge when needed" things.

mike95490

Re: Help with basic inverter construction

Another option, is an accessory relay wired to power an inverter 120VAC sinewave, that then powers a good 40A battery charger to quickly bulk up your accessory battery. The low amps of your 12v outlet may not give you a complete charge in the time allowed.

And a solar panel, if you want to park in the sunshine!

tnine

Re: Help with basic inverter construction

Thanks for the help!

Cariboocoot wrote: »

1). Insufficient time from the engine to recharge the battery. This will depend on how deeply you discharge the battery and how much you drive to recharge it. You will want some reliable system to measure its SOC before you get in trouble here. Voltage is not really a good method of judging this. Battery monitors cost a lot of money. A hydrometer is fairly accurate, but requires you to actually check the SG and that means dipping into the cells (acid is nasty stuff). No perfect answer here.

I purchased this case. Do you think the meter attached will be sufficient?

http://www.amazon.com/gp/product/B000MT95MI/ref=oh_details_o00_s00_i00?ie=UTF8&psc=1

Cariboocoot wrote: »

2). The current limitation of the accessory port. A discharged battery may demand more current than this is designed to handle, leading to frequent fuses popping and not getting the battery recharged. The only way around this is to be able to charge from the vehicle battery directly if the current demand is going to be high. Find out how much current the outlet can actually handle. With any luck you can install a small circuit breaker between it and the battery that will trip before the fuse blows.

The DC charger has a 10 amp fuse wired into it. Will that be a small enough fuse to blow BEFORE the one in the car?

Cariboocoot wrote: »

3). Voltage drop. Depending on the wire size and length from the alternator, the Voltage even at low current levels may be a tad low to bring the battery in the back all the way up. Again this is one of those "monitor it and give it more charge when needed" things.

Would checking this with my multimeter be sufficient, or is this something that can vary while the engine is running and various devices are plugged into the power system?

Thanks,
Todd

Cariboocoot

Re: Help with basic inverter construction

tnine wrote: »

I purchased this case. Do you think the meter attached will be sufficient?

http://www.amazon.com/gp/product/B000MT95MI/ref=oh_details_o00_s00_i00?ie=UTF8&psc=1

That looks like the "monitor" is a series of LED's that light according to Voltage level. Accuracy? Well Voltage level isn't accurate SOC indicator to begin with. I don't think that would be as accurate as this MidNite meter for example: http://www.solar-electric.com/mnbcm.html Same principal, but with some more "smarts" to it.

The DC charger has a 10 amp fuse wired into it. Will that be a small enough fuse to blow BEFORE the one in the car?

That would depend on what the one in the car is. There must be a spec (owner's manual) that tells you how much current the outlet is good for. 10 Amps would be quite reasonable under most circumstances, but a deeply discharged battery could draw more.

Would checking this with my multimeter be sufficient, or is this something that can vary while the engine is running and various devices are plugged into the power system?

Thanks,
Todd

Two things. Checking the battery SOC as a function of Voltage involves not using the battery for several hours: no current in or out will allow you to take a "resting Voltage" reading, which is as close to SOC based on Voltage as you can get.

As far as the outlet sourcing is concerned, the vehicle's Voltage will vary according to what speed the engine is at and how heavily loaded its battery & alternator are. For instance right after starting it has to recharge that battery. Adding a second to it at the same time will increase the draw. As such at that moment the Voltage is bound to be off, but it doesn't matter. The bigger concern is how much drop there is on that line towards the end of the charging process when current is at the lowest it will get too. If they've 'undersized' the wire then the second battery may only come up to 13.6 Volts instead of 13.8 for example.

You can check this function with a Voltmeter by connecting it to the outlet with a load that will draw a few Amps and seeing what Voltage you get there. The closer the current draw is to what your other battery will demand the more accurate the assessment of available Voltage level.

NorthGuy

Re: Help with basic inverter construction

Batteries require a little bit higher voltage than 12V. Typically, you'll need 14 to 14.5V.

If the outlet is connected directly to the alternator then it'll provide this voltage. All depends on the state of charge and size of the battery. If you connect decently discharged battery with 30-50AH capacity directly to the outlet, it'll most certainly blow the 10A fuse. Smaller battery (or a battery that is discharged only a little) will be Ok. Otherwise you need something to limit the current not to blow the fuse.

If the outlet is voltage regulated and produces 12V exactly, then such voltage is not enough to charge lead-acid 12V battery.

Endurance

Re: Help with basic inverter construction

If your inverter is other than a true sine wave, you might want to test things before you head out on your trip. While square wave or so-called modified sine wave inverters work well for many purposes, they seem to have a hard time running digital clocks and operating battery chargers.

I am guessing your accessory plug on your Subaru will be fused higher than 10 amps. If so, I think I'd stock up on some of the 10 amp fuses for your accessory cable and give the thing a try given that you don't plan to discharge your marine battery deeply.

As far as metering, Marc is exactly right about a proper meter. They really are the way to go for most purposes. But you are going to be charging and discharging pretty slowly. If ever there were a situation where a spot voltage reading would give you a reasonable feel for state of charge, this might be it. You can do even better if you watch voltage over time and get a feel for how it's trending. I think I would give it a try before I purchased the proper meter.

I think you have a reasonable chance of this working because of your modest power demands. It sound like you have a good option for a great trip.

BB.

Re: Help with basic inverter construction

I know you have an inverter already--But I would highly suggest you look at the MorningStar 300 watts (600 watt for 10 minutes) TSW inverter...

It has a remote on/off (real handy) and even a "search mode" (only turns on for a fraction of a second every 1-2 seconds looking for an AC load)--Both of these options are very rare for 12 VDC inverters--The price is pretty good, and the inverter has been very reliable (many folks here use it).

-Bill

tnine

Re: Help with basic inverter construction

Thanks for all the help guys! I'm all set up and I've tested it, but I think my inverter may be junk . I purchased it secondhand, and it doesn't appear to produce enough voltage. The connection from the battery to the inverter DC connection posts shows 12.44 Volts, which seems correct to me. However when the inverter is running, I only get 100.8 out of the AC sockets. This seems like it's simply too low of a voltage given the DC input. Could this be due to the battery, or is my inverter just scrap?

Thanks,
Todd

Blackcherry04

Re: Help with basic inverter construction

tnine wrote: »

Thanks for all the help guys! I'm all set up and I've tested it, but I think my inverter may be junk . I purchased it secondhand, and it doesn't appear to produce enough voltage. The connection from the battery to the inverter DC connection posts shows 12.44 Volts, which seems correct to me. However when the inverter is running, I only get 100.8 out of the AC sockets. This seems like it's simply too low of a voltage given the DC input. Could this be due to the battery, or is my inverter just scrap?

Thanks,
Todd

Thats low output. What kind of meter are you using. Without a true RMS meter it'd hard to get a correct reading. Try plugging a light bulb into it and see how bright it is. Some Inverters will give all kinds of readings without a load and have some kind of auto standby mode.

tnine

Re: Help with basic inverter construction

I'm just using a cheap Regal DT-830B to test. Everything I plug in runs just fine, but I'm a bit leery of using it if it's going to fry my appliances due to low voltage. I plugged a small fan into it, and it runs just as well as the wall socket.

BB.

Re: Help with basic inverter construction

If it is a MSW (Modified Square Wave) inverter, they will read low on a typical (non-True RMS reading) digital meter.

The reason is the "Peak" voltage of the MSW "square wave" is lower than the "Peak" voltage of a true sine wave. The "cheap" DMM meters just read the "peak AC voltage" an multiply by 0.707 [1/srt(2)] to convert from Peak Sine Wave to RMS (Root Mean Square--The DC power equivalent voltage) sign wave voltage. (other wave forms, such as MSW need a different voltage/conversion conversion factor).

MSW inverters will work OK on ~80% of the AC devices out there, and may cause damage/early death on 10% or so of the AC devices out there... The difficulty is knowing which is which.

Typically, small AC power converters (wall warts, wall transformers) are those that tend to over heat. And some appliances (such as refrigerator compressors and other heavily loaded induction motors) where the motors may use upwards of 20% more power (wasted as heat). Excessive heat is not good for any device.

As BC04 says--If a filament lamp plugged in looks OK, and the devices you wish to use run OK--Then the inverter is probably fine.

Otherwise, the Morningstar 300 watt TSW 12 volt inverter is difficult to beat (yes, they still cost a lot more than a typical MSW inverter).

All About Inverters
Choosing an Inverter - Home Power Magazine

-Bill

Cariboocoot

Re: Help with basic inverter construction

I think we can safely say that is an MSW type inverter. They often give low output Voltage readings. Especially the not-so-good ones. This can be a problem for a lot of things like battery chargers.

Cariboocoot

Re: Help with basic inverter construction

The Morningstar may not be the best choice for this application as its output needs to be "hard wired" and the OP probably just wants an outlet. So without any motors to run he should consider the Samlex PST series: http://www.solar-electric.com/samlex-pst-series-pure-sine-wave-inverters.html

BB.

Re: Help with basic inverter construction

Personally, I would put the MorningStar in a protected location hear the battery/engine space (thinking about putting in engine compartment--but probably not a good idea--perhaps above passenger's feet?)... And run an AC cable to where the power is needed.

Problem is even a small "700 Watt" inverter needs heavy/permanent/bolted up electrical cabling on the DC side--Those cables need to support 75 amps or more (if operated near maximum 700 watt rating + starting surge).

Cannot just pop open the hood and use alligator clips to connect the inverter.

-Bill

Cariboocoot

Re: Help with basic inverter construction

BB. wrote: »

Personally, I would put the MorningStar in a protected location hear the battery/engine space (thinking about putting in engine compartment--but probably not a good idea--perhaps above passenger's feet?)... And run an AC cable to where the power is needed.

Problem is even a small "700 Watt" inverter needs heavy/permanent/bolted up electrical cabling on the DC side--Those cables need to support 75 amps or more (if operated near maximum 700 watt rating + starting surge).

Cannot just pop open the hood and use alligator clips to connect the inverter.

-Bill

He's not planning to. He's building a stand-alone unit with its own battery that plugs into the vehicle for recharging. Not a permanent install.

tnine

Re: Help with basic inverter construction

Thanks for the help guys. I'm all set up, but I'm unable to get my car to charge the battery. However, I *think* I may have an incorrectly wired DC plug. I may be wrong about this, so please correct me. Below are the test results from my multimeter

Positive test side => + negative tester => -

+tester on + batter, - tester on -battery => 12.3 V

+ tester in center female power port in car, - on outside contact => 12.09V

+ tester in center contact of male plug, - on outside contact (when connected to Marine Battery) => -12.3v

Note that when I've wired the male DC plug on to the battery according to it's labeled terminals, I'm getting current backwards from what I'm expecting when I test the DC male plug. Since every DC female port seems to have positive in the center of the DC receiver, I'm surprised that my multimeter is showing that positive is reversed from the plugs in the car (I checked my other car as well, positive in the middle of the female plug). When I attempted to charge the battery in the car, it blew the 7.5 amp fuse in the DC male connecter. Is this connecter wired incorrectly, or am I just reading it incorrectly? I'm a software engineer, not an electrical engineer, so I know just enough to be dangerous

The wall based AC->DC charger that came with the cable has the + and - reversed from the male DC plug, hence my confusion.



Any help would be greatly appreciated!

Thanks,
Todd

BB.

Re: Help with basic inverter construction

Your car battery at 12.3 volts resting is not very good... Unless it is quite hot there--the battery should be 12.6 volts or more resting.

But what you are really looking for is what is the a) the car battery voltage with the engine running (and alternator charging). And 2) what is the voltage battery you are attempting to charge when plugged into the outlet (note--yes, you want + to + or you will have the batteries wired backwards and will (at the very least) quickly pop the fuse).

For any battery to charge (at ~77F/25C), you need (rough/typical numbers):

• 12.7 volts -- resting battery voltage (after a few hours of no load/charging)
• 13.2-13.6 volts -- "float voltage". Enough voltage to keep the battery from self discharging, but not much else
• 13.8-14.2 volts -- typical voltage a car alternator will recharge the battery at when engine running
• 14.2-14.4 volts -- typical voltage for recharging an AGM or GEL type battery
• 14.5-14.8 volts -- typical voltage for (quickly) recharging a flooded cell storage battery
• 15.0-15.5 volts -- typical "equalization voltage" for flooded cell batteries (done once every month or so for a few hours--depending on need).

If you are getting 12.3 volts at the lighter plug--you are not recharging the battery.

-Bill

NorthGuy

Re: Help with basic inverter construction

tnine wrote: »

+ tester in center female power port in car, - on outside contact => 12.09V

Is that when the engine is running? If so, you may have a regulated plug (see my post #6) which doesn't have enough voltage to charge the battery. If not, you cannot charge when the engine is not running.