Additional electrical loads

[sparkydave]

Does anyone have a feel for how much additional load an '05 Legacy's alternator can handle, especially at idle?

 

My wife is needing to use a laptop and printer in her car, so we bought an inverter that will run both, but she has little faith in her car's electrical system and doesn't want to become stranded if the alternator can't keep the battery charged while the inverter is running. We were going to use a deep cycle battery, but her dad shot that down and doesn't want her driving around with a battery on board.

 

The inverter will need 20 amps tops to run the computer and laptop, so obviously the accessory outlet won't work. I've installed my amateur radio gear (25-30 amps) in cars before by running fused wires from the battery, but those cars usually came out and said what the alternator was rated for and how much additional load you could add.

[porcupine73]

What accessories are you planning to run at the same time, like heat or air conditioning, headlights, rear defrost? If all those are off it will handle the 20 amp load alright.

 

Also, I am assuming the printer is an inkjet (not laser?)

 

One way to go is install a voltmeter. If while it's sitting there idling, you see the voltage keep dropping you know the alternator can't keep up. Usually you will see maybe 13.4 volts or so, but if it drops to like 12.8...then 12....then 11.7 the load is too high. Not sure how low it can go before the vehicle won't run anymore, but if it drops below 11.5V I would not shut the vehicle off, but rather drive or shut off accessories for a while before shutting off.

 

With radio equipment, the power used when receiving isn't usually excessive, but when transmitting it will eat up some juice.

[211]

I had a similar question started a while back.

 

http://www.ultimatesubaru.org/forum/showthread.php?t=78750&highlight=inverter+laptop

 

I had no problems running my laptop while on a road trip. Inverter, laptop, lights, A/C, and stereo. The kids had the laptop in the back seat w/ headphones and I had the stereo playing, A/C on chill, fan on high.

 

I don't think you'll have any charging issues. The car will handle the laptop just fine and with the "occasional" print job, I think you'll be alright.

She's not printing out an owners manual or anything right? Simple one or two page printout?

[porcupine73]

Above idle, like driving/cruising, I wouldn't worry about it at all. At idle when underhood temps are high is when the alternator's output capacity is lowest.

[jamal]

Yeah, I send e-mails and make toast in my car, while talking on the cell phone drinking coffee and brushing my hair as I drive 80 through traffic on the freeway.

 

Seriously, though, how necessary is it to have a printer in the car?

[nipper]

You may think of two things. One is install a amp meter and a volt meter to know whats going on (thats actually a must).

 

The second is install a second battery with the proper charge regulator. Even brand new cars will kill the batteries if stuck in a traffic jam from hell with everything on. You need to rev the engine every so often to build the system back up. ALso for extra insurance get your self a deep discharge battery as the 2nd battery.

 

nipper

[johnceggleston]

i have a 400 watt inverter which can handle my laptop. but i plugged in my cell phone charger (120v) the other day and the warning tone on the inverter would sound occasionally, mostly at stop lights. so i think my problem is one of 2 things, either the altenator is bad, or there is sever voltage drop in the wire run to the lighter. running heavy gage wires to the battery would be good.

 

if her laptop battery is good, it will run the laptop and the inverter will charge it. the printer is just a straight pull. how about adding a Uninteruptable Power Supply (charger, battery, inverter) this would give you power for a limited time and you could leave it plugged in while you drive around keeping it topped up. then it's just a matter of size and money.

[montana105]

I'm hoping she's not gonna be reading her computer and printing while she's driving,but if she is I'm glad she's in Ohio and I'm up here in Montana.but seriously it would help to hav a better description of the uses of the computer

and printer(how long and how frequently,car running or not- is she printing invoices or what?) we have numerous times been watching tv in a friends rv with one deep cell battery for a couple hours at a time with no effect on pumps and such but they have no relation to the starting system.I'm with Nipper here,a true volt meter for precise readings. My thinking is if she is just printing occassionally she should be alright

[sparkydave]

Yeah, I send e-mails and make toast in my car, while talking on the cell phone drinking coffee and brushing my hair as I drive 80 through traffic on the freeway.

 

Seriously, though, how necessary is it to have a printer in the car?

 

Clearly I neglected to mention that the whole reason behind this was her mobile veterinary practice for horses, and the need to be able to access client information, print receipts, and do billing on site. She wasn't planning on doing all of this while on the road driving, as some might assume. :-\

 

I'm going with the reasoning that not having the lights, defroster, air conditioning, and other accessories going will leave plenty of capacity to run the notebook and printer. Now it just remains to see if the lighter outlet will handle it, and in a pinch she could unplug the notebook to print if the two combined blow the fuse.

 

Thanks all!

[jamal]

Lighter outlet definitely cannot handle it. You would need to run a new plug from the battery with better wiring. For 20a, you would want to run 8awg wire with a fuse right at the positive terminal. Here's a convenient table:

 

http://ken-gilbert.com/techstuff/AWG_WIRE_TABLE.html

 

Other than that I don't see a problem. I have the section of the manual that show alternator output at specific rpm ratings at home so I could tell you the amperage at idle in a few hours if you'd like.

[sparkydave]

I actually put the whole getup on my 12 volt power supply, and found out the computer and printer together peak at just under 10 amps on the 12 volt side while the printer is printing. Looking at the car's fuse box it actually has a 20 amp fuse on the accessory outlet, so it looks like it will run fine out of the outlet after all.

[jamal]

okay, well if I were you I would run a separate outlet with bigger wiring regardless.

[montana105]

Sparky,don't know how serious jamal was but I was just kinda of kidding you,glad to hear she won't be driving at the same time. I think the thing to consider here is that if she is gonna be on the computer for extended times you may want to look at other avenues as has been mentioned numerous times here these cars do not maintain a battery at idle with just convenience items running much less a computer and printer,if it's only a couple minutes at at a time I see no problem, but you need to get real time readings on what your alternator is doing at various rpms( i'm assuming since you mentioned a/c and such the car will be running which also drains power just to provide spark and maintain vehicle computers which also causes additional drain on the electrical system at idle,not to mention if it has daytime running lights)

[sparkydave]

I'm afraid I'll have to disagree with the idea that "these cars do not maintain a battery at idle with just convenience items running much less a computer and printer". Even my old MG has no trouble maintaining 14.1 volts at idle with headlights and heater fan running (15+ amps of load), even with its pitiful 34 amp alternator. I checked the Subaru with all accessories running (headlights, foglights, air conditioning, defroster, radio, interior lights, and windshield wipers), and it was also maintaining 14.1 volts at the battery. So, given we'll be using the computer with most (if not all) of those turned off it should be fine.

[nipper]

I'm afraid I'll have to disagree with the idea that "these cars do not maintain a battery at idle with just convenience items running much less a computer and printer". Even my old MG has no trouble maintaining 14.1 volts at idle with headlights and heater fan running (15+ amps of load), even with its pitiful 34 amp alternator. I checked the Subaru with all accessories running (headlights, foglights, air conditioning, defroster, radio, interior lights, and windshield wipers), and it was also maintaining 14.1 volts at the battery. So, given we'll be using the computer with most (if not all) of those turned off it should be fine.

 

Again, you cant compare a car from one era to another. It just doesnt work.

 

Your pitiful MG had far less electrical load then a modenr car. Also that 34 amp altenator is rated at 2000 rpm (like all altenators are), not at idel speed.

 

Altenators do not put out a lot of power at idle speed, this is an established fact.

 

This lowly automotive engineer here can attest to that, and why new cars with lots of gizmos can kill batteries while idling in traffic with everything on. Gizmos mean the lights, climate controls, 4-5 compouters that are in the car, stero, radiator fans, solenoids, fule injector pumps. There is a hell of an electrical load at idle even in the simplest modern car.

 

http://www.aa1car.com/library/charging_checks.htm

 

At idle an alt may only be putting out 20-30 amps. In a modern car that is easly all consumed. your little MG may have been using 5 amps at idle.

 

nipper

[jamal]

Your pitiful MG had far less electrical load then a modenr car. Also that 34 amp altenator is rated at 2000 rpm (like all altenators are), not at idel speed.

nipper

 

Ratings are based on the speed of the alternator, not the engine. The alt turns about 3x faster than the crank.

[nipper]

Look up any altenator spec. You test the output at 2000 RPM engine speed. All alt's are rated at this speed, not at 750ish which is idle.

 

Ratings are based on the size of the alt, for the given car.

 

 

nipper

[jamal]

most alternator specs I've seen, such as those in the FSM, give a spec for multiple alternator speeds.

 

Yes, I know the alternator isn't putting out the max current at idle, and off the top of my head I don't know what the numbers are. At 5000 alt rpm it puts out 85a.

[sparkydave]

Again, you cant compare a car from one era to another. It just doesnt work.

 

Your pitiful MG had far less electrical load then a modenr car. Also that 34 amp altenator is rated at 2000 rpm (like all altenators are), not at idel speed.

 

Altenators do not put out a lot of power at idle speed, this is an established fact.

 

This lowly automotive engineer here can attest to that, and why new cars with lots of gizmos can kill batteries while idling in traffic with everything on. Gizmos mean the lights, climate controls, 4-5 compouters that are in the car, stero, radiator fans, solenoids, fule injector pumps. There is a hell of an electrical load at idle even in the simplest modern car.

 

http://www.aa1car.com/library/charging_checks.htm

 

At idle an alt may only be putting out 20-30 amps. In a modern car that is easly all consumed. your little MG may have been using 5 amps at idle.

 

nipper

 

Amps are still amps, regardless of what vintage vehicle it is. I'm well aware that the MG has way less electrical load than the Subaru, which is why the MG with its single ignition coil and carburetor gets along fine with its dinky 34 amp alternator, and the Subaru has a substantially more powerful 110 amp alternator. I'm also well aware that the alternator can't produce the rated output at idle. Proportionally though, the Subaru's output at idle is still considerably higher than the MG's output at idle. What you're overlooking is that in my comparison the MG's alternator was delivering whatever load the single ignition coil takes PLUS the 15+ amps to run the lights and blower motor, and that represents about half the rated capacity. Now if a 1977 vintage alternator made by a company that's notorious for its vehicle electrical systems can deliver at least half of that stated capacity at idle, then why wouldn't a modern car also be able to do deliver at least half its rated output at idle? Are you quite sure it's only 20-30 amps? That would be pretty pathetic.

 

My point is that if the Subaru's alternator is capable of powering all of those accessories in my test PLUS the background current to run the fuel injection, ignition, PCM, fuel pump, radiator fans, etc., then it is perfectly capable of supplying a 10 amp load to run a computer if all the accessories I can turn off are off. Believe me, those accessories combined take considerably more than 10 amps.

 

As for the battery not being maintained, the fact that 14.1 volts is being maintained while all those accessories are on at idle (again, in ADDITION to the loads you can't turn off), then clearly the alternator is delivering sufficient current (albeit not the rated output) to supply charging current to the battery, as well as supplying all the car's loads. If it wasn't, then the voltage would be lower. A charging voltage of 14.1 volts is normal for flooded lead acid batteries, because going much higher than that the electrolyte starts bubbling. Even 13.4 volts is sufficient for battery maintaining, though not enough to replace lost charge at any reasonable rate. If it was dropping to the battery voltage or lower then that would mean the battery is having to deliver current to keep up with the demand. You can't have a voltage of 14.1 volts on a battery with a nominal open circuit voltage of 12.6-12.8 volts (I measured 12.95 with the engine off) and have current flowing out of the battery. This lowly electrical engineer is sure of that.

[PAezb]

You'll be fine, the car battery acts as a large capacitor during peak loads anyway, and will certainly handle sustained loads within voltage tolerances from it's reserve capacity for longer periods - with or without the alternator's help. I use to run a high-powered car audio system (2 120w/ch x 2 amps, 1st gen disc-changer - Sony's disc jockey - not exactly power friendly) for well over an hour at a time, at high output with the engine turned off before starting it up and recharging for 10-15 minutes. That was quite frequent with a gathering of friends in the evenings at a local park. Never had a problem - and that was on a GM vehicle :-p (if that really makes any difference). Of course that was with the proper dedicated wiring supplying power direct from the battery.

 

To the main electrical system, an additional 10 amp load is nothing.... but I do have to agree with Jamal, the wiring on the cigarette lighter circuit, and even the fuse wire on the back of the lighter socket tend to be smaller gauge capacity than I'd like to see (on Subarus). But you'll probably be fine - and if you blow anything it will be most likely that thermo-fuse wire on the back of the lighter socket.

[montana105]

Guess theory prevails over practical experience.I'm out.