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Discussion Starter · #1 ·
In searching (yup, I used the "search" function!) the forum, I can't find any good advice on how much stuff you can add onto your stock electrical system before you need to upgrade the alternator (or the battery, for that matter). Is there any simple way to calculate the amount of amps you use with all of the additional equipment (radios, lights, winches, etc) and compare that to what the alternator is rated at? Or, do you just keep adding stuff until it starts to dim when you have every last thing turned on, keyed up, or flashing, and then you decide to push in the cigarette lighter?

If, in answering this question, anyone can reference a good "vehicle electronics for dumies" site/publication, that would probably do lots of good in my attempts to NOT have my car catch fire because I had no idea what I was doing wiring everything through 30A fuses with 16g wiring.
 

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If you add up the power each item requires and divide by 12, you'll get approximately how many amps it draws. For example, a pair of lights with 55W bulbs takes 9.2 amps (call it 10).

Divide the rated alternator output by 2 to determine how much capacity you have for added devices.

What kind of truck is this we're talking about? The older ones have notoriously small alternators (only 60A). You can have them upgraded very reasonably by Boyle Future Tech in CA.
 

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Discussion Starter · #3 · (Edited)
It's an '02 Double Cab Tacoma. The "divide by two" thing you mention: is that assuming that 1/2 of the alternator is used to supply the stuff that's stock in the truck, leaving the other 1/2 for add-ons?
 

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i run 2 amps and 2 procomp offroad lights. im sure my alternator hates me but it works. i also did the big 3 upgrade to help out.
 

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do what red runner tc told ya... and find out the amperage it draws...I know mine without even calculating it is drawing too much power.. My next upgrade will be a bigger alternator and a second battery (odyssey?) I have a large amp w/o cap, manual fan, more powerful headlights, larger cb (2amps...not much) and a radar detector.. and my interior lights dim when i hit the gas... no esta bien... search dual battery set ups. lots of people on here run them...
 

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Discussion Starter · #6 ·
OK. Now, when computing amps for stuff like radios/stereos, do you consider how much it can put out MAX? For instance, I have a Sony head unit that does 52W max output. Does that make it a 4.3A draw? What about the 50W Kenwood VHF transmitter? I just want to be sure I'm figuring this stuff up correctly. Thanks!
 

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no someone correct me if im wrong but you want the average... rms? but you should figure out the max just to cover your ass ...lets see some numbers..? what are you running totoal MAX and total average output of amperage..?
 

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wradar said:
It's an '02 Double Cab Tacoma. The "divide by two" thing you mention: is that assuming that 1/2 of the alternator is used to supply the stuff that's stock in the truck, leaving the other 1/2 for add-ons?
Exactly. It's kinda a rough number, but conservative if anything.

wradar said:
OK. Now, when computing amps for stuff like radios/stereos, do you consider how much it can put out MAX? For instance, I have a Sony head unit that does 52W max output. Does that make it a 4.3A draw? What about the 50W Kenwood VHF transmitter? I just want to be sure I'm figuring this stuff up correctly. Thanks!
Well, with audio equipment, it's a more dynamic power draw. You are very rarely actually making 52W. You could use the 4.3A and be super conservative, but it's more like 1A (divide by 4) continuous...
 

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Even RMS is still based on the max output, which you are rarely making, and even then in "bursts" which the battery can handle, however, the "divide by 4" roughly converts peak into RMS...
 

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Also, if you can get a volt usage gauge (voltmeter)... i believe summit has themm? I need one bad.. meh My buddy has one a RELIES on that damn thing since he has a first gen 4runner with a TINY alternator
 

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Discussion Starter · #11 ·
Here's what I've got so far:
Sony Stereo (52W max) 4.4A max or 1.1A (div by 4)
Kenwood VHF Radio (50W TX) 4.2A
Kenwood UHF Radio (35W TX) 3.0A
Whelen Strobes (90W) 9.0A (!!!! this is per the company)
Radio Shack Scanner 0.6A
TOTAL 17.9A

This, obviously, doesn't include the stock stuff, of which I have no real idea on how to calculate. (I assume that's why you divide the alternator's capacity by 2, so you can assume that 1/2 already goes to the starter, stock lights, electric windows, etc, etc?)
 

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wradar said:
Here's what I've got so far:
Sony Stereo (52W max) 4.4A max or 1.1A (div by 4)
Kenwood VHF Radio (50W TX) 4.2A
Kenwood UHF Radio (35W TX) 3.0A
Whelen Strobes (90W) 9.0A (!!!! this is per the company)
Radio Shack Scanner 0.6A
TOTAL 17.9A

This, obviously, doesn't include the stock stuff, of which I have no real idea on how to calculate. (I assume that's why you divide the alternator's capacity by 2, so you can assume that 1/2 already goes to the starter, stock lights, electric windows, etc, etc?)

The two Kenwoods are mostly listening, and when you transmit, is it both radio's...continuous? so you could probably cut a few amps there (they probably draw less than 1 each while in recieve....

even if you used up everything you have listed there, you are probably o.k.; at the top end, but o.k.

Just to add to this mess of a thought process... stock alternator should be 80A... but not at idle... so if you're using all that stuff and idling for a bit, you'll end up with a dead battery still!..... That's one of the bigger gains a high-output alternator gives you...the ability to run a reasonable amount of stuff at idle...

Lights are a big user of power... a few set's of over powered rock lights will run you into the red if you're not paying attention...as well as driving/fog/xxx lights...

Good Luck!
 

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Discussion Starter · #13 ·
So (and I ask this knowing it's probably a really basic question), are you saying that when your alternator is giving it all she's got, then the battery tries to pick up the remainder of the load?
 

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Mr Tacomi said:
a few set's of over powered rock lights will run you into the red if you're not paying attention
I can confirm that! Bump starting the truck on the bottom part of Moab Rim SUCKS!

wradar said:
So (and I ask this knowing it's probably a really basic question), are you saying that when your alternator is giving it all she's got, then the battery tries to pick up the remainder of the load?
Exactly. But the battery only has a certain amount of energy stored in it, so if you pull power over the alternator for longer than that amount of energy, you're done. But, if the power is intermittent, then the alternator has a chance to recharge the battery. This is kind of a slippery slope, because the alternator is at 14V and the battery is at 12V. The more you pull over what the alternator can provide, the more amps you need to draw to make the same power, which means the faster the battery dies.
 

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wradar said:
In searching (yup, I used the "search" function!) the forum
Since yer search skills leave plenty to be desired...

If and when you decide to upgrade... Here's some options

Thought I'd get yer next question answered... before it was asked... ;)
 

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Discussion Starter · #16 ·
RedRunner: thanks for the info!

Dell: wait, wait, wait... you mean to say that when I search, I'm supposed to READ the results? Ha ha. Thanks for that link!

I think I've got some time before I have to worry about upgrading the alternator, but I'm sure that time will come. ...that is, if I ever get to do all the stuff I'd *like* to do to my truck! :cool:
 

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RedRunnertc said:
...because the alternator is at 14V and the battery is at 12V. The more you pull over what the alternator can provide, the more amps you need to draw to make the same power, which means the faster the battery dies.
Well, - most of the big power draws typically are not constant-power loads but resistive loads. For example lights, winches (or anything with an electric motor). For those devices, as the system voltage droops, the current also drops, and the power consumed drops even more.

The behavior you describe would probably only apply to those electronics that incorporate a switched-mode PSU because they run from internal power voltages greater than the system voltage. Or to inverters for mains power.
 
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