high output alternator - help

[Monkers]

Hi team,

I have a subaru liberty 1996 lx. currently the alternator cant supply enough current to my speaker system, i'm worried its going to damage the amplifiers. I'm heading to a few festivals and camping trips this summer so it will be nice to have something working that i can leave running in idle. I'm looking for either a high out put alternator (120A<), second alternator installation or a alternator rewinding. I'm in Victoria and cant seem to find any auto electricians willing to give me a quote.

Also a thought was to get a length of 4G cable and wire my car to a few mate cars in parallel, creating one big power source so there's no strain on my car. Any harm in doing this?

Thanks.

[al_cam]

O.K. Sermon starts here. Apologies for the verbosity.
1. Not enough current is extremely unlikely to damage your amplifiers. Too much power being drawn by the system or too high a voltage is what causes things to blow. If the power supply is under rated for the load then all that happens is the supply voltage falls or droops. The effects would be audio distortion or lack of volume. Can’t imagine what would be damaged by an under rated supply. I’m an Electrical Engineer and have blown up quite a few things in my career. More than happy to be corrected though.

2. High output alternator at idle? Pretty hard if you are after a really high current. Alternator power output is almost linear with rpm. At idle they put out stuff all. If you have a flat battery and manage to start the car, you need to go for a drive for a while or put a brick on the accelerator to get the revs up. At idle you may not even be putting out enough power to charge the battery. – But if your demands aren’t really that great than a higher rated alternator with the idle speed raised a little may be enough. This is an Amps-RPM chart for a Bosch 120A alternator a typical curve.

alt chart.jpg (20.12 KiB) Viewed 3273 times

3. What is the condition of your alternator in your Liberty? If it’s 1996 it may have had it and may just need to be refurbished or replaced. Brushes worn, voltage regulator stuffed, drive belt condition, bearings &c... Put a digital volt meter as close to the output of the alternator at as you can get. At idle you should see around 13.8 volts. Turn off everything electrical and check, then turn on everything and see what it does. Voltage will drop with high beam lights on but you should get around 13.8V at no load.

4. Rating of a 1996 alternator would probably be around 80A or better which is enough for a normal car, so re-winding for higher rating may not get much increase – which is why you may be having trouble getting a quote. Higher rating alternators are certainly available, you just have to make them fit – brackets, pulleys, lining up the belts &c... I’m guessing but you’d be looking at $800 up. Also given that some of the things you say are a little odd, electrically speaking (not trying to offend you here), this may be putting auto electricians off as they think you will just be a “difficult” customer who may not be happy with whatever they do.

5. Second alternator? Would look at bigger single one instead. Maybe look at removing the air-con compressor and mounting a big alternator in that space if there isn’t enough room. I’m not familiar with the engine bay of a ’96 Suby.

6. Two cars in parallel? Yes it will work, but be aware that any difference in voltage will cause current flows until they match - And you have car batteries in the circuit. They can supply huge currents for short periods. Really dangerous power if you get it wrong. Flash, bang, quick trip to the burns department. Bits of Monkers missing.

7. Think about maybe installing a second car battery (in the boot?) to supply just the Amplifier. Hook it up with a diode so that it doesn’t supply current when starting your car, only get’s charged by the alternator. This is typically done for camping fridges or lighting. You should be able to pick up a “second battery” kit from a car parts shop, 4WD store &c... This is only a temporary supply; lead-acid batteries don’t like being discharged too deep. So better kits will have something to monitor the voltage so that they warn you or switch off the supply when the voltage gets low. So you need to by the highest rated battery you can ($$$$). Also the second battery kit should isolate the starter battery so that in the morning when the milk is still cold you can still the start your car.

If you can find the specs for a battery (not always as easy as you think): 480 CCA means it can supply 480 cold cranking amps (obvious?) for a few seconds - typically how battery specs are given. But another rating is say 38 Ah (ampere-hours) means a theoretical 38 Amps for one hour (the “1C” rate) – but it won’t. It will however supply 1 tenth for ten times longer i.e. 3.8A for ten hours (“10C” rate). Probably 8A for 5 hours - when new and fully charged. 8x12 = 48W. I'm a deaf old bugger and that would be enough music power for me, but I was a teen when the invented heavy metal. What you need to know is what your sound system power supply requirement is at 12 volts and work up from there. Put an ammeter in the supply line to you amp. turn the ignition off and crank up the volume.

Good Luck.
AL.

[bigBADbenny]

I have a 200 amp hour deep cycle battery if you need something for camping/doofing. It's a monster @ 56kg

[dr20t]

Fantastic post Al !

Might pick your brains via pm on a couple of things you you have mentioned there actually (I'm secretly a wannabe electrical nutter)

[al_cam]

O . k . he says warily. Fire away.

But...

't fix your computer.jpg (10.84 KiB) Viewed 3237 times

[BUDDAH]

This bloke had the same problem

[al_cam]

What's the speed rating of the wheels on that gen ?

[Ric]

O.K. Sermon starts here. Apologies for the verbosity.
1. Not enough current is extremely unlikely to damage your amplifiers. Too much power being drawn by the system or too high a voltage is what causes things to blow. If the power supply is under rated for the load then all that happens is the supply voltage falls or droops. The effects would be audio distortion or lack of volume. Can’t imagine what would be damaged by an under rated supply. I’m an Electrical Engineer and have blown up quite a few things in my career. More than happy to be corrected though.

Agree in general, but if the voltage droops, and the volume is wound right up, you can get clipping on the output, which can damage speaker voice coils if you leave it running.

2. High output alternator at idle? Pretty hard if you are after a really high current. Alternator power output is almost linear with rpm. At idle they put out stuff all. If you have a flat battery and manage to start the car, you need to go for a drive for a while or put a brick on the accelerator to get the revs up. At idle you may not even be putting out enough power to charge the battery. – But if your demands aren’t really that great than a higher rated alternator with the idle speed raised a little may be enough. This is an Amps-RPM chart for a Bosch 120A alternator a typical curve.

alt chart.jpg

Note the "RPM" on that chart is the alternator RPM, not the engine RPM. Typically the alternator is rotating at 4x engine speed, depending upon pulley sizes. So, you WILL be getting output at engine idle, but only about 40A.
(Although I'm curious why that chart only goes up to 6000 RPM.)
Also note there can be another complication. Modern cars don't have regulators inside the alternator. The field winding is controlled directly by the ECU, which automatically compensates for extra loads like headlights etc. and varies the charging voltage with temperature as well.
That probably doesn't apply to a 1996 LX though.

I agree, two alternators, and hooking multiple cars together are bad ideas.

[Deep Heat]

Note the "RPM" on that chart is the alternator RPM, not the engine RPM. Typically the alternator is rotating at 4x engine speed, depending upon pulley sizes. So, you WILL be getting output at engine idle, but only about 40A.
(Although I'm curious why that chart only goes up to 6000 RPM.)

Probably because the alternator is rated to 120Amps.