PDA

View Full Version : voltage drop



mk3phill
Friday 17th October 2008, 19:05
Hey guys,. got a query... since ive had my 850 with aftermerket HU and small power sub/amp,. ive knoticed some voltage drops when operating at night or with a few pieces of equiptment,..

now it didnt bother me much but have recently purchased some new subs,.. now ive only got one (out of two) running and at only half the power it really needs (due to using old amp) but the voltage drop is now more severe,. with the system on at adequit volume the whole interior lights flicker, along with headlights.

before i get the right amp and have both subs running at right power is there anything i can do to the electrical system,.. bigger altenator?? or would i be better to aid the use of a power cap for my subs

oh i have a 95/96 B5252s

t5 pete
Friday 17th October 2008, 19:22
myne runs fine with the standard alternator it sounds like your is going down and will need replacing if the lights are flickering at tickover with the sub its most likley the voltage stabaliser

mk3phill
Friday 17th October 2008, 20:37
sorry i must add the drop is due to the sub but its happening while i drive also. how much would a recon altenator be,. or voltage stabi

t5 pete
Friday 17th October 2008, 20:50
the beat place would probley be gsf or euro car parts they might be aroung £100 for a new one there are a few on ebay but you never know the condition and with items like a alternator i would rather have a new one

thebadger
Saturday 18th October 2008, 00:31
Reccomend you start the car, then take off the negative lead to the battery...

If the car konks out, your alternator is goosed!

I always reccomend a power cap when using any more than 1 sub! Bigger the better too!

I currently use an 8f cap with one 15" sub!

Hope this helps..

mk3phill
Saturday 18th October 2008, 00:49
thanks for the tips guys,.. ill try gsf,. know some of the lads there so get some decent discount,. an if its dry il attempt removing the neg lead,.. tho i still end up gettin a power cap aswell,. the electrics have always seemed a bit weak tbh

thebadger
Saturday 18th October 2008, 03:16
Most probably a dud altenator, or maybeys a dry cell in the battery....

The neg lead will prove the theory!

SyKo
Saturday 18th October 2008, 13:30
Reccomend you start the car, then take off the negative lead to the battery...

If the car konks out, your alternator is goosed!

I always reccomend a power cap when using any more than 1 sub! Bigger the better too!

I currently use an 8f cap with one 15" sub!

Hope this helps..

ok ... i thought that if you took the negative off regardless the car would cut out as there is no physical ground connection, current can only flow if there is a ground. I thought it was take the positive off, then if it dies its a dead alternator.

Secondly, power caps only give a little bit of extra power but only in an instantaneous draw. Your alkternator is what ios charging the battery and the power cap. If you dont have a big enough alternator then your lights will dim regardless of how many power caps you have or batteries.

I have 2 subs running at 2k each in my boot aswell as a jl a4300 driving the 4 door cards and a a2150 driving components and 6x9's in the parcel shelf, equates to about 5000w of power all in my boot.

P= I x V so 5000w = I x 12 therefore P/V = I, 5000/12= 416

i have a split charge system which i was running with the stock alternator (80amp) and my lights were dimming. (even with extra battery and power cap).

i went out and spent the best money ever i feel on a performance 200 amp alternator and my lights do not dim any more.

You have to think that if the alternator cannot put out enough current to charge the battery and run the car AND a 1000w+ stereo then your lights are going to dim.

The reason for cars with alternators circa 55-80amps is because they rate them at being big enough to run everything standard on the car if the battery terminal comes off whilst driving.

once you introduce an alien into the system such as a stereo them thats going to be drawing an extra say 25-40 amps which is putting extra strain onto the alternator, especially when you crank the volume.

In response to your lights dimming because of these new sibs, you amp is drawing more current to run the new subs, your old one was probably 4ohms. Your new ones could be 2 ohms. which will make the amp draw more current. (both my subs are running at 1ohm, and run at 2k)

thebadger
Saturday 18th October 2008, 20:54
The battery is there to start the car......

that's about it! However, bigger amp droppage will over tax any alternator, causing dimming of the lights etc, a cap acts like a resivoir for say, a hose, you're using the hose & opening & shutting the tap on it, the more you open the tap the quicker you use the supply & lose the flow.

A cap acts as an extra pot full of water to keep the flow steady!

Two batterys can do a very similar thing, tho you now have two batteries drawing on the alternator. (your uprated alternator was a good idea)

But for thoes of us with a limit on funds, a power cap is a really good alternative.

Also, the amps "stored" in the battery would not be available if the battery is disconnected (either terminal), however, any of my fellow boffins will know that electrons flow from the negative - to the positive, so a better idea to remove the negative to stop the additional strain on the battery. Electrons know nothing of circitry & go wherever they like, capacitors can do the same too!

P.s. SyKo, I'll assume you realise that RMS is what you go by for calculations of that sort & not peak, elst yer car would melt & the batteries would boil!

Ed.
Saturday 18th October 2008, 23:23
To check your alternator, use a multimeter set to voltage to measure the voltage of the battery. Note the voltage, it should be between 11v and 12.7v, now start your car. Voltage should rise to 14v+. If it doesn't then the alternator is not charging.

If you have a clamp meter, measure the positive cable from the alternator to the battery. This will show the current that is flowing between alternator and battery. As you turn on aircon, heated screen, lights, etc the current draw should increase.

If your alternator is healthy, and the battery is not discharged, then i suggest uprating the big 3. These are cables between the battery earth, the alternator to battery positive cable and alternator to ground cable. These should be uprated to 4awg cable or thicker. I would advise against a power cap, they do not help in the long run. A second battery or bigger alternator is the way to go.

Ed

mk3phill
Monday 20th October 2008, 00:05
thanks for the info guys,. had a quick look yesterday and it seems i already have a 100amp altenator,.. will check the actual figures through the week. as for the big 3,.. its something i done with my polo,. didnt think it would need doing on such a bigger car.

thebadger
Monday 20th October 2008, 01:37
The negative battery lead on my 70 is 0awg! If I replace that I'm gonna need a bigger boat!

Ed.
Monday 20th October 2008, 02:09
Back in the days when i used to compete in dB drag sound offs, i ran 4 or 5 lengths of 4awg for battery earth. It all helps.

Ed

SyKo
Monday 20th October 2008, 15:44
I've changed all the big power wires for 0 on mine. if anyone wants some 0 awg i have 5m of blue and about 3 of silver.

ps. thought the alternator couldnt generate a ground ... current wont flow if it hanst got anywhere to go?

thebadger
Monday 20th October 2008, 17:27
ps. thought the alternator couldnt generate a ground ... current wont flow if it hanst got anywhere to go?

The altenator, is a big dynamo, so it generates its own negative.

Why else would it need a ground lead?

Promise you if you detatch your batteries from the car (negative cables) the car will keep going!

SyKo
Tuesday 21st October 2008, 14:03
and if it doesnt? like mine...

thebadger
Tuesday 21st October 2008, 17:25
Then your altenator may need replacing.

My car runs fine with the neg off the battery when the car is running.

May be to do with your twin battery system tho, I have no idea how the relay system would respond to removal of a cable!

fast-t5
Tuesday 21st October 2008, 20:41
well when i had my system in mine i had to change the alternator as i had the same problem, steer clear of power caps there a waste of time honest, (i used to work in an audio shop) place a second battery in your boot mate as they charge quicker than a power cap, my stereo was two 15's and used to read 151.6 db so i no a little on the subject lol i had two ultima batterys in my boot running 0 gauge all round never had any trouble once alternator was changed.

mk3phill
Thursday 23rd October 2008, 01:15
thanks fast-t5,.. and to the rest of you guys,. unfortunately im giving up my twin 12" mtx subs in favour of a more hands on project,.. well my mate wants them for his type-r so the cash is gettin spent on my engine. but this is some good and usefull information,. for future reference!

SyKo
Thursday 23rd October 2008, 17:59
true. the second battery has to be grounded to the car chassis to be able to be charged. all i have is a 180amp relay attached to a switch, which when i am driving around i have turned on so it charges both, then when the engine is off i turn off the switch and it isolates the battery in the boot.

151 ... thats blooming loud! my friend spooted a concrete filled 240 with a massive 18inch jack hammer i think it was that was pumping 180db or something silly like that, apparently it was shaking the building to bits!