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Joolz

Are My Voltage Readings Correct

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Hi guys I just bought a 99 IS200 and the Battery died last week evne after a weekend of running it. So I had the batt replaced, the dash lights and headlights used to dim at idle with the old batt, but even with the new batt they do that still so I had the alternator (measuring 12ish V when car running) replaced too. Now wiht a new alternator (not brand new, but refurb off a younger car) the reading (on 1 of those voltmeter plugged into the cigar lighter socket, and also indicated on the lexus' own voltage indidcator dial on the dashboard) is 14 when car is running at normal driving speed (30-50mph) with all lights, fogs, stereo and air con on, the reading drops to 13.1-13.3 at engine idle and all elec off, it drops to 12.7-8 when headlights are on, fogs will make it hit 12.6, and if heated seats are also on, I get 12.5V. When engine is off and only ignition is on, with all lights, air con, etc switched off, the reading is 12.4. Now my question is, is it normal for the alternator to go above 13 only at 2k+ rpm? It seems to me, it falls below 13 when any electrical equipments such as the headlights and air con are swtiched on at idle speed, is that at all normal? A friend had mentionmed that the alternator should produce 14V even at idle speed, and it really should not be affected by the amount of elec equipment you have on as long as the engine is running, you should always get 14? So does it mean my Battery or the new alternator is not doing its job? How do I know if it's the Battery or the alternator? Or am I just being a paranoia? I do still notice a little increase in light intensity when I rev the car when car is not moving, but not as much as before now, if you don't pay attnetion you probably wont' notice the slight increase in brightness of the headlihgts and dash lights.

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I reckon you are probably OK.

The no-load V of a decent fully charged 12v PB Battery is around 13.8 - 14.2V depending on its quality and age etc.

The alternator may well be putting out 14V - but this will appear "damped down" as it were by the Battery itself taking in charge.

I wouldnt worry too much...my Kit car sits around unused for daysweeks, and in winter 6 months - and the Battery shows way less than 14v off load, but still works fine.

I have just started using a very low rate ( 200m/a ) trickle charger which I connect permanently ( Yes alright....except when driving clever clogs :D ) and these are available pretty cheaply these days and are a good idea.

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I have bought a solar powered Battery charging panel thing, and connected it to the cigar lighter socket. Now I wonder if it'd really work and charge the Battery during the day... but it's worth a try, I cannot risk having the Battery run flat as I really need my car when I use it, but I don't drive it everyday, and when I do it's mostly for shortish distances. Anyway I checked my dad's honda out and the Battery V level is a lot more stable, when the car is running I get 13.8V with or without all lightsand air con on, and at rev it's still 13.8-9, it doesn't really change at all! If engine is not running I get 12.7-8. It makes me think there is something wrong with the wiring or somewhere in the elec system, as my lights are still relatively dimmer when at idle and when car is idle with lights and air con on, the V reads 12.8, so doesi t mean it's really not charging the Battery much when the car is stationary or at low rev? Shouldn't the V be near 14 and stay there whenever the engine is running? Or am I still worrying too much... I wonder if I should take it to lexus for a check, but they'll probably charge me loads and not spot anyting... I have already tried a local garage who replaced the alternator and batt but it is still not giving me stable V...

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A friend mentioned about possible bad earthing, does that sound like something that might cause the fluctuating V readings?

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It's worth checking the earth connections are clean and tight but I think everything is normal.

An alternator will produce 14v at idle if the Battery requires charging but when you turn on headlights etc. you start to draw more amps than the alternator can generate at idle speed so the voltage will drop. Above 2000 rpm and the voltage should rise.

Only if the voltage drops below 12v (really 12.4) do you have a problem.

What size Battery did you install?

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Quick question.

After having the stereo running and the air con when the engine wasn't running i managed to drain the Battery until it read 6V thinking it would still have enough juice left to start the car. To my surprise it wouldn't even attempt to turn over, i left it for an hour and still it wouldn't turn over so i got some jump leads and problem sorted, its now reading 12-13 V.

is this normal? if so then why dont they have a warning area showing when the Battery isn't capable of starting the car.

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Quick question.

After having the stereo running and the air con when the engine wasn't running i managed to drain the battery until it read 6V thinking it would still have enough juice left to start the car. To my surprise it wouldn't even attempt to turn over, i left it for an hour and still it wouldn't turn over so i got some jump leads and problem sorted, its now reading 12-13 V.

is this normal? if so then why dont they have a warning area showing when the battery isn't capable of starting the car.

Yes, it would be normal. By the time the Battery reads 6v it is VERY flat. The voltage is not proportional to the amount of charge ie 6v doesn't mean it's at 50% capacity.

There's no warning because they don't expect you to do it, and besides, you find out soon enough :)

The voltmeter is to show you that the Battery is being charged but it doesn't really indicate the condition of the Battery.

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Quick question.

After having the stereo running and the air con when the engine wasn't running i managed to drain the battery until it read 6V thinking it would still have enough juice left to start the car. To my surprise it wouldn't even attempt to turn over, i left it for an hour and still it wouldn't turn over so i got some jump leads and problem sorted, its now reading 12-13 V.

is this normal? if so then why dont they have a warning area showing when the battery isn't capable of starting the car.

Yes, it would be normal. By the time the Battery reads 6v it is VERY flat. The voltage is not proportional to the amount of charge ie 6v doesn't mean it's at 50% capacity.

There's no warning because they don't expect you to do it, and besides, you find out soon enough :)

The voltmeter is to show you that the Battery is being charged but it doesn't really indicate the condition of the Battery.

cheers, aye its a lesson you only need to learn once. I can't say i'm well up on voltages etc. but i would like a line that shows minimum voltage required to start car. By the way what voltage would that be?

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cheers, aye its a lesson you only need to learn once. I can't say i'm well up on voltages etc. but i would like a line that shows minimum voltage required to start car. By the way what voltage would that be?

Well, I'm no auto-electrician so I can't give a definite on that, but it's current (amps) that you need, not volts. As the Battery discharges, the voltage will stay around 12v for quite a long time, and then fall away fairly quickly as it becomes depleted - that's why the voltage doesn't necessarily tell you how much charge you have. I think if you're much under 10v you will have trouble starting it.

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cheers, aye its a lesson you only need to learn once. I can't say i'm well up on voltages etc. but i would like a line that shows minimum voltage required to start car. By the way what voltage would that be?

Well, I'm no auto-electrician so I can't give a definite on that, but it's current (amps) that you need, not volts. As the Battery discharges, the voltage will stay around 12v for quite a long time, and then fall away fairly quickly as it becomes depleted - that's why the voltage doesn't necessarily tell you how much charge you have. I think if you're much under 10v you will have trouble starting it.

that a pretty good explanation. thanks.

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I have bought a solar powered battery charging panel thing, and connected it to the cigar lighter socket. Now I wonder if it'd really work and charge the battery during the day... but it's worth a try,

From memory these panels only work through the cigar lighter if it is a permanently powered one. I.e. when you turn off the ignition you can still get power from the lighter. If it is not you have to wire it in to the Battery. The Lexus does not have a permanently powered lighter.

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I have bought a solar powered battery charging panel thing, and connected it to the cigar lighter socket. Now I wonder if it'd really work and charge the battery during the day... but it's worth a try,

From memory these panels only work through the cigar lighter if it is a permanently powered one. I.e. when you turn off the ignition you can still get power from the lighter. If it is not you have to wire it in to the Battery. The Lexus does not have a permanently powered lighter.

Yes it seems so, thanks. I wonder if anyone knows where I could push the wires out into the engine bay from the cabin if i were to wire the panel to the Battery? I remember on my A4 (and GTO) there was a hole on the vertical section just above the floor behind/under the glovebox, I can't seem to spot one in the lexus...

Thanks,

Julius

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