Battery headlights dim

[Wheel2Turn]

Hi everyone 😎

I notice on my 2004 Rav4 XT3 VVTI 1998cc petrol when headlights are on my head lights are dim on low revs and then headlights go a little brighter on high revs is this normal like power surge from the alternator  ?  I've not notice it before.

When I got the car, the dealer put a Halfords Battery on it (3 year 380amps 45ah 12volts ) 

Done a test today sunny day

Battery reading 12.5 volts

Car engine running reading14.2 volts

Engine running at 2000 rpm with lights on rear heater on blowers etc reading 13.20/13.30 volts

So everything seems fine with Battery don't you think.

Just was wondering why was my lights doing strange things when the revs was low and high.

Another thing I needed to had in the cold weather the car did stall a few times on me and it did take longer to start not all the time just now and again,I did change the plugs today and starting seemed more positive,but like I said the sun was out for a change 😁

Just need someone on here to confirm all is well with my battery readings.😎 next step maybe a bad earth somewhere.

[IanML]

The Battery is fine.  The alternator volt drop under load is perhaps a little more than I would expect.  It may benefit from new brushes.

[Wheel2Turn]

2 minutes ago, IanML said:

The battery is fine.  The alternator volt drop under load is perhaps a little more than I would expect.  It may benefit from new brushes.

Thank you again IanML for the quick reply nice one 👍

Do you think brushes or new alternator because I just been reading it's a nightmare to get at it to take off,because I would have to take it off to change the brushes so i would change the belt why I was at it. anything else I would need to change my rav have done about 96000.

I read also somewhere the diesel rav Battery is better on the petrol rav when changing Battery from the original toyota one don't know how true that is. Don't know if it would make much difference. You guys on here would know more then me on that one.

See I brought the car with some service history so I don't know what's been changed or done on it, I love the car just getting through the things to do list ☺

I done the engine oil change and filters front breaks disc's and pads, I still not sorted that light out yet that a mentioned about when I first joined this forum, I think it was you that I was taking to when I first joined 😎

 

[Mooly]

Your voltage readings are fine. Stick with the next bit and it will start to make sense.

Although not 100% accurate (due to the non linear filament resistance with temperature) you can think of the bulb as a resistor. A 55 watt 12 volt bulb would have a hot resistance of 2.62 ohms. Just ohms law. Now increase the voltage to 14.2 volts. The wattage dissipated by the bulb is now nearer 76 watts. Its only a small voltage change but look at the difference of 55 watts vs 76 watts. No wonder it looks brighter.

(the above is over simplified because the filament resistance of a bulb actually increases the hotter it gets and that alters the absolute values, but the point is that a small increase in voltage gives a large increase in power dissipated in the load. So bulbs are brighter, motors spin quicker... compare your wiper or electric windows and see how much faster they run on that couple of volts difference).

A Battery aimed for Diesel engines will have a better CCA (cold cranking ability) which simply means the voltage will hold up better under the heavier starting current needed by the engine. So no problems there either :) 

 

[IanML]

If you use a different Battery, check the terminals are in the right place.  Some Toyota models (not sure which) have the terminals in the opposite sense, and the harness won't fit!  I have the standard Battery for the petrol engine, and it's absolutely fine, but Jersey is generally a bit warmer than Wales in Winter!

[Wheel2Turn]

On 20/04/2018 at 8:12 PM, Mooly said:

Your voltage readings are fine. Stick with the next bit and it will start to make sense.

Although not 100% accurate (due to the non linear filament resistance with temperature) you can think of the bulb as a resistor. A 55 watt 12 volt bulb would have a hot resistance of 2.62 ohms. Just ohms law. Now increase the voltage to 14.2 volts. The wattage dissipated by the bulb is now nearer 76 watts. Its only a small voltage change but look at the difference of 55 watts vs 76 watts. No wonder it looks brighter.

(the above is over simplified because the filament resistance of a bulb actually increases the hotter it gets and that alters the absolute values, but the point is that a small increase in voltage gives a large increase in power dissipated in the load. So bulbs are brighter, motors spin quicker... compare your wiper or electric windows and see how much faster they run on that couple of volts difference).

A battery aimed for Diesel engines will have a better CCA (cold cranking ability) which simply means the voltage will hold up better under the heavier starting current needed by the engine. So no problems there either :) 

 

Thank you for your reply and the explanation it makes sense , so everything seems normal then with my rav.

What I have notice that also when my car is ticking over and headlights are on if I switch my electric window on and off I can see the headlights dim a little bit,but I guess that could be the electric windows pulling the power.

That makes sense about the Battery on Diesel cars with cold starts.

Thank you again nice one 👍☺

[Wheel2Turn]

On 20/04/2018 at 11:31 PM, IanML said:

If you use a different battery, check the terminals are in the right place.  Some Toyota models (not sure which) have the terminals in the opposite sense, and the harness won't fit!  I have the standard battery for the petrol engine, and it's absolutely fine, but Jersey is generally a bit warmer than Wales in Winter!

Thank you

I been looking at the Bosch S4 069 Battery,If what I have googled so far it should fit.

Do you want to swap places lol 👍😀

[Mooly]

29 minutes ago, Wheel2Turn said:

Thank you for your reply and the explanation it makes sense , so everything seems normal then with my rav.

What I have notice that also when my car is ticking over and headlights are on if I switch my electric window on and off I can see the headlights dim a little bit,but I guess that could be the electric windows pulling the power.

That makes sense about the battery on Diesel cars with cold starts.

Thank you again nice one 👍☺

Yes, all normal. Car electric systems (not hybrids) are all basically low voltage and high current circuits, and the high current causes voltage to be lost along wiring (ohms law again).  

So the wiring from the Battery to the fusebox actually drops or loses a little bit of voltage across its length, exactly how much voltage depends on the current pulled through it and the thickness of the wire. Result... the voltage at the end of the wire falls the more current you draw, for example when you operate the windows.

Also the alternator does not maintain an exact output voltage under heavy loading, the voltage will fall a little, and more so when the engine is idling and the alternator isn't spinning all that fast. It should be able to maintain a high enough voltage at idle (so a voltage above the resting voltage of the Battery when the engine isn't running) to ensure that the Battery doesn't discharge when normal but heavy electrical consumers are in use such as being stuck in traffic with headlights and wipers on.

You might observe with a hot engine at idle that the idle speed increases if you turn on say headlights and heated window. The engine management system does that to slightly increase the output from the alternator by spinning it a little faster.

 

 

[Wheel2Turn]

18 hours ago, Mooly said:

 

Edited April 25, 2018 by Wheel2Turn
I didnt have me glasses on ooops

[Wheel2Turn]

18 hours ago, Mooly said:

Yes, all normal. Car electric systems (not hybrids) are all basically low voltage and high current circuits, and the high current causes voltage to be lost along wiring (ohms law again).  

So the wiring from the battery to the fusebox actually drops or loses a little bit of voltage across its length, exactly how much voltage depends on the current pulled through it and the thickness of the wire. Result... the voltage at the end of the wire falls the more current you draw, for example when you operate the windows.

Also the alternator does not maintain an exact output voltage under heavy loading, the voltage will fall a little, and more so when the engine is idling and the alternator isn't spinning all that fast. It should be able to maintain a high enough voltage at idle (so a voltage above the resting voltage of the battery when the engine isn't running) to ensure that the battery doesn't discharge when normal but heavy electrical consumers are in use such as being stuck in traffic with headlights and wipers on.

You might observe with a hot engine at idle that the idle speed increases if you turn on say headlights and heated window. The engine management system does that to slightly increase the output from the alternator by spinning it a little faster.

 

 

Thank you again, you have been really helpful 👍

All what you have said makes sense now and yes i have notice an increase in the idle speed when turning on the heated window.

I might change the Battery for next year winter to help with the cold starts but it does seem a lot better now that i have changed the plugs.

Thank you again nice one 👍☺😎

 

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