battery charge voltage

[PeterAllen]

Just a quick one. What should I be reading at the battery at idle? I am getting only 13.95 volts and thought it should be charging in the 14 plus numbers. Can anyone clarify this for me. Is my alternator on its last legs?

[Keithy P38]

When I installed my dual battery system I got 13.95 at the battery as well. My alternator is 2yrs old.

[TheTree]

Peter,

It depends on how long it is since you started the car.

I have a new battery fitted and after it has been sitting and is started, the voltage goes to around 14.2V while it charges the battery, once the battery has been topped up the voltage will drop to around 13V.

So if you never see your voltage hit at least 14V then you may have a bad regulator or alternator

Steve

[drivesafe]

Hi Peter and what is your vehicle, and how often do you drive it and when you drive, how long ( on average ) is your drive

[TheTree]

I meant to add this link, this lays it all out pretty clearly.

Sticky: Electrical Troubleshooting Updated 2010 (http://www.rangerovers.net/forum/7-range-rover-mark-ii-p38/28663-sticky-electrical-troubleshooting-updated-2010-a.html)

[PeterAllen]

Thanks all. The car has been sitting for a day . Usual trips of 30 to 40 minutes per day but had drained the battery thru manual overriding the compressor without running the motor. Charged it overnight with a cheap charger . Starts car without a problem . Just checking the battery health as I know low battery causes all sorts of weird stuff.

[Keithy P38]

Just reading the document, mine is not getting full charge either by the looks of it. I think I know why too.

I always accepted that 13.95 was good! Seeing as my batteries (both crank and aux) are the exact same, and both less than 6months old, and my alternator was rebuilt 2yrs ago.

I'll have a look too...

Cheers
Keithy

[p38arover]

Coincidentally, I was taking to Greg Hind (who wrote that) last week.

He asked about my readings so I did them.

Here are the results I emailed to him. I can see I need to look at the lead from the alt to the battery.



Worksheet

This is a cut down version of the tests, see above for detail on how to complete each test.

1. Engine off, all accessories on for 30 seconds, then off. What voltage 12.96
2. Voltage across battery at idle (no electrical load) 14.46
3. Voltage across battery at 2000rpm (no electrical load) 14.43
4. Voltage across battery at 2000rpm (everything electrical switched on) 14.23
5. Voltage at ALTERNATOR at 2000rpm (everything switched on) 14.43
6. Voltage drop between alternator body and battery negative (YES negative). Measure at max electrical load and 2000rpm 75 mV (millivolts please)
7. Voltage drop between alternator positive and battery positive. 2000rpm, max electrical load 220 mV
8. Check Voltage drop from Battery Negative terminal to 1st groundpoint on vehicle 24 mV
9. Check voltage drop from 1st ground point to block 52 mV
10. Check voltage drop from Alternator Housing to block 2 mV
11. Voltage from alternator positive to alternator housing I thought I’d checked this.

I might have to do this again.

Whatever I do, I need to fix the drop between the alternator and battery.

Ron

[drivesafe]

Hi again Peter and I should have realised what vehicle you had by the section you were posting but I came to the thread via the NEW POST section and didn’t bother to see where you had posted.

As Ron has posted, you would normally see at least 14.0v while driving, but at idle, a number of factors can effect the alternator’s voltage level.

Best time to take a voltage measurement while the engine is idling is after you have driven for at least 30 minutes.

After a 30 minute drive, pull up, leave your motor running but turn off all non essential electrics, like your headlights, sound system, A/C and so on, then pop the bonnet and take your measurement.

[p38arover]

I had a call today to help out a chap whose P38A wouldn't start. He owned a Fault Mate but his laptop was dead so I took my laptop which has Faultmate installed.

When I got there, the car was on charge. I plugged in Faultmate and, without doing anything else, the car started. But it smelled very rich and was running very roughly. No faults had been recorded so I had a look at fuelling but, having no idea what I should be reading, it didn't help.

However, during the checks, I noticed the alternator voltage looked too high. It was 16 volts. I wonder what the affect of that would be. He said the alternator was an aftermarket replacement and the battery had been replaced.

I suggested he have the battery tested. I wonder if the battery was failing, could its impedance rise and the charging rise commensurately?

[drivesafe]

I had a call today to help out a chap whose P38A wouldn't start. He owned a Fault Mate but his laptop was dead so I took my laptop which has Faultmate installed.

When I got there, the car was on charge. I plugged in Faultmate and, without doing anything else, the car started. But it smelled very rich and was running very roughly. No faults had been recorded so I had a look at fuelling but, having no idea what I should be reading, it didn't help.

However, during the checks, I noticed the alternator voltage looked too high. It was 16 volts. I wonder what the affect of that would be. He said the alternator was an aftermarket replacement and the battery had been replaced.

I suggested he have the battery tested. I wonder if the battery was failing, could its impedance rise and the charging rise commensurately?
Hi Ron, and 16v is WAY too high.

As long as your mate has only been doing short trips, no longer than about 10 minutes, his battery may be OK.

But even one long trip at 16v is going to damage the battery, and if it is an AGM cranking battery, it will quickly destroy it.

P38s are pre SMART voltage alternator operation and were set up for wet ( flooded ) type batteries and as such, 15v would be the absolute maximum voltage, and this is still too high for continuous operation.

A runaway alternator ( No earth connection ) will charge at up to 18v, so it is more likely a poor earth connection somewhere, or a faulty voltage regulator.

I would start by checking the engine block to chassis earth first, and I have no idea where this is located on a P38.