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What's your voltage from the alternator?

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Hi all,

1996 300TDi 110 CSW

I've got a problem with batteries not holding-up a decent voltage and I wonder if I'm not charging at a high enough level.

When running with little load the voltage sits at 14.3. I monitor this with a little digital meter in the dash. At the the direct from the terminals. I have checked with with my voltmeter too, at the battery.

However it drops down to 13.8v with loads like headlamps, HRW and wipers.

As a result(?) I never see more than 12.8v after switch off and this is pulled down quite quickly (two days?) by the alarm unit, to 12.2v. It would go lower but I'm normally driving again by then.

Is this behaviour normal :blush: ? The alternator is an A127 65A, was an exchange unit about twelve months ago. The belt / tensioner is good and doesn't squeak. Done about 9,000 miles but really never charged better than this. The pulley is off the previous unit but not sure if it is the smaller one or not and I don't remember the belt size but I have a spare and I will check the part number.

The battery is Odyssey PC1500 so its an AGM type and could be charged at slightly higher volts than a normal lead acid (and when fitted, the aux is the same but its not fitted currently). I'm not happy with the batteries (in spite of the £££) but I wonder if a different alternator (another 65A or maybe a 100A) would be better.

I guess the charging volts are the best indicator of this - hence the question.

Can anyone help by quoting their normal volts during charging?



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The aux battery has already been returned to the supplier who agrees that it isn't holding charge now. The starter battery was returned to me after their check, as OK. The Aux is now with Odyssey who apparently destructively test them to establish the cause of the problem. So I'll either be getting a replacement or there'll be nothing!

I checked my split charge system before removing the aux and the charging volts with both in circuit are the same as I quoted above.

So I guess there's no benefit to me from a 100A unit then, if the volts from this 65A look about OK.

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Disconnect a battery terminal and put an ammeter (normally just the amp setting on a multimeter) between the battery leads and the terminal, with everything turned off. Make sure the ammeter is up for at least 5-10 amps and don't turn anything on as it's very easy to fry a cheap ammeter. That will measure the drain you have when parked (alarm, receiver for central locking, a tiny bit for the radio, plus any leakage you're not aware of). You can quickly do the maths to see if the loss on your battery is very light, implying your battery is faulty, or more than you thought, which means the battery may be fine but it is time to look for a current leak. A drain of even half an amp will cause a problem over a few days.

Something like a multi-power socket adaptor with an LED light could draw more than you think if it doesn't turn off with the key.

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Thanks good advice. I have a 10A Fluke so will do just that.

It's true that as well as the normal alarm I do have another bit of electronics in there as well as a dual USB outlet with the tell tale blue led. Can't recall as I sit here if it's IGN controlled or not ( and I just drove it for over an hour).

Must be an age thing!

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I mentioned the power adaptor because I fitted one, permanently plugged in via my main power supply, and found it interfered with the radio signal. That main power supply goes through an isolator switch, which is off when I'm not driving, while the radio and internal light have an independent connection to the battery (it is intended as a camper). If the radio is on and I turn on the "main" switch, the radio picks up a strong signal from the USB outlet thingy. Therefore, I concluded, it must be using a bit of power even when nothing is plugged in or turned on.

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