Is something else going on? Spec is Powervamp 25 battery and Ford 75 amp alternator plus HD cable connecting the two. Battery charges to 13 volts ( open circuit out of car ) then reduces to 12.5 over 24 hours. Then seemingly holds steady for several days. Engine spins over for 'instant' start at this voltage. Digidash shows dynamic voltage on initial running as 11.7 +/- 0.4 . Voltage measured over battery terminals ( engine running ) 11.7. Alternator output shows 13.4 +/-0.2. During subsequent driving voltage recorded by Digidash fluctuates considerably from 11.0 to occasional highs of 12.5. The situation has been on a downward trend resulting in a non start at Snetterton this week ( Bummer! ).
Mechanical engineer needs help!
PS. Battery was on the shelf for 5 or 6 6 years before being put into service last year. Car has done approx. 200 miles of testing since then.

Most garages can test a battery for you - they put a meter with 2 prongs onto the terminals, and measure the voltage across a load. Charge the battery before taking it for the test.

As for the rest, I'd expect a voltage of around 13.8v across the battery when the engine is running at anything much above idle. If you're seeing 13.something at the alternator and a lot less at the battery, I'd start by checking every connection - dismantle every joint and wire-brush to make sure it's clean (including the earth).

The fluctauting voltage indicates an arcing connection somewhere between alternator and the battery

The voltage at the battery post with engine running at 2000-2500 rpm and head lights on the voltage at the should be be a more or less constant 13.4 to 14.8 volts. Anything under 13.1 volts won't charge the battery it will just about break even break even. It iseems unlikely to be a battery problem altough the battery may have had its charge capacity severly damaged by the (suspected) charging problem.

There are a couple of checks you can do on the alternator.
(1) Examine the alternator connections (this is a likely cause especially if the alternator has a 3 blade connector) .
(2) Check the cable between the alternator and the battery positive for volts drop, with the engine running the volts drop between the alternator connector and directly to the battery +ve post. The drop voltage across these two points should be less than 200 mV.
(3) I know this is obvious but check the drive belt tension and belt and pulley condition. I had a couple of cases where the alternator pulley was spinning on it's shaft.
(4) Connect a 12v bulb test light to the battery positive. Use the other wire of the test light to connect to the WL terminnal on the alternator. to the battery. Engine off the test lamp should illuminate engine running the light should go out.

I don't think the battery is deffective, the old fashioned prong style testers are fairly brutal they place a huge current load on the battery and have the potential to cause the battery to explode so I would be reluctant to use one on a smallish battery that might be OK. This type of tester can be found on eBay for under £20.

More modern battery testers measure the batteries AC impedance and estimate life remaining in the battety and effective CCA and amp hours. The basic models of this type of tester can be boiught for £40.

There are another DIY couple checks than can be done on the battery,
(5) The simplest one is to charge the battery fully let it rest overnight, then measure what happens to the battery voltage after the lights (side and tail ) have been left on for 15 to 20 minutes. If after the lights are switched off and the battery is allowed a couple of minutes to recover a voltage below 12.4 volts is bad news.

(6) The next is a cranking voltage test this is a DIY version of the prong style load tester. This best done immediately after test 5. Disconnect the injectors to stop the engine from starting and measure the voltage accross the battery while cranking the engine normally the voltage will drop to 10 volts below 9 volts is bad news for the battery.




[Edited on 1/5/16 by britishtrident]

if your battery (with the cell covers removed to allow the cells to be seen) "fizzes" when put under a big load (cold cranking, or battery tester or, and this does work but is a brutal test... shorting the terminals with something big and chunky like a wheel brace. BUT ONLY FOR A FEW SECONDS.....)...if any of the cells fizz, the battery is bin fodder. this test is only for "wet" lead acid battery’s....the gel types need proper equipment to check them.

This Youtube video by ScannerDanner may help

Alternator testing 3 Wire

Also this one

Battery Keeps Going Dead Testing

Beware dead shorting a car battery -- major danger of hydrogen gas in the battery exploding showering acid all over you.



[Edited on 1/5/16 by britishtrident]

yeah i know the battery’s can go pop.....nervous folks should definitely get someone competent to check their battery