8 replies to this topic

[JonnyBMX]

Hi guys seem to be dropping a volt between the alternator and the battery any ideas??

[Barman]

Hi guys seem to be dropping a volt between the alternator and the battery any ideas??

Poor Earth or corroded connections on the live.

[JonnyBMX]

Just bypassed both and got to 13.8v still drops alot more then I would expect

[Jordie]

the distance and resistance will always have an effect, how big or small I dont know.

[JonnyBMX]

Yea ofc, I'm just trying to work out why my battery always goes flat, the battery drain is like nothing , good earths, connections now all good, what sorta current are you people getting at the battery on Tickover?

P.s it's an spi and has a 70 amp alternator on

[Jordie]

you checked the drain with everything off. what was it?

might be a duff battery

[Ivor Badger]

The actual voltage of a lead acid cell is 2.2v. So a 12v battery is actually 13.2v when fully charged. The potential voltage of the charging sustem has to be greater than the battery to force the charge into the battery. So the alternator output is between 13.8 to 14.6v. Above 14.6v can boil the battery dry and below 13.6v fails to charge the battery. So your readings are quite normal.

[tiger99]

Actually the battery ideally wants to be at least 13.6V if fully charged, but it does vary with temperature. If it can still deliver 13.2V after an hour with the engine off it is probably ok.

Now to diagnose the problem. Put a long lead on the multimeter and clip it to the battery positive. Put the other lead, which can be normal length, on the alternator positive. (You can usually remove the back of the alternator plug to get at the terminals.) Run theengine at medium revs. How much voltage do you see between the two points? (Ideally zero, but it will not be of course. 0.1V would be as good as you might get.)

If you see a volt, most of your drop is in the positive side. Check again, between alternator positive and the battery terminal of the starter solenoid. If you see much voltage drop there, the thick brown wires are in effect not thick enough. The standard wires are totally inadequate for a 70 amp alternator. Remedy obvious, provided that you can get suitable terminals and crimp them properly with a high quality ratchet crimp tool.

Check also the earth side, alternator fixing bolt to battery negative. If the figure is higher than you would like, check again from alternator fixing to battery earth bolt in boot, and again from alternator fixing to somewhere on front bulkhead. Remember that you are measuring voltage drop and should only see small fractions of a volt.

I would be very interested to know what you actually measure.

[JonnyBMX]

Sorry guys, should have probably pointed out the fact I'm an electrical engineer and I did know exactly how a battery and alternator works and at what voltages, thanks for the good coents though, and with it being an spi I think it was standard on 70 amp?

I will obv course let you know some exact figures, I may even get the pen out and calculate the exact values I should be getting taking into account resistances of the wire etc!

Edited by JonnyBMX, 08 December 2012 - 09:07 AM.