Battery Going flat

[RPWC]

My Dispatch battery keeps going flat overnight, I had it tested by auto electrician a couple of weeks ago, batterybis brand new about .7 weeks ago, no discharge , alternator charging ok. At first we put it down to faulty split charge relay,, removed that and all,was fine for 3 days, fitted new one, battery went flat, 3 nights on the trot. Have now fitted standard relay, which is connected to the ignition,so once the engine is off,there is no power to the second battery, came out this morning, battery flat again. Can an6one throw any ideas my way please?

[white exec]

If battery is a good one, there must be some discharge from it when left overnight.
Test this with a test-meter (on Amps/milliamps) inserted into one of the main battery connections.
With everything off, drain from the battery should be no more than about 30mA (for radio, alarm, ECUs etc).
Overnight 'flat' suggests discharge is several amps.

One cause of this (unusual) is an alternator that has been incorrectly wired in - where the permanent Batt+ and IGN-switched + have been accidentally reversed. This causes battery to charge ok, but leaves a discharge which can cause the battery to go flat overnight/in a couple of days. The meter test above will show this up.

[GiveMeABreak]

It is possible that you may have a defective diode in the alternator. Normally this is one-way, like a valve, so that it charges the battery, but prevents the current from flowing back and discharging the battery.

If you have a multimeter, take off the connectors and the leads from the alternator, then put the multimeter into diode testing mode.
Connect the red probe to the case of the alternator and the black probe to the lead post. The reading should be anywhere between 500 to 800 millivolts typically.
Then reverse the probes with the black on the case and the red on the lead post. The reading should be 0 or 1. If it is reading significantly anything over this then the diode is knackered and is causing the drain. When connected back up to the battery will soon drain.

Obviously check the for other items causing a drain - but this is a quick test first to rule the alternator out. If it is not alternator drain, then you can focus on narrowing down the drain by going through the fuses one by one.

[Gibbo2286]

Alternator regulator diode blown is a fairly common cause, also after market alarms if fitted are notoriously 'leaky'

[Peter.N.]

You can also check the alternator by disconnecting the thick terminal and connect your meter on the 'amps' range between the end of the wire and alternator terminal, I have known them draw an amp or more.

Peter

[Pug_XUD_KeenAmateur]

Have a google of 'Battery Parasitic Drain', there's lots of useful info on this, which I used for my own benefit just a few weeks ago.

[white exec]

Sorry to repeat, but an ammeter (amps, milliamps) inserted into one of the main battery connections - at the battery - will tell you immediately whether battery drain is going on, or not.
It there is excessive current outflow, that's when you can start looking for what's causing it.
You should do this test first.

[RPWC]

I am not clued up on how to use the amps side, so don’t want to try it and then post up incorrect information, but I have disconnected all the wires from the split charger, so there is no connection at all to the second battery. My auto electrician friend ,said to do this, and report back. He said if the battery doesn’t drain, then the fault lies with the split charge system,if it does,then he will need to bring it in for further in depth testing on the charging system. If it’s this diode that you all speak of, is that an easy fix, or replacement alternator job? When I had no s c relay wired up, the battery didn’t go flat, this was for 3 days. As soon as a new relay was fitted, battery drained overnight..

[RPWC]

I disconnected all the wires from the relay last night, so there was no connection between the two batteries at all, tried to start it last night at 10; after it had stood for 4 hours,it did. Came out this morning , put the meter on it, 12:57v, but it wouldn’t start.

[Peter.N.]

Check the voltage across the battery with the starter engaged.

Peter

[GiveMeABreak]

As Richard says, the battery may be showing the correct voltage, but doesn't have enough welly to crank the engine - so the battery really needs to be fully charged back up and then tested after it has been left overnight and not connected to the secondary battery to see if it is holding a charge. If it is I suspect it's that other gizmo you've got hooked up causing the problem.

[RPWC]

As Richard says, the battery may be showing the correct voltage, but doesn't have enough welly to crank the engine - so the battery really needs to be fully charged back up and then tested after it has been left overnight and not connected to the secondary battery to see if it is holding a charge. If it is I suspect it's that other gizmo you've got hooked up causing the problem.

The split charge relay was disconnected last night Marc, so there was no connection from the second battery at all, the wires from ignition and main battery, to the relay were disconnected, so there was nothing attached to it, other than the earth lead and the connector to the second battery. Got the RAC coming, so hopefully they will test the battery and charge system with an ammeter.

[white exec]

Get them to measure the residual current drain* with a multimeter, if they carry one.

* Current that is leaving the battery, with everything switched off.

[RPWC]

Update. RAC been, told him all tha had been happening,he put the clamp tester on the battery lead, and said 12:41v, that is plenty to start the vehicle I suspect something else, he told me to try and start it, turned the key, as per, not even a click. He said ,that’s the starter motor, not the battery. He turned the key and held it, after about 4-5 seconds, it started, no jump leads or power pack used. I asked how come it was only doing this after standing overnight, and why in that case it was starting off the jump leads. He then said, let me check your engine to body earth, we found it and the bolt on the body end was quite corroded. He told me to remove both ends and clean them up then refit, and to do the same with the connections on the starter motor. Apparently the reason the van would start from jump lead was because it was then provided with a good earth connection. He told me that after cleaning the connectors up, if I still had problems, it was worn brushes in the starter. Hope it’s not that, as it’s a pain in the arse to get off by the looks of it.