Volkswagen Eos Forum banner

1 - 12 of 12 Posts

·
Registered
Joined
·
21 Posts
Discussion Starter #1
I’m trying to diagnose an issue where my battery runs low everyday. As far as I know, nothing in the car goes dead(radio, lights), except the car will not start. It dwindles down throughout the day until I’m not able to crank. The radio is on fine, and I believe my lights work fine. Battery is brand new . As far as I can see everything works, but will not crank at the end of the day. Obviously, the car is living off the battery.
I’ve been keeping it on life support (battery mainter).
Should I start by Replacing the battery cable? Any suggestions?
 

·
Registered
Joined
·
5,334 Posts
You need to do some tests first. The first test is to put a digital voltmeter across the battery terminals. Measure the voltage then start the car. If the battery voltage goes up from 12ish to about 14.5 your alternator is charging. When the battery is fully charged with engine running, the voltage should drop to about 13.5V. Battery maintainers won't fully charge a near flat battery in a day, you need a proper charger and charge the battery disconnected from the car. Just because lights work doesn't tell you much about the battery state of charge or condition

If you put your voltmeter across the battery and crank the engine, the battery voltage shouldn't drop below 10-11 volts if it is fully charged to start with. If it does then either the battery isn't fully charged or it's no good. If a battery is fully charged and you don't use the lights, blower, defroster or roof pump I would expect the car to last longer than a day unless there is a bad current drain.
 

·
Registered
Joined
·
21 Posts
Discussion Starter #5
Voltage on the battery while car running...13.2V. Voltage on the battery without car running...12.7...
 

·
Registered
Joined
·
5,334 Posts
You should measure the battery voltage when it has rested from charge or discharge i.e Before starting the engine or after key off (not charging) for half an hour.
Engine running: Is that with all the loads on I said and the engine fast idling at about 3-4k rpm?
What about the other test for voltage drop whilst cranking?
 

·
Registered
Joined
·
21 Posts
Discussion Starter #7
Drop while cranking, drops down to 11.2. That 13.2 is not all loads, but I did not trick the car to lock either, so there was some type of load in it.
I checked all the fuses and all seems to be fine. No volt boost in any of the fuses.
symptoms - the car would turn over approv. 13 times before it would start to dwindle. I remember losing power and I would move/shift battery cables (positive). I didnt remove, just slightly moved them, and the car would start up. How Can I test cabling?
 

·
Registered
Joined
·
227 Posts
I remember losing power and I would move/shift battery cables (positive). I didnt remove, just slightly moved them, and the car would start up. How Can I test cabling?
That's a different issue than a current drain or failing battery. Its always difficult to diagnose an intermittent issue like that but the principles are, you'd test as much as you can without disturbing connectors etc, and visually inspect things thoroughly. But its possible its (part-)broken internally and you'd not know until you eg cut it open.

If its suspect and the part isn't too costly, it makes sense to change it especially if your car is an auto.

If its a current drain, then there's not much you can infer from the above measurements except to say the battery isn't totally knackered. So you'd need to investigate further, eg by putting an ammeter on wires to measure the current draw over time. I'd use an inductive clamp ammeter, initially on eg the battery cable; then refine it down by following a wiring diagram and using a "fuse buddy" in place of a suspected circuit's fuse.
 

·
Registered
Joined
·
5,334 Posts
You can have an intermittent bad terminal connection anywhere from the battery positive post to the starter terminal or the battery negative to the chassis ground connection. A d.c Hall effect clamp meter for 500 Amps may be quite expensive to buy for this one job?

Based on your cranking voltage result, the battery seems o.k so the next step is to clean the terminals and ground points between the battery and starter. If you want to try measuring a battery lead fault, clip one end of your DVM to the starter terminal and the other end to the positive battery post (the lead part NOT the clamp) and have somebody crank. If you measure more than 0.5 to 1 volt or see this voltage changing a lot, you have a poor connection which you will fix by removing each of the connections and cleaning the parts of any oxide or corrosion. Repeat the cable voltage drop test to confirm cleaning has reduced the voltage drop and stays fairly constant whilst cranking.

If whilst cranking you see the voltage drop change to below 0.5-1 volt when the starter appears to give out on a fully charged battery (13.2V) this suggests your starter relay has bad contacts or the starter motor is faulty. Non of this relates to parasitic battery drain, but that's what you are left with if the starter, relay, battery and wiring are confirmed good.
 

·
Registered
Joined
·
21 Posts
Discussion Starter #10
Looks like it’s the harness from the Alternator to the fuse box. Anyone have the diagram for this this harness? it Has multiple connectors. i know point A is alternator and point D is the engine fuse box, what’s in between?
 

·
Registered
Joined
·
5,334 Posts
The wiring diagrams change with vehicle VINs, month, model, year of production and options. Do you have a workshop manual for your EOS version which should include the correct wiring diagrams?

Somebody here had melting problems with their ABS fusebox (underneath) which you can only see by removing it.
 

·
Registered
Joined
·
35 Posts
I had a problem like yours and it turned out to be a bad cell in the battery. I let it go too long and it resulted in wearing my alternator out trying to keep the battery charged. I should have dealt with it the first time battery was too low to start the car
Solution was a new battery and alternator and VW Bosch alternators are $$$$
 
1 - 12 of 12 Posts
Top