Electrical / battery issue

[Duke Caboom]

Hi,

I have called out two mobile auto electricians but neither has replied so I'm going to the PH expertise.

Mercedes B Class '06

We noticed that starter seemed to be turning slightly slow - no issues re starting but a worry as my wife lives in fear of getting .

I drop tested battery. Its right on the limit. Its branded "Mercedes" so it may well br the original. 14 years old and 90k miles old so I replace it with new battery.

Everything is normal for a few weeks. then we have lock down. Leave car in garage for a week, go to start it and engine is very slow to turn over. I put the battery conditioner on and give car long charge.

A week performing normally - we are not using the car but I check it every day and it starts well.

Use the car to go to work for a day. Works normally.

Go to car next day, its so flat that the electronic steering lock won't come off. Showing 1.5 volts across terminals. "Its a duff battery", I decide and put the old one back in after long charge on battery conditioner.

All normal for a while then it suddenly also very low. (this is the old battery, removed because I suspected a fault.)

No obvious drains that I can see.

What would you do next? Thanks

[DuraAce]

Duke Caboom said:

No obvious drains that I can see.

Have you measured the actual battery drain with an ammeter/amp clamp?

If not, then do it.

[Penelope Stopit]

As above, something not visible to the eye could be draining the battery

Wouldn't bother attempting to use the old battery as there can't possibly be much life left in it after all those years of work but......

[NMNeil]

Take off one of the battery terminals, hook up your multimeter on 10A current measurement setting and see the actual current draw with the doors closed and everything off.

[Duke Caboom]

Thanks for all the replies.

I removed the lead from the positive terminal and connected the multimeter on 10a setting between the lead and the positive terminal.

After about 5 minutes it settled down to reading 0.16.

I pulled each fuse one by one but nothing changed the 0.16.

0.16 doesn't seem enough to flatten a 70AH battery, does it?

Confused.

Thanks

[GreenV8S]

Duke Caboom said:

0.16 doesn't seem enough to flatten a 70AH battery, does it?

Assuming that's 0.16A, that corresponds to almost 4AH per day and would flatten a fully charged 70AH battery in just under three weeks. It would be too flat to crank the engine some time before that.

[Duke Caboom]

Thanks. Yes, in not a multimeter expert but I think it is amps. The battery was flattening in a few days. Too flat to open doors, let alone crank engine.

What would a typical car be showing, do you think? I guess the alarm has some draw.

[GreenV8S]

Duke Caboom said:

What would a typical car be showing, do you think? I guess the alarm has some draw.

The battery maintainer on my Mondeo says it discharges at a rate of about 50 mA (0.050A) - that covers current drawn by the car's electronics, and internal leakage within the battery. I think that's typical for a modern car. That would fully discharge the battery in about 40 days, but it would probably be too flat to start the engine after 3-4 weeks.

[spikeyhead]

I wouldn't trust most multimeters below 1A on a 10A range.