Merc E320 CDi - battery/electrics

[Observer]

1. Last February was getting "Electrical consumers switched off" warning on first start after a cold night (which indicated low battery charge). Car was on Merc maintenenace contract and eventually persuaded the service agent (Merce dealer) to supply new battery. So battery is ~6 months old genuine Merc part.

2. All okay since then up until ~4 weeks ago. Then started getting the same warning.

3. Then car failed to start one morning - starter motor turned for a second or so then nothing. Started okay on jump leads.

4. My journey to work is ~20 miles/45 minutes so plenty of time for starting charge taken out to be replaced.

5. Voltmeter on battery terminals (leads not disconnected) is >12v with engine off and ~14.2V with engine at idle. Drops to ~13.8V with high load consumers switched on (lights. heated rear window etc). Except in one case where meter showed <10V.

6. Car has failed to start last 3 mornings and yesterday evening after sitting in car park all day. Started okay on jump leads.

7. Meter on battery this morning, immediately after it failed to start, showed 12.4V.

8. Nothing is being left on overnight whilst engine is off. Lights are automatic (and I have had them on manual control anyway). Radio switches off automatically and I have been turing it off manually anyway.

9. No other warnings seen.

10. I have taken to Kwik Fit twice for battery check and they say nothing wrong with battery (according to their test meter) and charge from alternator appears okay.

11. I want to try some fault finding myself before risking open-ended investigation at dealer labour rates of ~£100/hour.

Questions:

(i) Clearly (I think) something is draining the battery whilst the engine is not running OR the battery is never getting fully charged. Any suggestions for fault finding?

(ii) I have a battery charger and thought would try putting battery on charge overnight or during the day whilst in the office. Is it (a) absolutely necessary or (b) advisable but not that necessary, to disconnect battery leads whilst charging.

 

[Spark123]

If you think it is the battery causing the problem, can you disconnect it overnight? Reconnect it in the morning and see if that is your problem?
The voltages you are measuring all appear to be fine - it should be around 14v when the alternator is running. 12v is fine at rest - if there was a drain on it then I'd expect it to be lower. I take it quick fit did a drop tested it - (applies a load and measures voltage) ?
iirc there was a fairly large recall of merc batteries but that was a few years back now.
When it fails to start - do the lights etc go dim?
When did the meter show <10v?

 

[davieboy]

some of the e-class mercs actually have 2 batteries. the main starter battery and a smaller reserve battery, looks like a motorbike battery. if this second battery is on its last legs it can drain the main battery fairly quickly. not all of them have a secondary battery though.

its usually under the plastic covers at the passengers side rear of under bonnet area. depends on year (or if, indeed it has one) as to where it may be.