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Background
Due to the recent coronavirus lockdown, my car hasn't been getting the mileage it's been used to.
For reference, it's a 2007 Mercedes E280, with the 3.0L 6-cylinder turbo-diesel & 7-speed tiptronic.

The battery is only a year old - the original having lasted 12 years. In normal circumstances where it gets a decent couple of hours' run at least once a week if not a lot more, there is no issue at all.
Since lockdown, it was at first getting no more than 15 minutes every couple of weeks for a stock up at the local supermarket. Six weeks of this & it was flat. I live right next to a garage, so getting it jumped wan't a problem.

I've since determined that I need to run it at least once a week to keep it happy. For this particular exercise I'm not particularly interested in fuel saving or efficiency, just a 'best practice'. Question
What's the best method to get a 'reasonable' charge into it every week?

Options, as I see them…

  1. Idle it for 30 mins [650rpm]
  2. Play half an hour's stop-start on roads with traffic lights & junctions with periods of artificially hard acceleration off the lights [4000rpm]
  3. Find somewhere I can pootle along at 30-40mph constant [1000 rpm]
  4. Run it up the motorway
    a. at its normal 2000rpm it does at 80mph
    b. artificially drop a couple of gears & rev it at 3000 or above

In short - is it the revs or simply the duration that is going to do the job? Will it charge 'better' at high revs or am I just wasting fuel for nothing?

After comments; A mains charger is not an option. I live in London. I have absolutely no way to park the car anywhere near the house or any electricity supply.

  • Or use a charger to top up the charge once a week or two weeks. – Solar Mike Jun 27 at 17:22
  • A charger is not an option, otherwise I would have included it in the question [or not even asked at all :P. I live in London. I have absolutely no way to park the car anywhere near the house or any electricity supply. [I should actually add that to the question] – Tetsujin Jun 27 at 17:30
  • Well, good answers come from good questions, leaving critical stuff out makes life difficult for all. – Solar Mike Jun 27 at 17:34
  • I presented the options. I didn't present things that weren't options. I didn't include a solar charger either, or pedal power... – Tetsujin Jun 27 at 17:35
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In short - is it the revs or simply the duration that is going to do the job? Will it charge 'better' at high revs or am I just wasting fuel for nothing?

Mostly, for cars with a reasonable alternator, it's the duration. Although the alternator will produce more power at high revs, if you don't turn on all the electrical accessories, the power produced at idle is more than enough to charge a battery. Thus, it won't charge any better at high revs as it's battery-limited and not alternator-limited. However, you may be trading a problem for another if you let it idle for 30min. Car engines are not designed for idling. You will get more engine wear if you let it idle. When idling, it warms up slowly, thus running for a longer duration at low temperature.

Thus, if you have the possibility, go for a half-hour long drive on the motorway. If not, I would be looking at microprocessor controlled battery chargers as a non engine wear adding replacement for idling. Do not use a cheap non microprocessor controlled charger unless you have the possibility to disconnect it once the battery is full!

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  • OP is clear that external charging is not an option. And, having put ammeter and voltmeter on alternators, running at idle is usually sufficient to put the light out. To get 40, 60 or more amps output means running the engine above idle. – Solar Mike Jun 28 at 9:18
  • Thanks. Based on this I've picked a route I can vary for interest, but includes a quick blast of 7 miles or so on the motorway; basically streets for a while, down the motorway for 1 junction, then back on streets. Distance 20m or so, duration almost an hour. [Yup, that's about the best you can do in London even without much traffic ;) – Tetsujin Jul 2 at 14:43
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4 is the only option, many times I have advised clients that a minimum of 10 or 15 minutes at around 2000 to 2500rpm is needed just to put the starting energy back into the battery and that is without having wipers, headlights rear window heater etc

So you need to give it a good run once a week making sure that you minimise electrical use. If you can’t then it just takes longer...

Much like charging a laptop, if it is doing nothing else all the charge, or most, goes into the battery but if you max out the machine doing tasks then the battery can take a long time to charge.

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  • 10 minutes at 2500 RPM is probably about right for a small engine that revs at 2000 - 3000 RPM most of the time in normal driving, but it seems a bit high for a big diesel that is only doing 1500 RPM at legal cruising speeds (off motorway in the UK). – alephzero Jun 27 at 22:28
  • @alephzero read 4b as posted by the OP. – Solar Mike Jun 27 at 22:29
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I left my car unused for 9 months and then put the dead battery on a good microprocessor controlled desulfating charger overnight and it was fine. Get a wheelbarrow and a good book. Dont charge car batteries indoors.

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  • I'm afraid this doesn't work as a frame-challenge. I have no charger, no wheelbarrow [or anywhere to keep one] & no way to get electricity outdoors. Even more so, I don't want to have to take the floor out of the car every week, when I could just drive it around for a while. – Tetsujin Jun 30 at 10:29
  • @tetsujin maybe a different battery chemistry like a RV or boat battery or a really heavy duty, parcel delivery grade battery. Have you seen those parcel guys? Must be 300 horse diesel and glow plug setup and they literally stop the engine every 100 ft and restart. – Andyz Smith Jun 30 at 12:41
  • I'm not buying a new battery, charger, wheelbarrow or anything to solve a problem I can just drive around to solve. – Tetsujin Jun 30 at 17:19
  • @Tetsujin I would be putting some monitoring in then. Hook up to the OBD-2 port and capture the battery voltage on a graphing live data reader. You should get a good idea what is required to keep the battery topped up. – Andyz Smith Jul 1 at 11:52
  • I can drive round once a week, or I can throw money at a problem that doesn't really exist. I'll stick to the driving around, thanks. – Tetsujin Jul 4 at 9:32

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