Stupid battery/jump start question...

[ptruman]

My W212 E Class battery is due to get swapped on Weds, as it's been having issues starting recently, and getting worse (esp this w/e)
As I'm not driving today/tomorrow - sods law says it won't start when I'm due to take it to get it swapped.

I'm assuming there aren't any issues with my using the wife's '09 A150 to jump it? (other than hooking up, letting the A run for a bit first, as the E is going to require a fair amount to crank...)

 

[W1ghty]

My W212 E Class battery is due to get swapped on Weds, as it's been having issues starting recently, and getting worse (esp this w/e)
As I'm not driving today/tomorrow - sods law says it won't start when I'm due to take it to get it swapped.

I'm assuming there aren't any issues with my using the wife's '09 A150 to jump it? (other than hooking up, letting the A run for a bit first, as the E is going to require a fair amount to crank...)

I would favour buying a cheap battery charger to leave on instead ,
something like the Maypole
if you decide to jump start , have a good read of the cars manual first

 

[markjay]

Jump-starting from one car to another is not recommended.

https://www.halfords.com/motoring/battery-maintenance/jump-starters/

If you absolutely have to, then make sure the ignition is OFF on the donor vehicle (but don't run the battery down), and also switch on the headlight on that vehicle.

Also, the A-Class battery might not have sufficient cranking power to start the E-Class engine.

 

[geoffus]

Just make sure you connect to correct terminals and you’re away no problem.
The wife’s B class has the same battery as my E class so you won’t have any trouble starting it.

 

[ptruman]

Well, I've taken a punt on a cheap jumper (due tomorrow) as I need to move the car on Weds. Place bets now....

 

[W1ghty]

Well, I've taken a punt on a cheap jumper (due tomorrow) as I need to move the car on Weds. Place bets now....

Might be the glow plug relay module as well as the battery that’s causing starting issues .
Im voting with the charger on from tomorrow it will start first time (disconnect the charger first, obviously)

 

[ptruman]

It varies in behaviour.
Sometimes I get a "click" and everythings cuts out/back in and then starts.
Or it just cranks slowly, lights dim as it cranks etc - which to me says "it's buggered". I had to do a couple of small runs (house to garage, circa .5M, then garage to shop (another .5M), then back home (1M) and it didn't want to start from home, nearly didn't start from the garage - did start at the shop (just) and now it's back home waiting....

Wife's '09 A Class had a new battery a few months ago, and was doing similar behaviour.

 

[Stratman]

My wife's tiny Hyundai i10 has no trouble jump starting my C200cdi diesel.

 

[jasonh]

Whilst pretty much any running car will be able to perform the jump start the problem is "load dump". When you separate the two vehicles electrically the change in load with running alternators creates a huge voltage spike (for both cars).

In the old days that was no problem, no fancy electronics in a car. These days with multiple ECUs it's a risky business. I know on some makes Bluetooth modules, light control modules and glow plug modules can all go pop.

 

[ptruman]

I've jumped the A from the E before (just before we had the A's battery done) - and had no issue - just assumed the smaller car may well have a smaller battery and just "not cope"

On a side, I will add that originally I had bought a battery from Halfords, with fitting - but they "couldn't do it" as they didn't have the Merc tools to check the car was ok after the change....
(so, in case anyone thinks of getting Halford to do that - there you go...)

 

[190]

We need a bit of reality here. There is no difference electrically between jump starting from another vehicle and jump starting with one of these starter battery packs. They are both jump starting and both are nominally 12 volt sources.

According to my handbook MB do not preclude jump starting from another vehicle if it's done safely.

It's simpler and safer not to start the donor car, it's not necessary if the donor cars battery is good and it's safer for the donor cars electronics when the jump leads are disconnected.

Connect positive lead to donor first then positive to dead car.

Connect Negative lead to donor car first and then to the dead car.

Wait 2 or 3 minutes to get some charge into the dead cars battery then Start the dead cars engine.

Disconnect the jump leads in the reverse order negative lead first at both ends and then the positive lead at both ends.

 

[geoffus]

Whilst pretty much any running car will be able to perform the jump start the problem is "load dump". When you separate the two vehicles electrically the change in load with running alternators creates a huge voltage spike (for both cars).

In the old days that was no problem, no fancy electronics in a car. These days with multiple ECUs it's a risky business. I know on some makes Bluetooth modules, light control modules and glow plug modules can all go pop.

I think you will find that a “voltage spike” is an old wife’s tale.
The battery in the car would act as a suppressor in any case, but it’s not physically possible to create a spike from removing jump leads.
Connecting the wrong polarity would cause issues though.

 

[jasonh]

I think you will find that a “voltage spike” is an old wife’s tale.
The battery in the car would act as a suppressor in any case, but it’s not physically possible to create a spike from removing jump leads.
Connecting the wrong polarity would cause issues though.

You are wrong; it definitely is the case.

If you use two cars you have two alternators running flat out to pump charge into the flat battery. Alternators in modern cars are controlled by databuses so they can control their output. An alternator on an E350 is 180A and 2.4kW. When you disconnect the two cars, this happens in microseconds. The alternator response will be much slower: 10 of milliseconds at best. So on the donor car you'll have a massive surplus of energy with nowhere to go, so the potential of the whole system rises - a voltage spike. All the ECUs should have surge protection but sometimes it isn't enough.

On the car being started when you disconnect the donor car there is a massive step change on the load too. Charging of the battery was being performed by two alternators. In microseconds assistance from one disappears. The alternator ramps up as fast as it can often overshooting as is typical in control loops - and you get a spike. This isn't as bad as the donor car but it still can cause problems.

If I can dig up a BMW service bulletin on this I'll post it here. They banned jump starts on diesels for quite a few years because it blew the MULF or Glowplug module almost every time (despite the cars having jump start terminals under the bonnet!).

 

[geoffus]

What has the current rating of the alternator to do with the voltage ?
The alternator of the either car will not produce a voltage spike while the batteries are still connected, you may get a rise in voltage if you disconnect your cars battery while the car is running, but not by just removing jump leads.
The cars battery acts as a smoothing dump, just like a capacitor to absorb any high voltages, but your alternator has its own regulator and cannot produce sudden high voltages to damage components. Once the donor battery is disconnected no voltage will flow and no excess load will be created.
Every auction house and large dealerships still jump start their flat batteries.

 

[jasonh]

Let me clarify a bit further:

The problem primarily arises when you use a running second car to start the car with the flat battery. This always was the recommended way to jump start a car. Why because it gives maximum power and ensures you don't also kill the battery in the good vehicle. If for example you had one flat battery and one good battery and you connected them for a long period of time the charge would even out and you'd have two half flat batteries. If you are turning over and over a car that won't start, your donor battery is not 100% (which it almost never will be) then without a running donor car you risk two flat batteries.

With regard to the car battery absorbing the voltage spike. If you are familiar with Ohm's law V=IR then you will know that current and voltage are related. The wiring to and from the battery has resistance (albeit very low), the car battery has internal impedance (resistance). A car battery should have a low impedance, say 5 milliohms or, if not in great health, may have quite a high internal impedance, let's say 100 milliohms. From V=IR with a current flow there is a potential difference across the resistance. If you are expecting the car battery to absorb the voltage spike it won't absorb all of it. The voltage at the car's ECUs could be very different to the voltage at the battery (momentarily). Let's say there's 100A flowing and the battery internal resistance is 100 milliohms, the difference is 10V, that's 10V on top of the 14V that was there at the moment of disconnection, i.e. 24V which can be too much for some ECUs.

What is safe, and is done routinely now by car breakdown services, auctions and garages is the use of jumper packs. This is just a second battery, which if consumed doesn't stop another car from starting.

 

[optimusprime]

Or

 

[Stratman]

When the leads are connected there's no problem. When the leads are disconnected there's an open circuit, even at the precise moment of disconnection. As per Ohm's Law V=IR, therefore I=V/R. When R is infinite I=0.

 

[Tonye]

Don't start the donor car at all, the chance of it ending up dead is zero as long as the dead car starts after a couple of cranks and no spikes as it will just act as a starter pack.

 

[clk320x]

Or View attachment 143857

This!

 

[BTB 500]

The other risk is causing a spark that can potentially ignite the hydrogen & oxygen vented by a lead/acid battery when charging. Hence at least one connection to an earth point away from the battery, and the specific order for connecting/disconnecting.

The handbook will give the recommended jump start procedure for your own specific car. If you follow that exactly you shouldn't have a problem.

As an aside I did a jump start yesterday for the first time in 20+ years. Although it always starts, our Vito has been cranking rather slowly for some time ... the battery is over 16 years old (with 160k miles on it) so an obvious suspect! I hooked up a spare battery in parallel as a test, and sure enough it spun over much more enthusiastically. The garage took the battery out and tested it last time they had it in but the results were inconclusive ... all the cabling (incl. earth strap) was fine. They did mention something I'd not heard of before as being a possibility - a 'lazy starter motor' (apparently modern BMWs in particular are prone to this). Hence my quick test to rule this out.

Jump starting the Vito involves taking the cover off the fuse box (under the bonnet) to get access to a +ve connection point, covered by a red rubber cap. There's an earth connection on the bodywork next to the fuse box, covered by a black rubber cap. You need decent (fairly hefty) jump start cables for a 3 litre diesel as the current required to turn it over is high.