Actually modern cars are generally not supposed to be jump started. Our MBs have a sticker on the rear battery stating not to jump start.
It is quite safe to jump the car from the front battery. That is stated in the manual and that's good enough for me as long as normal precautions are taken. Running any number of batteries in parallel will massively increase the available current, but the starter circuit will only consume what it needs so you can't overload anything. It's always still 12V.

Those hand-held jumper packs are the way to go. Brilliant little piece of tech.

 

Great that you brought up the topic of Super Capacitors as a way to jump start a car with a weak battery. For those not familiar with these, they will charge up from even a weak battery for a period of 5-10 minutes and can then crank an engine for 10 seconds or so at full 12+ voltage.
The advantage is that you can leave one in your car for years and it will work when needed. In contrast a "forgotten" battery jump pack will likely be discharged after a year of just sitting.
I read many of the reviews of the $100 SuperCapacitor on Amazon. Vast majority liked it, but disadvantages have to be considered too. Mainly it takes significant time to charge up; the claimed 5 minute might be true if your battery is at 11 Volts, but it increases to 30 minutes at 8 volts and won't charge up at all below 8 Volts. Next if your engine has trouble starting with 10 seconds of cranking, they you are again out of luck. A battery jump pack can power for 10x that.
Ideally one should have both, keep the battery jump pack charged every few months and the Super Capacitor in case you forget.

As I understand it, the R230 is supposed to be able to start even if the rear battery is weak; i.e. the BCM engages the K57 relay to let the front battery power the car. However this has never happened for me; if the rear battery is weak, the starter will not engage or not fully engage. (It might just spin without engaging the engine.) Therefore, I have repeatedly put a jump box on the rear battery and started the car. I have also had both batteries discharge for some reason and have then even used two jump boxes to start the car. I know, not recommended, but in some situations you just have to do what you have to do. I know, it is not recommended.

One concern about jump starting a car is the EMF (back voltage) that the starter briefly creates when it is disengaged. (The spinning motor briefly acts as a generator.) Normally a healthy battery just absorbs this power spike. However lets say the battery terminals are corroded, effectively disconnecting the battery. While the jumper battery should be able to absorb the power spike, its circuitry might not allow accepting power causing the power spike to run through the car's electronics. The EMF power spike is often much higher than the original voltage. To see this, plug in a box fan, and while it is running pull the plug; the resulting spark sure looks like a lot more than 110Volts. (This is also why in ancient cars that the "ignition points" wore out - they only switched in 12Volts but hundreds of volts shot back when they opened; the points capacitor did absorb most of it.)

 

Realistically, if the batteries are well charged, remain healthy, and are replaced on schedule, there should be no need for a jump. But that's just theory. In real life, you have to be prepared for the worst. From a hobby perspective, gadgets are great. Have both a supercap and a lithium battery. The more the merrier. From an ownership perspective, it doesn't make sense to spend several hundreds of dollars being overly concerned over a $250 part and being prepared means having a AAA membership.

 

I have said this many times, there is no other battery then the original Mercedes Dealer battery for our cars. You may find that other batteries will do the job but I promise use they are not the same, the OEM is Varta and even if you buy another Varta battery it is not set up the same for out cars, even if the CCA and Ah is the same on the label. I have used 3 other brands and all them did the same thing that after 1-2 months I would get the 'Inconvenience Functions Temporally Unavailable' on the dash. Then you have to wait 13mins for it to clear.
The Mercedes dealer battery is about the same price as other brands, so just get the best, I have had them on my car front and back for 5 years and never have issues with the car even after leaving for a month without starting,
Also please avoid using any trickle charges, these have been known to damage the BCM which monitors the current and voltage to the front and rear batteries, a failure can cause them to ignite unless you have preformed the BCM safety mod: If you have to use a trickle charger unplug them frequently and do not leave them constantly plugged in.

 

While I greatly appreciate your excellent YouTube videos and contributions on this forum, I am respectfully very skeptical of two of your three points above. The first is that a MB/Varta battery is any different from any other high quality AGM battery. I've done electronics and automotive electrical systems for 40+ years and I've never heard of a valid reason why a similar capacity and type battery can be electrically different from any other. Cheap batteries could fail early due to vibration and I notice that some batteries do not charge at all below about a 13.6V alternator voltage. I've been active on different brand auto forums for 20+ years and notice it is mostly BMW and MB owners who claim that their dealer batteries are better than even high quality store brands. It is certainly not my experience with many cars. On my SL I have a MB dealer front battery because it was the same price (with my shop discount) as anything else in that unusual size. The rear is a DuraLast Platinum AGM bought from AutoZone. Since we forgeo to disconnect it when the car went into storage for a few months, the first one went dead. I simply took it back to AutoZone and with no questions, no testing, no receipt, only my phone number, they gave me a new one.

Second, I am also skeptical that trickle chargers are going to damage the BCM. I'm pretty sure the consensus is that the BCMs fails because the capacitors in them fail over time, short out and cause a high current draw through the module. Even a claim of "everyone who has long used a trickle charger has had the BCM fail" is separating cause and effect as these basically require a trickle charger unless you drive them at least weekly. Granted, connecting even a trickle charger if the battery is disconnected could have an effect on the BCM. I suspect claims about a trickle charger damaging a BCM are due to something else entirely.

Now you point of fusing the BCM is absolutely correct and I have been suggesting here in many times.

Again, I respectfully am not trying to start an argument as there could never be a resolution. Just wanted to present a different opinion. Everyone needs to make their own choice. And MB dealer batteries are really not that expensive, but not sure of their warranty period.
Thank you!
​​​​​​​

 

Granted, connecting even a trickle charger if the battery is disconnected could have an effect on the BCM

If the battery is charged while disconnected, then the BCM is not in the circuit, so how can it be affected?

 

My car came with mounted trickle chargers (front and back) directly wired to the battery terminals with power cords ready to plug in. I was imagining some one removing the battery, or disconnecting it with the disconnect you now have, and then accidentally plugging in the trickle charger in which case there is no battery to absorb the voltage spikes some trickle chargers use to recondition the battery. Crazy electrical things happen due to accidents and misunderstandings.

 

Hi

Thanks for the kind words on my videos !

You raised a good point with the MB battery, inside the battery is a PCB and from what I understand Varta were employed to manufacture a battery for Mercedes as that that is there specialism. From my experience of automotive when I worked for Honeywell, I was involved with the BMW 7 series oil pressure sensor, the level of quality they went to on 3 small solder joints was incredible, they just would not accept anything less than perfection. Mercedes quality criteria for the Varta batteries would be equally as stringent. My experience when I installed a standard of the shelf Varta battery was obvious, it would go below voltage after about 2 months, the same was for 2 other brands of battery too. Once I installed the Mercedes battery, no issues for 5 years.

In regards to the BCM, you are right that some causes have been down to components failing such as the capacitors. I've read posts that have said when the trickle chargers when connected for long periods have been a possible cause, however until I see a lab test with data then this is up in the air, but from an electrical stand point with power going through the BCM constantly it may cause issues, but I cannot prove,.

Therefore in summary the first point on the battery is from personal experience and I know worked for me. For the trickle charges if it were me, I would use sporadically and not leave on all the time, ergo, trickle charge the battery, once full remove the charger, wait for a month and put it back on again. I go with a month as I can leave me car for that time with no battery issues.

Hope this helps and thanks again for your respectful reply.

 

Yes you are correct, a lot of people do not disconnect though, if you disconnect the its fine.

 

Now I see what you mean. I connect the battery charger directly to the battery terminals. Hence, it doesn't matter if the disconnect switch is on or off. The charger will always see either the battery alone or the battery in parallel with the BCM. But it will never see the BCM alone. On the other hand, if the battery charger was connected to the negative power cable from the car and the disconnect switch was in the off position, then the charger would see BCM alone. However, I don't see the problem. Neither voltage nor current levels from a trickle charger are going to be more than what the large BCM electrolytic capacitors should be able to tolerate. After all, the purpose of an electrolytic power supply capacitor is to receive, store, and deliver power.

 

I have never heard of a PCM (electronic circuitry) in a lead-acid/AGM battery. Therefore I Googled the concept, but see no such confirmation. Getting specific with just Varta auto batteries, only their Lithium and Ni-MH batteries have any circuitry inside them (as is true for most (all?) manufacturers to reduce chance of fire.) However if indeed any MB lead-acid/AGM batteries contain circuitry, I would love to learn that and change my buying decisions.

On the subject of R230 batteries, I had thought of replacing the rear battery with a Lithium iron phosphate battery (LiFePO4) battery, as they won't be killed by draining. (Their internal circuitry shuts them off.) I even bought one, but then doubted that the alternator could properly charge these (actually 13V) batteries. Between that, worries about the BCM, having a Lithium battery catch fire, and members here discouraging me (!), I decided against it.

Back to the BCM, it is plausible that the slightly higher voltage due to constant charging might cause the BCM capacitors to degrade quicker. In that case the OP's disconnect switch might then lengthen the BCM life.

Thank you for bring up topics which gave me reasons to learn ever more about auto batteries.
While some people might dread these cars, I am finding them to be an educational challenge.

 

The genuine-or-not battery debate will probably never be settled but it has been mentioned repeatedly by different users here and on the other forum that genuine MB batteries seem to last much longer and discharge far less quickly.

All anecdotal of course but probably count for something.

 

Yes, the near consensus is that the MB branded batteries are best, which is great. I'm just questioning the "why" to determine what might be 2nd best for those who do not have easy access to a MB dealer or hope to save a little with another brand. The batteries are a bigger issue on these cars than any other I have ever had or even heard of. Seems even in the best cases, the rear one will drain in a month or less.

 

My BMW battery lasted 12 years. BMW systems charge batteries differently as they age. That must have affected its longevity. But it probably was a better battery from day one, even if it came from the exact same factory as similar batteries that lasted only 5 years. Why? Because the same battery made in the same factory, can be specified differently over a range of tolerances. Mercedes specifications and quality control will reject all but the cream of the crop. Same battery/manufacturer. Better specifications and quality standards.

 

I believe the greatest stress on a power supply electrolytic capacitor, other than overvoltage, is ripple current followed by heat and humidity. the highest voltage a charger can output is 15.5V, which isn't considered overvoltage. Ripple current doesn't apply to 12V DC circuits. That leaves environmental stresses as the most likely culprit to BCM capacitor failures. Is it true that most BCM fires are in the UK and other wet climates vs cool and dry climates?

 

Forgot to mention the other culprit, which is age. Electrolytic capacitors are made out of paper, which becomes brittle and deteriorates - if for no other reason - over time alone.

 

Yes, my 2013 M3 battery has lasted 8+ years; probably an old Sears DieHard AGM. BMW owners complain about having to "register" a new battery with a scan tool (or horror's the dealer), but evidently changing the charging characteristics over the life of the battery increases its life.

I suspect the BCM capacitors are not the higher quality "Tantalum" type. The M275 engines have an Ignition "Transformer" which is an electronic switching power supply for the 24 spark plugs. It similarly fails due to its capacitors. Electronic repair shops have noted that MB did not use MIL or even automotive quality capacitors in them. These repair shops will upgrade them (and the hyper expensive coil packs) with tantalum capacitors. So, I suspect the same capacitor issue applies to the BCM. Still, looking up the life expectancy of tantalum capacitors, it is listed as 20 years. (Knowing that I have hundreds of electrolytic capacitors that are over 20 years old from my electronics days, I will toss them all.)

UK or not, fuse your BCM. If you are willing to cut a few access slits into the rear carpeting, the BCM is easily accessed and the slits are barely visible.

 

The nine capacitors in the BCM that leak are polarised electrolytics. Each one is 470uF. Tantalum capacitors top out at around a tenth of that and are typically much smaller than that..

You simply can not obtain tantalum caps that large.

The caps in the BCM leak and short the board. Read up on the capacitor plague from around that time to see the carnage.

If I could get the circuit I would know for sure but these are almost certainly used in the output to smooth the DC charge for the front battery.

 

Yeah all of my (old) tantalum capacitors have small values, but interestingly on the Digikey.com site, they list 49 Tantalum capacitors of 470uF and 20+ Volts; of course style and size matter too.
Tom, do you know of a thread or YouTube that lists exactly what part number capacitors to buy for a BCM rebuild? Come next winter when the car is in storage, I might preemptively rebuild the BCM.
Thanks.

 

The BCM includes a DC/DC converter, which requires large capacitance at both input and output. But so what if they are prone to failure for environmental reasons more than electrical reasons?

 

The BCM is a DC-DC converter and smart charger - that's pretty much its only job. The issues arise when those electros leak and short the board.

Modern fresh capacitors should solve the problem - replace them with name brands like Panasonic and you should be right.

 

My understanding is that the BCM has other power management functions too. See page 19.

 

Yes, but many would say that the only significant/important function of the BCM is the DC/DC converter to charge the front battery as needed. The clever after-market BCM replacement only performs the DC/DC function and satisfies CAN bus messages.
Turning off accessories when the rear battery is a bit weak is just annoying.

 

The R230s went to the dual battery system because of the SBC brakes which have very poor mechanical backup. While probably overkill 99.9% of the time, all of the power management is built around priority 1: Braking and priority 2: limping home, or at least to a safe stopping location.

 

I forgot to connect the consumer battery the last time I got in the car. Just to see what happens, I tried to operate several things in the car before starting the car. The cluster, seats, and infotainment system worked. The steering wheel and convertible top did not work. The alternator is not providing power because the car is not running. That can only mean that the starter battery was providing power and the BCM was selecting what it would and would not power. Hence, the belief that the starter battery is only for starting the car is false. It also powers consumer functions when the consumer battery is not providing adequate power. It is my understanding that it also works the other way - the consumer battery starts (or helps to start) the car if the starter battery is not providing adequate power. Hence, the BCM is doing more than what you may think it is doing. I would not want to use a modified BCM that acts only as a DC/DC converter.

I am now keeping both batteries fully charged at all times. For convenience, I ran DC extension cables to the cowl. May seem counterintuitive, but if the voltage loss over the longer cables is excessive, you have to use a lower power charger. My 1A charger is working fine whereas my 20A charger failed. Because 20X more current means 20X more IR drop (voltage loss). Good example of less is more.