No start problem that does not happen often - PeachParts Mercedes-Benz Forum

JHZR2 · #4 06-20-2023, 10:03 AM

Quote:

Originally Posted by Diesel911

My case was not the typical issue.

In my case what I had to find out is if the battery if the battery could be charged and if it did get charged if it had enough umph to start.

What I found out was it was not accepting a normal charge. Likely because the newer style battery charger was detecting the voltage and that was telling the charger that it was charged. A battery being charged and having the amperage to start is a different thing.

Not sure on new batteries but in the old days the batteries amperage was determined by the surface area (number of) plates and if the surface aera was still good. In my case I am betting after 13 years the plates in the battery so shot there is only enough working areas to produce a soft weak spark when I short the battery terminals with a wire. (Another way it can happen is if the plates lose their connections and are no longer connected inside of the batter.)

How to do that without much equipment is what I was trying to show and also how the battery having a normal voltage can be deceptive.

I am not familiar with the other thing that kills batteries which is a buildup of crap on the bottom of the battery that shorts/bridges the battery plastes and renders those plates useless. If there is a separatee unique symptom for that I don't know it.

A battery can show “full voltage” (12.7v after sitting a while after a full charge) and either have issues or no energy behind it (the plate issue you allude to)

Any charger can be “tricked” into thinking that the battery is “full”.

If you had a shorted cell, you would see around 2.1V less than expected when the battery has sat idle for a while. Or lower. Very easy to identify.

Can you report the lowest voltage observed when you try to crank it? And just before? This is the first important diagnosis. The rest is below.

So the primary things to do are as follows:

1) disconnect battery, take voltage, report.
2) attach battery charger. Note current charge rate and time. Monitor to see when it’s done. Report all. (If the charge time is excessively short, that gives you some insights too).
3) using fused 10a leads for meter, attach battery ground via 10A multimeter leads. Measure parasitic current. Report observed current at connection and 10 mins later
4) reconnect battery correctly. Measure voltage, report.
5) crank car, watching voltage while car is started, report lowest observed.