Connecting battery charger fly leads

A modern code reader will reset the car's adaptive charging module to a new battery - no need to visit the dealer. I think in any event a reset will occur over a short time period.

I’m not sure if this will inform the discussion in any useful way: **LINK**

https://bimmerscan.com/bmw-intelligent-battery-sensor-ibs/

My BM is too old for such fripperies

MichaelG.

And I think BMW may hold one of the answers to the original question. My old 5 Series (E39) had a small explosive device on the battery terminal that in the event of a serious accident would sever the battery connection to try to reduce the risk of fire. There were many dire warnings about jump starting and charging in the manual, I think it suggested all these things would be done via the dedicated terminals under the bonnet. The battery was in the rear wing.

In the large multinational company of field engineers I used to work for we used to have (about once per year) a 12V battery explode when jump starting, sometimes with fatal results.

Some of the explosions probably came from crossing the leads but others were correctly connected and still exploded presumably because of huge currents circulating between a fully charged battery and a fully discharged battery giving rise to hydrogen gas evolution ignited by a spark from a jumper lead breaking or making a connection.

The solution, which appeared on stickers in every vehicle, was to ensure that the last jumper lead to be connected and the first lead to be disconnected was to a chassis earth. Thus any sparks were hopefully distant enough from any source of hydrogen to keep things (relatively) safe.

I believe that in general the advice not to use the battery negative terminal for charging is just to reinforce best practice for the use of jumper leads.

Thanks for the link, very interesting. My BMW has electronics that I don't pretend to understand but they are very clever. Most of my miles these days are on a long stretch of roadworks with an average speed limit of 40mph. Easy, just set the cruise and relax. However, last week my car told me my battery was low and to start the engine, something that has never happened before. I concluded that because I was using cruise, the car wasn't charging. It only charges when you take your foot off the accelerator or when braking.

I have a smart charger and connected it to the charging points in the car, and getting back to the OP, unless you are competent in modern auto electrics, just follow the instructions, that is what they are for.

Thank you Michael G - one of the few to address my 'why' question, rather than retell war stories about batterys exploding; I have one of those myself. So it seems that there is a battery management module between the battety earth post and the vehicle, and I can see (imperfectly) that connecting a charger to the earth post may disrupt the module, at least temporarily. I have of course connected my fly leads as advised by the manufacturer, but now have a explanation of the reason for doing so. We can close this thread now.

With respect, the emboldened (above) is a far different situation than a likely far less risk than proper use of a 5A battery charger. Modern valve-regulated batteries are likely to retain the stoichiometric (some may need to search out the meaning of such terms) ratio of explosive gas, just waiting for an internal source of ignition. Further good battery chargers will not supply a current to a battery exhibiting a low voltage.

The old (almost banned?) open-cell batteries would soon lose any hydrogen produced, simply by it diffusing at a much higher molecular velocity through the vents or with caps loosened/removed. Also, I expect those that operated that type of battery were more savvy wrt safety or the risks - not like a large percentage of the younger generation that assume it is someone else's responsibility ?

I always assumed that the "live connection first" policy when jump-starting was for a different safety reason. If you connect earth first, then start waving the live clip around the engine compartment then any accidental contact with metalwork will, at best, give a big spark and maybe a lot worse. Connect live first and you would be touching an earthed connection to earth - no problem (apart from a much smaller spark representing balancing charge current). Never worried about hydrogen build-up - I would be doing this kind of thing by the roadside with an open bonnet and while hydrogen is certainly inflammable-going-on-explosive it is also very light and will quickly disperse.

I have no idea exactly how my current car electrical system works. It is a so-called "mild hybrid" so has a 48V battery plus motor-generator principally used for a very aggressive start-stop facility, plus a more conventional 12V system. That battery, I believe, lives under the boot floor but there are terminals under the bonnet for jump-starting. Reading that BMW write-up, I can understand why (my car is Audi but I assume that these manufacturers are pretty much in step when it comes to these things). I think that there is also an inverter setup that does something with cross-charging these batteries, plus regenerative charging of one or other of them. It's a long while since I did things like change an ignition coil or reset contact points by the roadside - done both in my time. It's all a bit of a mystery to me now...

As a genuine request for information I have a 300A Jump starter which is a wonder starting traditional (ie less electronic) vehicles with ease. Its star performance was starting a 3.2L diesel minibus that had been in a garage for 18 months over lockdown where there was not enough oomph in the battery even to pop the door locks.

Any suggestions should I ever need to use it on my current stop/start Skoda?

Is the question really, "will my 300A jump-starter overpower my Skoda?" To which the answer is, probably not. This "max current rating" thing often seems to confuse, but think about plugging a table light into a nominal 13A socket. The socket may be capable of delivering 13A, but the light will only take as much as it needs. Ditto jump starter - if the car starter motor needs that kind of current to turn it over (and it may - car starters are very high-current devices) then you have the means to deliver it but in itself it is not going to overload anything.

As for how to connect - that comes back to reading the manual! I wouldn't place bets on second-guessing that kind of thing with modern cars...

For anyone tempted to dismiss the risk of battery explosions, THINK AGAIN

As already said, a Lead Acid battery that has been charged will have produce a mixture of hydrogen and oxygen.. If any heat source, flame or spark is applied, even inadvertently, it will ignite the mixture and there will be an explosion.

I have seen it happen, fortunately on a small scale.

Lord De Lisle and Dudley retired and bought himself a boat. Having charged the batteries, he checked the acid level with a naked flame. It put him in hospital for some time.

Treat any battery with great respect. A vehicle battery, apart from the explosion potential after charging, and the sulphuric acid content, can deliver a current several hundred amps

The starter motor can have a current draw, on cold cranking, of over 400 Amps

The winding are usually copper strips. so not designed to minimise current flow, rather the reverse..

You can electric weld with about a tenth of that, so be warned.,

Howard

Mike is correct, this is all about the adative charging system.

A tiny plastic "block" on the negative terminal connector with only a couple of thin wires connected to it can continiously monitor the battery state of charge and health, even when the car is off. One name for this is a Intelligent Battery Sensor (IBS). The data is stored internallly on some models. First read the manual. If jump-start connection points are provided it is typical to use those. If no information use chassis and battery positive.
Some of the sensors have a quick release for the ground wire. This allows the car to be made "safe" for servicing while keeping the module connected.

Note that some cars have a simlar lump in the negative connection that is a one time pyrotechnically operated circuit cutout that isolates the battery in a crash.

Robert G8RPI.

Edited By Robert Atkinson 2 on 13/07/2022 19:19:36

For 99.9% of the time, there will not be enough explosive gasses present when connecting batteries to give trouble. Look at how many jump leads are sold. But never forget that hydrogen is not the only gas produced by batteries, if it was there would only be small flames. Hydrogen comes from the cathodes and oxygen comes from the anodes. From the breakdown of the water content of the battery acid. Produced in a perfect explosive mixture, 2 hydrogens to 1 oxygen.

In my earlier post about surviving a beaater explosion, our quick excuse which saved our bacon from the boss was that out firm pioneered the use of "catylators" which were plastic pots which screwed onto each cell vent in place of the cap. The battery had these fitted. They contained a tiny ammount of palladium chloride which recombined the hydrogen and oxygen to water which dripped back into the cell. Modern maintenance free batteries must have some way of achieving this.