Connecting battery charger fly leads

Modern cars are power hungry when locked, and will often show low battery after a few weeks. For this reason it is prudent to charge the battery occasionally, and for convenience permanent fly leads and a plug, for say a CTEK charger, may be attached. It is always recommended to attach the permanent fly lead earth to the vehicle body rather than the battery terminal. Why is this? I have seen it said that direct connection to the battery terminal can fool the adaptive charging system which modern cars employ - could someone explain this in terms appropriate for a mechanical rather than an electrical engineer.

Very simple. Any spark that might be generated at connection is well away from the battery - avoiding any risk of battery explosion at connection time.

Yes, I know about explosion risk, but a permanent fly lead could just as easily be connected to the battery post.

another one of those myths that people accept as fact after a period of time.

Hi ndiy,where do you connect charger earth lead when battery is on floor in shed ?

For last 50 years I’ve connected leads across battery with no problems.

I assumed connecting charger to chassis was due to electronics in modern cars

Maurice

In some cases the battery is under one of the front seats so using the remote terminals under the bonnet would make sense. Stop start batteries use the more complicated charging system, but no one I can find says you can't connect direct, but you do need an Auto battery charger, so the old Winfield charger needs to be binned

Many car batteries these days are sealed, so they shouldn't vent hydrogen unless grossly overcharged. So no risk of explosion. I always connect my charger across the battery terminals.

Andrew

I use a Motobat smart charger intended for motorcycles, which has only about 1 amp output. They are small and waterproof and I have used it for years to keep the bikes battery in working order by once a month connecting for 48 hours. It was also useful for the car when I was unable to drive after knee joint replacement. The trickle rate is only about 50mA when it senses a charged battery. The charger gets switched on after connecting and off before disconnecting.

Edited By old mart on 12/07/2022 21:02:37

I have read all the above posts and note that none mentions this. All connections to the battery or other terminals should be made before switching the charger on. Then there will then be no spark. Explosion is very unlikely as there would need to be a substantial build up of hydrogen and oxygen trapped within a building or shed, garage. That sort of quantity of gases would likely build up after perhaps several hours of charging. However, it would always be best practice not to smoke inside a charging room and avoid sparks, naked flames, just in case.......

Edited By DMB on 12/07/2022 21:09:16

0n my car (newish Audi, but I imagine it will apply to a lot of modern cars), the current flow in and out if the battery is monitored by a module connected between the battery negative lead and the chassis. This is part of a sophisticated battery management system which includes recharging from regenerative braking. If you connect a charger directly to the battery terminals you bypass the current flow sensor, and allegedly confuse the system. A special chassis terminal is provided for battery charger connection. On my car this is right next to the battery, so no lower explosion risk than a direct connection ti the battery terminal. Incidentally my smart charger can’t be enabled until the leads are connected to the battery, and the power is shut off if the connection is intermittent, so In theory no risk of a spark. I imagine the CTEK is the same.

Mike

I had an incident when I worked at a motorcycle workshop. I removed a new battery from a Honda 90 which had been sold and put it on charge. There was a large two voltage charger and selected the 6 volt pair and connected and switched it on. Another worker placed a spanner on the same bench which disconnected one of the crocodile clips and caused a spark. I could see a flame travel up the clear plastic tube connected to the battery vent and ran. The battery exploded spraying acid etc over the new bike and other stock. No one was hurt but only just.

David

We are getting off the point of my post. Mention of explosion is a red herring; please assume the PERMANENT fly leads are at least 1 metre long, so the the connection point is well away from the battery. The manufacturers of both vehicles and smart chargers say to use an earth point away from the battery - is it because of adaptive charging, and if so what does the remote earth connection achieve that a direct ( battery post) connection does not.

When I was a teenager in electroplating, we had a duff plating transformer rectifier which was attached to a 6V lead acid battery. The battery cells were about 1 gallon each, not a tiny car battery. I said to Ray, "try shorting the battery", so he did with a 1/4" diameter nickel plated rod out of the drying oven. As he shorted it, I was leaning over him to get a better look ans a cell exploded. All the acid and bits of the plates and separators and the top of the cell just dissapeared, and neither of us had a scratch. We could have been killed, that cell was about 300AH. We hastly concocted a plausible story and got away with it when the boss appeared seconds later.

I have had a healthy respect for batteries ever since.

Not so, hydrogen is explosive in air from 4% to 74%. When I first started working on electric vehicles in the early 90s we were using lead acid batteries. We had to have hydrogen sensors fitted in our battey room and in each vehicle. One of the experimental battery packs we were asked to look at had clearly had an internal fire, suspiciously just above the battery vent.

Andrew

Thankyou Mike - that seems to explain it.

That's a new one on me. I've designed the battery management hardware for a number of electric vehicle battery packs, including regen capability. But this was always for the main battery pack, not the SLI battery?

It turns out that regen is not as useful as one might think, not least because the battery needs to be at a relatively low SOC to accept the short term high currents without being over-voltaged.

Andrew

Yep, I’ve done it safely for 60 years, or so, too. A friend had a battery explode - he was angle grinding the other side of his workshop. The gas from a battery is not just hydrogen. It is water that is electrolysed to hydrogen and oxygen - the perfect proportion for recombining it all back to water.

Charging a battery direct with a decent ctek charger would not really affect any fancy electronics - neither of our cars is like that. I’ve charged literally hundreds of batteries with anything from a trickle charger to a 100A engine starter. 10 starts (or more) on machines, in quick succession was the order of the day at one time. Batteries needed boosting to start the engines, then they were run a while to charge the battery from its generator.

The principle is still the same - that of keeping any sparking well away from the battery.

The sensible ones amongst us do not connect or disconnect the battery charger to the mains until after the leads are connected. Just another KISS principle to follow. The Ctek charger instruction is explicit in this respect. Common sense to those that understand there is a risk - but many are totally oblivious to it.

Even on a ‘modern’ car (both ours are modern but not that new) if the battery needs charging, it needs charging. Full stop.

The adaptive charging will be taken care of when the engine is running. Manufacturers make it difficult for even renewing the battery these days - must go to dealer for them to do it and they make a nice healthy charge for it.

Dont bin it as it is useful for anodizing/rust removal/testing stuff etc. The new types do not work for those uses.

regards

Andrew

According to the technical blurb for my car, the battery management system controls normal charging so that it never charges the battery to more than 80% to leave headroom for regenerative charging. The system does seem to be pretty sophisticated, and, for example, if the battery charge is getting low, or drain is high, it progressively shuts down power consumers so that essential items are kept going as long as possible. It also adapts to allows for battery ageing. One complication is that when the battery is changed the relevant ECU has to be reprogrammed with the details of the new battery or the charging characteristics will be non-optimum. So unless you have the necessary software, a simple battery change turns into an expensive visit to the dealer!

Mike

Double post

Edited By Mike Crossfield on 12/07/2022 22:30:57