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Battery Charging - Disconnect terminals?

Discussion in 'off topic' started by snowflake, Jan 18, 2021.

  1. snowflake

    snowflake Former Albino Ape

    Paging Motoring tecs,

    My start/stop light came on permanently the other day, and I know in these modern times it can be a sign of a discharged battery, or something catastrophic, so hopefully the former.

    Anyways, bought one of these here smart chargers from Halfords. Its does all manner of vodoo before it charges and then drops to a trickle, it alleges.

    Like a good prole read the instructions in the car manual (Hyundai i40) that said disconnect the battery terminals before charging - which I did, and then had to re-connect them to lock the car, then disconnect again (haven't figured out where the key goes in emergencies, think you need to prise a bit of the drivers door handle off - possibly).

    So anyways, question is on a modern car can you just connect the battery charger without disconnecting the terminals? I have my bike wired up to an optimate all winter without disconnecting the terminals and it seems to survive.


  2. martin clark

    martin clark pinko bodger

    Yes, you can do just what you propose.

    The manuals are now written that way as a get-out to people hooking up giant-capacity brute-chargers in extremis to get-going, esp with vehicle live and potentially spiking some ecu somewhere. That won't happen in practice, - esp with ign off, and very esp with modern charging algorithms, which tend to measure existing voltage, see what happens under a small-current for a few seconds, then pick /ramp-up to the selected charging profile.

    (ETA: vehicle electronics are designed to withstand far more energetic things like alternator load-dumps, than a small smart /even a classic 5-10A ish dumb charger can ever apply.)
  3. snowflake

    snowflake Former Albino Ape

    Was my exact fear! - think it 6 amp max (apparently 1-6 amp pending 'thirst' of battery and 0.8 to 1 amp on a trickle). Also have a BIG note on the dash saying 'battery charging' to prevent any potential ignition/drive-away while plugged in mishaps :)


  4. Mike P

    Mike P pfm Member

    It can be an issue with some classic vehicles. The owners manual and Haynes manual for my 1978 Honda CB550 both warn that the rectifier can be damaged if the battery is charged while still connected.
  5. martin clark

    martin clark pinko bodger

    Bikes are different, because of they ways the regulators work (which is very crudely, esp of that period!)
    It is safe to 'jump' start from a 12v car battery - with the car's engine off and do not start the car, you'll cook the bike's voltage 'regulator' (which will be trying to pull down the cars alternator output to the voltage on the bike battery by trying hard to short the car's alternator out ...)
    Mike P likes this.
  6. cutting42

    cutting42 Arrived at B4 Hacker Erg \o/

    With the sad lack of use on our motor vehicles I bought a lovely CTEK charger and leave the fleet plugged in overnight on a rota to bring the batteries up to snuff every month or so.

    Not the Tesla though, that has its own special charger ;-)
  7. davidsrsb

    davidsrsb pfm Member

    If you don't move a car so long that you need to charge the battery, you will have many other problems with brakes, engine, air con etc
    Best thing is to just drive weekly it until the engine is fully warmed up
  8. cutting42

    cutting42 Arrived at B4 Hacker Erg \o/

    Agreed, however I don't think that exercising your car falls into the permitted lockdown activities currently.
    Mike Reed likes this.
  9. Ponty

    Ponty pfm Member

    You can leave the terminals on, just make sure you connect the charger to the positive only. The negative should be connected to an earth point on the vehicle and not the battery. This should ensure the vehicle electronics are bypassed. I use a CTEK MXS-10, anything more than 10A is not generally recommended for modern car home charging.

    If leaving standing for a while, leave the handbrake off if you can and pump the tyres up a bit.
  10. PaulMB

    PaulMB pfm Member

    That sounds strange to me. I would think it is the same if you connect the Earth to the battery or the chassis, since they are joined to each other with a strap connector. In all the 50 years I've been charging batteries I've never noticed any difference if I disconnect the battery or not, apart from having to re-set the radio stations.
  11. awkwardbydesign

    awkwardbydesign Officially Awesome

    If I understand it correctly, some modern (unlike mine) cars with computers, disconnecting the battery can cause serious problems! But I charge car and bike batteries without disconnecting them.
    I also use a solar panel trickle charger inside the windscreen permanently wired to the battery on the cars. And a garage door mounted one for the bikes.
  12. Pete MB&D

    Pete MB&D Pete Maddex, the one and only!

    I had to charge mine last night, Aldi/Lidl smart charger, I didn't disconnect the battery just conected it straight across the terminals.
    Its off for a new battery today, 7 years on the old battery!

    I had a Kawasaki Z650 with a mechanical regulator, the type that shorts the excess to ground, it worked but it was old technology even then.

  13. stevec67

    stevec67 pfm Member

    Correct. Battery earth is the same as chassis earth, as you say if you trace the cables the battery neg is connected by a big black wire the size of your finger to a convenient body earth point nearby.
    I didn't know motorbikes had a different system though, that's interesting.
  14. Pete MB&D

    Pete MB&D Pete Maddex, the one and only!

    Only the old ones, my 1992 ZXR750J has a typical alternator/regulator.

  15. Ali T

    Ali T pfm Member

    Spurious faults can also be a sign that you need to replace your battery.
  16. Ali T

    Ali T pfm Member

    The main reason is to prevent sparking and a possible explosion if there are fuel fumes or gas present.
    Colin Barron likes this.
  17. stevec67

    stevec67 pfm Member

    That makes sense. Mind you, the risk of sparking is very small and the same with the terminal removed or attached, but I can see why they would go that way.
  18. Ali T

    Ali T pfm Member

    Well I don’t know about anyone else, but I almost always get some sparking when replacing the battery terminals if I’ve had the battery disconnected.
  19. dweezil

    dweezil pfm Member

    Sparking in the vicinity of a recently charged battery can cause a surprisingly large explosion. I have a friend with a few bits of battery still embedded in his forehead, safer to leave it there and he's lucky he was wearing glasses.
    Ali T likes this.
  20. Mike Reed

    Mike Reed pfm Member

    Mine was replaced in November after 11 + years and all local but frequent hops until Covid. They attached a battery to something under the dashboard/steering wheel 'to keep the info. intact' before removing the old and fitting the new. First time I'd seen that.

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