Advice on jump starting

GrumpyGel · Sep 5, 2020

My son's Beemer 320 has a flat battery and he has asked me to jump start it tomorrow using the L Series Freelander.

Am I safe to jump it, or is it likely to kill some electrics on the car?

Should I disconnect my cables from the battery and jump his beemer while its not connected to the Freelander - and cross my fingers that the radio will work and the rear window won't throw its regulator on the way down or up!

kermit_rr · Sep 5, 2020

Attach leads then start beemer? Leave landy switched off

Colthebrummie · Sep 5, 2020

If you listen to the experts, jump starting modern cars is a mine field. I expect to be shot down on this and I take risk I don't expect other to do but here goes, connect the jump leads to the flat battery, then with the engine off, connect to the freelander, start the freelander and the the BMW. Disconnect the BMW before disconnecting the freelander. Someone will be along soon to give alternative method.

Col

kermit_rr · Sep 5, 2020

I'm not an eggspurt, but if you leave leads connected with landy running for 10 mins, then switch off and disconnect, that should give beemer enough to start? Surely that can't hurt anything

Beneagles · Sep 5, 2020

If not needed until tomorrow send him to Halfords to buy a charger. Charge it over night. problem solved.

Alternatively, when you go along tomorrow, disconnect his battery totally from the car and connect jump leads from yours. Run Fl for 20-30 mins to charge battery then disconnect leads and reconnect Beemer.

kermit_rr · Sep 5, 2020

Beneagles said:
If not needed until tomorrow send him to Halfords to buy a charger. Charge it over night. problem solved.

Alternatively, when you go along tomorrow, disconnect his battery totally from the car and connect jump leads from yours. Run Fl for 20-30 mins to charge battery then disconnect leads and reconnect Beemer.

Easier said than done if it's a modern one with all the junk on top of the battery

Colthebrummie · Sep 5, 2020

Life is so much easier with a lithium jump start pack. No shunting of vehicles for leads to reach, no unraveling of dirty cables, no accidental short circuits when clips touch together.

Col

jamesmartin · Sep 5, 2020

i cant see the issue with jump starting,give it 10 mins with your engine running and leave it running when you go to start the beemer, fit neg cable last and remove first

kermit_rr · Sep 5, 2020

RAC don't mention anything about smart alternators and suggest the tried and tested method of jump starting.
You would hope they know what they are talking about and know the workings of modern cars.
https://www.rac.co.uk/drive/advice/car-maintenance/how-to-jump-start-a-car/

paul99 · Sep 5, 2020

Keep an eye open for dedicated connection points for the jump leads. Some modern cars have them, instead of connecting to the battery positive and a ground point.

kermit_rr · Sep 5, 2020

paul99 said:
Keep an eye open for dedicated connection points for the jump leads. Some modern cars have them, instead of connecting to the battery positive and a ground point.

On the beemer, the positive is under a red cap in the engine bay, battery is in the boot

Nodge68 · Sep 5, 2020

GrumpyGel said:
Am I safe to jump it, or is it likely to kill some electrics on the car?

It'll be fine. Just don't connect the L series battery to the BMW with the L series running, but start it once the two batteries are connected. Leave the L series running for 10 minutes, before attempting to start the BMW. Once the BMW is running, disconnect the negative jump lead at the BMW end, then the L series end. You can then remove the remaining positive jump lead, this time it doesn't matter which end you disconnect first, but as you'll be nearer the L series battery, you can disconnect that one first.

Don't attempt to use a charger to charge the BMW, while the battery is connected to the car, as it upsets the BCM, don't ask how I know.

guineafowl21 · Sep 5, 2020

The problem, as far as I know, is modern alternators.

Once you’ve jumped the dead car, and the two are still connected, the good car’s alternator is putting out full whack to charge the dead car’s battery. If you disconnect at this point, the good car’s electrical system will see a sudden drop in load, and will be unable to respond quickly enough. Result is a ‘load dump’ and consequent voltage spike in the good car.

This doesn’t happen to the dead car, because it still has the dead battery load.

Best advice I can find, and have never seen a problem with, is to shut off the good car before disconnecting leads.

GrumpyGel · Sep 5, 2020

Thanks for the advice everyone.

Bit of a mixed bag of answers, but it looks like I'll be OK to jump it from the Freelander. I'll portion out the repair bills for you

He lives in another part of town and the car has been here for months as there are major road works outside his so they are just using his girlfriend's car. I spoke to him yesterday morning about it and was going to put it on charge and leave it charging up till he got here today. However, yesterday's primary job was to fix the pull start on the strimmer/line trimmer/weed eater, which I did, but it still wouldn't start. Was only then I realised the fuel lines had dissolved, so had to get some replacements. Got those fixed and was feeling chuffed, so strimmed some of the bits needing attention before dinner. Then, last night while playing cards Mrs Grumpy says "weren't you supposed to charge up Michael's battery" - doh!

guineafowl21 · Sep 5, 2020

Colthebrummie said:
Life is so much easier with a lithium jump start pack. No shunting of vehicles for leads to reach, no unraveling of dirty cables, no accidental short circuits when clips touch together.

Col

Can you recommend a good one? I’ve never owned one, but have known many crap ones, usually Ring or Maypole. Typically, they sit there ‘just in case’, then when the owner goes to use them they fail, with an output below 7.5V, then the battery inside puffs up and they get thrown away.

GrumpyGel · Sep 6, 2020

So took the advice, including GF's to turn Freelander off before disconnecting cables and all good.

The BMW had been sitting for 4 months, the battery was completely dead, central locking would only work after connecting up to the Landie. It started immediately though. We took it for a 20-25 minute drive and it put a lot of power into the battery, but not enough to turn it over again after standing for 15 mins.

So we'll do it again in the week when it can be taken for a longer drive.

Thanks for the advice

Nodge68 · Sep 6, 2020

GrumpyGel said:
The BMW had been sitting for 4 months, the battery was completely dead

The battery will be scrap, as the Lead Acid battery is seriously damaged by being left in the fully discharged state. The fact it didn't start the car after a 20 minute charge is pointing that way too.

Buy a replacement and fit that.

Colthebrummie · Sep 6, 2020

guineafowl21 said:
Can you recommend a good one? I’ve never owned one, but have known many crap ones, usually Ring or Maypole. Typically, they sit there ‘just in case’, then when the owner goes to use them they fail, with an output below 7.5V, then the battery inside puffs up and they get thrown away.

The one I've got is a Tackllife I bought from Amazon for about £60. It will start 5 litre engines and has plenty of power to start 3 or 4 times with plenty of power left over. It also has a couple of USB sockets for charging phones etc. It has been a godsend. I've only had it for a couple of years so can't comment on its longevity.

Col

kermit_rr · Sep 6, 2020

Is it a start stop engine? Those batteries aren't cheap! You can have start stop disabled and put a normal battery in if you want to keep the cost down but either way the car will need to be told it has a new battery and how many AH it is

guineafowl21 · Sep 6, 2020

GrumpyGel said:
So took the advice, including GF's to turn Freelander off before disconnecting cables and all good.

The BMW had been sitting for 4 months, the battery was completely dead, central locking would only work after connecting up to the Landie. It started immediately though. We took it for a 20-25 minute drive and it put a lot of power into the battery, but not enough to turn it over again after standing for 15 mins.

So we'll do it again in the week when it can be taken for a longer drive.

Thanks for the advice

The battery is buggered. You might revive it somewhat with an intelligent charger in ‘recon’ mode, but the end is nigh. It’s also a lot of strain on the alternator, charging up a flat battery, so I would go for a bench charge rather than driving about.

Advice on jump starting

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