Very very very bad idea. Modern cars use the battery as a smoothing capacitor to take out voltage spikes etc. When you remove it the alternator will be told to go to full as there is no charge in battery, so it will, and will get hot, normally the battery starts to fill up and the alternator is signaled to back down, but with no battery there it never will be so normally it blows its diode pack

It also voids your warranty and the BCU records the event so I hope you now don't ever need to go to Ford to claim on any electrical warranty. It also mucks up your battery age and charge calculations. The resulting voltage and current spike can blow anything connected at the time

Its expressly mentioned and forbidden in the Ford owners manual and even Haynes says its a bad idea. I've seen a lot of expensive damage done this way and you are really really lucky your cars are still running.

On cars since about 2007 its been a bad idea to even disconnect the battery when the engine is dead due to ECU memories and that many systems never actually shut down. Again the owners manual says not to do it unless changing battery or working on something electrical and there are procedures to go through to re-establish the battery charge calculations etc

Very very very bad idea. Modern cars use the battery as a smoothing capacitor to take out voltage spikes etc. When you remove it the alternator will be told to go to full as there is no charge in battery, so it will, and will get hot, normally the battery starts to fill up and the alternator is signaled to back down, but with no battery there it never will be so normally it blows its diode pack

It also voids your warranty and the BCU records the event so I hope you now don't ever need to go to Ford to claim on any electrical warranty. It also mucks up your battery age and charge calculations. The resulting voltage and current spike can blow anything connected at the time

Its expressly mentioned and forbidden in the Ford owners manual and even Haynes says its a bad idea. I've seen a lot of expensive damage done this way and you are really really lucky your cars are still running.

On cars since about 2007 its been a bad idea to even disconnect the battery when the engine is dead due to ECU memories and that many systems never actually shut down. Again the owners manual says not to do it unless changing battery or working on something electrical and there are procedures to go through to re-establish the battery charge calculations etc

are you talking about disconnecting battery when engine is running?

I take it its safe enough to use the booster packs on run down battery's in emergencies?

Whoops, yes, answer to someone else somehow posted to forum rather than PM, sorry guys

Booster pack fully safe as long as earth is last connected and first disconnected and preferably connect earth to body work or earth point and NOT the earth on the battery

Very very very bad idea. Modern cars use the battery as a smoothing capacitor to take out voltage spikes etc. When you remove it the alternator will be told to go to full as there is no charge in battery, so it will, and will get hot, normally the battery starts to fill up and the alternator is signaled to back down, but with no battery there it never will be so normally it blows its diode pack

It also voids your warranty and the BCU records the event so I hope you now don't ever need to go to Ford to claim on any electrical warranty. It also mucks up your battery age and charge calculations. The resulting voltage and current spike can blow anything connected at the time

Its expressly mentioned and forbidden in the Ford owners manual and even Haynes says its a bad idea. I've seen a lot of expensive damage done this way and you are really really lucky your cars are still running.

On cars since about 2007 its been a bad idea to even disconnect the battery when the engine is dead due to ECU memories and that many systems never actually shut down. Again the owners manual says not to do it unless changing battery or working on something electrical and there are procedures to go through to re-establish the battery charge calculations etc

Can we just check something ? ! ? Are you saying that if you run down your battery absolutely flat the alternator can then can damage the car`s electrics ?

If you`re not saying that and it was just an error I think we should remove all references to this answer because it`ll be worrying people, it certainly worried me when I saw it !

If you are saying that then that is the most ridiculous thing I`ve ever read. Who are these idiots designing these cars......

No I'm not saying that at all, running your battery absolutely flat will only irreversibly damage your battery itself. It wont harm the car other than to upset the battery age and condition algorithms, and most times the battery will recover once its charged (albeit with less capacity due to sulphation)

Mail was a reply to someone who wanted to pull their battery while engine was running !!!!!

Having said that on the Mk3 Focus, there are caveats about when you should disconnect a battery. Basically your owners handbook says don't do it unless changing the battery or its needed for maintenance purposes. On the Mk3 there is a ECU attached between the -ve terminal and the earth point that measure all charge flow in and out and computes the battery life, health and state of charge, disconnecting the battery drops all those calculations and means the car makes wrong assumptions until its learnt them again. That's why you always connect the boosters to the body earth point that ford so nicely provide, so the ECU can measure the charge changes you make

The handbook says its fine to charge the battery and boost it in car

No I'm not saying that at all, running your battery absolutely flat will only irreversibly damage your battery itself. It wont harm the car other than to upset the battery age and condition algorithms, and most times the battery will recover once its charged (albeit with less capacity due to sulphation)

Mail was a reply to someone who wanted to pull their battery while engine was running !!!!!

Having said that on the Mk3 Focus, there are caveats about when you should disconnect a battery. Basically your owners handbook says don't do it unless changing the battery or its needed for maintenance purposes. On the Mk3 there is a ECU attached between the -ve terminal and the earth point that measure all charge flow in and out and computes the battery life, health and state of charge, disconnecting the battery drops all those calculations and means the car makes wrong assumptions until its learnt them again. That's why you always connect the boosters to the body earth point that ford so nicely provide, so the ECU can measure the charge changes you make

The handbook says its fine to charge the battery and boost it in car

Good to have cleared that up !