Practically Classics Forum

Had an interesting problem yesterday, have a Renault Clio of W reg vintage which has sat on my garden for close on 12months which is going to a new home. Problem 1 the battery won't hold a charge and is scrap, 2 jumping it with my good battery just wasn't enough for it to start. Even if I disconnected the duff battery.

I even got out a spare battery which I took of my car 2 years ago because it wouldn't turn over my Jag in the winter, still wouldn't turn the clio over parrelleled to the other battery.

Then I remebered back into my youth when the mechanic who looked after our fleet of vans at work used to use 2 batteries in series breath life into reluctant starters on cold wintery mornings.

Managed to connect two batteries in series which gave me close on 22volts, I had drained some of the life out of the good one, any way jump leads on and a turn of the ignition key and viola, definitely whizzed the starter round and the engine burst into life and a quick disconnect of the batteries and it kept running.

So the reason for the post not knowing a whole lot about ecu's and thinking that alternators diode packs have a peak inverse voltage of about 30v any thoughts of on my cowboy attitude towards car electric's, is this a good idea!

One of those things where no-one would advise that you do it but plenty of us would give it a quick go when stuck.

Main thing is to be in a position to disconnect them batteries damn quick as soon as the engine starts or you see any smoke. Best done as a 2 man job with yourself at the battery end and your assistant on the starter. If anything does go wrong then it will be you getting injured and your "daft idea".

Not that I'm advising you should ever try it. Nope.

In my youth, people used to convert 6 volt cars to 12 volts, and it was normal to retain the original 6 volt starter, which would whizz the engine over at a good rate. These days, however, you risk subjecting electronic components to over voltage stress, either due to having 24 volts, or losing the smoothing effect of the battery when you disconnect the pair in order to re-connect the single one. You got away with it this time, but I wouldn't make a habit of it with any vehicle with an alternator or electronic ignition.

Got to be honest and say that this isn't my normal opus operandi and made even worse because I am an electrical engineer and very safety aware, sic.

I am aware that back in my halcion days I had a mate with an Escort Rally car which utilised 24v starting and 12v running but implemented relays solenoids etc etc which I obviously didn't have on Saturday.

Was banking on the ecu power supply being tolerant enough to manage the input, switched mode power supplies being pretty forgiving on input over voltage but i didn't know if 1.5 to 2 times would be too much. Then reasoned that car electric aren't the cleanest things going and can be badly regulated so guess ecu are well protected.

So that's the reasoning applied before doing the deed and it did suprise me that after so long idle the engine exploded into life, I was only interested in driving it off the garden and onto the road for pickup.

For info: according to the 'net' ecu's will handle between 25 and 36 v across its terminals for about period of time and infact handle the same reverse voltage for a period of time, it's in the design spec, we are talking about 5 mins, my guess is the power unit can't disapate the power for any longer.

Minimum voltage about 6 volts, but then again a normal automative relay stopped working about 2 volts ago

The only other comment is don't be surprised if the interior light blows, assuming every thing else is off, might not do the radio ice much good either.

Wont it blow the alternator?

Bandylandy wrote:Wont it blow the alternator?

Not unless you somehow manage to reverse the polarity of the supply during the operation.