ronnie_rotten Posted February 8, 2011 Share Posted February 8, 2011 i've been told by 3 people who i think should know that no damage would occur to the alternator in the time it would take to swap batteries while the engine is running. i've also spoken to some who assure me it will knacker the alternator as the charge isn't going anywhere. everyone agrees that having no battery attached for more than a couple of minutes would be a bad idea. anyone know for sure what the actual risks are and what science backs it up? Quote Link to comment Share on other sites More sharing options...
MrKev Posted February 8, 2011 Share Posted February 8, 2011 read this: http://en.wikipedia.org/wiki/Load_dump Quote Link to comment Share on other sites More sharing options...
ronnie_rotten Posted February 8, 2011 Author Share Posted February 8, 2011 errrrrr, ok. so that's the science bit covered! i'm reading that as being not such a great thing for modern engines with nasty little computer brains, but not too damaging to a clunky old 300tdi? or am i just not taking the hint that a 'load dump' has the same painful results on man and machine......? =@ Quote Link to comment Share on other sites More sharing options...
Bowie69 Posted February 8, 2011 Share Posted February 8, 2011 There's electronics inside the alternator, namely diodes which can go pop. Quote Link to comment Share on other sites More sharing options...
simonb Posted February 8, 2011 Share Posted February 8, 2011 No battery, alternator will not get a reference voltage, regulator will then wind up output voltage till either it or the diodes go pop. Why do you want/need to change the battery with the engine running? Quote Link to comment Share on other sites More sharing options...
Tanuki Posted February 8, 2011 Share Posted February 8, 2011 I certainly wouldn't recommend doing it: any electronics (ECUs, immobilizers, radios) that are receiving power run the risk of getting "spiked". There are electronics in places-you-do-not-expect: intermittent-wipe timer, the relay that cuts your heated-rear-window after it's been on for a while. The alternator itself will also be at-risk. If you really *must* do it, I'd suggest that you turn on some significant electrical loads (headlights, HRW, heater-blower) to help absorb the more traumatic of the electrical spikes. Quote Link to comment Share on other sites More sharing options...
Disco-Ron Posted February 8, 2011 Share Posted February 8, 2011 Can you not connect a slave battery whilst you're swapping the battery...... on a 300, the main power goes straight from the battery to the back of the starter, and the earth, to earth, hows about opening the bonnet, having another battery sat somewhere with jump leads on it, and then swapping the one under the seat, but to be honest, i'm puzzled as to why you would need to swap the battery with the engine running, unless you have some sort of radio system that you don;t have a code for or similar...?????????? Quote Link to comment Share on other sites More sharing options...
western Posted February 8, 2011 Share Posted February 8, 2011 Why do you need to swap battery ? why do you need to do the battery change with the engine running ? your alternator will hate you forever as will your wallet/piggybank Quote Link to comment Share on other sites More sharing options...
Litch Posted February 8, 2011 Share Posted February 8, 2011 I also am dying to know why you want to carry out the battery change with the engine running, to what purpose? An FIA battery master-switch has a double pair of contacts on the rear to ensure that while the engine is spinning down and the battery is isolated from the alternator there is a ballast resistor in circuit to take the momentary discharge from the alternator. That is only for a second or two, I certainly wouldn't want to risk my alternator for a deliberate act like removing one battery and replacing it with another. Quote Link to comment Share on other sites More sharing options...
Smego Posted February 10, 2011 Share Posted February 10, 2011 If it is just to stop the radio code getting lost, just connect a simple 9V pp3 to the car for a few seconds while you change the battery, will give enough power to protect the codes, old trick. Quote Link to comment Share on other sites More sharing options...
ronnie_rotten Posted February 10, 2011 Author Share Posted February 10, 2011 ok ok ok! i wont do it i promise! thanks for the info, all i really helps to have science to back up advise. as for why? well i'm currently off and about the uk with the missus and baby visiting old mates in our old merc camper and the landy. the van's got a split charge system but all our batteries for power are past their best. half the time i end up rotating the landy battery with the one we use for tv, lights etc. a few times the battery i want to switch to doesn't have the umph to start the landy, hence i wondered if i could switch on the go. you see? the whole problem would be avoided by buying new batteries, but that'll have to wait til i start back at work again. until then it'll have to be a 20 minute top up with jump leads before the switch. ho hum. keeps me busy i suppose..... Quote Link to comment Share on other sites More sharing options...
Disco-Ron Posted February 10, 2011 Share Posted February 10, 2011 why not swap the battery and then use jump leads off the good one? Easier than leavin the leads on for 20mins. Quote Link to comment Share on other sites More sharing options...
ronnie_rotten Posted February 10, 2011 Author Share Posted February 10, 2011 that has worked sometimes, but as i say these batteries are well past theiir best. we were in powys during the cold snap and my mate recorded minus 21 degrees! one of the batteries froze solid, which i imagine didn't do it any favours. Quote Link to comment Share on other sites More sharing options...
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