Alternator damage

[WindermereColvic]

Hi, every now and again for various reasons i have had to connect the battery terminals (engine bat) to another battery carried as a spare to start 'er up. In the past i have then (carefully, but as quickly as possible, say 5 seconds) swapped the terminals back to the other, dead, battery. However the more i find out about these things the more i think i may damage the alternator, so my question is this, should i abandon the practice all together and use jump leads, or am i ok having a break in the circuit for a few seconds. oh and the engine is a yanmar yse 8 with original alternator.
Oh and when i say terminal, i actually mean the ring on the end of the wire that goes over the battery post, not sure of the proper name, but we always refer to them as terminals.
Thank, Matt

 

[ShipsWoofy]

Your question shows a perfect need for a 1-2-both switch on board.

 

[VicS]

Do not disconnect the battery from an alternator while it is running. It will cause a voltage surge as all the electrons suddenly have nowhere to go except pile up in a massive heap. The voltage surge this causes can, and very possibly will, blow the diodes in the alternator in an instant.

Take great care with jump leads. The spark when you make the final connection, or more likely break the first connection, when you disconnect them can and very occasionally does, ignite the hydrogen gas that may have been given off from the battery. The chances of it happening may not be great but the consequences of a battery exploding in your face could be very serious indeed.

If you follow the instructions for using jump leads on say your car they give a sequence in which they should be connected and disconnected the most important part of which is that the last connection made and the first one broken is always well away from the battery.

1, 2, both, off, battery switches are mentioned. They have a poor reputation. Any one fitting one, and they are undeniably convenient things to use, should buy the best they possibly can.

When using such a switch it is important if the engine is running to always switch from one battery to the other via the "both" position so that the alternator is not disconnected from a battery while it is running.

 

[Zephyr362]

Not a good idea, some alternators will not regulate very well under small or no load so you could get a higher voltage out than you expect which could damage any electronics, as Dogwatch suggested put in a 1-2-both switch.
It's far easier to join them all together once the engine is running to charge them all up at once, and of course you can leave one battery as a dedicated engine start so you should not have a 'dead' battery.
Plastimo seems to be the standard switch at around £20

 

[sailorman]

A "Junp" Lead is cheaper then an alternator repair /forums/images/graemlins/blush.gif

 

[Tranona]

Better to fit a split system such as the BEP cluster which gives you separate switch for each battery bank and a parallel switch to use the house bank if the engine battery is low. Using a VSR to split the alternator charge would also be a good idea to reduce the risk of not charging the start battery first.

 

[oldharry]

As VicS has pointed out, diodes will blow in an instant, and disconnecting the battery while the alternator is running is one of the quickest and easiest ways to do it!

Two way switches MUST have 'make before break' contacts, for the same reason. The cheaper ones dont.

The only 'safe' way to do what you are doing is to arrange it so that you reconnect the low battery BEFORE you disconnect the spare. Jump starting the car you would just connect the fresh battery across the existing one until it starts - why not do the same here?

 

[Guest]

I've never understood why alternator manufacture's don't fit a pair of suitably rated back-to-back zener diodes across the terminals - a few pence invested like this would protect the diode pack from the back-EMF caused by disconnection.

 

[Pete7]

That might reduce the number of alternators they sell and they are in the market to sell alternators, not provide a service.

Pete

 

[Refueler]

Ok you have two possibilities - and your reply will decide what is most likely.

On your start panel - do you have an ignition / charge indicator light ? Normally a red light that comes on when power is switched on. The light then goes out once engine started and alternator charging .... This is the field energiser lead powering the light.

So question is - If you have this light, when you disconnect battery you've used for starting - is the red light still on just before you disconnect ? Or has it already gone out indicating charging has started ?

If the red ignition light is still on just as you disconnect - then charging has not started and alternator is not energised. Then no damage is done. But if light goes out when you start engine and then you disconnect - then you risk blowing the diodes in the alternator regulator.

Many engines need to be revved a touch after starting to get the light to go out - my engine needs a reasonable blip of roughly over 1500 rpm to put the light out.

The answer is that really you should never do what you do ... Only disconnect batterys when engine is stopped and alternator is not spinning.
As another says - your situation is a prime case for a 1-both-2-off switch where you can have the two batterys and avoid the break in connection. If not that - then use jump leads. It is advisable to have a remote way of connecting as battery box's may contain Hydrogen vapours but if vented H is lighter than air and should dissipate. Best is to connect to boat battery first and then emergency start battery second, unless its a power pack - then you have no choice but boat battery second.

If you think that alternator diodes may be damaged or your charging suspect. A simple Charge / Battery tester for less than a tenner in Halfords etc. will tell you. I use one and it diagnosed my poor regulator. Took alternator to service guy and he fixed and all was excellent.

 

[Refueler]

in fact I believe modern alternators not only having higher ouput voltage than older ones - now also have break protection. But as I was told by auto-lecky .... don't rely on it ! Still act as though it's not there and make before break.

The strange thing is alternator regulators themselves do break circuit when full charge reached - so I believe. So how can they do it - or do they have a way of bleeding of the power to stop damage ?

 

[VicS]

[ QUOTE ]
The strange thing is alternator regulators themselves do break circuit when full charge reached

[/ QUOTE ] They dont disconnect the alternator from the battery, they regulate the alternator output and they do that by controlling the field current!

You can if fact safely switch an alternator off switching off the field current.

I think you are right about alternator surge protection. It can be fitted but the usual advice is to not depend upon it. I have seen it suggested that it might work once but in so doing be damaged and so useless for a second time.