Thread: Prevent damage from battery connected backwards

The six 30 cent diodes in a diode bridge in the alternator is the cheap insurance.

The $300 replacement cost for six 30 cent diodes is the problem.

Master Splinter,

Thanks for the reply.

Will educate myself further on this before going back and discussing with the chap who did the job.

I currently know and understand nothing at all about cars and engines.


Bill

The alternator diodes are often buried inside the casting of the alternator (so that they have some form of heatsinking), so to replace them you have to get the alternator out of the car, disassemble it, unsolder the diodes, replace them, solder in the new ones, put the alternator back together (using new bearings if required) and reinstall.

Then you've got the case where they are built into a voltage regulator circuit as well, so you are no longer replacing a few diodes, but an entire encapsulated electronics assembly, and often these are priced like they are made of gold for reasons that are only known to parts wholesalers.

Assuming that Landrovers use good old Lucas Electrics (aka the Prince of Darkness), high parts cost would not surprise me!

Here's a video of how an alternator is dissasembled.
How to repair a 3g alternator | 3g rebuild kit - YouTube

Thanks Master Splinter,

The guts of the issue for me is this.

Is there a simple sacrificial gadget that I can insert into the cars wiring system so that if I ever do this again my cheap simple sacrificial gadget dissappears with an appropriate puff of smoke and sizzling noise - leaving me in no doubt that I have been silly again - but my $300+ alternator is safe and well.

One solution is for battery manufacturers to make one post square and the other one round - then the terminals will never fit the wrong way round. Maybe I could file it down myself.

Off to talk to Landrover mechanic, and auto electrician and do some more googling.

Thanks for your help.

One post is bigger than the other but not square, there may be some sort of diode gate available but I'm sure you will be more careful next time anyway. Just be thankful the computer didn't go, then you could pop another zero on the end of your $300 (and then some) . You probably got off lightly

As Claw Hama said, the terminals are mechanically different, which is generally considered sufficient protection against reverse connection.

While a suitably rated diode would provide reverse polarity protection, the slight voltage drop it would create would possibly confuse the rest of the electronics in the system. Try some of the caravan and off-road stores, there might be something there for vehicles with two battery systems.

Originally Posted by Master Splinter

While a suitably rated diode would provide reverse polarity protection, the slight voltage drop it would create would possibly confuse the rest of the electronics in the system.

A suitably rated diode would leave you on the side of the road fairly quickly as if connected to allow the battery to supply power to the car, it would be preventing charge current from the alternator charging the battery. Sooner or later the battery will be flat and the car won't start. It would need to be a darn good diode to cope with 500A+ for a starter moter as well.

A fuse or fusible link won't solve the problem either, one of the first things we were taught about semiconductors during Electronics Engineering Degree course was that semiconductors protect fuses, rather than fuses protect semiconductors. One early demonstration of this for me in the 70's was a prototype 3 phase powered switchmode battery charger for railway loco battery packs, nominal output around 48V@400A. Someone setting it up for testing twiddled a pot just a bit too far and sent it into linear mode. Interesting to watch the tops of 100+ TO3 power transitors blast into space taking out the plasterboard ceiling, but all the fuses etc and the cabling survived unscathed.

Preventing charging. Damn. Hadn't thought of that!

You must have had it connected some time to burn out the diodes! Most EFI modules have either protection or diodes designed to burn out if connected backwards. If you need to jump from or to an EFI car leave them connected and running for about 10min before you disconnect. I could explain but it is lengthy - just trust me on this one.