Export thread

I thought I had a bad battery so I bought a new battery. I nearly cooked it. It started OK but quit after a few seconds of running. The voltage dropped quickly and the battery started smoking. Both cables were super hot. There's something shorting out. Where to start checking? I've never had anything happen like this on a car so I haven't a clue what is the problem. It doesn't seem like the starter solenoid is sticking.
There's no sparking when I first connected the battery but I got some sparks after I disconnected it. I didn't want to leave it connected after his happened fearing damaging the battery or frying any wires.

Sounds very much like either the battery is in backwards or the rectifier is wired up backwards or the rectifer is defective and pumping AC back into the battery

Battery was hooked up right. The rectifier has just a plug so it can't be wired up wrong Is there a way to test the rectifier?

If the rectifier is the culprit and enough current was flowing to make the battery smoke and the battery wires super hot, why didn't that rectifier plug melt?

I think you've got a dead short to ground, somewhere in the circuits that have BIG wires. I can only think of one--the starting circuit.

There was a suggestion in my other thread that the starter could be at fault. I'm just not all convinced since it still seems to work fine but I'm no expert. I have to figure how to test it myself.

cpurvis said:

If the rectifier is the culprit and enough current was flowing to make the battery smoke and the battery wires super hot, why didn't that rectifier plug melt?

I think you've got a dead short to ground, somewhere in the circuits that have BIG wires. I can only think of one--the starting circuit.

Click to expand...

If he has a dead short to ground then
1) there would be a big spark whenever the battery is connected or disconnected
2) the battery would go flat when the engine is not running
3) the wire on the rectifier is big enough to take the maximum current the alternator can produce + 25%

Pumping pulsed DC or AC into a battery will kill it in no time flat as will pumping in too high a voltage
If the rectifier is pushing 25 V AC ( or more ) into the battery the wires will get very hot and the plates will gas badly

Not discounting that there may be a short in the solenoid.
Or a short in the power wire to the solenoid from the battery
All are easy tested.
Disconnect the + wire at the battery and at the solenoid
Run a light jumper from the battery + to the solenoid the start the mower using a heavy jumper from the battery directly to the + on the starter, remember you need 12V to open the fuel solenoid
or
Disconnect the stator wires and run the mower

I don't think its overcharging since the voltage at the battery dropped, not increased. I did get gassing when the problem occurred the first time. I was mowing for some time after I started it, maybe 5 or 10 minutes ans then it just died. I thought it was out of gas but I had just filled it. It didn't even click when I tried restarting it. I didn't check the cables then. I hooked up the battery charger without disconnecting the cables to just jump it and it just pegged. I thought then it was a shorted battery but found otherwise this morning with a new battery.
I'll try your test method Bert next time I have a chance, maybe on Monday. I don't do any work Sundays.

cpurvis said:

If the rectifier is the culprit and enough current was flowing to make the battery smoke and the battery wires super hot, why didn't that rectifier plug melt?

I think you've got a dead short to ground, somewhere in the circuits that have BIG wires. I can only think of one--the starting circuit.

Click to expand...

Because the plug is designed to carry the full load of the alternator's output.
The alternator is not putting out any more amps than it would be so there is nothing to make the plug hot.
The plug can take 25 amps but the alternator only puts out 15 so well withing the designed capacity of the plug and for that mater the fuse.

Heat is generated by a resistance such as corrosion ( all metal oxides are resistive ) or too much current for the wire.
Neither is happening at the alternator plug, so nothing to make it hot.

25 amps is not enough to heat up battery cables or make the battery smoke.

But a bad rectifier could reverse the charging polarity and heat up things fast. That's a easy check by simply unplugging and see what happens then. That's my next plan.

cpurvis said:

25 amps is not enough to heat up battery cables or make the battery smoke.

Click to expand...

25A AC is more than enough to melt wires if connected to a battery.
A 10 Amp ( 60 Watt ) dynamo can happily melt the wires on old Pommie bikes and generate enough hydrogen to cause the battery to explode.
This was a bigger problem with the old hard rubber battery cases which litterally did explode while the latter plastic batteries just pop the top off .
See it dozens of times.
And I have a spot welder that melts steel while only drawing 5 amps.
It ain't the amps and I can not understand why you are fixated on Amps.
It is the resistance that creates the heating .
You can melt wires with a single D cell battery, if it does not explode first.

I'm "fixated" on amps because with the very low resistance of large wires, you have to put LOTS of amps through them to make them hot. Those wires don't know or care whether you're asking them to carry DC or AC current--I have yet to see a wire labeled "For DC only" or "For AC only." All they know is whether you are exceeding the amperage capacity for which they were designed. If you exceed it, they get hot.

cpurvis said:

I'm "fixated" on amps because with the very low resistance of large wires, you have to put LOTS of amps through them to make them hot. Those wires don't know or care whether you're asking them to carry DC or AC current--I have yet to see a wire labeled "For DC only" or "For AC only." All they know is whether you are exceeding the amperage capacity for which they were designed. If you exceed it, they get hot.

Click to expand...

Quite correct but the wire connected to a big resistor will get hot and if you are pumping AC into a battery, it is a big resistor.
The wies running to your room heater are quite capable of carrying the amps the heater draws, but they get quite hot and the heat starts from the heater end.

If the amperage rating of a wire is not exceeded, such as is the case in the house/heater example, the wire may get warm--not hot.

In Bleach's lawnmower's case, he said the battery wires got HOT. That will not occur unless they're overloaded, such as a dead short.

cpurvis said:

If the amperage rating of a wire is not exceeded, such as is the case in the house/heater example, the wire may get warm--not hot.

In Bleach's lawnmower's case, he said the battery wires got HOT. That will not occur unless they're overloaded, such as a dead short.

Click to expand...

First of all, I can see where you are coming from.
Second, one man's warm is another man's hot.
Third the main wires are rated at 100A continious .
The only thing that can supply more than 100 A is the battery
So by following your over current load logic, it has to be a short in the main power wires because any other wire passing enough amps to heat the main power wires would melt or blow the fuse.

But the fuse does not blow and problem only happens when the mower is running.

There is no part of the main power supply that is in circuit engine running and out of circuit engine off.
The only time you get a heavy load is when the solenoid is energised and if the solenoid was shorting to ground after being de-energised, ( engine running / starter off ) it would stay short, ignition or or ignition off.
Electrically the only difference mower running to mower not running is the secondary power supply coming from the alternator.
Thus if the problem only happens when the engine is running the most logical source of the problem will be the charging system.
Now batteries get hot when one of 2 things happen
1) connected to too high a voltage ( 16V +)
2) connected to a reverse flow of current (Wired backwards or getting AC )

If the battery plates are getting hot, the battery terminals will get hot .
If the battery terminals are hot then then heat will also travel back through the terminals and into the wires so they will get hot.

As mentioned earlier this was very common in the 30's to the 60's with Lucas dynamos fitted to motorcycles that would reverse polarize when left not in use for a long time, flatten the battery and then boil it or explode it when the bike was running by pumping current backwards into the battery. And we are talking 5A to 10A on a 6V system or 3A to 6A on a 12 V system.

OK I figured it out this evening. It turned out it was a very easy fix and it turned out I have to admit I am a total idiot. The terminal where the positive cable attaches to the starter was shorting out on the exhaust pipe. It was obscured by the rubber cover on the terminal. It had been attached that way for quite some time and I didn't even think to look there. It may have only been only time before the cap wore through to expose the terminal and I may have hit something to cause it to shift just enough to cause a short.
When I hooked up the battery again it sparked and got hot. I had a feeling to check the cables again so traced the cable and sure enough that's when I found it. I pulled the exhaust pipe down away from the the starter hooked the battery to start it and this time no sparks and now hot cables.
I think my prayers to St. Anthony were heard! Thank you St. Anthony for helping me find this simple problem.
Now may I have my serving of crow too?

That is good to hear
looks like it is cpurvis 1 bert 0 at full time

Don't sweat it, Bert! The real score is now probably more like Bert 25 cpurv 1.

Either way you guys are awesome!