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battery polarity has reversed

Posted By: Cbeard

battery polarity has reversed - 06/07/19 3:30 pm

My 1968 A65L battery seems to have reversed polarity. Its always been used with pos. ground connection. What would make this happen? Thanks in advance for any reply.
Posted By: Gordo in Comox

Re: battery polarity has reversed - 06/07/19 4:16 pm

What makes you think that the battery has changed polarity?

Gordo
Posted By: Cbeard

Re: battery polarity has reversed - 06/07/19 4:24 pm

She wouldn't start. A friend that is better at electrics than me swapped the battery connections and she started 1st kick. The battery is old and won't hold a charge for long. About to install a new one. I checked out the wiring with a multimeter using instructions from britbike and it is a neg. ground system. The battery had enough voltage left to check its polarity and now it appears normal. Go figure.
Posted By: Cbeard

Re: battery polarity has reversed - 06/07/19 4:26 pm

Sorry I meant to say it is a positive ground.
Posted By: MarcB

Re: battery polarity has reversed - 06/07/19 5:29 pm

I don't think I understand what you're saying here:

Originally Posted by Cbeard
She wouldn't start. A friend that is better at electrics than me swapped the battery connections and she started 1st kick. The battery is old and won't hold a charge for long. About to install a new one. I checked out the wiring with a multimeter using instructions from britbike and it is a neg. ground system. The battery had enough voltage left to check its polarity and now it appears normal. Go figure.


Originally Posted by Cbeard
Sorry I meant to say it is a positive ground.


What does "it appears normal" mean? Normal on a BSA is positive ground. Sounds to me like you had the battery connected incorrectly to begin with.

Can you describe exactly what you found? Do you mean that you found the ground wire (you checked that it was really the ground, right?) connected to the battery's positive terminal, moved it to the negative and the bike started? How did you determine that it was the ground?

We need clearer details to help you out here.

Posted By: BSA_WM20

Re: battery polarity has reversed - 06/07/19 11:16 pm

Batteries do not , in fact a lead acid battery can not change polarity.
And as mark hs already stated, more accurate detailed information is needed.
You are there looking at the bike scratching your had.
W are here looking at a screen
Posted By: quinten

Re: battery polarity has reversed - 06/08/19 12:47 am

A battery can be "forced" to change its polarity

But it will-not/can-not happen on its own ... the change would have to be forced.

One way for this to happen is ...
A fullly flat battery
... will have both plates coated with lead sulfate ....PbSO4 ... in a water solution. .. the battery no longer has a polarity .
( just a + & - mark next to the terminals)

So if a charger is hooked up backwards ... the battery can charge backwards. ( the plus and minus labels will be backwards.)
( modern batteries are not formatted with neutral lead plates , able to switch polarities
without substantial penalty . )
So whatever charge you can force into a dead-battery , if done backwards , will likely only be a light charge




Posted By: BSA_WM20

Re: battery polarity has reversed - 06/08/19 3:13 am

Originally Posted by quinten
A battery can be "forced" to change its polarity

But it will-not/can-not happen on its own ... the change would have to be forced.

One way for this to happen is ...
A fullly flat battery
... will have both plates coated with lead sulfate ....PbSO4 ... in a water solution. .. the battery no longer has a polarity .
( just a + & - mark next to the terminals)

So if a charger is hooked up backwards ... the battery can charge backwards. ( the plus and minus labels will be backwards.)
( modern batteries are not formatted with neutral lead plates , able to switch polarities
without substantial penalty . )
So whatever charge you can force into a dead-battery , if done backwards , will likely only be a light charge


In theory it is possible, in practice it can not.
The actual positive & negative plates are made from different alloys and even without any paste will retain polarity.
Next the paste is never ever fully spent.
The surface just gets passivated by the sulphate.
This is how all those "miracle battery revivers" work.
They drastically over charge causing the plates to gas and knock the passivated surface off the plates and the battery is "good as new"
Well to be honest , if the sulpate blown off the plates that drops to the bottom does not end up shorting out the plates then you will have a goos as new battery of about 10% to 30% of the original capacity.
Most chargers will either blow the output transistors ( cheap ones ) or trip a safety relay ( expensive one ), if hooked up backwards.

Grid alloys are under perpetual development .
The last time I was associated with battery making was in the 80's but even then the alloy compositions changed almost with every batch.
We were experimenting with plating an overcoat over a Pb/Sb grid which looked very promising so the + & - plates could be cast from the same alloy then electroplated to turn them into + or - plates and the plating could be done simultaneously with the heat treatment.
The amount of over voltage required to force a battery into reverse polarity, even is dead flat would be quite substantial and way over what either the alternator or most automative charges could put out.
FWIW using a Pb/Sb grid allows more scrap lead to be used in the casting alloy which was the aim of the exercise, it also allowed Pb of a higher Cu level ( Cu is an impurity is grid alloys ) to be used and removing excess copper is a very expensive & chemically dirty process.
Posted By: Mark Z

Re: battery polarity has reversed - 06/09/19 12:41 pm

This is all very elucidating, but is in response to a confusing description of a problem and probably unrelated to what really happened.

I would think that hooking up a battery backwards would cause a dead short through the rectifier, and would either blow the fuse (if fused) or start melting wires... unless, that is, there wasn't enough poop left in the battery to generate heat.

Posted By: Allan Gill

Re: battery polarity has reversed - 06/09/19 5:26 pm

Originally Posted by Mark Z
This is all very elucidating, but is in response to a confusing description of a problem and probably unrelated to what really happened.

I would think that hooking up a battery backwards would cause a dead short through the rectifier, and would either blow the fuse (if fused) or start melting wires... unless, that is, there wasn't enough poop left in the battery to generate heat.



Did it accidentally many moons ago when running a bike on points. It ran although sounded a bit laboured, then when I revved the engine the fuse blew. If it was EI then who knows. I did fit a CDI unit to a bike which was supposed to be positive earth (for a negative earth bike) no mention of it in the bullet point instructions although had I followed the wiring diagram instead then it would have shown me. This nearly melted one of the wires to the box (low amperage wire too) was ok once set it to +VE
Posted By: Mitch

Re: battery polarity has reversed - 06/10/19 12:27 am

guessing the reason it started after the battery was switched was due to cleaning enough oxidation off the terminals to get a go. and ya, being reversed would have made a mess of the electrics if the battery had much ooph. but then, maybe it did and you just don't know it yet
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