Battery Polarity

[2414.ginger]

I took a battery that had been on trickle charge for probably about 10 days, out to put on an electric fence, however it would not run fencer. When I got back to workshop and put voltmeter on it registered -12.53, but with positive to negative terminal it was +12.53.
This to me indicates a reverse in polarity, is this possible, how?, why?, WTF.
Is it possible for me prove this is what has happened?
What can I do to change it back again? not that it matters particularly as it is only for fencer use.

Thanks Pete

p.s. I will take it back to fencer and see if it will run with conecters on wrong terminals.

[Wazza]

I took a battery that had been on trickle charge for probably about 10 days, out to put on an electric fence, however it would not run fencer. When I got back to workshop and put voltmeter on it registered -12.53, but with positive to negative terminal it was +12.53.
This to me indicates a reverse in polarity, is this possible, how?, why?, WTF.
p.s. I will take it back to fencer and see if it will run with connectors on wrong terminals.

Are you sure you have your test leads inserted into the multimeter correctly ,black to common .
Some electric fence energisers have a low voltage cutout ,I have mine running on a 100 amp deep cycle battery charged by an 80 watt solar panel with charge controller so the battery is always fully charged .

[jackpreacher]

Are electric fences reliant on polarity?

[Stock]

Are electric fences reliant on polarity?

Probably are Jack I think they have a large capacitor and timed circuitry to generate the heigh voltage low ampage pulse of electricity ....Now as to wheather the solar panel would work with this rain and flooding is a different story...............

[2414.ginger]

Yes Jack , well at least my fencers are reliant on the correct polarity,
Yes Wazza I do have the leads the right way round on voltmeter.
Anyway I took it back out today and it works fine with the leads put onto the wrong terminals, but wouldn't work with them the right way round.
This takes me back to my question, how did the battery reverse polarity,why, and can it go back to normal?
This certainly has got me puzzled.
Thanks guys, all the best.
Pete

[ianoz]

Probably are Jack I think they have a large capacitor and timed circuitry to generate the heigh voltage low ampage pulse of electricity ....Now as to wheather the solar panel would work with this rain and flooding is a different story...............

Sick of the rain already ?

[Wazza]

Just read and article that suggests that if you completely drain a battery and then accidentally reverse the connections and charge it you will reverse the polarity ,so maybe it can be done ,but a battery cannot change polarity all by itself .
General thought is that the battery wont last long or work properly after being reversed.

http://www.batterystuff.com/blog/bat...-polarity.html

[jackpreacher]

...methinks we need a chemist on this one. - it's all about the Ions.

[Bobthebuilder]

How interesting. I would have said this was impossible but thinking about it a bit more, it makes sense. I'll tell you all something else you may find interesting to do with lead acid batteries too. Many years ago I use to work in an electrical test and repair department for a large manufacturing company and we regularly received gear back for repair where the internal lead acid battery appeared to have given up. The somewhat unique physical size of them meant they were a bit tricky to get hold of after a while so we use to 'revive' them.

Generally the problem was that battery would not take a charge which was indicated by a low current reading when on charge even though the battery was completely flat. In these circumstances we use to connect a DC power supply to the battery with a big ammeter in series and wind the voltage up very carefully while monitoring the current. At some point, the much higher than normal charge voltage starts to overcome the reluctance of the battery to take the charge and the current will start to rise sharply. When this happens you need to swiftly turn the voltage back to 'control' the current. Now that the battery is taking the charge it can be left on charge for 12 hours or so at 13.8V in the normal way. We must have revived hundreds of lead acid batteries like this - they just needed a big kick up the arse. The main cause was thought to be the fact that most of the batteries were constantly on trickle charge inside the equipment and had never really worked until there was a power cut. This is not a problem you are likely to see on construction plant but none the less its an interesting fact I thought I would share. If any of you try this you need to be very careful and always cover the battery incase the case should split (it happened a couple of times to us). It's a great way to revive a stubborn battery but you need to be careful and don't take your eye of that ammeter while you are doing it!

[Muz]

Interesting points there. From my limited chemistry knowledge, I'd always been led to believe that the ionic discharge that provides the power for battery output is an optimal process on a one way flow. The by product which is sulphation, which cannot be reversed per se.

However on a fully discharged battery this leads to the electrolyte (sulphuric acid) being largy converted to water. If youve left a battery the wrong way round on a charger that can withstand this perhaps youve reversed this process to some degree

The WIKI on the subject is this


In the discharged state both the positive and negative plates become lead(II) sulfate (PbSO4) and the electrolyte loses much of its dissolved sulfuric acid and becomes primarily water. The discharge process is driven by the conduction of electrons from the negative plate back into the cell at the positive plate in the external circuit.
Negative plate reaction(Anode Reaction): Pb(s) + HSO−4(aq) → PbSO4(s) + H+(aq) + 2-e
Positive plate reaction(Cathode Reaction): PbO2(s) + HSO−4(aq) + 3H+(aq) + 2-e → PbSO4(s) + 2H2O(l)

The total reaction can be written:Pb(s) + PbO2(s) + 2H2SO4(aq) → 2PbSO4(s) + 2H2O(l)

The sum of the molecular weights of the reactants is 642.6, so theoretically a cell can produce two faradays of charge from 642.6 g of reactants, or 83.4 amp-hours per kg (or 13.9 amp-hours per kg for a 12-volt battery). At 2 volts per cell, this comes to 167 watt-hours per kg, but lead-acid batteries in fact give only 30 to 40 watt-hours per kg due to the weight of the water and other factors.
Charging


Fully Charged: Lead and Lead Oxide plates


In the charged state, each cell contains negative plates of elemental lead (Pb) and positive plates of lead(IV) oxide (PbO2) in an electrolyte of approximately 33.5% v/v (4.2 Molar) sulfuric acid (H2SO4).

The charging process is driven by the forcible removal of electrons from the positive plate and the forcible introduction of them to the negative plate by the charging source.

Negative plate reaction: PbSO4(s) + H+(aq) + 2-e → Pb(s) + HSO−4(aq)

Positive plate reaction: PbSO4(s) + 2H2O(l) → PbO2(s) + HSO−4(aq) + 3H+(aq) + 2-e


Overcharging with high charging voltages generates oxygen and hydrogen gas by electrolysis of water, which is lost to the cell. Periodic maintenance of lead acid batteries requires inspection of the electrolyte level and replacement of any water that has been lost.
Due to the freezing-point depression of the electrolyte, as the battery discharges and the concentration of sulfuric acid decreases, the electrolyte is more likely to freeze during winter weather.




this bit

Due to the freezing-point depression of the electrolyte, as the battery discharges and the concentration of sulfuric acid decreases, the electrolyte is more likely to freeze during winter weather.


is interesting too as it explains why older batteries fail faster in cold weather

[Bobthebuilder]

You're getting a bit carried away there aren't you!

[Muz]

You're getting a bit carried away there aren't you!

Oh I dunno .. I see I double posted though as my electrolysis reaction got blown to bits when I copy and pasted it .. so I had to rebuild it but didnt remove the wrong 'un

I'm pi$$ed off batteries only seem to last 5 years is what I'm really saying