Just when I thought my battery problems were solved I hit a storing, which has broken my spirit and pocket.  My 12V Willards have been dying slowly to the point that I removed 4 (out of 8), now the last 4 are shot to the point that I have to cut my load as soon as the sun sets (off grid).  I scored a luck and bought 6 x 8V used Chinese batteries for R500 each.  Their voltages all checked out at 7.96/7.95 so i connected them up in series and got 47.6 odd volts.  Hooked up to the Axpert, and nothing happened, dead, nada.  Checked volts again, down to 4. something.  Checked each battery and found one is at -36,6V.  As soon as I break the circuit, that battery reads 7.8 V again.  I suspect that battery is shot, but these are very weird symptoms in my limited mind.

Any suggestions, other than trying to swap it with a good one, which might not be possible?

I do have a backup genie, but don't really want to run it just for a fridge and a few lights.  I have a 12V backup light circuit, so luckily I'm not completely in the dark.

Thanks in advance,

Peter

So sorry to hear about this Peter.

35 minutes ago, dax021 said:

As soon as I break the circuit, that battery reads 7.8 V again.

That is the reason I go so gatvol and bought a Hawkins tester that one day. Volts tests perfect till it smells a load.

That Batt is gone. Try and return them, it was a dud.

Time has arrived to go Pylon's I would guess.

And remember, if you still have all the batts, sell the as scrap lead, it is a few rands.

Howzit Jaco, thanks, i'm going to see the dude tomorrow and see if he has another one.  If not, i'm going to insist on getting a full refund.  Luckily he lives just around the corner and is actually a decent bloke.  He is workshop manager in a golf estate, so gets quite a few batteries.  

I dream of Pylon's every night, same as I dream of winning the Lotto, but unfortunately it aint gonna happen until i rob a bank, or meet a rich chick, which at my age is also very unlikely.

Why the negative voltage though, that confuses me?  I have about 10 dud batteries lying around.  Not sure if the scrap value will even pay for the fuel to get to the scrap yard.

1 hour ago, dax021 said:

Not sure if the scrap value will even pay for the fuel to get to the scrap yard.

+- maybe R9.20 per kg = 1 x 28g battery = R257.60 ... I think you will just just make it with one batt up for sale. 🙂 

-36.6v looked to me like a typo, or maybe the leads of the volt tester was wrong way around to give a negative.

Edited February 6, 20197 yr by Guest

2 hours ago, dax021 said:

Why the negative voltage

It's so dead that it reverse-biases easily. It is one of the dangers of a poor battery in a string of strong ones, that as the others discharge through it they actually reverse the voltage across it.

Another way to explain it, is that to reverse the polarity on a battery, you have to completely discharge it (during discharge current flows in the opposite direction compared to when it is charging) and then continue to pass current through it in the reverse direction after it is completely dead. If the other batteries in the string are still good... that is precisely what happens.

The moment you take it out of the circuit, it returns to being a bad battery (good voltage, no capacity).

Thanks Plonkster, that clears it up for me.  Still amazes me that an 8V battery can show -36V.  I have gone back to my old bank till i can sort this thing out.

1 hour ago, dax021 said:

Still amazes me that an 8V battery can show -36V

I thought I'd draw a picture. Why? Cause I wanted to be sure of my own claims 🙂

The top picture is normal operation. If you measure from the left to the right (assuming negative is on the left and positive on the right, and assuming conventional flow, clockwise), then the voltage adds up with each series battery and you get a nice 48V. The inverter, or whatever else you might have in the circuit, acts as a load and is indicated as a green resistor. It allows the full current to pass.

Now consider what happens if you have a bad battery that acts like a high impedance resistor (second picture, here indicated in red), and consider the electron flow in anti-clockwise fashion. Depending on how much current is passed by the green bit... you could get as much as -40V!

Edit: In fact, in the healthy picture, if you measured across the inverter in the same way as you measured across each battery (ie the probes of your DMM will end up inverted) you will see -48V, that is to say when you add all the voltages you're back at zero. If you apply that knowledge to the bad bank, I bet you the missing -4V (to get you from -36V to -40V) is sitting across the inverter 🙂.

Edited February 7, 20197 yr by plonkster

1 hour ago, dax021 said:

Still amazes me that an 8V battery can show -36V. 

And in an electric vehicle, a single 3.2 V cell that goes high resistance can end up with about -360 V, depending on the situation. That sort of voltage can do bad things, to the cell and its Battery Management Unit. 💀

Edited February 7, 20197 yr by Coulomb

Thanks All, I have learnt something.  The seller also replaced the faulty battery with 3 x 8V Trojans.  Haven't connected them up yet, tomorrow will tell.  I'm also kicking myself as when I reinstalled my old batteries this morning, I added my newly aquired battery balancer (thanks Chris) to see what transpires.  It is now almost 21h00 and my old bank is sitting on 50.1V.  Maybe I don't need new batteries after all.