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Battery polarity reversal.

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Anyone have an explanation for this, 12v lead acid battery, totaly discharched(lights left on MAYBE).


Recharged battery OK.


Ran down again, MAYBE something left on.


Tried recharging again, it quickly blew the fuse in the charger.


Invesigated further and tested battery with a battery dischage tester, found needle deflecting in wrong direction(yes I confirmed the correct connections) swapped connections around red to negative of battery and black to positive of battery and meter of discharge tester indicated 7 volts in the correct direction.


It appears the battery has swapped polarity !!!!!!


Battery failed discharge test immediately.


I am a lapsed electrical engineer from an industrial background and dont have a great deal of DC experience. Could this be a short within the battery?.


All testing carried out without any connection to the vehicle.



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Simple explanation. A lead/acid battery consists of several cells , each producing circa 2 volts and wired in series to give say 12 volts. With a used battery there will usually be one cell slightly stronger than the others, and if the battery is left connected to a load the stronger cell will attempt to charge the remaining cells in the reverse direction via the load once the other cells are fully discharged. This can even happen if the load consists of conducting moisture between the battery terminals.

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Took in seven the other day got £35 for them! Just oridinary fiesta type things. Prices up at the roof at the moment!


Good news! Battery worth in scrap nearly as much as a new one!


Can you get me some new batteries from your supplier then.. :cool2:

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Actually hunting around for a six volt, there are two places have come up worth a closer look, one is Blackheath Batteries, but haven't had a chance to get up there yet. On the net


Seriously considering an Optima for Katy, as she stands for long periods. Expensive but have a reputation or keeping a charge over lay ups. Anyone any experience of them?

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Back to some sense! I've been on the phone to Optima UK today. The batteries are in short supply at the moment as the British Army are taking everything. However I have found one six volt Optima. The battery is the RTS 2.1, it 's 50 amp hr but with a crank of 815 amps! Size wise it is tiny! 250 mm long x 200mm wide x 90mm high. The advice I got was very candid , including that though the battery will stand, a charge every 8 to 12 weeks is recommended to keep full performance. Considering the size of the lead acid that has come off and that it only cranks at 630 amps, the Optima looks good value. Cost quoted is £138 inc VAT and UK delivery. Considering the size alone, must be worth looking at for some armoured applications.

Edited by Tony B
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