Lead acid batteries page 2
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4 August 2006

Hmm, the same cell cell shorted itself out again (voltage drops to zero almost immediately charging voltage is removed). I unwrapped it again, and it seems the glass fibre mat is not a good separator. It had already gone mushy, and came apart in places under finger pressure while unrolling the lead. So next attempt is some curtain liner, which looks like nylon, which has a thickish and dense weave. Its much more pleasant to work with than glass fibre!Two layers were put in between the plates, and they were rolled up again, and back into the electrolyte (filtered through coffee filter paper again).

I took the opportunity to cut a corner off each plate, and looked at them under the microscope. The spongy texture is clearly visible on both the negative and positive. There were also long clear crystals on the surface of the negative plate. Perhaps sulphur, or sulphate? Amazingly, I could capture an image by holding my phonecam to the eyepiece of the microscope - see below for the negative and positive plates respectively, at about 12 000 X magnification.


 
And the same under a brighter (but more yellow) light:
 
 And here are the long "crystals" - on second thoughts I think they are actually glass fibres

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After the first charge, the voltage stayed above 5.5V for 314s - looking much better! One interesting thing was that in the two cells that had been left in a charged state for a day or so, a deep red clear pigment came off the positive plate for the first few minutes of charging.

 

5 August 2006

The nylon(?) separator seems to be working really well - the re-made cell is catching up. When discharged,the two plates are very similar colours - dark grey. When charged, the positive plate goes rust coloured, and the negative gets a silvery sheen. In one cell (cell 1), the small exposed part of the negative plate does not go dark grey (although some patches of it do). The silvery sheen on this plate is probably crystals of sulphur or sulphate.

A hot water bath during discharge doubles the shorted out current (eg. from 40 to 80mA) within a minute or so.

 

  6 August 2006

One of the other cells started to misbehave (voltage changed when moved around) , so I unwrapped and rerolled the remaining two cells using the nylon(?) separator. The 'leg' of the positive plate of cell 2 was weak, so I replaced the plate with a fresh sheet of lead. While busy, I couldn't resist jabbing holes in both sides of the plates with a clean compass point, and also scratching crossed diagonal lines into both sides. Hopefully this will increase the initial surface area, and allow the sponge-forming to penetrate more deeply.

The new positive plate is quite a different colour to the other positive plates - here are photos (left) of it fully discharged (after the first charge/discharge cycle), and then when it is charged (right).

And here is a graph of the latest test results - now getting just over six minutes of motor running time.

13 August 2006

So by now have got up to 10 minutes (600s) running time on the motor. At an estimated 750mA, this is 0.13 AmpHours. Ten minutes is too long to watch a motor spinning, so I switched to a 50W 12V light bulb, which ran for at most 120s drawing about 2.85A (before the voltage dropped below 5.5V), which makes 0.10 AmpHours. This suggests better ratings at lower current, and to be fair, 2.8A is probably a bit much to ask from these small test cells. It would be ideal to set up a test at 100mA, which would need a current limiting circuit and electronic timer. Any collaborators welcome!

The test times have hit a plateau, so in desperation I am trying to reverse charge cells 2 and 3. This will hopefully change to shapes of the surface of the plates, exposing more surface area. Using the battery charger to reverse charge all three cells results in the ex-positive plate being blasted from inside (flakes of PbO2 start to fall off), so I used a lower power DC supply set on a nominal 4.5V to charge just the two cells. Cell #1 is streets ahead - it can run a batch of LEDs overnight on its own and still deliver 1.8V+ in the morning.

The performance of the cells at high current is highly temperature dependant eg. 17 seconds cold, 112 seconds in a hot water bath.

28 August 2006

There doesn't seem to be much change with repeated charging now - I get just over 3mins at about 2.85A (50W bulb), making about 0.15 AmpHours.

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Please bear in mind acid is dangerous, and safety and disposal precautions should always be observed.