The charge level seems to have levelled off.
Note the big jump after the third entry of the graph. This happended after I removed some electrolyte from cell #2 (about 50ml or so) and replaced it with fresh battery acid (30% H2SO4). I did this after discharging the cell completely, which means the plates were sulphated, i.e. the acid would be at its weakest. I picked cell #2 because I measured the cell voltages during discharge, just after the total voltage dropped to below 5.5V. Consistently, I found cell #2 would be at 1.8V, whereas cell #1 was at 1.86V. As the discharge continued, cell #2 would drop rapidly down to zero volts (and then go negative due the effect of the other cells), while cell #1 was still up around 1.8V. I recorded the voltage of each cell as above during discharge, as well as after full charge (but before disconnecting the charger).
This gives a very good indication of relative cell performance (and generated huge volumes of data, which I have not recorded here).
Since drawing 3A from these dinky cells may be giving skewed performance, I also used 2 50W bulbs in series to draw less current. This was to test if the AmpHours rating increased (as measured by time taken for the voltage to drop to below 5.5V). This drew about 2A - see the 18th reading on the graph, where the time jumps up, and the mAHours also increases, but not as markedly.
BTW the 16th point (449 seconds) was measured after the batteries had stood discharged for a few weeks - it seemed to do them good!
So now some estimates can be done to see what surface are of plates would be needed to produce a battery of say 10 AmpHours. Bearing in mind, of course, that this is using plain sheets of smooth lead. Based on the approx. 3A load for 449 secs (the highest reading obtained), it would need about 26x the surface area per plate, or 0.72m2, or 85cm by 85cm. Based on the 2A load and 400secs, we get 0.7m2, about the same. The 1.65A loading gives 0.63m2, or 80cm by 80cm.
Cell #1 still seems to perform streets ahead of the others - when #2 and #3 are flat, it it still putting out 1.8V. I really need to test the electrolyte somehow...
More to follow...
Please bear in mind acid is dangerous, and safety and disposal precautions should always be observed.
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