Can anyone explain what is happening to this battery?

I just changed a 14 Ah, 12 V lead acid gel battery, and I thought I would see how bad the old one had got.

I charged it at 13.8 V overnight, and then plotted the voltage with a 12 V 21 W bulb connected, and this is the graph:-

I connected the lamp at 8 am. I did have a loose connection a few minutes later and corrected that, which you can see at just after 8 am. I disconnected at around 4:40 pm.

Can anyone explain the jumps up and down in voltage? They are not due to the lamp being disconnected.

I know the battery is scrap, but I just wondered what make the voltage rise after the initial dip, and jump up and down at times.

There is an equivalent cct. of 6x 2V cells and each cell has another equivalent cct. or a large C, and a series ESR and leakage parallel R. ( with more Rs+C||Rp in parallel called the electric double-layer effect) These parallel RC circuits have a larger C value but also higher ESR value are responsible for the memory in all electrolytic capacitors and batteries and even the ones with so-called "no memory" The batteries with memory have lower ESR in the secondary charge which is why Ni_Cads were often deep discharged to restore more capacity.

Then there are defects called dendrites which act as parallel shorts or a change of particles or slivers of low resistance that drop the voltage 1.5V suddenly or so the voltage of the defective cell then fuse open from heat. Then later short out again and fuse open like a bi-metallic switch inside some thermal-chemical activity. The other changes in slope are similar defects but higher resistance close and open of inter-cellular defects in insulation.

All the drops are approximately 2V steps - as each cell is exhausted at different times during the discharge cycle.

rjenkinsgb said:

All the drops are approximately 2V steps - as each cell is exhausted at different times during the discharge cycle.

Click to expand...

I agree with that, and I have seen the 2 V steps before on 12 V and 24 V lead-acid batteries.

It was the sudden drops and the rises that I couldn't easily explain. I guess that dendrites forming and breaking down might well be the reason.

rjenkinsgb said:

All the drops are approximately 2V steps - as each cell is exhausted at different times during the discharge cycle.

Click to expand...

The cells are not getting exhausted ( low SoC)
One is internally shorting out as I thought I explained. Some spikes are > 4V but then return steps are < 2V.