Yeah, they are dead :(

So I think I'm coming to finally accept that my batteries really are not in a good state of health. I tested them today with a 4A load from a headlight bulb after giving them a hard charge last night. Here is the graph of the results. Ignore the current line, I forgot to connect the load after the shunt and so it wasn't recorded.

Testing the battery with a 12v headlight bulb (4A)

As you can see, the battery is sitting at quite a high voltage still... over 13 volts. When the load is added you can see the voltage drop dramatically. Even if the voltage had started at 12.8v the voltage drop down to 12.65v the first time (16:00) and 12.6v the second time (19:30), is not a good sign.

Voltage under 4A load

I stumbled across this bit taken directly from a Rolls battery manual:

Corrective equalization needs to be performed if symptoms arise such as a constantly running generator (low capacity) or the battery bank will “not hold a charge”. These symptoms are typical of a heavily sulfated battery. If a battery is not being fully charged on a regular basis or limited equalization is performed using a generator (see Bulletin 611, Generators, Inverters and Equalization) sulfation will occur from “deficit” cycling. This undercharge condition can take months before it becomes a major and noticeable problem. This under charge condition is caused when batteries are deficit cycled. The bank receives less of a charge each cycle and starts to sulfate. Eventually the sulfate will cause a resistance to charge and a “false high voltage” reading will occur. The “false high voltage” is measured by the charge controller, which further lowers the charging current to maintain the voltage set point. This further increases the undercharge condition. This is one reason why specific gravity measurements are so important as “false high voltage” readings can be misleading. See Bulletin 609, Voltage, SG and State of Charge for information on how to correctly interpret voltage readings.

I'm thinking this sounds like what has happened to my batteries. Thinking about it, my main charging source, the Sterling B2B only runs when my engine is running. I think I do many trips in which the full charging cycle will not have completed. This means that the batteries may not have reached the absorption stage of the charging routine. That combined with the B2B getting a bit confused by the solar panel keeping the voltage high means that the charging has not been idea. So as described above I think the batteries have a "false high voltage" and that has been fooling both the chargers and myself that the batteries are charged when they are not. So they are now badly sulphated and hence the high internal resistance.

If only I had a battery monitor that was able to log this information over time ;)

Well the next set of batteries is now going to have that device. That is what I'm building here. So the next set of batteries should last much longer. I'm also going to make sure I get normal flooded lead acid batteries that I can get the caps off to top up / test the electrolyte. I had that type last time and they worked well. This time I  didn't realise these batteries I have are sealed 'maintenance free' ones until after they arrived. They were a good price so decided to stick with them.

Now that solar panels have come down considerably in price I'm planning to buy an additional 80W panel to go with my 64W one. I will check the roof space on the van, might even get two 80W panels. That should mean that the solar regulator will be able to do a better job keeping the batteries charge up.