Odyssey Extreme 25 - no-load discharge?

[SLR No.77]

Earlier this year I had to replace my Odyssey Extreme 25 due to it becoming completely unreliable, finally after the winter layup it was completely dead and needed jump starting, subsequently I replaced the battery with a new Odyssey (same spec). The old battery had been used with an Optimate and also had a CTEK Battery Smart fitted which connects to my iPhone to log the battery condition. Being aware this is a parasitic load which AGM batteries don't like, I've not used the Optimate nor the Battery Smart on the replacement Odyssey, just removed the FIA cut-out key after each drive, wired to disconnect everything. So far all good but the car's now tucked up for winter.

So, the old battery sat around for another month or so then on a whim I decided to investigate a way of trying to recover the charge, and connected the battery to a traditional car battery charger. It took a couple of days but gradually the battery started to show some life and did eventually charge fully, I then disconnected the charger and measured the voltage, 13.93 volts.

Since then I've periodically measured the voltage across the battery (no additional charging or use):

27/5/21   13.93v
29/5/21   12.78v 
31/5/21   12.73v
6/6/21     12.68v    
27/6/21   12.57v    
27/12/21 11.50v

Rough analysis is that the first couple of days resulted in a 0.2v drop, the first month was approximately a 1.4v drop, with the following 6 months showing a fall of only 1 volt, so clearly a reducing loss.

Is this normal for an Odyssey? It's certainly holding more charge than I expected but I'm unsure whether it would be reliable if used in the car. Also the traditional battery charger definitely had more impact than the Optimate!  
Any thoughts?

Stu.

[Jonathan Kay]

That drop over the first few days is more than I'd expect for any car battery in good nick.

...

But to test whether a battery is safe to use in a car I'd use its output under load. And the most convenient way of measuring that without specialist equipment is the minimum voltage seen during cranking. 

Apart from more being better there are some components on some Sevens that won't work properly at  < 10.5 V. This is one of the reasons for failed or poor starting even when the battery will turn the engine over at a reasonable speed.

So I'd suggest measuring that minimum voltage with each battery. 

...

I also suggest that everyone measures that periodically as an early marker of deterioration.

...

Happy Christmas

Jonathan

[Jonathan Kay]

"Also the traditional battery charger definitely had more impact than the Optimate!"

Yes. There are lots of reports of small AGM batteries being restored to useful function in that way, or by jumping and charging off the alternator.

Jonathan

[SLR No.77]

Thanks Jonathan.

I'll not be using the Caterham for a few months yet and it is of course on the new battery now, but before spring I think I'll reconnect the charger to this one for a few days and measure again to see whether it has any further positive effect ('scuse the pun). 

I'll then refit it and measure the cranking voltage to compare to the new battery which has never seen an Optimate. Compiling data on the two could be very interesting.

Stu.