I bought my GPX 4500 second hand in 2010 (was new 2009) from a fellow detecting full time. My use has been 3-4 months/year except last year when it was not used at all.
I have always kept the battery charged, but considering its age, 3-4 years ago I did a deal on a dead battery and rebuilt it with new 18650 cells. This gave me a good spare, not having to worry when camped remote.
At the end of each season I charged both batteries and did a periodic check in between.
However, due to several other issues, the checking of the batteries was completely overlooked last year until yesterday, when starting to get ready for this winter decided was time to look at them.
Imagine my surprise when the control box showed both batteries (original 2009, 1-rebuilt ~2015) 8+ volts.
Put them on charge, both started blinking within 5 minutes and fully charged within 1/2 hour.
Have to wonder why the vehicle industry is still not using Lithium ion batteries. (other than hybrid models)
I think that when the price on lithium batteries suitable for the automotive industries come way down to affordable levels
then we may see them.
I've looked at the Deep Cycle LI batteries but for a cost of $2k plus put me off!!!
Last year I purchased a 800CCA lithium jump starter pack [$200] for the Tojo & was amazed for its size that it could easily crank over the chevvy.