Airdrik, in some wargames, where fighting battle is the core of the game itself (e.g. Battle Isle 2, BI2 for friend

), a unit can gain experience level
a) spending time at training camp
b) winning a battle against enemy
In BI2 you can gain as much as 10 stars (levels) AFAIR
You can loose experience, too.
Because units usually has a strenght of 10, you loose experience every time you repair a damaged unit:
a unit damaged to a strenght of 4 can be repaired to full strenght of ten, but will lose till 6 stars of experience (rookies must lern hard lesson again).
A badly damaged unit can cost too much to repair (resources are needed, as a percentage of building cost), because will loose any experience anyway: better rebuild it from scratch.
Loosing experience by time passing wasn't a problem, because of shorter timescale (months, not years).
If you like this level of details, you can start from some like SMAC model of experience, raising experience easily (e.g. every time you defeat a unit not already less than 50% of your strenght) but adding a way to loose it also:
1) by long time passing without a fight (e.g. ten turns = one experience level lost)
2) by repairing process (e.g. every time your unit is repaired from less than half strength it loss 1 experience level)
Instead of disbanding unit you should merge two equal type stacked on same square, with a resulting experience level mix of the two (e.g. a
6/10 point strength unit experience 2/5 merge with a
5/10 strength unit experience 5/5 resulting in a
10 point strenght (rounded to 10 limit) 2*0.6 + 5*0,5 = experience 4 (rounded up).
Repairing the two units separately must result in
2-(0.5 loss experience*4 repaired points)=0 experience
5-(0.5 loss experience*5 repaired points)=3 experience
(rounded up)
Don't forget to compare this by resource (shield) cost of repairing unit (it must be included, for balancing), as fraction of unit cost of building.
Well, I don't check carefully the math, please correct me if I'm wrong
Disclaimer: I'm not sure all this is needed in a CIV game, but if you
really want this level of detail in units management, it can be a good model to use.
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Admiral Naismith AKA mcostant