So if it tastes bad, and my battery is still good - it can hold a charge, and is well charged. Then I know my atomizer needs to be cleaned or replaced.
What happens when a cartridge gets stale if there is no change in taste? Less vapor?
Less vapour normally indicates a lack of power: the contacts need cleaning or adjusting or the battery is going flat but has not yet reached it's cut-off.
By "cartridge" I suspect you mean a cartomiser - it has both the liquid and the atomiser in one unit. These don't normally go stale in time frame that we use them, though if you have a badly burnt atomiser it can contaminate the taste because it leeches into the filler. refilling doesn't always get rid of the bad taste even if you leave it a while in which case you can try boiling it. (nothing else to lose).
You can also get poor vapour if the atomiser is burnt and has a layer of carbon over it. They still work, but seem to need more time to get up to the proper temperature. On an automatic battery this can be a real problem since every drag is cold, vapourless, and tasteless. Boiling cannot remove a carbon layer over a coil, but it can still help with the flavour. I read that 6% hydrogen peroxide can be used to clean a coil but I tried it and had no luck. You see, I've had these problems with cartomisers running heavy liquids myself.
If you actually have a cartomiser with filler or the soft ceramic these can not be disassembled. Just boil the whole thing and make sure it is dry before re-filling. Any silicon rubber parts (eg soft ceramic cartomisers) will not be damaged by boiling. If you have some kind of tank cartomiser or a separate tank/atomiser system then remove the atomiser element and boild it on it's own.
I started out with cartomisers with filler and soft ceramic cartomisers but it looked like an expensive proposition if they need replacing every few days. I changed to a CE4 tank cartomiser (1.6ml) for may main vape and bottom coil clearomisers (1ml) for testing flavours and for stuff I only use occassionally. Using these has got around most of my problems because you can see when they are running low and just fill them up or switch them out. Cartomisers with filler and soft ceramic cartomisers only hold a tiny amount of juice, and you are more inclined to use them until they taste bad at which time it's too late - the damage is done.
I liked the CE4 style clearomiser so much I've ordered two different ones from another supplier to find the best one. There seem to be a whole swag of options out there. Try some as you might find as I did that other options work better and are more satisfying.
Chris.