I'm brand new at filling my own carts, and my first attempt has been an abject failure! My WOW blank carts and juice arrived, and I filled them according to V4L's vids. I was SO disappointed! They all taste burnt, harsh and disgusting. I used both the drip method (18 drops), and the condom method (15 drops). Did I use too much juice? Too little? Have I ruined the carts and/or my batts? I need expert help!
Since you mentioned your batts, you've used the carts on your batts to some extent. If you've used them on your PT, which has a higher voltage and also burns juice faster, it does seem like you're not filling the carts with enough juice.
A coolcart can hold 3 near full condom fills with spinning. If you can't get that much juice in one, you might want to try using a plastic syringe or use a safety pin away from the center piece just to put extra holes in the filler. You have to get the filler wet near the bottom/connector end. One reason that can prevent that from happening is the juice not spreading evenly and vaping on it with a PT - the juice has nowhere to go and collects in 'pockets'. To resolve that, a very thin metal (safety pin) will open up the filler so the juice can spread correctly in the carto.
I add a few extra holes to all my carts before filling them having had a hard time learning to refill and reading some fillers are packed too tight. The filler can't be removed on these without wrecking the entire cart. I haven't had a problem since opening the filler a little with a safety pin(a straight pin will also work as long as it's thin) and a better vape on near every cart. - Just be sure to not get too close to the metal center(atomizer) piece if you try this. If you move it off center or don't move the pin with a very light touch, it's a very thin wire connector the pin can break. Then you end up with a busted cart.
Also make sure you can get air through the center hole by blowing on the plastic cap and putting your hand on the connector end. If that airway is blocked, you have to open it before it filling the cart (easier on a dry cart) or the cart won't work no matter how much or how little juice you fill it with. It just means the filler has 'collasped' into the center and can happen when the juice isn't spreading evenly. You need a paper clip to fix that. Simply push the filler away (slowly) from the center to the sides of cart by putting the paper clip through the connector. Again, avoid the metal center piece. Hold the cart up to a light and you should be able to see the light on other end. That doesn't happen very often on the coolcarts but, it can and doesn't mean your carto is busted. By 'collasped filler' I mean it's over expanded which is why adding a couple extra and small but top to bottom extra vent holes with a safety pin, before adding any juice, is a good way to prevent that from happening.
I also put my carts half way on a clean, empty condom while dripping. Those extra holes make excess juice come out of the threaded end faster....so you don't waste any extra juice, if you're adding drops from the top. I let it sit for 10-15 minutes like that and from time to time, part of the condom ends up with juice in it. But, it's still fresh e-juice you can use when you top off and a lot less messy that way.
I could be wrong but it really seems like you have some carts with tightly packed filler. If you work with it, it's a plus because you got some carts that can hold more juice than normal/you won't have to refill them as often. The 'watery' thing is pulling juice from 'pockets' and the burnt taste is from the dry filler that isn't open enough to evenly absorb the juice and just burning every time you pull. I can't think of anything else that would cause both to happen but, it can give you a longer lasting cart once you open up the filler a little.