Battery discharging

The battery in my 2014 Dart went bad last summer and I bought a new one for it at Walmart. Everything was fine until one morning 3 weeks ago, I went to start it, and it barely turned over. Third attempt it started. Since then has started each time, but after sitting all night or for the day while I’m at work, it shows signs the battery is weak. It’s fine when warm or when it’s only been parked 4 or 5 hours.
I went to Walmart the other day and told them I was suspicious the battery was no good, but they tested it and it passed.
I checked the terminals and they look clean, and connections tight. Only thing I noted was evidence a mouse had visited the insulated battery cover, left some seeds and a few droppings behind, so I put a few drops of peppermint oil on the cover to hopefully repel him.
Is there any possibility the battery isn’t good despite what the stores tester showed? If so, how does one further test?
I don’t have a multimeter, working all day the next two days, then having foot surgery Friday. I’m not planning to try to drive for a week afterward, and betting it will be dead when I try to start it late next week. Beyond the battery I’m pretty limited what I can do now.
Any ideas would be appreciated, I’m thinking if it can’t be the battery I’m going to have to take it in to see if something is stuck on and discharging the battery when it’s parked.

Mice like to chew on wires so may be some low current short from damaged wires. Worth a good look all around.

Grab any sort of small 12 volt automotive light bulb. A test light works great if you have one. Remove a cable from the battery (either one) and somehow wire the light between the batt terminal and the cable you removed. If the bulb lights you have a draw.

At this point you can start pulling fuses one at a time, and putting them back, until you find one that causes the bulb to go out. When that happens you have at least found the circuit that has the draw.

Did the store use a digital tester? Those things have their place but they are for sure not the best way to test a battery, especially one that is having issues.

The battery could have a bad cell and when you driving it enough its proping the voltage up enough to start the car OK, after it sits a day or two that is no longer the case and it cranks slow. Low cranking voltage is very hard on electronics so try to track it down ASAP of course. You might go by an Interstate Battery Distributor and ask them to do a load test for you. If you can carry it in to one of them they can keep it, charge it, and properly test it to make %100 sure its good. We do that at no charge here, I am not sure about others.

A bad cell is very obvious, even with just a multimeter on it.

Not once did you mention that you jumped the battery to start it so I would go with a new starter

Huh? Just start throwing new parts at it?

I haven't had to jump it yet, but it barely turned over quick enough to finally start after several tries a few weeks ago. It has turned over at what seems to be a normal or near normal rate every time since then, though with a few exceptions when the starter seemed to lag for a second at first. It was 12° this morning and I had my jumper/air pump unit charged up and ready to go, but it did start thankfully.
I can tell the starter unit is not turning over normally often after the car has been sitting 9 or more hours in recent weeks. Plus when the battery was on its last legs last summer, I'd noticed the instrument panel does a strange thing when you open the door, and when you first turn the key. It rattles, and a faint white screen comes up momentarily in the screen where the odometer/MPG/MPH info can be displayed. It wasn't doing that after I replaced the battery until that morning it almost didn't start. And it doesn't do that when I go to start when the engine is warm or the car has been driven in the last few hours. That rattle in the instruments seems to be a telltale that the battery is weak.

Get it to someone(besides Walmart) that can load test the battery (a battery can have a bad cell that only fails under load), an alternator output test and a starter current draw test. Once all that has been tested and verified it is OK then a parasitic draw test is next(and a light bulb is NOT the correct way to do it) Good luck!!

I had a new battery that acted just like yours when tested it showed good I took it back and shook it good before the test and it showed bad brand new battery with a short in it.

We have a 16 Dart my wife drives every day. Whenever I drive it(about once a week) I notice the same thing. It's always done it. I don't think that is a sign of a battery problem. I'd like to hear from a tech though that's familiar with these cars to see if they all do that. I haven't checked on a Dart specific website to see if they mention it at all. When the dash "rattles" the gauge needles swing wildly for a second before they settle down to their stops.

If you are having surgery soon I'd drive the car so the battery is good and charged. When you park it, disconnect the battery. When you come back to it, see if it still starts hard. If it starts normally, then there is a drain. If the battery is dead or the car starts really hard, then I think you can say the battery is junk. By disconnecting the battery after you've driven it for awhile, it should still have full charge after a week.

You need to test it for ignition off draw. Like said in a previous response. A voltmeter or test lamp installed in between the negative cable and the negative battery post. The bulb or meter may show a draw for 30 minutes or so. Depending on radio and USB hub programming. If it doesn't shut down in that time frame it will kill the battery. My job encompasses these type repairs on prototypes at FCA . Sometimes they can be tough to track. If the CAN BUS stays awake the messaging will never shut down. Easy to tell if its the battery. Disconnect it for a few days. Reconnect it. Does it start? If yes you most likely have something pulling the battery down. It may take a scanner to locate the issue. You can try pulling fusses one at a time but it can be a long process. Start with the radio and USB. Most likely suspects. Make sure to trigger the door latch shut if you are testing with the door open.
Doug

You suggested to check for a drain..did he say he jumped the battery to start it?