I've killed many a minnow in my engel cooler!

I don't use minnows anymore but used them a lot last year when my parents visited to fish. What I did to keep the water cool was add those frozen plastic containers that you typically use small "lunch containers". This can cool the water down somewhat without introducing chlorine.

Those batteries last a long time! i've run them 24/7 for four days and never missed a beat. mine also came with a 12v cigarette adapter for the boat. works great. you still need to change the water out periodically but those coolers do a great job!

I use a plug in aquarium bubbler. Works great but you need to change your water every couple days.

the oxygen is critical. but they dont require as much as most folks believe.. keeping the water cool is the key. some frozen water bottles with the cap ON, placed in the bait cooler will make all the difference..
on cool fall or winter days you can keep minnows alive for 3 or 4 days, with NO bubbler in a cooler.,,. stress and heat kills minnows faster than lack of additional oxygen being pumped into the water.
over oxygenation can be as bad as under oxygenation.,
the air you are pumping into the water contains heat from the ambient air temp, and may also get a little extra heat from the pumping mechanism inside the bubbler. the little rubber pump blatter is driven by the motor that sits right next to it, in an enclosed box, so it may increase the temp of the air entering the cooler a couple degrees.,., so in a nutshell, on a smoldering hot day, you could be pumping 100 plus degree F air into your bait cooler. your cooler is insulated.. it will hold in heat or cold. the more hot air u pump into the water the hotter the water will get...
if you take notice in a bait shop, they keep it nice and cold in the bait room. so the pumps are pumping cold air into the bait tanks, or they have refrigeration systems that cool the water.
so try you some frozen water bottles or the frozen gel packs, (put them in a zip lock bag as they occasionally leak)., and your minnows will appreciate it!

I have the Engel and a Frabill. The Engel bubbler will go several trips on a pair of Dollar Tree D cells. The Frabill bubbler will burn a set of batteries in less than a day. They look almost identical inside, but battery life is vastly different. If I put a quart of shrimp in my 13 qt. cooler, they will die quickly if I don't change out the water. But a pint will survive all day. I guess that's too much shrimp urine for that amount of water. I have the mesh bag that lets you catch shrimp or bait easily. Worth it. IslandJim

Interesting, our Magnum Bait Station® 13 will last 3-4 days on batteries. It comes with an adapter for cig lighter plug and I bought an ac adapter so we just plug it in overnight when camping

https://frabill.com/products/magnum-bait-stationr-13-999FRBBA213

POWER SOURCE: MULTIPLE OPTIONS:
TWO D BATTERIES
12V ADAPTER
110V ADAPTER PORT (110V ADAPTER NOT INCLUDED)

also,, most aerators will run on one battery, if you put it in the correct slot. if u get sufficient aeration with one battery, this will double the time,, but to your original question 2 D cells will run abt 3 days straight

Im a cheapo when I saw what my buddy paid for a minnow cooler. I bought a $10 cooler and 2 AA battery aerators for total of $22.....works perfect...change batteries every 2 or 3 trips...

I’ve been using one of these type pumps for many years in the cooler. I like the fact that this one has adjustable output whereas my old one does not. A 12 volt deer feeder battery or 12 volt accessory outlet adapter to run it.

https://www.amazon.com/dp/B000N9MI6W?ref_=cm_sw_r_cp_ud_dp_1KAAE9G3H9DG1RQE55VV

To the original post.....2 D batteries last all weekend for my Engle Cooler.