The less than a year old batteries in our 2001 Diesel are going flat. I have to admit that we hadn't started it in 3 weeks due to emergencies. Got it started and drove it for 6 miles. Tested the voltage yesterday and the the left was 12.3v and the right 11.8v. How can I determine what is causing the drain? Appreciate the help.

 

I'd recommend going to YouTube and pull up videos that discuss the parasitic draw test. Are you sure that no headlights or interior lights were left on?

If the parasitic draw test doesn't find an electrical draw that may be causing the issue, might want to take those batteries back to the purchase point for testing. I bought a brand new battery recently and determined that it had a dead cell in it due to electrocyte leaking out from a small hole in the battery case after testing.

 

Remove both ground cables from the battery. Remove key from the ignition switch, close the doors, make sure the underhood light is not lit. Get a H7 halogen bulb (or similar), wire it so that one cable goes from bulb terminal to ground cable terminal, and the other goes to negative post of the battery. If there should be any draw, bulb should be lit. The brighter it is, the more there is draw. Then start pulling the fuses to locate the faulty circuit.

How long trips you usually drive, and what's the temperature on your location?

 

On Left Right
12/10 12.34v 11.8v
12/11 12.05v 12.09v
12/13 11.94v 12.04v Disconnected right battery cable
12/14 11.85v 12.06v
We are in the Olympia Washington area. Highs are in the 40's and lows in he low 30's.

 

Three weeks idle is not enough to drain a decent battery (set). I think lead acid batteries lose about +-1%/day.
Did you try isolating a current draw as suggested? The circuit causing the problem is usually pretty easy to find, what on that circuit maybe not so easy.
If one battery fails, or you replaced the pair independently of one another, mixing capacities and ages, a weak battery will continuously try to charge itself from the stronger one if that is possible. Charge them, check them, load test them independently. Schumacher makes a decent load tester, can buy at WalMart, Amazon, NAPA. Not expensive.
The seatbelt interlock module on the '97? up has been known to fail and just drain a battery slowly. It's under the center jump seat on the extended cab models. I had one fail on a '99 gasser, killed an otherwise good battery when I was out of town working for couple months.

 

After getting the truck started yesterday afternoon I drove it about 6 miles. After I parked it I disconnected both ground cables and check the voltages. The left was 12.49v and the right 12.7v. This morning I check the voltage and the left was 12.19v and the right 12.04v. More than 1% is less than 24 hours. Both batteries were replaced in April. Shouldn't be any draw down with the negative cables disconnected, right? So I guess I'll take them back to Costco and have them tested. Originals were purchased on 2/28/21 and the replacements on 4/13/23. So don't know how much is left on the warranty. Thank for the info. Very useful.

 

Lead-acid batteries have a surface charge effect which shows a higher voltage right after being charged and disconnected. A small voltage drop in the first 24 hrs is normal but if the voltage continues to drop that's an issue.

Driving 6 miles likely didn't fully charge the batteries, you might want to put them on a charger or drive a little longer. My old school way of testing the battery is letting the truck sit with the headlight on for 15-20 minutes and if it starts the battery is still serviceable.

 

I put the batteries on the charger for 2 hours yesterday. At 4 pm the left read 13.65v and the right 12.52v. The negative cables are not connected. At 10:30 am the left read 12.24v and the right 12.27v. The drop is more that 1% the way I see it. Did the parasitic test and on my meter it didn't show anything with the red lead in the mA slot. In the normal right slot it showed 2? So I am thinking at this point the batteries are ad and there is no parasitic drain. I wonder is the fuel heater is bad. Turning the key on and waiting for the "Wait to Start" light to go out might be draining the batteries?

 

Have the batteries load tested! IF they are fine then look for a drain issue. I think you can take your truck to different auto parts stores and they can check them for free.

 

After purchasing a better meter and not finding any parasite draws, I followed advice and took the battery to O’Reilly’s Auto Parts. They found that it needed more charging before it could be tested and asked me to come back in an hour. I called an hour later and they said it had tested fine and was OK. So I went to pick up the battery. They put it back on the tester to show me the results. It now showed it NEEDS RECHARGING. So the battery WAS bad. Took them both to Costco and they replaced them under warranty. Put them in yesterday and everything works like is supposed to. Both batteries show 12.62v constant for the last 8 hours.Thanks to everyone for your help. Don’t think I would have solved the problem without you all.