Battery discharge indicator question

Typically I charge my batteries (6mos old 31s) after every drive. I have noticed something that hopefully someone here can help me understand.

I just drove 3.6 miles and the indicator was down to showing 84%. I got home and checked the cables (one was hot, looks like a bad connection) and test voltage (all were right around 12.8).

I turned the car on about three minutes later and the BDI showed 100%.

What am I missing? Why the large drain after less than 4 miles and why the sudden jump from 84% to 100%?

Edit: Batteries started at 13.2. All flat ground, no hills.

The Battery Gauge or SOC “State of Charge”
Meter on your dash will do exactly what you found out it’s totally normal.

Example you drive to a location starting with a full pack 100%. Let’s say you drive 5 miles you pack shows 90%. You stop “go in and get a drink” come back and put the key in “Poof” Magic! suddenly your back to 100% Yes unfortunately this is (Fake Battery News) you will find out that after your flooded Acid battery’s rested the voltage creeps up and they gave your cars SOC a false Reading. You will find out that as soon as you start driving that 100% will quickly return back to 90%

Yes it’s great that you found the loose cable. Consider yourself lucky. That’s a great way to burn up a battery. But unfortunately voltage sag is a part of the flooded lead acid game.
Just keep a eye on it and only use your SOC gauge as a suggestion of actual range left. Don’t count on that Gauge to be super accurate after a prolonged rest.

I think the SOC gauge is most accurate right after you stop. Just after putting a load on your battery’s stop look at the gauge. That’s a pretty accurate indication of actual Battey strength.

As always, thank you @grantwest for the help.

I’m tuning up a 07 e4 to sell. The 8G31 gels all tested good. Fully charge up. Take it for a ride up and down around my hilly neighborhood. 2 miles and 1 bar down then it drops to 1 red bar and Turtles. 73-74 vdc at the controller with 2 different DVMs. Load test all batteries (Carbon tester Harbor Freight) all but 1 shows 12vdc analog. 1 battery was lower on load than the rest. Turn the key on, still 1 red bar at 74vdc?
What your thoughts @grantwest @Inwo @Old_Houseboater

On my e6, the dash will not show any battery level. Shows 82 then it shows 81 and wrench.
@Inwo @grantwest @Old_Houseboater

In a message dated 4/11/2020 7:30:07 PM Eastern Standard Time, writes:

Do a brake on load test of each battery. I . think you might find a dud with a bad cell. Remember all connections bright and tight. 1 corroded connection can through you for a loop

In a message dated 4/11/2020 7:30:07 PM Eastern Standard Time, writes:

All are bright and tight. Charging and seen 3 bars then 100%. No progress just 30% to done. Driving 90% then to 10% turtle. 74.2 vdc at controller showing 10% on dash.
1 battery is lower and slower to charge but the display is weird.

put your volt meter on your controller and run it . you need to see what the voltage is just as it goes into turtle , not after . I think 65v triggers turtle and 60v is controller shut off .

I have a few new 200 volt amp digital displays that I was going to connect to the contactor out. I think they will fit right of the glove box.
What I find weird is the dash showing 30 % then done at charging? Same when master off 30 seconds then on showing still showed 10% while 74.1 at the controller?
Checking connections at the dash leads at the contactor. But I don’t believe that effects the Turtle mode.

Your dash probably needs recalibration