I remember when I first got my 21 there was lots of concern about the battery and its state of charge. With all of the electrical things going on in the background, even with the car off, the voltage slowly decreases. I am still on my original battery. I did kill it once when an OTA update failed. Jumped the car and carried on.

Now nearly 4 years later, and never a hesitation to start, I added a bluetooth battery monitor to watch what happens. After I did this for a bit, I remembered that my Noco genius 10 has a repair, or desulfation mode. Did a bit of research and gave it a go. Disconnected the battery from the car and let it go for about 4 hours or so. Here is a before and after chart of the voltages.

For reference, the voltage on 8/14 is 12.82V. On 8/9 it was 12.6V









Not sure if it will mean much, but it appears that running the repair battery mode did indeed give me back some capacity.

I also used it on a trailer mover battery. Before even after charging, it was only about 12.6V. It was 12.8V after repairing it and waiting a day to measure the voltage.

 

Before I bought my 2025 C8 HTC 3LT I was reading in other forums that the AC Delco Lead Acid batteries were an issue in several ways. I skipped around looking for alternative ways of handling the challenges of keeping the battery charged. Short of leaving the battery tender plugged in nothing else was offered as an answer as to why they slowly discharged (electronics drain). With a little research I found an answer. Replace the Lead Acid battery with a Lithium Ion battery, namely an Antigravity Battery. They are pricey, around $1100.00. Considering that this was a 100K car the $1100.00 seemed like a no brainer for the added peace of mind. But what does that give you?

• H6/Group 48 OEM Automotive Case size (directly replace stock battery).
• LxWxH: 10.75 x 7 x 7.5 inches.
• Amp Hour Options: 24 Ah, 40 Ah, or 60 Ah.
• High Power: 24Ah=1000CA, 40Ah=1500CA, 60Ah=1800 Cranking Amps.
• Exclusive RE-START Technology: Wireless Jump-Starting built-in; just press the button on your Keyfob remote.
• Complete Battery Management System built-in.
• Ultra Lightweight: Drop up to 40 lbs instantly! 70% lighter than lead.
• Safest Battery Technology using full BMS and Lifepo4 Lithium chemistry.
• Threaded Terminals: M8x1.25

I took the plunge and replaced my lead acid battery with this model. My C8 starts with a roar every time. I have taken several extended trips with not one single issue with battery strength. I highly recommend upgrading to this battery. It took all of 5 minutes to swap it out, and instantly saved 20 pounds of dead weight. ​​​​​​I chose the 60Ah version. Only an initial charge with the optional tender was required. I hope this helps those of you that want to upgrade to a "no more issues with battery power" solution. No I don't work for Antigravity but do know that most professional racers choose this battery for the reasons above.

 

Yes that should work fine. While I do not have that battery, I did research it, and it is very clear that the AG battery is a drop in solution and there is no adjustment required to the charging system. Others have also reported using it successfully as well.

As for the parasitic draw, I have measured it. Once the car has been off and undisturbed for 15 minutes or so, the draw drops to 10 ma which is very low. FLA batteries, even good ones, are going to lose charge with time, even with no draw. An AG battery will too, though not as quick.

 

^^^
Yep, the C8 (and frankly started with the C7) has a VERY low parasitic current draw after about 10 to 20 minutes max. As @Andybump measured it was 10 milliamps. For those that don't relate that is 0.01 amps X 12 volts = 0.12 Watts! Can't find a bulb that small! From GM published data in my C7 4 volume service manual (see Pic) the car can sit over 30 days and the battery still start the car.

Been on vacation for 3 weeks with the C8 and my 2020 C8 and do not use a charger. Too many late afternoon thunderstorms in our are in summer. Frankly it's also my DD and always drive at least 3 times a week the 60 mile round trip to town other than vacation that keeps the battery charged. BUT mostly not needed.
However lead Acid batteries have a fixed life. I measure battery voltage periodically and when it starts to drop (typically 4 max 5 years) cheaper to buy a new one than get stuck!


Note GM says and 80% charged battery (all they charge a c8 in normal day driving to save fuel) it will start a car with 25 mA parasitic draw even after 33 days. With Andy's measured 10 mA even longer. However after 4 to 5 years I would replace the battery. If it ever had it drain completely (yea desulfurizing can help) I would replace at 4 years! The damage done is not 100% reparable. You're living on borrowed time!

Note: OP is right, need to measure ~12 to 24 hours after shutting the car off or after charging. There is a surface charge that will show higher than actual until it depletes.