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No. The longevity of the 12V AGM battery can be affected by excessive discharging and excessive heat. In a conventional 12V lead acid battery vibration from the engine compartment can also shorten the life of the battery. AGM batteries are much less succeptible to damage due to vibration. If my understanding is correct, in the ELR as in the Volt the 12V AGM battery rides in the rear, away from the engine compartment. When it needs to be replaced, or if you just want to proactively replace the 12V AGM battery due to the age of the battery, the ~$200 replacement cost is reasonable.
 

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Discussion Starter · #3 · (Edited)
Thanks for the quick response. I understand all of your logic.

Since the "hybrid" nature of the electrical system is so complex, I'm still wondering if constant short trip discharge is going to adversely effect the battery life. Of course, in a traditional ICE configuration, battery discharge is greatest at start-up, with the alternator quickly kicking in to stabilize voltage. Can I assume there's no dramatic draining of the 12v AGM battery during short trips, so it's charge/discharge cycling is less dramatic? (I don't want to assume anything!)

Thanks again, and for further clarification.
 

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When you start up an ELR, Volt, Bolt or really any EV/PHEV you are booting up several computers and other 12V systems. When the Volt is powered on it consumes ~500W. Doing the math, Power = Voltage x Amperage, 500W/12.6V = ~40 amps. Compared to a typical ICE engine turning over the 12V AGM battery only needs to handle about 1/5 of the starting ~200 amps that a 12V starter motor can draw while cranking. In the ELR (Volt and Bolt too) once the computer system and other 12V systems have booted up (about 5 seconds, maybe shorter period) the ELR's accessory power module (APM) takes over and provides 12V power from the 300V traction battery while maintaining a charge on the 12V AGM battery. If you park, don't drive the ELR for an extended period (more than 30 days) the owner's manual probably includes instructions for connecting a 12V battery minder to the 12V AGM battery.

When the ELR's gas engine is started, the 12V battery is not providing the power. The 300V battery pack powers one of the ELR's two high power electric motors; the electric motor cranks the gas engine via the planetary gears and a clutch.
 

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It's worth keeping in mind that those short trips are the very worst kind of driving for your ICE also. The engine doesn't get hot enough long enough to boil condensation from the oil and the same goes for the exhaust system. Plus, the alternator may not run fast enough long enough to recharge the battery. Finally, an ICE gets horrible millage until the engine is throughly heated. All-in-all, an EV is a much better vehicle for many short trips. This is reason enough to keep your Volt charged up. It much easier on the car if you don't have start the fossil burner for those short trips.
 

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If planocaddie is in Plano Texas, then heat is a big factor in battery life.

I got about 3.5 years out of the OEM 12v in both my Ford and Subaru here in Texas. The Ford is driven about 10 miles per day whereas the Subbie is about 100.
 

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Discussion Starter · #7 ·
All good answers.
fofomot1: it's PIANOCADDIE, and I'm in Ft. Worth. Car in question came from Tulsa, garage kept, lifetime MPG of 171, so I'm sure it's first 4 years were all short trips. At 4+ years, I may simply proactively change the 12V AGM battery.
jcanoe: thanks for the clear explanation.
Overvoltage: Your logic is exactly why we prefer our ELR to our CMax and Avalon Hybrid. ICE in both of them kicks in way too often, too early, for too short a period of time. Our ELR and Leaf are our daily drivers. Both CMax and Avalon Hybrid get 38-40mpg on the road, so that's what we use them for.
Thanks, all.
 

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All good answers.
fofomot1: it's PIANOCADDIE, and I'm in Ft. Worth. Car in question came from Tulsa, garage kept,
Hey I was only 1 letter off (still looks like an "L" to me due to the font/size) but "fofomot1" is WAY off! LOL

Anyway, I wanted to mention both the Ford and Subaru are garaged too. The Ford is usually parked in partial shade during the day but the Subie usually gets full sun. I also had a new Nissan battery fail at 14 months.
 

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Short trips won't hurt a PHEV at all. That's actually better for the lithium battery. Full cycling and time under the curve are the enemies of batteries.
Lead acid batteries either cook off water and the electrolyte dries up and the plates get damaged, or they over heat and the plates get damaged and a shorted cell develops and overcharging the other cells to make up kills the battery.

Li-ion batteries slowly build fingers of precipitate ( rock content ) that damage or penetrate cells and cause them to fail. The worst thing for a lithium is full discharge and overcharging or maintaining a high charge at high temperature extremes. The ELR and Volt lithium have cooling and battery management to keep the battery from ever charging beyond about 90% of full capacity and below 14% of capacity. The only problem appears to be cell balancing over time. nothing you can do about that's it's a lottery.

The AGM is not a big deal either. Those basically have a 5 year life. It's in the trunk, under the floor. It's not like a regular car that's in the engine bay getting baked every single day.
If anything, I think a lot of the agm batteries in the volt and elr die from not being used enough. The APM also puts out 14.5v which is slightly above the "safe" float voltage for an AGM battery. AGM really needs to get cycled and heated to work well. The gel around the plates doesn't circulate like a fluid in a flooded lead acid until it's heated up, but again since the battery is internal to the cabin it needs to be an AGM. I put mine on a 100 amp load test and hit it with a 60 amp smart charge with a temp sensor to keep from cooking the electrolyte off about once a year or so to keep it perky.
 

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The APM in the Gen 2 Volt does not output a constant voltage. Depending on the load and ambient temperature I have observed voltages from 12.4V up to 15.4V (winter temperatures.) Typically the APM voltage is ~13.0V
 

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Discussion Starter · #11 ·
fotomoto1: At least my typo wasn't MOFO!!!!

I had a single incident when the door locks wouldn't unlock with either keyfob. Windows DID open when the unlock button was held, so I was able to reach in and manually open the door with the floor lever. Made me consider the 12V is weak.

All good info, folks. Both Chev and Cadillac dealers near me have confirmed the info above. Thanks.

(I don't understand why the title of my thread didn't include the "12V AGM questions" that I typed in. Do I need to ask a moderator to edit it?)
 

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I had a single incident when the door locks wouldn't unlock with either keyfob. Windows DID open when the unlock button was held, so I was able to reach in and manually open the door with the floor lever. Made me consider the 12V is weak.
Pop the key out, pry the back off and remove the battery in the keyfob. bend the little clips a tiny bit to make them touch better and put it all back together. Don't let the spring or the tab you press to remove the metal key fall out and escape! I've had the same thing. It's not a weak battery, its the battery in the fob not getting good contact.

For me, I could always open the trunk, but the door unlock buttons or the passive unlock didn't always work. Trunk opening means the receiver is ok and the fob is ok. The battery is under the trunk button directly so when you squeeze there it makes a good connection.

Try it out.
 

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fotomoto1: At least my typo wasn't MOFO!!!!

I had a single incident when the door locks wouldn't unlock with either keyfob. Windows DID open when the unlock button was held, so I was able to reach in and manually open the door with the floor lever. Made me consider the 12V is weak.
My door locks (from the door handle button) stop working for 6 weeks although I would test them periodically) then when I tried them once (unlocked rear door with hatch up) they started working again and have been so ever since (several weeks so far).
 

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Discussion Starter · #14 ·
Aseras: Fob was working sufficiently to roll down the windows (holding the unlock button)
Fl Spyder: No rear doors...HAHA!

2014 ELR...
 

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I'm just waiting for that day when some engineer comes up with a good reason to put the 12v aux power bus on a DC-DC converter (probably to smooth things out for the computers), and completely isolates the charging demands of the aux battery from the rest of the car.
 
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