12v battery replacement 2021, worth getting the (smaller) OEM?

Hi all,

I have an aftermarket 12V battery from Advance Auto Parts (Diehard) that looks to be having trouble holding a charge. Had to replace it under warranty after 2 years, and now the replacement is getting weak (11.5V when starting). I drive a lot less these days but I tried to treat this replacement better with weekly driving, carefully monitoring the voltage on startup (via cigarette lighter port), and trickle charging as needed. Maybe this brand of battery just isn't very good?

My battery was reading 12.6V new, but the voltage it holds after charging has been steadily dropping every few months - 12.4V, 12.2V. Now even after an hour of driving it won't go above 12.0V at best. It's always in the 11.* range now. The lowest I've seen it was at 11.3V and I put it on a trickle charger immediately, but maybe it was already permanently damaged/sulfated at that point? In ready mode I get 13.9V-14.1V so the charging system seems to work fine. I was considering trying a battery-reconditioning charger like a CTEK to try and revive it, anyone had any luck with that?

If not, any recommendations for a good battery replacement in 2021? I know the Optima yellowtops used to be popular (prior owner of mine had one) but apparently quality is way down now. Should I just go with an OEM replacement these days? I have the non-SKS Gen 2 Prius so I don't like how small the capacity of OEM is, but maybe the 7-year warranty makes up for that (and it looks like Toyota will cover towing/labor if need be).

I was also considering trying a cheap $20 U1 wheelchair battery but wouldn't want to end up damaging some electronics or losing power while driving. But it would be nice to not have to shell out $150-250 every year or two if my Prius (and/or driving habits) just chew through 12V batteries...

I thought about parasitic drain but have no idea where that could be coming from (since the cigarette lighter ports aren't powered after the car turns off and I always turn off headlights, shut all doors, etc). I did check the voltage change overnight and saw maybe a 0.05V decrease over 12 hours. Is that normal? I know at that rate sitting 2 weeks would totally discharge the battery (12.6->11.4V).

 

So I have been using one of these $20 3A maintainer/trickle chargers from Walmart to charge it occasionally when it's gotten low: EverStart 3A Portable Smart Battery Charger and Maintainer with Digital Display - Walmart.com - Walmart.com

Before that I'd just let the car idle (fortunately the ICE itself doesn't usually need to turn on) about an hour (or two), but that didn't seem to charge as well as letting the trickle charger work overnight (3A*12hr = 36Ah). This worked great before but now the charge just isn't really holding as well as before. Seems to top out at 12.0V (after letting surface charge dissipate)

I hadn't thought about a mouse-chewed wire; maintenance records for this car (it's a retired fleet car) mentioned that's happened before. I guess I could try to hunt that down with a voltmeter on the fuse box, like this? Testing Parasitic Draw via Fuse Voltage Drop Replacing the wires shouldn't be too hard, but it might turn into a wild goose chase finding it.

The other thing I was thinking was to install a secondary 12V (maybe a cheapo U1 lawn mower battery, ideally 30Ah+) and wire that up in parallel with my main 12V, just to give it a little extra capacity so I don't have to micromanage the voltage quite so much. Reasonable idea? Or would that just ruin the newer one since the two don't have balanced voltages? (not sure that applies to lead-acid batteries)

Something like this (minus the relay since I probably won't try to use it as a camping power source):

I guess all in all it's not a huge problem since I carry a lithium jump starter battery with me. It just pains me a bit to have such an expensive 12V battery die so quickly :/

 

So I've been driving the car a few times a week regularly and on longer trips the last ~7 months, but sadly I've lately been seeing the voltage (under starting load) drop to 10.9V. Winter's probably not helping.

I've been using a 3A maintainer to fill any gaps if I don't have a longer drive every few days, and I've noticed that the charger reaches "90%" or full much faster than before (<2 hours). The resting voltage drops rapidly from "full" and settles down to 12.45V, and that drops steadily to 12.1V just sitting for 24 hours.

So the real capacity of the battery is just almost nothing now, not long for this world now at just 16 months old I actually took much more care to keep it either regularly driven or on a 3A charger than my previous battery (AAP Autocraft battery).

Putting it on a maintainer every other day is a chore and I assume it's badly sulfated by now; I could try to recondition it but then again its capacity was pretty mediocre even when new, so I figure I should just let it drain down and look at replacements at this point? I do have a jumper pack in my trunk so not a big deal if I have to jump it for a while.

I'm really micromanaging my 12V at this point if it's not obvious from the post, so going forward I'd really like to not have to worry about my battery every other day. I really wish that putting it on 3A charger overnight (or doing a long drive) could buy me at least a couple of weeks of capacity. In the winter, I rarely drive on any longer trips and may have holiday travels, so it would be a battery-killing circumstance pretty much every year.

There has to be a better way? Thinking I need a battery with a lot more reserve capacity to avoid these kinds of problems and mitigate any parasitic draw issues I might have on this thing. Thinking these options at this point:
1) A LiFePO4 battery (saw Jack's DIY 12v lithium LiFePo4? less drain thread).
+ Much better depth of discharge and won't suffer permanent damage if it had to drain down to say 25% capacity on occasion.
+ Small, light, compact
+ Could DIY with enough cells to probably reach 70+Ah(?) capacity and still fit in the battery well
- unclear safety, although LiFePO4 should be
- unclear longevity, whether the Prius inverter will charge a 12V lithium battery properly over years. may need separate BMS hardware?
- most difficult to DIY, will need to solder unless I buy something aftermarket like a Ohmmu (but that's only ~40Ah capacity and $350, though it has a 5yr warranty)
- raises eyebrows at resale
2) A 55Ah AGM mobility battery per this thread: Fitting mobility 12volt AGM battery. | Page 26 | PriusChat
+ cheap ~$100 upfront, high capacity to handle situations above without sulfation
+ other posters claim lasting 6+ years
+ same chemistry as OEM
+ cheap to replace in the future
- oversized, need to DIY something with brackets and posts to get it to fit in the tiny battery cavity.
- risk of unvented battery
- still have to be careful about sulfation if SoC goes below ~50%, but there's much more buffer before then
- quality of these AGMs unknown, may be hard to find exact replacements again in ~4 years
3) an OEM battery
+ great warranty, free tow and install 2yrs, 50% 2-4yrs
+ possibly better quality AGM, may handle deep discharge better than AAP batteries did? AAP batteries may be lower quality like recent Yellowtops
+ best option for resale, convenient, no DIY needed
- low capacity, non-SKS battery (30Ah?)
- upfront $240 at dealers around me (WA)
- may also just get sulfated again quickly, not necessarily better than AAP batteries if sitting long periods. AAP batteries are built by Johnson Controls (quality brand)
- actual dealer may balk at warranty especially if more than one replacement is needed

I'm leaning towards trying the 2nd option. Hopefully the modifications aren't too painful and still keep the battery and connections secure. EDIT: made a post in the mobility 12v thread, install doesn't look too bad! Hoping I can get 3-4+ years out of this next one (better yet 6-8+ years like others in the thread), but for $110 it's definitely worth a shot.

 

I have the non-SKS (but with expanded battery tray) so there definitely is some other parasitic drain going on; likely some old/chewed wiring but probably not worth going down a rabbit hole to figure it out; much less replace.

No garage at the moment; that would make this a lot less of a pain. Lugging out extension cords and fiddling with the terminals makes this really troublesome; it would be nice to just plug it in like a phone someday.

Thinking with work-from-home, I may as well buy some better 12v hardware (CTEK, load tester) since it looks like even newer cars are having this problem. Just seems to be worse for the Prius given its tiny 12v; I guess big SUVs with beefy 12Vs 100Ah+ have enough reserve to not think about it?

Solar charger is an interesting idea, but living in WA I'm skeptical if I'd get enough light in the winter when I most need it. Still, maybe a $20 5-10W trickle charge is all I need to offset the parasitic drain; and at <1A current, it's probably safe to charge with the electronics still connected, right? Seems to be according to this thread: 12V Solar Battery Charger | Page 2 | PriusChat

Looking at it now, the panel is smaller than I thought; 7x11 or 9x13 would be a handy size to keep in the hatch, would it get enough light in there though? Sounds worth a try, though not sure I can rely on it year-round. (Plus it could come in handy for summer camping, maybe powering a cooler?) Looks like others have had success with this in NY/Canada: 12V Solar Battery Charger | PriusChat

 

Glad to hear they both seem to be working out for you so far! I can't see what you linked, but I just bought a 10W unit from Renology, is that what you got? It's on sale for $16 on their direct site so hard to pass it up at that price: 10W Solar Battery Trickle Charger Maintainer | Renogy Solar It's my first foray into solar; seems perfect for this use case.

I may still end up starting over with a new 12V since this one probably has ~10Ah of actual capacity at this point. Will see how it goes! If all goes well, my next 12V will probably last for the remaining lifetime of the car.