12V Battery Replacement Recommendation

This thread is a little old, but I thought I'd mention a few things for others that might come here.

I have a 2010 Prius, and have replaced the 12V battery with an OEM battery from Toyota which I've then installed myself. It's very easy to do, as others have mentioned. The Prius doesn't use the 12V battery to "crank" a starter, it just powers the computer, so the 12V battery tends to last a long time. The OEM battery from Toyota -- though on the expensive side -- is a very good battery, fits perfectly, has the proper vent tube opening, and "tiny" battery posts which fit the tiny battery terminals on the cables. Another battery might not have those same battery posts.

To keep any settings from disappearing, I just connect an old 12V battery (that still has some charge left in it) to the + jump start post and to the ground under the hood. (Make sure you connect positive to the + post in the fuse box, and the negative to a ground.)

Now during COVID, since the car is not driven very often, I've added a battery maintainer and plug it in when I know the car is likely to sit for a while. And yes, the battery is still connected to the car.

 

Doing the same as your last paragraph; no problems, with the car often sitting 4~5 days.

 

Those are the go-to solutions to address both the hold-over and storage issues.

 

My 6y old Corolla/auris hybrid used 3rd gen powertrain. The battery is still good 12.3V at -5C. All interior lights turn off automatically even if the door is opened or someone forgot to turn the vanity light off. The headlight won't turn on if the Ignition is off, but parking light and RDL still turn on with IGN off.
It is probably why the 12V battery last long. But my 06 Prius battery also last really long 7+ years at 200k miles when the first first 12V battery dead. But it still went flat if someone forgot to turn the vanity light off for a whole day.

I am afraid if a bad trickle charge makes my battery lasts shorter than just the DC to DC converter does. Overcharging is more harmful than undercharging because hydrogen formation and lost of electrolyte from overcharging.
Does anyone have good long term experience which chargers are really good

 

12.3 is NOT "still good". That is on it's way out.......probably.

What you do NOT want is a cheap trickle charger, mostly for the reasons that you mentioned.
You can't go wrong with the Battery Tender brand but there are other major brands that are good too.
Stay away from Harbor Freight for battery chargers. JUNK.

 

It is still close to 12.6V after a long trip. 12.3 V is only after many short trips or parks for many days. The fully charged open circuit is 12.8V, and the Toyota manual said 12.5V is the limit when we need to charge the battery for any electronics tests (DC to DC converter, inverter, reprogram the ECU, etc.).

 

Those are two entirely different situations.
Unless your short trips are REALLY short, that should still keep a fully charged battery charged to at least 12.6.
The parking for many days depends on how many days that is.
A fully charged battery should remain at or above 12.6 after sitting for a week or less.

Are you still insisting that your 12 V battery is still "good" ??

 

Yes, it was like that since 2 years ago in winter. Nothing much change. After i parked for about 5 days, it is 12.3V from the fuse box.

If I have 2 hours or more in ready mode, usually it has 12.5- 12.7 V.
The most important part is how much the voltage drop under load. As long as it is more than 10V under heavy load like starting the car, it is still good. At least that is the standard for non hybrid car. Prius will also give a CEL code if the voltage drop less than 10V at any moment.

It is indeed probably on the way out, 6 years old. At around 4 years, I usually start to check the 12V battery in all of my cars.

 

Toyota Prius: How to Check the 12v Auxiliary Battery Health - PriusDIY.com

Easiest check that also works for 3rd gen.
The bottom limit of usable battery (normal or new):
1 press power button to ACC: 11.5 V (11.9-12.6)
Head light On (Halogens): 10.6V (11.3-12.2)
Headlight, defroster, blower motor On: 10.2 V (10.7-11.3).
2 power buttons to IGN ON, everything else off: 11.3V (11.7-12.2)

Occasionally, we can still start the car on dying battery after the brake booster sound stop for few seconds. It is useful in emergency situation when the can cannot be turned on right away.

 

Which does NOT apply to a hybrid because starting does not apply a heavy load on the 12 V battery.
If it reads 10 V at any time, it is way PAST dead.

 

Exactly, that's why we get CEL in hybrid at any time it drop below 10V even just a split of second dip, not just slow starting or dim vanity light like regular car. But it still can give us the last legs to get moving.

 

If you are depending on that alarm function to tell you when your battery is getting too weak to perform adequately........you likely will be disappointed. There are MANY people posting here who have had erratic operation caused by a weak battery and never had any alarms.

 

2012 Prius two with original battery. Toyota stealership wanted $280 here in Bend OR. Got the premium NAPA battery online and picked up curbside 199.99 with a 20% online rebate so I paid 159.99.

Part number: 9851p - NAPA The Legend Premium AGM Battery BCI No. NA 325 A AGM (Absorbed Glass Mat)

Installed myself in the parking lot in like 9 minutes. Super easy.

 

My old 06 Prius had a sudden plain flat battery without warning either. That's why I start monitoring my battery these days because it drops occasionally below 12.1 V in the early morning after not driving it for 2 days. After 2 hours trip, it was 12.5V, 11.7V with headlights ON~ 10Amps Load. I am just curious what is the cut-off open voltage and 10Amps load voltage when it will not start the car anymore. The Lowest I had tried was 11.9V open voltage, and 10Amps load 10.8V, It still can start the car. Another important parameter is how fast the voltage drops from 12.5 V to any number in 1,2, or 3 days.

 

And.....how old IS it now ?

NO amount of monitoring will tell you when it is about to go "plain flat" because the moment of death varies wildly.
Tenths of a volt are important. After a 2 hour trip, it should be around 12.8. After sitting for 2 days, it should be 12.5 or 6.

Your numbers tell ME that it is time to get a new one.
And keep it fully charged, as much as possible.

 

Thanks for the advice. Yes, I agree, the variance is too large to make any conclusion on 12v AGM battery behaviour. It is exactly 6 years old now. I never charge it externally, exclusively by the DCtoDC converter.

Sitting for 2 days start from 12.5V, usually 12.4 V. I have smart keys system. It is indeed aging, and max voltage i ever measure now is 12.6V.

I remember my 06 Prius in ACC mode was less 10.8 V just before I replaced it. But this number is too late. I only can start it if I park less than 6 hours after 10 minutes driving.

My current hypothesis, 11 V in ACC mode or 10.5 V IGN ON is the absolute limit to replace the battery, 12 hours later after 2 hours driving.

I probably will by NOCO 5 charger but it must be directly connected to the battery from the trunk. I cannot use jump start port on the fuse box for charging because it will not charge the battery. We add MegaOhm serial resistance in front of the battery circuit.

 

What exactly is the thing that you added ......and why ?
You absolutely should be able to charge the battery from the jump points.
If you can't, something is wrong.
AND......it is probably too late now to accomplish anything useful with a battery tender on the old battery.
AND.....the "absolute limit" for replacing the battery is when it stops working. Any readings that you take likely will be misleading.
Second time I've said that.

 

I show a basic electronics serial circuit. In the serial circuit, all parts gain the same current. Power (watts) = Current x current x resistance. If you charge a car battery connected to the car and you charge it from the jumper ports, You supply most of the energy to the Car body, and effectively not so much charging power goes to the battery. Fusebox is exclusively connected to a + battery terminal with almost zero resistance to battery + post. However, the negative terminal is connected to the entire car body that may have large resistance (~1 Ohm) on some paths. I did measure the resistance from the engine area to the positive terminal, it is > 0.1 Ohms. Battery internal resistance is in order of milliohms. But if we can find a ground that gives miliohms resistance, charging from jumper port still work.

Ideally, we should disconnect the negative cable on the battery from the car body therefore no parallel circuit that allows the small current flow to the car body. However, battery internal resistance is way smaller than the ground/negative terminal connected to the car body. Most of the current will flow to the battery instead of to the car body when you charge directly from the battery terminal.

My goal is not to waste a usable battery but will not experience a flat battery. Most people including me in the past, replace the battery once it won't start anymore. Some of us replace the battery too soon although it may still be usable for 1 or 2 more years with normal usage.

From the cost perspective, running on READY mode for 30 minutes if the battery is below 12V is cheaper than buying any good battery charger with a microcontroller like NOCO5. It cost less than 0.5 L of fuel per 30 minutes.

 

Absolutely, totally WRONG.
No part of the main power distribution constitutes a series circuit......except for the path through any main fuses that are present.
At no time and under no circumstances is any energy "supplied" to the car body.
You have a strange understanding of electronics........and most of it seems to be dead wrong.

 

This is a short sighted conclusion to arrive at.

If/when the battery gets weak enough, it will not go INTO ready mode.
The charger will always work......well almost always, depending on the model and how sick the battery is.
The charger will also put more charge into the battery and will do it without spewing any CO into the garage.

THEN......most people are happy to "throw away" some theoretically usable battery life in trade for a much more reliable car. YOU are, of course, free to run it down to the very end......and hope that it doesn't eventually give out on the highway somewhere.......where the cost of a tow might be MORE that a new battery.