12 volt battery charging voltage when fully charged?

i doubt it is agm in a truck, but idk. local gas station does the oil. only 28,000 miles, so i don't ask them to do anything else. but they might check it, i never thought of that

i thought modern batteries were maintenance free. in almost 50 years of car ownership, i can't remember the last time battery water was part of my maintenance routine

 

A bit more info:

At ~90% of plugged in charging, the battery was switching back n forth between 12.75 and 12.79 volts.

After charging was finished by ~30 minutes, with the charge handle Still Connected to the chassis, battery voltage was 12.60 volts.

After charging was finished by ~2 hours, with the charge handle Still Connected to the chassis, battery voltage was 12.50 volts.

Then the second the charge handle was removed from the chassis at the ~2 hour mark, battery voltage dropped to 12.45 volts.

All of this was done with the car locked, key fob far away from chassis, and the hood up so no electrical systems would wake up and spoil my results.


Rob43

 

Interesting, but not surprised.


Rob43

 

All of them are NOT.
Maybe you should look at yours.

 

thanks, what would i be looking for?

 

Removable caps on top of the cells. If they are removable, unscrew/pry them off and you can see the electrolyte level. If there are no removable caps, then your battery is "maintenance free". Here is what Interstate Battery has to say about water level in batteries:

A conventional flooded battery will experience water loss as a normal condition when the battery is being charged. The reason for the water loss is due to the charge current splitting the water into oxygen and hydrogen gases, the gases will escape out of the vents. All of the automotive cranking batteries that Interstate carries are considered to be maintenance-free under normal conditions. What that means is that the battery loses water at a very low rate. Some of the batteries have removable caps so they are maintenance-possible. Under normal conditions you do not need to check the water levels but if the battery has been over charged, you live in a hot climate or the battery is well used, it is a good idea to check the levels and add distilled water as necessary. Maximum fill level should be 1/8” below the bottom of the vent well tubes.

Conventional deep-cycle batteries do require regular watering to keep them working properly. Check water levels every 4-6 weeks. The minimum level should be ½” over the plates to a maximum of 1/8” below the bottom of the vent wells. Do not over fill because it can cause the electrolyte to get pushed out of the vents causing corrosion. Use distilled or deionized water.

Here are just a few reasons why batteries may use more water than expected:

1. Over-charging or keeping a battery on a trickle charger for months at a time
2. Age. New batteries and well used batteries tend to use more water
3. Hot climates and batteries running hot due to the location of the battery
4. Frequent battery use. We recommend charging the battery after each use but be aware that this will speed up water loss so more frequent service will be required
5. Deep discharges will cause more water loss because of the longer charge times needed to bring the battery back up to a full state of charge.

​

 

I think you can safely conclude that your car is functioning normally. Several of us with the same model have seen the same behavior. I drove yesterday using an OBD tool to monitor the voltage reported by the ECU, and saw exactly what you see.

Since the resting voltage of your battery after driving is similar to others with the same model (12.5 to 12.6 volt range) and since this is typically the range of a resting, fully charged 12V lead-acid battery, the car appears to be fully charging the battery during operation.

I also measured the voltage across the battery terminals while the car was plugged into the wall and actively charging. In this mode, the battery charge controller under the rear seat also provides 12VDC power. During wall charging, the voltage at the 12VDC terminals was 12.79 volts, which is similar to what a previous commenter noted on his Prim and what you see when driving.

 

OK, I give up.
Apparently the Prime's work differently.
That also may be true of any other models that have gone back to a conventional battery too......if any of them did.

Just one more comment: Using a scan tool to monitor the voltage might NOT be seeing the actual battery voltage.
The power coming from the "cigarette lighter" power socket should be very close though.

 

thanks!

reading this tells me that it is probably fine. it only has 28,000 miles (low battery use)

and it is not in a hot climate.

the 12v on my home generator has a float charger and was still going strong after 14 years just like the truck battery. it only cranks once a week plus when the power goes out.
unfortunately, the pc board on the generator crapped out.

 

NO. It doesn't tell you that at all.

Here are just a few reasons why batteries may use more water than expected:
Age. New batteries and well used batteries tend to use more water

You are reading only the part that you want to hear.

 

i read the whole thing, not 'only the part that i want to hear'.
without more reasons, i don't know if it would be low

 

What happened to that first line of your post. It didn't show up until I did a quote ??

Anyway, it is absolutely true that "you don't know".

After 14 years, it certainly has lost SOME water just from normal evaporation through the vented caps.

 

i agree completely

 

I used the 12 V power socket during my test. You can see my results in post 56.

 

If someone is still with this thread and doesn't know about watering their battery....

A flooded lead acid battery (the usual car battery) has a liquid electrolyte mixture of sulfuric acid diluted with water. Under normal usage some of the water is slowly vaporized out of the battery. A 12 volt battery has 6 cells, and each cell needs to be checked periodically (yearly is OK). If the level is low, it needs to be replenished with distilled or deionized water. Tap water might be OK or might contain minerals that damage your battery. Use distilled or deionized water. The minimum acceptable level without damage is just above the tops of the lead plates visible in the fill holes. The maximum level is at the bottom of the fill tubes in the fill holes. Filling to the top of the fill holes might result in an acid spill when the battery warms. The fill tubes' bottoms have slits in the sides so the meniscus (curved top of the water in the tube) is easily visible. Do clean off the top of the battery before removing the caps to avoid dirt getting into the innards. If one battery post is growing green fuzz, you probably have a slow acid leak where the casing is supposed to seal around the battery post. The green stuff can be cleaned with a small wire brush and maybe a cloth soaked in ammonia--never get any of this stuff inside the battery fill ports. There are treated felt circles that go around the post to help absorb the acid leakage for a short time. Left uncleaned the acid will damage the battery cable clamp.

 

i took a look today. there are two rectangular caps with this warning: MAINTENANCE FREE BATTERY, DO NOT PRY OFF CAPS, DO NOT ADD WATER!

 

Just to be safe, get out the pliers, pry those bad boys off and check the water level. Just don't add any if low...

 

Confirmed on my 2018 Prime that the voltage at the battery terminals matches the voltage reported by the OBD reader from the ECU. The OBD reports only to the nearest 0.1 volts, while the digital multimeter hooked to the battery reports to the nearest 0.01 volts. The OBD truncates the reading downward, so 12.99 volts on the digital is shown as 12.9 volts on the OBD. When the digital goes to 13.00, the OBD jumps up to 13.0.