AGM Battery

Why do you need a new battery so soon?

Unlike the Gen 3, there is nothing special about the Prius 12V battery in a Gen 4. Mine was replaced (under warranty) with a standard off-the-shelf battery

 

Ask your Chevrolet dealer for an ACDelco LN1AGM battery (GM part number 88865429), as used in the Chevrolet Spark EV. At this writing, RockAuto offers them for $144.79.

For future readers, note that unlike on older Prius cars, which require an AGM battery, the fourth-generation Prius and Prius Prime use a flooded (wet-cell) battery, discussed in many previous threads. I don’t know why an AGM battery, of suitable size and characteristics, couldn’t be used, but it’s not the originally-installed type.

 

I'm not sure what the advantages would be. The main points seem to be that ther're spill-proof and less vent less (plus have a vent port, typically): moot points with the battery in the engine bay?

They're usually more expensive, and longevity about the same?

 

AGM batteries have a slightly different charging voltage requirement.

Specifically, a normal flooded lead acid battery should be charged with a fixed float charge of 14.4V, whereas an AGM should receive only 13.8V. Furthermore, AGM batteries are far more sensitive to overcharging. Those two facts in combination, make an AGM battery a bad idea in a vehicle that has a charging system designed for traditional batteries.

 

Strange, my CTEK 4.3 manual recommends a charging mode with higher voltage for some AGM:

 

what benefit did you get from the oem that died after a few years?

 

That's primary charging voltages, and that information is correct for that. Once the battery is fully charged, the charging system switches to a float charging methodology to maintain the battery charge at 100%. It's that float charging that has a lower voltage with AGM batteries.

 

NO, not right.
They usually are a bit more expensive than a conventional style but should last about 30% longer.
And there is no "water" to check.

Why did Toyota go back ?
They saved maybe $20 per car.
There really is NO advantage to NOT using AGM.......except cost.

 

Since when ? (hint: never)
14.4 is the fast charge or absorption voltage and at that point in the charging cycle, it will work just fine for an AGM too.

 

OP is in Texas or more accurately, southern Texas (da' red zone).

Battery Testing | Interstate All Battery Centers

 

Since always. They're different battery designs and have different requirements. We're specifically talking about the float charging here, as we already discussed the fact that higher voltages are fine prior to that (in reference to an external battery charger). But don't take my word for it, let's take the first hit on Google...

"As with all gelled and sealed units, AGM batteries are sensitive to overcharging. A charge to 2.40V/cell (and higher) is fine; however, the float charge should be reduced to between 2.25 and 2.30V/cell (summer temperatures may require lower voltages). Automotive charging systems for flooded lead acid often have a fixed float voltage setting of 14.40V (2.40V/cell); a direct replacement with a sealed unit could overcharge the battery on a long drive. (See BU-403: Charging Lead Acid.)"

Source: Absorbent Glass Mat (AGM) Battery Information - Battery University

So... 6 cells in a 12V battery...
6x2.3V=13.8V, exactly what I said above, and it also warns of overcharging at higher voltages during float charging, specifically referencing the exact scenario described in the original post.

 

That’s a position not shared by Bosch, who state, “AGM batteries can be installed in ANY Auto/Truck/SUV where there is a fitting size available. AGM batteries can replace conventional flooded batteries.” Similarly, VARTA endorses the idea of using an AGM battery in retrofit applications.

Perhaps that’s just marketing—and to be clear, I don’t recommend such a substitution, or any other modification—but I’m also not sure I’d agree that an AGM replacement is inherently unsuitable.

 

1) You said: "Specifically, a normal flooded lead acid battery should be charged with a fixed float charge of 14.4V,"
and that is just NOT true. Just because the regulators often are set that way does not make it "right".
Many, many modern electronic regulators are set to "float" down around 13.4 to 13.6.
They work pretty much like a battery tender......and lower the voltage as the charge level builds up.

2) Wet cell batteries can be damaged by high charging voltages too, especially if you never check the electrolyte level.

I have been using AGMs in EVERY vehicle I've owned since they first came out......20 years ago......30....more ??
Cars. trucks, tractors, lawn mowers and motorcycles.
I have enjoyed the near-to zero maintenance and roughly double the life.

To each his own.
But you should NOT be telling people to "float" their batteries at 14.4, no matter what type it is.

 

OK, I somewhat agree. The term "should be" is a poor choice of wording. Perhaps "are often" would be better, as that's the point I'm really trying to make. All of the sources I can find, be in pretty graphs for charger design research, or the specs of automotive or external chargers, all use/say 14.4V for float charging flooded lead acid batteries. I'm not saying that's a good voltage or bad. But that is what seems to be used by a lot of car and electronics manufacturers. Hold that thought...

We're not talking about batteries with low electrolyte, so let's put number 2 to the side for now. I do, however, agree that any battery can be damaged by too high a charging voltage. Hold this thought too...

The issue here, as you alluded to yourself, is that many chargers DO float charge at 14.4V, or a too high a voltage in general. We both seem to agree on this. That high float voltage is the issue. And it's an even BIGGER issue with AGM batteries (given they're sealed, and gas at slightly lower voltages of 13.5V to 13.8V, depending on temperature).

So, bring those two thoughts back to mind... What's going to happen when you drop an AGM battery, which is easily damaged by overcharging, into a vehicle that floats at a commonly used, but (as we agree) too high a float voltage of 14.4V? Especially on a Prius that is probably float charging most of the time? That's the point I'm trying to make.

I would check to find out what float voltage a Prius uses, and take it from there. If it's higher than 13.5V, I would avoid swapping in an AGM battery.

 

So what will likely happen then ?
You will forget to check the "water" level and cook the battery to death.
I am pretty sure that the people who are saying that "excessively high" charging voltages will harm and AGM battery are NOT talking about 14.4.

At any rate, I put 3 deep cycle AGMs into my toy hauler two years ago when I got it.
I was surprised to see that the "charging" voltage coming from the engine alternator is near to 14.4 ......ALL the time.
It is cheaper to make a regulator that just limits the voltage to one specific value.
Mine are still doing fine and I am not worried in the least.

 

Is that linked diagram correct? It shows extreme cold (i.e. the north of Canadian boarder) to have battery life expectancy of 51 months while northern US (cold regions) to have 56 months. Dose extreme cold actually shorten the life of battery compared to being just plain "cold"?

 

yup. and performance too.

That's why knowing the vehicle's operating location is of immense value to the discussion. Same goes for fuel economy threads. For example, I get great mileage in the winter whereas you not so much.

 

i wonder what data they collected to come up with the chart

 

As AGM batteries are normally sealed, you can't normally check nor top up the water level.

As 14.4V is well beyond the gassing voltage of an AGM battery, it'll start gassing at that charge voltage once it's fully charged. The pressure will increase due to it being sealed, and the battery will either explode or the pressure valve will pop if this happens for too long. Either way, you're battery will need replacing.

While I'm sure there's anecdotal evidence to the contrary, I stand by my recommendation of not putting the wrong battery technology into a charging system designed for something different enough to make that choice potentially dangerous.