12V battery group size

Unlike in previous generations, the original auxiliary battery in a fourth-generation Prius is a flooded (wet-cell) type, not an absorbed glass mat (AGM) type.

Toyota’s Repair Manual calls for checking the electrolyte levels and says to add distilled water and check the specific gravity, if necessary. It may be difficult to see in the photos, but the battery itself is marked:

●爆発の恐れあい、液面はLOWER以下で使用しない
To avoid risk of explosion, do not use if liquid level is below LOWER.

●液漏れの恐れあり、UPPER以上に補水しない
To avoid leakage, do not refill with water beyond UPPER.​

Toyota’s FAQ answer about the auxiliary battery (in Japanese) for the Prius PHV in Japan, which the parts catalog says uses the same 28800-37130 battery as the Prius Prime and liftback Prius built for the U.S. market, also says it has “各セル排気タイプ (液線あり),” i.e., individually-vented cells and a liquid line.

None of this would apply to an AGM battery, in which the electrolyte is bound into glass-fiber mats, rather than being a free-flowing liquid, and water is never added.

This is also one reason I’m not sure the Toyota 00544-21171-325 is the most direct OEM replacement, since it is an AGM battery used on older Prius cars. I don’t mean to suggest that an AGM battery would be unsuitable for a fourth-generation Prius, of course, just that it’s not the originally-installed type.

Flooded (wet-cell) and AGM batteries are two types of lead-acid batteries. “Lead-acid” refers to the battery chemistry (PbO₂ + Pb + 2H₂SO₄ ⇌ 2PbSO₄ + 2H₂O), not to the form of the electrolyte.

 

Thanks. Looking at the battery manufacturer's link you posted earlier, it seemed to me they added some special design to minimized spillage and, I think, minimize explosion. The Google Translate was poor and all the accompanying graphics were still in Japanese.

Here is the page: クルマ用バッテリー｜株式会社ジーエス・ユアサ バッテリー

 

I think we forgot about the hydrogen escape...

 

Under the hood? It would just ride up into the sky, unless it is airtight under your hood. Then the engine would overheat.

 

yes, i suppose i should have. please accept my apologies.

 

I meant from the recharging, but I suppose that's a different formula

 

Recharging under the hood is OK. There is enough ventilation to dissipate the gasses.
On older generations with the battery in the cargo area need ventilation.

 

Because......lead acid has proven itself to be a pain in the backside.
AGM is not just more expensive, it is better in several respects.

 

not the ones toyota uses.

 

Some of your comments are just ridiculous.

 

Not really. The AGM batteries Toyota uses are "sealed", meaning you can't add water (but they still vent if necessary to prevent explosion). Hence, after 3-4 years of "normal use" they start to run out of water.
So here there is an "advantage" with a "normal" or non-AGM lead acid battery. You can add water to extend its' life. If you have a gen 4 and the battery starts to exhibit signs of failing, DO check the water level. Top up with distilled water to the indicated marks on the battery. You may think I'm exaggerating, so for example, my 2007 FJ Cruiser, with a "normal" Panasonic lead acid battery is still using the original 2007 battery, after I added distilled water when it started to indicate if was about to fail, - four years - ago.

Some further info. for battery enthusiasts. There are several types of "lead acid" batteries.

The "standard" or most common in automotive use uses a lead antimony plate support grid, holding the "spongy lead". The antimony is added to increase the strength of the lead, so the plates don't sag and short. The antimony causes extra electrolysis, causing the battery to loose water over time. Easily fixed by providing caps to refill with distilled. But definitely NOT "sealable".

The AGM type has the added support of the plates pressed tightly together, so less or no antimony is used, reducing the hydrolysis and water use. You -can- add distilled water to an AGM battery, IF there were readily accessible caps to remove, but most batteries of this type don't provide that option. I've done it with success with batteries where they didn't hide the caps well enough. There are still caps, they have to add the electrolyte before shipping. But sometimes they hide them or glue them in place.

Then there is the AGM with "ultra pure" lead, which extends the life of the battery by reducing the hydrolysis even further, and a further "improvement" to the AGM involves including a catylist device inside to recombine the H2 and O2, thus allowing the "emergency vent" to be sealed, or almost sealed. Check out the "Odyssey" line of lead acid AGM batteries to see these. Very expensive, but very nice!

And finally, for now, the Lead Acid battery with Calcium plate grids. These are used in some "stationary batteries", used at radio sites and for solar power accumulators. The Calcium grids don't conduct electricity, so don't add hydrolysis. These batteries can sit for a year or more and not require water. You can even buy catylist caps for them.

I'm sure there are more technical innovations to come for lead acid batteries. They ain't dead yet!

 

From my reading of the manufacturer's page I believe you can add distilled water to the Gen 4 battery.

 

AGM info by Yuasa, an interesting read.
AGM & EFB Automotive Batteries Explained - Yuasa battery

 

Yeah first thing I did when I got my Eco was to check Die Hard's website to see if they had a replacement battery. None listed.We either have to wait a few more years or somebody has to be a guinea pig for the rest of us.

 

i would guess they will all be under warranty until at least one replacement is available. but not for those simply replacing a good battery with a 'better' one.
you can make just about anything work under the hood, as compared to the hatch.

 

Thanks - yes. One of the reasons I was given by an auto-electrician in the '90s for "low maintenance" batteries, and probably later iterations, is because, with cars generally not needing as much checking as 20+ years before (when you'd check the tyres weekly, the oil twice a week, the radiator sometimes daily) - many people stopped checking their batteries at all.

 

Nearly a 100 posts.
Glad we kept it simple.

 

If it wasn't simple, I'd be lost on here!

 

Well sure the OP's question seems simple enough on the surface - what is the group size on the Gen 4 battery.

Simple if this information was published by Toyota, but it's not.

Simple if third-party battery resellers listed a replacement for the Gen 4, but apparently they don't. Leading to a discussion of whether the battery even belongs to an existing standard group size.

Simple if it was the same battery used for many years in Prius, but it's not, there was a change in 12V strategy in the new model. Since the batteries in Gen 4 are still under warranty, the few that have failed are being replaced by the dealer, so no one has any experience yet with DIY battery replacement.

Also the the premise of the OP's question would be simple if they simply had a dead battery that they wanted to replace themselves, but they added a couple of caveats:

- they want to "upgrade" the battery. Leading to a discussion of whether this is possible, considering we don't know much about the existing 12V, or what is the requirement for the car.

- the reason they want to upgrade the 12V is because they are running a dash cam when the car is not running and it's draining the current battery to the point that they sometimes have to jump start the car. Leading to a discussion about whether upgrading the 12V is the correct route or whether they should be looking at a secondary battery to power the dash cam.

I don't think the answers were overly complex, instead I think the OP's question had an oversimplified assumed premise. Combined with the fact that this seems to be the first detailed discussion about the Gen 4 battery, and people are not just speculating but also researching and trying to come up with definitive answers.

 

The key words there are "if necessary". In normal operation, that should never happen.

I respectfully ask for some reference that supports the claim that they lose water under normal operating conditions, over a normal expected life span.

I think that claim is NOT true. But I've been wrong before. Educate me, please.