Oem battery lost 5 pounds of electrolite

The battery does not have 5 lbs of electrolyte to start with. That would represent 4 pints (.5 gallon imperial). The Prius battery is supposed to be an AGM battery. This means no free electrolyte (liquid) the only liquid being absorbed by the glass mats that are 95% saturated. Topping up the battery with distilled water would effectively weaken the acid.
However many people including myself have reported free liquid in the OEM battery making the claim that the battery will not spill acid if the case is broken in an accident somewhat invalid.
The most likely reason for the weight difference is the amount of lead used in the construction of the battery. The amount of lead in different forms in contact with the acid directly affects the capacity of the battery.

John (Britprius)

 

5 lbs loss of electrolyte corresponds to 2.3 liters of water volume. That would be about what I would expect the battery to hold, if it was of flooded cell type. One does have to work a bit to refill these batteries, but it can be done. I have rehydrated gel cell types as well, to get additional life. But once the plates get dried out, it is often not possible to get back to a usable state. A desulfating, pulsing type charger is needed to reclaim the battery. A standard charger just heats things up and boils more water away, as you noted.

The Prius could do with a three state charger, which would taper off to a float voltage of 13V after the bulk and absorption phases. That is how it is done in places where batteries are meant to last.

The idea is that these batteries are spill proof for safety reasons. But being lead acid, they lose water just like the standard flooded cells.

 

Not if it is a true AGM it won't.....unless it is SEVERLY overcharged......or very poorly designed.

The H and O produced recombine to produce water again.....and they are completely sealed, up to a certain point, so there is nowhere for a "loss" to occur.

A well designed AGM battery should last 7 years or so if not in severe service.
I suggest that you forgo the OEM and go for another name-brand real AGM battery this time.

 

aye. i wonder if the vegas heat has any effect?

 

28 lb per my digital bathroom scale, a GS Yuasa battery with July 2009 production date. This battery is currently uninstalled and fully-charged, and I believe it to be AGM since there are no fluid sloshing sounds when I shake it.

It appears that some shipped GS Yuasa batteries are actually flooded lead acid, even though their external appearance is the same as the GS Yuasa AGM batteries. Not sure why Toyota accepts this practice from their supplier since the Toyota technical literature is very clear that only AGM batteries are acceptable.

 

In theory it should make little or no difference. A VRLA-AGM battery (Prius battery) is effectively sealed.
Each cell has a pressure valve on the top that only releases if excess pressure is built up. This can occur if charging rates are to high. When on charge at a safe rate the battery gasses hydrogen and oxygen, but these gasses are turned back to water by a catalyst in the top of each cell.
Pressure does not build up, the valve does not open, so there is no water loss. Even if the battery is heated there is so little air space in the battery for expansion to build enough pressure to open the pressure relief valves, and if they do release any pressure as the battery cools a vacuum is set up inside the battery as the valves work one way only. Making any further release less likely, and also stopping external oxygen entering the battery accelerating sulphation of the plates. The valves themselves are self sealing and although it would be possible to get a valve failure it would be most unlikely for all 6 to fail.
An AGM battery should have no free liquid inside the cells. As the abbreviation in AGM suggests A stands for absorbed G for glass M for mat. The mat is only 95% saturated in electrolyte.

John (Britprius)

 

this is interesting news since we usually advise not to replace oem with flooded lead acid.

 

Yes the strange thing is these OEM batteries with free liquid acid have AGM labels on them. I still have the original battery that was fitted on the car, (no longer used on the Prius) and this has enough free liquid to be able to use a bulb type hydrometer on it.

John (Britprius)

 

i wonder if toyota engineer's have done enough testing and are comfortable with this? perhaps poor agm results/cost have forced them to reconsider.

 

The battery concerned was from a 2008 original battery.

John (Britprius)

 

is there any possibility that toyota had one foisted on them and never recognized it?

 

Real AGM batteries were not too common 5-6 years ago.
It might be a transition technology.

 

The gen2 Prius has been around since 2004 and has always been specified with an AGM battery. However true AGM batteries have been around for a long time used in alarm equipment, UPS , as well as mobility equipment. The battery I use in my Prius is a mobility battery rated at 62 AH against the OEM batteries 35 AH or 45 AH. It is only 10 mm deeper from back to front all other measurements being the same fitting in the OEM battery tray.

John (Britprius)

 

Since model year 2001, all Prius were supposed to be equipped with 12V AGM batteries. I removed the top of the original battery from my 2001 several years ago and saw the fuzzy white mats separating the lead plates. No free liquid within.

 

I was not sure about the gen1 as the battery was in the boot "sorry trunk", and was not sure if in that position it required an AGM.

John (Britprius)