Want 12 Volt Battery Specs.

The cables are made for small terminals, and any battery used there should be vented. Optima makes a replacement battery; these guys sell that plus an adapter kit. Together it costs about the same as a Toyota battery but ought to be much less wimpy.
Parts kit for installing Optima battery on 2004, 2005, 2006, 2007 & 2008 Prius

 

It is, approximately, a 30 amp-hour VRLA with a vent tube fitting installed. The valve behind that fitting is not supposed to open during normal operations. If something unusual happens, the valve can open and the hydrogen gas (etc.) gets vented outside the car.

The terminal posts are of uncommon dimensions, which can cause slight problems with substitutions. There have been a few alternatives mentioned previously here. Seen those?

 

Note that the Optima is a 38 A-Hr battery, very close to the capacity of the stock battery. It's a better quality design.

I -suspect- many of the reported problems with the stock battery are caused by two things:
1. The stock battery is nearly or fully discharged during shipping from Japan and/or sitting on the dealers lot.
2. The owner shut off the car but runs with the headlamp switch on all the time. He/she sat in the car with it off waiting for someone for a while. The headlamps will stay on in this situation, unless you remember to open and close the drivers door. This will drain the battery (Optima or stock).

Fully draining -any- battery, deep discharge or not, will shorten its' life. It actually reduces the charge capacity of the battery a little every time you do it. A "deep discharge" battery suffers less from this than a "starter battery", perhaps only 10% as much as a starter battery.
Note that the Prius battery is a perfect use for a "deep discharge" battery, as the current draw is about 50 Amps max. A "normal" starter battery is designed for current pulses (under starting) of 150 Amps or so. This would shorten the life of a deep discharge battery, about as much as deep discharging a starter battery. So you make your choice of compromises when you choose the battery.

 

The Mazda Miata battery will fit with minor modifications. An Optima battery will also fit, as others have mentioned

I had trouble with my Prius battery the first winter. Bitter cold temps, headlights on, Max Heat (No winter front so almost froze), electric rear defrost, the Prius 12 vdc system couldn't keep up. I put in a VDC Battery Minder and that solved the problem

If you intend to leave your Prius sitting for extended periods of time, a battery tender is a must. Never use a trickle charger, as they can damage a battery

 

As I understand, it really is a completely different design than a traditional wet lead acid battery, even a so called "maintenance free" or vented one. A sealed VRLA (valve regulated lead acid) is important for two reasons in the Prius's application. First there is little to no liquid acid inside. The acid is soaked into a fiberglass matting, making it also refered to as an AGM or "absorbed glass mat" battery. This is important since the battery is mounted inside the vehicle. In the event of an accident, you don't want liquid sulfuric acid splashing around the inside of the passenger compartment. Second, sealed lead acid batteries vent no hydrogen gas under normal operating conditions. Traditional starter batteries can vent quite a lot, hence the concerns about blowing up your car when jump starting. "Maintenance free" or vented starter batteries vent less hydrogen, so they generally don't need to be topped up with water. Again, since the Prius battery is in the passenger compartment, they really want to make sure that there is more or less no possibility of Hydrogen gas accumulating in the car. There are certain failure situations where even the sealed batteries will vent Hydrogen (permenantly killing the batery), so Toyota also makes provision for venting this to the outside in the unlikely event the dc:dc converter goes berserk or something. The battery really has much more to do with the ones used in computer UPSes, home alarm systems, or electric wheel chairs than they do any sort of traditional car battery.

The Optima mentioned is generally a high end automotive version of the same type of sealed VRLA type battery, so should be a pretty good candidate. They are pretty expensive, but very popular with the offroad and import car crowds due to their high current output, high capacity per size/weight and good deep cycle life span. Unfortunately the yellow top mentioned (D51) has I believe been discontinued because of either a problem with the design or the quality control issues resulting in failures. The other yellow tops are generally very robust, but are sadly too big to fit in a Prius as far as I know. The D51 may be fine in the relatively benign environment of the Prius. Most of the failures I have heard discussed were from the import crowd, who would use them to run their x000 watt sound systems for hours with the engine off, and then start the ICE off them too. The bigger yellow tops excel at this sort of abuse, but the smaller D51 didn't quite seem to cut it. Of course at $150 the D51 isn't really much cheaper, but it may be more tolerant of abuse than the factory battery. You could probably use UPS batteries, but the challenge would be wading through the huge number of available models to find one that would provide the correct specs (max discharge, charge voltage, float voltage, internal resistance, cycle life, temperature range, terminal style, capacity, peukert number, size, weight, etc, etc). Wouldn't be so bad if we new what the specs of the original battery were. A possible example of the right sort of thing would be the 35 Ah UB12350:
http://www.universalpowergroup.com/specs/D5722.pdf
It seems smaller than the D51, and they spec it up to 105A so it might work. No idea if the terminals are the right sort of thing though, or if the charge/float voltage are ok. At $65-$80 it might be worth looking into more. UB gives no spec on number of cycles though hey are generally intended to be deep cycled. Here's another thats still smaller than the D51, 39Ah, spec'd up to 400A and claims 350 cycles at 100% DOD (~10V as I recall) :
CSB Battery Technologies
Price is still pretty good at ~$100, but again no idea if it would really be as good as or better than the OEM battery. FYI, both of these batteries are slightly larger versions of the ones a number of folks are using in their PHEV conversions.

Lastly, using a regular wet lead acid battery would not only be dangerous it probably wouldn't work very well. I believe the sealed batteries are charged at lower voltages, so the output of the dc:dc converter would probably be too low to do a good job of charging a regular wet battery.

Rob

 

The Prius 12V system runs at 13.8 V, which should be fine for either "normal" lead acid batteries or deep discharge types, either wet cell, AGM, or gell type. It's a nice general charge voltage, which shouldn't "overcharge" any of the types of lead acid 12 V batteries.

The Optima yellow top is a deep discharge type and the Optima red top is a "starter" battery.

The only problem with the Prius system is it isn't temperature compensated. That is, when the ambient temp drops below freezing, the charging system -should- increase the voltage slightly (going up to about 14.5 V at -30C). The Prius doesn't. This is one reason the battery looses charge at extremely low temps. Also, when the ambient temp rises over 40C the charge voltage should drop a bit (going down to about 13.5 V). Again, a reason those in hot climates may experience short battery life.

I suspect the main reason the Prius battery is expensive from Toyota is the lack of competition. It uses non-standard terminal sizes and has the vent tube. It may actually be intended for motorcycle applications.

 

Optima must have corrected that because they still sell it.

 

With constant use of the battery tender, you should be just fine for 6 days. Wouldn't hurt to disable SKS if your car has it

The VDC Battery Minder I use will also desulfate the battery as well. At the hobby farm, after I disconnect the Battery Minder, I'll leave the neg cable off but recheck the battery voltage after 24 hours: 13.1 vdc

With any battery, once you get into a limited market the price really goes up. Price a battery for a Cessna 172/210 and you will grab your chest

 

Thats good news. For a while you could still find them on the market, but there was no data available on their own website. Looks like its back on there now.

I think you may be right about the motorcycle thing. After some googling, it seems like the Gen I battery at least is a re-badged Yuasa. Yuasa makes a lot of SLAs for motorcycles, jetskis etc. Couldn't find as much info on the Gen II battery right off hand, anyone know if they are same/similar?

Here is where I found some info on the Gen I:
12V Battery in Classic Prius

I'm guessing it could be the same, as the 34 in the part number probably denotes a nominal 34Ah 20 hour capacity. From the few pictures i can find of the Gen II battery it looks similar. If thats the case, there may may not be that much markup. The 3x Ah 12V Yuasas I can find seem to run ~$150.

Rob

 

Hi Rob,

There are two possible 2G 12V auxiliary battery choices. For the base model without Smart Entry/Smart Start, the GS battery is the same size and case style as that found in Classic, except that battery terminal polarity is reversed. The larger capacity 12V battery for 2G with SE/SS has an all black case and is longer.

Since the Classic battery has an AH rating of 28 at 5 hours, my guess is that the comparable AH rating for the larger 2G battery is in the low-30s.

I posted photos of the larger 2G battery compared to the Classic battery in the following string, msg. #10:

http://priuschat.com/forums/prius-technical-discussion/44500-sudden-12v-battery-failure.html

 

how long would you say if i measured 12.5 volts with the all the interior lights off (including the hatch door light, genius pressing on it enables it to be on or off!) after the car has been sitting for at least 5 hours + and had a small ev mode swing so i can work on the car?

also ordered the optima yellow kit, however the batteries themselfs are very expensive... considering getting a 2nd hand one and bring my voltmeter along.

 

While you're exploring, be sure to read Hobbit's 12V adventure. You will see a liquid electrolyte battery with a bulge, among other items of interest, like dimensions.

 

I don't know what you guys are complaining about, Toyota want us to pay $470 (=$450 US) for a replacement 12v battery here in Australia. I have been looking for an alternative as I don't think the guys selling the Optima one will post to Aus.

 

If you can get the Optima battery in Australia, maybe all you'd need to order from the States is the adapter wiring. The Optima and Toyota battery posts don't match.

 

Interstate also sells the Optima batteries, badged accordingly. There may be minor differences in construction, but they are made by Optima for Interstate. So that's another possible source. Interstate dealers will often give motorcycle clubs good discounts. Check it out. My experience with Optima is they support their dealers rigorously (will not sell to non-dealers), and so far I haven't found a dealer who was willing to discount.

 

That is a really nice adventure by Hobbit!

 

Is the 12v battery in my 08 Prius maintenance-free, like the gel-type? Or do you periodically have to check the electrolyte level?