12v dying.

If the battery terminal voltage at Ready is 14 V, and the resting voltage of the battery (AGM type???) is 10.5 V, that is per the FSM a bad battery, and needs replacing.

 

I posted the battery test instructions in another thread. 6 amp is too fast for the original size, but you have a Group 25 battery which is bigger I believe, so I’m not sure. Here’s what I recommend to test if it is too fast:

Charge the battery with the charger. Once the charger says it is fully charged. Unplug the charger. Then plug the charger back into the outlet. If the charger starts charging again let it charge until full. Keep repeating this. If it keeps going on and on and on and on, (my 6A noted roughly 50% charged each time I repeated), either the battery is bad or the charger is too strong. If at some point you unplug and then replug and it still says full, the battery has reached fully charged.

Notes on the test when in situ (connected to the vehicle) say to then turn the high beams on for 30 seconds. If at that point it says 8 V, I believe the spec is less than 11 V, that battery is a goner.

 

I would say that is a very good guess.

 

Checking my third gen I got 16~20 milliamps, with periodic short spikes to 40 something. 20 milliamperes is same as @Patrick Wong 's .02 amps.

Consider getting a smart charger, 3~4 amp range. Also an electronic battery tester, like Solar BA5 if you can budget: very handy.

 

Your battery is beyond redemption.

Yes, exactly.

Don't bother, just go buy yourself the correct AGM, or direct drop in replacement.

 

Take it back to where you bought it and have them load test it.

 

Well done!

 

I discovered mine this morning was down to about 4 volts. Ugh. And despite being the size of a lawnmower battery, is $200 to replace! Uggggghhhh. I just posted more detail in another thread, but I discovered you can use a standard Type 51 if you replace the terminal lugs and get a vent adapter. $80 at Lowes, or $120 at Wal-Mart with a 5-year warranty.

There's an electrochemical reaction that occurs below about 12.5V that causes plate sulfation; let it go too long, and it damages the battery irreversibly. Charging (and resting) about 12.5V will usually dissolve the sulfation. You always want to keep voltages about 12.5V if you can.

 

I gather that that group 51 battery is a regular flooded Pb/A, being so cheap. The reason why the OEM battery is more expensive is that it is an AGM construction and as such runs slightly higher voltage and lasts longer. It also tolerates more frequent deep cycles.

You will find that regular flooded Pb/A batteries do not last as long in a Prius, and even less if you are not driving it a good 45 - 60 min per day, the latter being true of any battery.

If your drive cycle is on the lower side, invest in a good charger that goes into a maintenance mode once it has charged the battery and use it often. Unless you are happy to spend $80 every 2 - 3 years replacing your 12 V battery.

 

try some auto parts stores

 

Wow, for $80. Who'da thunk!

 

Type 51 is a form factor, a lot of different companies make it. You can get it in multiple technologies, but AGM is generally going to be the cheapest. Many stores have different supplies regionally, but any Type 51 should fit.

Wet-cell are less sensitive to Arrhenius degredation and generally last a lot longer than AGM in hot environments if you keep them topped off with distilled water, but they also constantly off-gas during charging, and I wouldn't put one in a Gen 3 Prius because it would be dangerous if the vent line came loose or case cracked. AGM are only going to off-gas if there's a failure in the battery or charging circuit that causes one of the valves to pop open, and don't usually leak any acid if the case cracks. Regardless of AGM or wet cell, it's very dangerous if hydrogen gas is release but not properly vented, but you'll notice it very quickly because hydrogen sulfide gas is also vented and has a horrible and unique stench. It would be also difficult to service a wet cell in a Prius due to the location of the battery (hard to check & top off acid levels) which may be why people's experience is that they die so quickly: you should be checking them once a month or so. AGM are basically maintenance-free.

This (or similar) will work well for the negative terminal. My existing pigtail was long enough to re-use despite cutting it a half inch shorter to install the new terminal. Very easy to install.


The positive terminal uses a different style, one with a screw, and is very easily replaced. I used a new heavy-duty terminal very similar to this one below, but I found the wingnut fouls the relays and it was difficult to install, and I had to slightly enlarge the hold. A lighter duty one with a nut, like the 2nd photo, should be fine and easier to install.