Can a weak 12v battery cause this much trouble?

Greetings and welcome to Prius Geekery!

Because the 12v in a Prius doesn't power the engine starter and the HV Battery does, the criminals who bought Optima and moved the manufacturing plant to Mexico turned one of the highest rated batteries of the 1990's into total garbage and they get away with it because in Prius' case the 12v system needs no cold-cranking amps rating and functions much like a back up power supply for your home computer.

As such, the very best 12v battery you can buy for your Prius is the most common 12v AGM battery for wheelchairs and backup power supplies that they sell at Batteries Plus for ~$140, which is $100 less than auto store battery for Prius. You do have to stop by the hardware store to get some longer bolts and bushings to adapt the terminals to fit these batteries though. But at 55amp hour these batteries are way, way better than anything you'll get an auto parts store.

As for a new 12v solving your problems, that is possible, but you usually get way more crazy on your dashboard and way more crazy on your OBD2-based apps or Toyota Techstream when that's the case.

As for the car running ok for a little while, that's most likely the 12v being disconnected for 5 minutes, which clears the codes and resets/reboots the cars system. So it takes a bit of driving for the car to remember it has a bad HV pack, or in some cases it takes the HV battery getting up to 100' degrees before the bad HV symptoms are obvious to the sensors.

What you're experiencing with the ECU running, or not, or losing power, etc is the computers gradually eliminate your access to HV pack as the pack's conditions worsen till you get to the point you lose use of HV battery entirely because the car is trying to protect itself from further damage. But then just disconnect the 12v battery again and let it all cool down and you can limp down the road a little bit again. But don't do that if you can avoid it. I overheated a pack doing that once.

As for repairs, first step is eliminating the least expensive fixes for bad hybrid battery symptoms, which is replacing the 12v and making sure the HV battery cooling fan isn't clogged, which is primary cause of over heating in most cars, especially dog owners cause the air intake to the right of the back seat isn't very well filtered.

Next step is to brush up on high voltage safety protocols and remove HV battery pack (see youtube for that), then pull off the bus bars, which leads to the next least expensive fix, which is the corrosion on the voltage sensors and bus bars.

Then if that doesn't fix it it's time to do a voltage test on all 28 modules, with less than 0.3v being the most amount of difference between healthy modules. Of course even bad packs will show you positive results, which is why you need to then do a load test on each module, which means connecting the car's headlight bulb to one module for two minutes and record the amount of voltage drop, usually .4volts or less of a drop is common. And usually that's enough to identify a bad module.

The next step is to buy a used module that's better performing than the existing modules(more on that when you get to that point) and install it. Then go thru a process of battery reconditioning which means using headlight bulb to discharge the battery down deeper and deeper amounts and then recharge with a NiMH smart charger, which are commonly used for charging RC cars and drones. Most of us prefer the Imax SkyRC B6AC V2. Or alternatively, you can do battery reconditioning on the whole pack at once with the pack still in the car via this much more expensive equipment: Prolong® Battery Systems FAQ – Hybrid Automotive

The re-conditioning discharge numbers for each module by way of electrical engineers at Hybrid Automotive are:

Stage 1 discharge: 4.8v
Stage 2 discharge: 3.0v
Stage 3 discharge: 0.6v

The benefit of doing reconditioning on each individual module rather than the whole pack is that even the a perfectly matched set of modules will show noticeable variations during the charge and discharge process, which makes the hardest to find bad module identifiable. The concept behind all this is that Prius Batteries function between 60-80% capacity in the car and that lack of full range of use of battery power cause tiny well conducting crystals in the battery cell to grow much larger into crystals that don't conduct as well. Because of this Toyota designed an HV pack to continue to work normally even after it's lost 50% of its capacity. More info on that here:
BU-807: How to Restore Nickel-based Batteries – Battery University

Some things to keep in mind is that Toyota and most mechanics won't replace individual modules in an HV because it's too problematic when it comes to having a set of modules match each other well enough when the pack gets up above 100' temp and there's 90+ amps pumping through it. The good news is that, while this is definitely true for early Honda Hybrid packs, there's a high enough of a success rate with Prius packs that just about every major city has a Craig's list mobile service entrepreneur making money rebuilding & swapping out packs, some doing a much more thorough job than others.

 

the battery pack is 13 years old and over 200,000 miles.. so that would be the cause of your problems..
BUT when you first start the car dont try to drive it until the ICE comes on and goes off on its own, if you drive the car before it cycles the charge will drop like a rock also keep the ac/heat off when starting out.. your battery is weak not point in killing it.
as mentioned you need the correct code scanner to find out whats going on, odds are you have 2 or more weak or leaking module inside the battery pack... if you open it and see "rust" its actually the acid leaking..
watch a few youtube videos on rebuilding the packs and follow all the safety steps..
as for the 12 volt take it to a auto parts store and get it load tested..

 

Oh yeah, I forgot to mention, you can force charge battery by putting car in drive and put foot on the brake while flooring it. But you problems probably won't be fixed by that.

 

Maybe. Maybe not.
Odds are that BOTH batteries are bad.

But it would be rather foolish to chase any other leads without first replacing the 12 V battery.
And any AGM battery can be used......as long as it will physically fit in the space, has a similar capacity, and you can "adapt" the posts to what the Prius has on the cables.

 

Exactly... But Prius doesn't need a battery designed to pump alot of amps to start an engine, it just needs alot of amp hours to last as long as possible, which is why standard wheelchair/backup power supply batteries at 55amp hour or higher is the true geek way to go!

 

What you are is a flipper. The bane of every hobby. At least be honest about what you are.

 

how has the wheel chair battery been working? any issues? thinking of going that route when this yellow top dies

 

TOTALLY unnecessary, useless and uncalled for nasty remark.
Oh, and with no quote, we don't really know WHO it was directed to.

Not me who has done that.
Maybe someone who has will respond.

 

Put one in back in 2013, haven't once had an issue, even with headlights or dome light accidentally left on a couple times... Might put it on a trickle charger for the first time soon when I do maintenance on the hybrid battery that has 232K miles. So far no battery problems, just little loss of capacity of HV battery.

 

Gen 1 packs still use the same type of battery’s inside the pack I think. In case you were thinking of repairing the pack you have.

Gen II Prius Individual Battery Module Replacement | PriusChat

 

Point taken. My apologies.

 

You can actually fit a much, much larger battery in that spot if you pull out the battery mounting hardware and replace it with something that holds it in place from up top... As for Torque app, I haven't got it to work and it hasn't been supported/updated since last Spring. And if you're focusing on a the HV pack you'd be better off with Hybrid assistant and Dr. Prius apps... Of course there's like a 5 to 1 ratio of OBD2 readers out there that don't work and myself and many friends have bought several in order to find one that actually has good enough hardware, not to mention free of manufacturing flaws, which seems to be way too common with these things. Here's the best article I've seen on buying one that works: Hybrid battery diagnostic and repair tool for Toyota and Lexus

As for Bosch, they make some of the best home gardening power tools, but they all run on EU 220V and because of lawsuit happy US folks, they don't even sell the battery powered garden stuff in the US. They make one electric wood chipper that works really well for small projects. If I lived in Europe I'd own a ton of their quality gear!

 

Gen 1 cells are the same voltage and there are more of them in the pack than a Gen 2, but they are also much older. For best reliability, get the newest un-rebuilt battery you can get and use all the cells so they are matched.

 

The Gen 1 modules ARE NOT identical to the Gen 2 you need. They are identical in capacity, but not physical dimensions. Nor do they match in how they interlock together.

Did your friend send you a photo of the serial number sticker from the battery? Sometimes you run into a recently manufactured Gen 1 style battery. The manufacture date can be decoded from the serial number.

If this is the case, since Gen 1's have 38 modules, you could use 28 to do a complete swap out in your Gen 2. Not the ideal scenario, but it would be a cheap way to freshen it. Also, a full swap out semi-negates the interlock tab difference, except for the outside modules where they meet the clamp end caps. The module mounting holes still align correctly with the base plate, just the module length of a Gen 1 is a bit shorter. The wiring may have a weird little wiggle in it due to the difference in module size but should still work.