I’m trying to find out of the “normal” auto battery in Mom’s Prius is slowly discharging backwards into the “battery pack”, i.e. the LiIon pack that is part of the Hybrid drive.
The car is from 2003. Original battery pack. Dealer confirms that several in the pack are failing. For reasons not worth going into, the car will remain in Mom’s driveway. It DOES drive. She is no longer driving it.
Here’s the problem. The car won’t start unless it’s driven every day. The normal battery ( located in the left side of the trunk ) will take a charge from an external charger and the car will start and drive fine. Since we are only at Mom’s every few weeks, the normal battery is always dead.
Not an emergency, just a hassle. I have to charge it before tooling it around on errands.
So, the question. Is that normal battery slowly discharging into the " battery pack " or is it just dying because the car is not used daily.
Ignorant of the wiring, I turn to The Teeming Masses.
Here’s my idea: Put a switch in line in the + side of the normal battery. That way, I can charge it up and drive it around and if it IS being slowly drained by the battery pack, the switch will stop the slow discharge from happening. ( things like car radio channels etc are irrelevant, so cutting the power from the car itself won’t mean a thing to me ).
If the battery pack under the car is NOT slowly discharging the car battery, then a switch won’t help and I’ll just have to continue to trickle it a bit when we visit Mom, so I can drive it around some.
Until recently I owned a Highlander Hybrid, which is probably nothing like a Prius, but when I bought it I asked the salesman if the “normal” battery died for some reason could I use the hybrid batteries to start the car, and he said no. The “normal” battery is not connected in any way to the hybrid batteries and is considered a separate system. Based on that my WAG is that the “normal” battery is no longer holding a charge and driving it every day simply keeps it charged enough to start the car. It’s easy enough to check. Take out the “normal” battery and take it into any car parts store and have them check it.
I don’t know anything about Prii, but it would be inconceivable if the two batteries were electrically connected. If there was any advantage of having the two batteries in parallel, they would make a big deal about it in their marketing. The batteries are completely different chemistries and voltages (the drive battery is high voltage), so I have to think they are completely separated.
It sounds to me like you have the typical “Phantom” drain, which has plagued car owners forever - a dome light, USB charger, or door lock (or some such thing) causing a slow but steady drain.
As an aside, I had my truck do this, and eventually found out I had left my OBDII-WiFI adapter plugged in, which drained the battery in a few days.
I had a Prius with this problem. The two batteries are not connected at all. How old is the normal battery? Mine lasted for about 8 years, which is damn good. I’m assuming that driving the car doesn’t charge the battery sufficiently. There are plenty of computers in the Prius so that there is a constant drain, which can make a weak battery worse.
I got a new battery, but the problem persisted, and the Toyota people found no problem. I finally took it to an independent mechanic, and he stress tested the battery enough so it died, still under warranty. I got a new one, he replaced it, and all was fine.
It is certainly possible that the hybrid battery is dying after almost 20 years, but that would wipe out the hybrid capability. But that’s not the problem. I’m betting time for a new regular battery.
Just to note that there are special chargers called “maintainers” that will keep the battery topped up. It shouldn’t be losing that much charge in a couple weeks, but you can keep it so that it starts a d doesn’t lose all of its settings.
I have a 2007 Camry Hybrid, a close cousin to the Prius. The batteries are not connected in any way. We just had to replace the starter battery (“normal” batter) because it was doing basically what you described. Batteries don’t live forever. Sometimes a bad battery can be charged up enough to start the car. Once. If your normal battery has never been replaced, consider yourself blessed that it lasted this long. If it is older than a few years, it may also just be at end of life.
A shop can test the whole electrical system to confirm this.
This car went unstarted and undriven from late Jan to roughly August of this year.
It is only charged and then driven about 10-15 miles once every few weeks. I will try a more robust drive with it next time to see if the battery can indeed hold a charge.
The “battery pack” in the floor of the car is 2003-era and has been deemed partially dead by her dealer. ( Mentioned in the O.P. )
It does appear I’ve just got a regular old battery issue to deal with. Which is a relief.
Sort of related, this is a story from Jalopnik (and accompanying YouTube video) about what happened when a Tesla’s 12V battery died after the car sat for an extended period. (Bet it didn’t occur to you that a Tesla has the same sort of 12V battery your ICE vehicle does).
Apparently the big battery pack in a Tesla is what charges the 12V automotive battery and in this case the big battery was fully charged, so when the car sat, it failed to keep the 12V battery charged.
I think this is true for the Prius, at least older models, but our Kia Niro has a 12V reset button that will revive the 12V battery (which is rather wimpy) if the car has sat so long it’s dead.
I’ve been told [ETA: by car mechanics] that modern vehicles need to be run at least every two to three weeks, for long enough to charge their batteries up; because, unlike vehicles from say the 1970’s, they’ve all got minor power drains that are on all the time. If the hybrids run all that stuff from the starter battery (about which I have no idea), then that in itself means you can’t let it sit for months and expect it to start.
I can let a 1950’s tractor sit for months in the winter and it’ll start right up in the spring (though not, of course, from a 1950’s battery.) But the 1998 work van needs to be run every three weeks or so, and the 2007 Subaru I’ve been told to run about every two weeks.
I have a seldom driven '16 Toyota Avalon. Last spring the 12V battery would not hold a charge. Replaced with one from the local auto supply store and no more problems.
Yep. If the car is barely used the failing drivetrain pack(s) is probably not a big deal. You’ll continue to have a working car, with a diminished or non-existent fuel economy savings.
I’ll add to the chorus about the batteries being disconnected. This happened to my wife’s Prius quite a bit and we could always trace it back to the “normal” reasons this happens in any car: in our case one of the kids leaving a dome light on or a door open. We got one of those portable jump starting devices from Costco, so its batteries all the way down.
The Tesla article was interesting. I hadn’t put any thought into when the regular 12V battery was charged on a pure electric car. Does that imply that it isn’t charged while driving at all? Only when piggybacking on the drivetrain battery charging?
I know zero about cars- but doesn’t the normal battery take a charge off of the alternator? And if so, that has nothing to do with the Prius “pack”.
Seems I am not driving it LONG enough when I am in front of it every few weeks. Need to give an hour to getting out onto the highway and ripping up a bit. 60 minutes of real driving may help the battery in a way that driving around the block twice will not.
Right. For a regular Prius ICE / battery hybrid the 12V battery is alternator driven same as a regular ICE car. I was commenting on Tesla or other completely battery powered cars. These don’t have alternators as far as I’m aware and rely on DC to DC convertors (400V to 12V in the Tesla case). I was curious about when the 12V battery charging happens in this case… is it just when the drivetrain battery is being charged or does it draw power from the drivetrain battery during driving as well?
My understanding is that the 12V battery is charged by the traction battery whenever the 12V’s voltage drops below a certain level. Because the 12V battery is running all of the automotive electronics (because those are designed to be run at 12V), it might happen during a drive.
My rule, which I made up, but seems to work, is I put the motorcycle on a battery tender when I haven’t ridden it for two weeks, and don’t plan to ride it in the next week. I put the truck on a battery tender if I haven’t driven it on at least a 10 mile trip in the last month, and don’t plan to do so in the next week.
My guess on the Prius is the 12V battery was already old, and leaving it sit for so long let it discharge completely, which is very bad for a lead acid car battery, and that finished it off.