Is it the stock battery or after-market? Even if you bought it one year ago, when was it filled with acid? Many auto parts stores sell sealed lead-acid batteries as "new" when they've been sitting on the shelf for a year and the clock starts ticking when the acid goes in. You can get higher quality AGM batteries that will last far longer, but know they are quite expensive. Alternatively, you have a parasitic battery drain going on with your car.

This is true of basically every car ever. Lead acid batteries can’t maintain a charge forever. Get a battery tender if you aren’t going to drive for long periods of time. If you only drive once a month do you even need to own a car? Just rent/uber when you occasionally need a car.

My Honda Hybrid is the same way, sometimes I go weeks between drives and if I park it for more than a couple weeks, I plug it into a 12V battery charger to keep the 12V battery topped up.

But I blame the car's smart features (that use the cell modem to allow remote start, etc) rather than it being a hybrid. I suspect that the non-hybrid model would be the same.

My 1991 Honda motorcycle does this too. I can promise you that it has no smart features. It doesn’t even have a fuel pump. Lead acid batteries just don’t have long shelf lives.

Well I get that lead acid batteries experience self discharge, my 2003 era car will go for over a month with around 20% discharge (it'll be around 12.4V), but after 2 or 3 weeks, my 2020 Honda will be at 50% discharge (around 12.0V).

I'd like to think that at some point the parasitic drain devices will shut down before they drain the battery completely, but I've never tested it beyond that point, I always plug in the charger if the car's going to be parked for more than a couple weeks. I installed a charger plug that comes out the front grill so I can plug it into the battery maintainer without even opening the hood.

> I always plug in the charger if the car's going to be parked for more than a couple weeks.

To this day, since ~1980, I disconnect the +ve battery lead if the cars not going to be used for a few days or parked up in the bush waiting for when we get back from a walk.

Stops parasitic drains and Heisenbug discharges when thermal expansion during a very hot day causes a hard to track short.

My 2019 vehicle's manual explicitly says not to do this, and that if the battery fully discharges you should make a follow-up service appointment to frobnicate the computers. I miss older cars.

I have heard that sudden voltage spikes can set off airbags.

Lead acid gel batteries hold their charge MUCH better than standard lead acid batteries. The difference is gel lead acid often self-discharge around 1% per month but standard lead acid batteries self-discharge on the order of 10% per month. I only use gel batteries on my small engine equipment like snowmobiles, motorcycles, and snowblower.

Happened to my wife's ex-Toyota CHR hybrid (not plug-in hybrid) after 5 weeks of vacation. I hooked a CTEK charger to it for 8 to 12 hours and all was good again. I didn't leave the car on the CTEK charger for five weeks because it was parked outside and not at our place.

Otherwise CTEK chargers are really nice. Certain car brands like Porsche while sell you a "Porsche charger" for two or three times the price but it's just a CTEK charger re-branded with the Porsche logo on it.

If you've got a garage with electricity, it's an option. If you don't want to let the car on the charger during 5 weeks, just connect it as soon as you come back and in a few hours the car is ready.

I have a VW bus that doesn't see a lot of use and with a bit of fiddling it was easy enough to run a (fused) connector to just behind the grill with a little rubber lid to make it waterproof. That allows me to plug in a trickle charger without having to open the hood. That little trick has saved me multiple batteries already.

I can use the cigerette lighter on my tt roadster's trickle charger. Does the bus have one?

I could have gone that route but it would require the cable to go into the interior from the outside so that's why I hardwired it to the front. It also helps that I park it with its nose to the garage so the wire is very short and I just let it dangle down when it isn't in use.

One thing I never understood is why hybrids have the 12 volt battery at all. Couldn't the entertainment system etc be powered from the 200V EV battery (after stepping it down obviously)?

Everything on the 12V system essentially is powered from the traction battery (the big one that can power the motors), once the car is turned on.

In Toyota, at least, the traction battery is completely disconnected (via a relay) when the car is off. The 12V battery is needed to power anything on the 12V system up until the car starts-- that includes the car's computer, which is what (after doing all its self-checks and whatnot) activates that relay and connects the traction battery to everything else in the car.

The traction battery isn't always connected probably mostly for safety reasons (having 400-someodd volts energized across the whole car even when it doesn't need it isn't great), but that also keeps it from getting excessively drained if something in the car malfunctions. It's pretty cheap to replace a 12V lead-acid battery if it's overdischarged after you left the lights on... the big hybrid battery, not so much.

Some EVs will monitor the 12V battery and periodically connect the traction battery to the DC-DC converter to maintain the 12V battery when the car is parked for an extended period of time. (On the older Smart EVs, this doesn’t have a limit, so the traction battery will kill itself trying to maintain a weak 12V battery. There’s a firmware patch for it.)

My Kia EV6 does that, and when the 800V battery pack is charging the 12V battery while parked, an orange light on top of the dashboard goes on.

This had me worried the first few times it happened. Then I found out it is a warning to mechanics/tow truck drivers/first responders that the high voltage system is energized.

Thats interesting; do they put the light somewhere that it isn't a nuisance to look at when you're inside the car?

The light is on top of the dashboard, right in the middle. It's intended to be very visible, especially from outside the car.

It is only on if the car is turned off and parked and the high voltage system has turned on to charge the 12V battery. When you're driving or just have turned on the car while parked, the light goes off.

So the only occasion the light might be annoying is if you're taking a nap in the car at night. And then you could just throw a towel on top of it.

Is it visible to the driver, while driving?

The light never goes on when you are driving.

You expect to have the high voltage system energized when driving, otherwise the car wouldn't go anywhere. There would be no purpose for this light to go on.

The light is specifically to indicate that the high voltage system is on when the car is parked. It's a safety warning for that situation.

Makes sense, thanks for the explanation.

Because so much of the real deep-down legacy bits of the car assume a 12-volt battery.

https://www.caranddriver.com/features/a38537243/electric-car...

There's a lot of Tier 3 supplier…stuff…in a car that was architected for 1978 and has never had a thorough re-think, because the profit margin is like nothing.

12v is such a common top-rail voltage for electronics of all kinds, I think its a bit more that than "car makers are too greedy to manufacture 400v headlights."

No. Or, at least, not easily. The high voltage battery isn't generally connected until "the computers are happy" with the state of things, for various hybrids/EVs/etc.

There's also a legal requirement that "marker lights and such" still work (presumably, also power locks and such) after a prime mover failure. Basically, if the engine quits, you should still be able to signal, get over, turn your 4-ways on, etc.

The easiest solution is to just put a lead acid 12V battery in the car for that. Lithium, in particular, is a problem below freezing because you can't safely charge it, whereas lead doesn't have that problem.

Even if they don’t have a 12v battery they still have to have a 12v system for reasons others mentioned and because it’s a requirement to sell cars in the USA. My Porsche Taycan has jump start lugs for the benefit of other cars and so I can have my low voltage systems operate with some help from another car.

My mild hybrid Audi is the same way. Gas engine, 48v accessory belt run battery, and that 48v battery works in tandem with a small 12v battery for the low voltage system.

Because they want to completely disconnected the big battery when not in use to prevent phantom drain, and you need a little power to run passive systems that need to be on still (like remote keys etc).

Some EVs like Tesla now use a separate lithium ion 12v battery that should last forever, instead of a lead acid 12v battery with a limited life span.

Maybe they hired whoever designed the original Tesla Model S 12V battery management system. Or the original Tesla low-power sleep system.

Oh man, that was a train wreck!

I did some analysis of the Tesla 12V system back in 2016, and I'm amazed the batteries lasted as long as they did with how badly they abused the lead acid battery!

https://www.sevarg.net/2016/10/30/tesla-model-s-12v-battery-...

I hate to admit it, but Kia got this right.

I own 3 Kia’s and they all have various problems, including really big problems, but this is an area they got right. On my daily driver, the Kia Niro, it has a battery button that instantly charges the 12v from the HEV battery. I’ve only needed it once, when I left a light on camping, but it was like magic.

What big problems have you had? I’m considering a Niro phev.

Not a Niro PHEV, but they have had... a couple of serious motor issues:

- https://www.kiaenginesettlement.com

- https://www.kiaengineclasssettlement.com

Edit: A friend of mine just got a free engine swap for her... 2012-ish Optima. It threw a CEL on the highway, had the appropriate trouble code, and bam, new engine from the dealership when she told them the code.

I had something like this happen on my non-hybrid vehicle. The battery was replaced under warranty, for free since it was less than 18 months old.

to the fusebox with a multimeter batman!

Unplug the USB chargers first.

Welcome to the motorcycling world! Today's cars continually start the engine - that's a huge draw. If you're going to leave your car sit more than a couple weeks then you should put your battery on a tender. Motorcyclists have been doing that for decades.

Yeah, I (not really) joke that everything on the hill gets plugged in. It makes life a lot easier.

Even with no idle draw, lead acid batteries self discharge over time. Life got way easier when I just accepted it, and now I've got a pile of 6V/12V battery tenders that go into just about everything (I have no shortage of 6V vehicles out here too). That and block heaters. The tractor and truck both appreciate them.

Why is that an issue for motorcycles?

Tiny batteries have less headroom against phantom drains. Also, most motorcycles aren't daily riders, so the problem is exacerbated.