Charging 12 volt battery while terminals connected to car

that is incorrect info. as long as you are driving the car every few days, the 12v is fine.
but yes, charging the 12v while connected to the car is fine, you won't hurt anything, but you are just wasting your time.
btw, you're a/c runs on the hybrid battery, not the 12v, and the inverter is constantly charging the 12v, even when the headlights are on, as long as the car is in ready.
also, don't read battery threads from prius before 2016, the gen 4 is a whole different beast.
if you can't sleep at night thinking about it, you'd be well served to buy a digital volt meter for 20 bucks or so, and measure the 12v every morning before starting the car.

 

I'm wondering - with the HVAC - I know the compressor is powered from the HYBRID battery - but what about the dashboard fan (even the condensor fan?) - I'd have thought that would be powered from the 12V system (like everything else under the dashboard).

 

or perhaps the inverter supplies the voltage?

 

Sounds like we need a wiring diagram - which I don't have. They don't supply them any more with the car, unlike in the 1940s . Probably because they're much more involved than back then.

 

OP, there is a DC to DC inverter that converts the high voltage 200+ volt DC current from the main traction battery and motor/generators to the low 14.x volt DC current that runs most of the auxiliary motors and electronics (wipers, radio, ECU computers, steering assist, brake booster, fuel pump, etc.), and recharges the auxiliary battery, like an alternator would in a non-hybrid conventional car.

There is no harm in charging your 12v auxiliary battery when it's connected to the car.

 

Our car sees light usage, sometimes only short trips every 3rd or 4th day. I'm charging our (with a CTEK 4.3), fully connected to the car, pretty much all the time. Get home, hook up the charger, leave it on till next usage. It's ok to leave it on continously, just trickle charges once it's done with charging session.

 

I have a Schumacher 6/4/2 amp smart charger with float mode:



But I have been reading a lot about plate sulfation leading to early battery failure lately.

Here in Hawaii, batteries never last very long due to the heat, and many fail within the warranty period. I just ordered this Battery Minder maintenance charger with high frequency, low voltage, battery desulfation to see if I can revive my 2 year old group 24 battery in my Camry.

I will also be using it on the AGM Optima Yellow top battery in the Prius to see if we can get as long a service life as possible:

 

fwiw i am using a 2amp lead acid charger but even then i notice the 12v battery doesn't draw the full 2amp when charging. usually around 0.4-0.8amps. i think it's because my battery is relatively full already...?

 

As the battery charges up, the voltage difference between the charger voltage and battery voltage decreases which reduces the current. A "smart" charger will then measure the battery voltage by periodically reducing the current to zero and if the battery is judged to be fully charged, will only produce a "Trickle" charge current to maintain the battery.

JeffD

 

I look at it this way. If I was designing a car that had a big honkin' battery that can actually power the car and just needed a little 12V battery to boot up the computers and close the main power relay to the big battery, which battery would I use to power the lights, air conditioner, etc since I already need to convert the HV voltage to 12V to charge the little battery?

As others pointed out, there's no harm in putting a smart charger on the 12V battery, but if it actually needs the charger, that probably means it's dying. And keep in mind, every time you hook it up, you'll need to remind yourself, "red to red, black to frame" and make really sure because getting it wrong is expensive. Sounds simple, but anyone can have a brain fart at any time. The more often you make that connection, the higher the chances of cranial flatulence at the wrong time.

 

Ctek's come with an intermediate connector, and an extra one with pigtail connectors, for permanent wiring. I got tired of popping off the fuse box lid, fussing with clamps: installed the pigtail connector. Now it's just a matter of popping the hood and using a connector, more convenient, secure and no mix ups.

 

Car 12V lead-acid batteries, of any flavor, can loose charge over time, even when the car is driven every day. For example, even in a Prius (Pearl S), due to short trips each day, the battery is never fully charged. If you only replace 98% of the charge you used each time you drove the car, eventually the battery charge level will get too low. Granted this is a "how long is a piece of string" situation.
Pearl S is driven every day, but many days there are two trips of only 3km or so. A general rule of thumb is if you don't drive the car for at least one hour, you probably haven't replaced all the charge in the 12V battery.

Above I said "even in a Prius" because we don't draw the heavy current needed to crank over an engine at 12V. Our 12V batteries use very little charge to put the car in "ready". They can use a lot if the operator insists on using "accessory" to sit and listen to the radio (especially if the headlamps happen to be left on - which can happen more often than you may think).

Why am I convinced the charge level can drop over time? Because I've seen it in Pearl and in Pearl S.

Like Mendel, I use a battery maintainer device (in my case a "Battery Tender") if the car is going to sit for longer than a few days. Occasionally, I will connect it even if the car is being used every day (my Scangauge readout tells me when it's time).

Installing the pigtails that often come with battery maintainers is a good solution for ease of connection. Just remember to disconnect it before driving away! Running the extension cord through or over the door handle is a good way to ensure you don't.

 

Yup. Bottom line: it doesn't hurt, leaving it hooked up regularly. Once the main charging session is complete, it's just ticking over, giving it a small boost very intermittently. It helps to have the car garaged, second best is in a nearby driveway and a permanent connection, perhaps with the charger secure and completely under the hood.

 

I agree, if you're making lots of short trips, that's it's good insurance. And especially if you have a pigtail to foolproof the connection. I'v never had a problem because it's rare to me to drive less than 25 minutes at a time.

 

There's a positive, bolt-style connection in the fuse box. I had a bear of a time getting the plastic cover off it. @NutzAboutBolts helped out immensely: end up making a video how-to. Once you've done once it's straightforward, but it can be confusing. At least to me lol.

For negative connection just look for something solid and grounded. I went with nearby bolt holding bracket that supports inverter coolant reservoir.

 

This.
I live "right down the street". Mine is a bit over 3 years old and is doing fine in the same circumstances as yours.
It is perfectly OK to top off the charge on the 12 V occasionally.......but if it's driven every 3 or 4 days that should be totally unnecessary.

Edit: And I see that you have replies from some of our more vocal "purists" who insist on giving you details that you don't need. Ignore it.

 

Like they say, it's a free country, lol.

 

um i clamp the clamps of my charger direct to the terminals of the battery instead of the negative to the engine block. is that inadvisable?

 

The only reason it is recommended people connect the negative last, to the chassis or anything but the battery is to ensure IF there is a spark, and IF there is O2 and H2 gas at the top of the battery, you won't ignite it because the spark is away from there. If you did have the gas there and ignited it, it would result in a very bad day (the battery case can explode). The batteries we use in Prius tend to be "sealed" and very likely there is no gas venting at the top of the battery. But it's advice that is good and a good idea to follow.