What charges the battery?

I found this:

 

You can do it on other cars with just a 9 volt battery - the kind smoke alarms use. I don't know for a fact that you can do that with the Prius, but eight 1.5 volt AA or AAA batteries, or even one little A23 12v battery should work. There may even be a device... Here's a mediocre YouTube vid on how to make such a device from an AC adapter:

It does tell you to install a blocking diode, although it doesn't explain that this is to prevent the car battery from frying the adapter (or 9 volt battery, I assume).

 

Thank you Michael. That is the cheapest solution yet.
The diode prevents the current running from 12V battery to 9V, while the 12V is connected. It could cause 9V battery to overheat and explode otherwise.

 

Yes I have. I just attached a 12v battery to the connecters under the hood like I was going to jump start the car and then I took the old battery out and put in a new battery. The car never lost power or its settings.

By the way, it is better to test the battery directly in the back of the car. All of the wiring leading up to the front f the car can affect the reading. It is not hard. I have done it.

Allan

 

I too check the voltage at the battery however since the voltage is to be checked when there is virtually no load the wiring won't make a measurable difference. If you check the voltage when there is even a modest load you will get a reading that doesn't correctly reflect the state of charge.

 

Where are the jumper terminals in the PIP, again? I don't want to have to wade through the owner's Bible to find the info. It's kind of ironic that I can't use my higher-tech lighter socket "jump starter" cable on a dead Prius, but have to use actual jumper cables...

 

Inside the fuse box under the hood, there's a red cap that pivots open to access the positive terminal.

 

The manual also shows a bare metal ground stud over on the left side of the engine compartment.

 

Thanks for the pics. Hope I never have to use it. Is it ok to use a battery tender on the aux battery?

 

Good question! And if so, could the connection be done under the hood or should it be connected directly?

 

You can use a tender, and you can connect it either place, Just don't use the negative battery terminal - use a metal ground elsewhere. Thanks for the pics!

 

Thanks for the info, will be buying tender soon.

 

I have both a .75 amp Battery Tender Jr. and a Diehard 2 amp charger/maintainer, which I got for my Leaf. IIRC, they both use the same hardwire connection, which is convenient. If you want to have just one, I suggest the larger unit, as there are times when you actually want to charge the battery, and the little BT-Jr is just too weak for that.

 

I would think you could also use one of those Voltage meters that plug into the cigarette lighter to keep an eye on it. If you check periodically in ACC mode after sitting overnight (ideally "unloaded" with lights, AC, etc off) it should give a good sense of how the 12V battery itself is doing at maintaining charge. Absolute value may not be super accurate, but watching for a decline over time would give you a heads up that the battery is getting weak. You might also be able to get a useful reading in the moment between starting and when the high voltage relays close after the safety checks are run, you'd have to try it and see. You may see a lower number representing the loaded battery voltage (which would likely be lower than the resting voltage) that then jumps up when the traction battery and dc:dc converter are engaged if delay is long enough and the meter/your eyeballs are fast enough

Something like this maybe?

 

The only problem I see with the socket-voltmeter is that the accessory socket is only live with the car in Accessory or On or Ready mode, so it will always register a load on the battery.

 

Agreed, but with everything off that can be off it's probably not too bad an estimate and way more convenient...

 

Pardon my ignorance. Why are you so careful, to measure the 12V battery with as little load as possible? In my opinion, one should check the voltage when the battery is supplying a typical load*. If the battery is unable to maintain 12V under normal load, the battery is bad or something is wrong with the load, isn't it? Either way it would be a warning sign not to take such a car for a drive.
I am trying to suggest, the battery is not necessarily fine if it passes the easiest test. If it passes the stress test, it's sure OK.

* Of course, to have measurements comparable, one would have to load the battery approximately the same each time (ACC with lights, radio etc. on).

 

Battery state of charge (SOC) tables assume no load. The purpose of measuring voltage is to determine state of charge, not capacity, which is much harder. To have the capacity measured, you can try an auto parts store. Checking voltage will only tell you how charged the battery is at any given time.