ONE GOOD WAY TO KEEP 12V BATTERY CHARGED UP

Actually the method you propose is not a really GOOD way to keep the battery charged either.

Driving the car has more advantages.......but that has been covered in other threads too.

I think this thread really accomplishes nothing useful.
Sorry.

 

I seem to recall protracted idling can actually run the 12 volt DOWN (excuse cap lock, highlighting is a pain on phone). Maybe more so on conventional car.. And, idling a Prius is an exercise in futility, it’ll maybe shut down in 2~3 min. If a dedicated charger takes at least 3~4 hours, what does 5 minutes idling do...

 

The engine. 12 volt battery keeps charging though, as long as the READY light's on. Doesn't matter if the engine's running.

 

I'm finding this thread useful since I don't have a convenient place to plug anything in at home.

Unless it's a last resort I won't be leaving the car in ready at home. What I may do is leave the car in ready mode while I'm waiting or between errands such as curbside pickup. Typically I used to turn the car off when "idling." Habit from ICE only cars I guess.

 

Ooops... Disregard!

 

If you stay in the car beween errands or while waiting for curbside delivery, then you only worry about the thugs who would drive off with you in it.

 

I believe this is one of the more important posts on this thread. It points out how difficult it can be for even well informed people to really grok the entirely different purpose of the 12V battery in the Prius/Prime. Of course precautions are necessary to prevent excessive discharging, but as said, if it starts the computer it has enough charge.

I was following with interest another thread about low voltage on the accessory port while driving. I haven't done my testing under load yet, but here is a graph of the voltage directly on the 12 Volt battery terminals beginning after an overnight charge just before I took the car for an EV drive that never required the ICE to start. The X axis is seconds with measurements made every 2 seconds. It covers roughly 2 hours of running errands with the time I was parked evident as flat areas of no activity. Before READY the voltage was at about 12.45 volts. The voltage varies from about 14.3 to short spikes as low as 11.6 VDC during the drive. When I parked it had apparently charged to about 12.6 volts.

So I hope you find it interesting. I will do this again with a substantial, 60 watt or so, load on the accessory port and will post that data on the other thread about low accessory port voltage.

 

'if it starts the computer, it has enough charge'

this is no way to measure battery health

 

True, that would require a load test, but that was not the point. The point was to relax and not obsess over the exact voltage. The battery can be perfectly healthy if the system has chosen to leave it a couple of tenths of a volt lower or higher. As long as it will start the computer it is functioning properly within the system. It's overall health is another issue.

 

I have a CTEK charger but wasn't sure how to hook the two permanent connections to the battery. The manual suggests that the negative be attached to a spot under the engine cover, which wouldn't work. Would the two red spots work? Or do you have another suggestion? Thank you.

 

With a 2020 I believe the battery is under the hood? Which greatly simplifies things (why am I disappointed...). You can connect at those points, and it'll be fine.

The alternative I used, having a third gen with battery at back of car, and still wanting the wire to be in the engine bay, accessible by popping the hood, was to connect the positive CTEK quick-connect cable wire where the main 12 volt positive wire connects at the under-hood fuse box. The negative connection can be any convenient/safe/solid bare metal point on the car body.

This video shows accessing 3rd gen under-hood fuse box positive connection:

4th gen may be similar. One thing I found, with my install (at that fuse box point): the pig-tail on the CTEK quick-connect positive wire has a plastic shoulder that needed to be mostly trimmed off, because there's very little room where it connects, it needs to bend down 90 degrees immediately beyond the connection point.

For negative connection, I first was considering loosening an existing bolt and sandwiching the CTEK neg pigtail connection. Then I noticed an unused threaded hole, quite close to the fuse box, and used that, with an extra metric bolt I had.

 

I would connect at the nut behind the clamps and tie wrap them down to follow the harness - before bundling them back together to plug into the charger. IMHO, it'll look cleaner and doesn't interfere if and when you need to replace that battery.

 

Thank you Mendel for the quick and detailed response. You are correct that my battery is under the hood. I also agree that the CTEK connectors are problematic in many cases but they are nice little devices when you do get it connected.

I'll take a look at the video and the videos in your signature area. I've only had the PP just under two months so I am certain I will find some very useful tips.

I had one more question. When charging the 12 volt battery can you also charge the battery pack or should you just perform one or the other?

The date code on my battery indicates that it is five years old. Yesterday my wife had an issue where the Prius would not start so I had to go and give her a jump. After charging with the CTEK it is at 13V now. Think I'll order the Solar BA9 you suggest and see if I need to replace it. Thanks again.

 

You're just charging the 12 volt battery battery. For the hybrid battery periodic charging is not really necessary, nor trivial.There are "chargers" for the hybrid battery, but they're more for battery balancing/restoring. The main thing that's really detrimental to the hybrid battery is a protracted idle time, like multiple months. I think Owner's Manual cautions, to not leave the car without use for more than "several months" IIRC.

That said, our '10's build-date was August, 2009, and we bought it, with about 10 kms on the odometer, in November, 2010. So far no problems with the hybrid battery. Knock on wood...

Good idea.

You're very welcome.

 

You may charge both batteries at the same time. When the traction pack is charging, the DC-DC converter is active. It is a 'smart' unit, so seeing your CTEK will reduce it's output to near zero. Make sure whatever maintenance smart charger you go with can output at least 3-4 amps to overcome the Prius's interval self-diagnostic current draw.
Also check and top-off the battery electrolyte levels. They're probably low, if nobody has been checking them. Use a nickle or quarter to gain access.

Hope this helps.....

 

I thought the MF code shown on my battery meant it was maintenance free. Would I still want to top off the battery in that case?

 

If you want it to last past it's projected end-of-life, you can just leave it alone and buy another one. The maintenance charger isn't going to do you any good if the cells are dry. IMHO.