Floating the 12V battery

Why do you need to replace the 12V battery on a 2008. There must be a story behind this one.

The best way to float the 12V system is to connect another battery to the jump points under the hood, or use a 12V battery minder on the jump points.

Tom

 

"Float?"

As in charge the 12 vdc battery?

As in retain vehicle memory when replacing the 12 vdc battery?

 

Just connect a known good 12VDC source to the positive cable and the ground cable, before disconncting aux battery. ie: a battery of about the same size. I use a 7AH gel.

 

"What else floats?"

"Batteries."

"Batteries?"

"Well, small ones..."

<sorry, that last Monty Python thread is stuck in my head>

Tom

 

:doh:

I really wasn't looking forward to leaving for another week.

It doesn't seem so bad now ....

 

The easiest and safest way to "float" the battery as in the OPs meaning, would be to connect a 12V battery to the boost terminal under the hood. This would ensure any sparks generated would be far from the Prius battery. You wouldn't need "jumper cables" to do this. The Prius will draw a maximum of 50 Amps from the 12V system, in approximately 1/2 second pulses. It WILL do this even when not turned on, so be careful and don't be surprised if it happens. It will draw one pulse if the brake pedal is touched, for example. It will also draw significantly less for 10 sec. or so some time after shutdown, when it checks the evap. system. It's very difficult to control this, so expect it.

This being said, it's probably better to just not bother. Changing the battery with another 12V source connected is more dangerous than just disconnecting the battery and letting the Prius forget. You only have to reprogram the radio presets and the drivers side window auto-up/down. Both are easy and fast. You will also loose any codes stored but who cares about that? If there's a problem they will quickly be re-stored (no pun intended).

 

Hello David, Very interesting comments. I have now done what the OP asked on several occasions, mainly to sub other aux bats just for experimentation purposes. I have never drawn a spark, no less an arc. I would naturally never dsconnect a powered up vehicle. Have you personally ever experienced a 50 AMP arc when replacing batteries on your Prius? And exactly where is the danger factor. Certainly not excessive hydrogen. Your idea of connecting up front under the hood would be a better idea and no memory would be lost. Andy.

 

Absolutely, NEVER have the car "on" in any mode when messing with the 12V battery, and especially if you are messing with the "traction battery". That would be courting disaster.

The danger with having a 12V source connected when changing the 12V battery is if you manage to short the positive 12V Prius battery connector when you disconnect it. Even though the Prius battery is "sealed" and "vented outside", don't rely on that to protect you from igniting the H2. Seals fail, especially on batteries that are "in trouble" (which is presumably why you are disconnecting it). If you do manage to spark the +12V line to ground it -could- generate enough of a pulse to damage some part of the computer systems in the Prius. Best to avoid this.

The -SAFE- way to work on batteries (i.e. disconnect and connect them) in a -ve ground vehicle is to disconnect the -ve post first. If you short the wrench to ground nothing happens. If you drop the cable to ground nothing happens. -THEN- if when disconnecting the +ve terminal you manage to short the wrench to ground, again, nothing happens, as the -ve has already been disconnected, so you have no circuit (though it's possible you could discharge some capacitors in the Prius systems). -EXCEPT- when you have another 12V source connected. Then you have to be extra careful that you don't ground the +ve.

Further, if the 12V battery happens to be discharged, you WILL get a spark when you connect another 12V source to the car. It will try to charge the dead battery.

I have measured the 50 Amp pulse (not a spark) when you touch the brake pedal, and one when you press "Start". The brake related one is current to run the brake hydraulic booster. The "Start" related one is current drawn to switch the computers to run mode and close the relay in the "traction battery" box. I phrased it this way because the computers are actually always running, just not in run mode. When you shut the car off they go into a hibernation state which draws very little current. The systems can still see you approach the car (the FOB signal), and will actually "get ready to start" -before- you press the "start" button. This is mentioned in the owners manual, but I noticed it first then read the manual to see what was up. Hey, I'm a guy. We don't read the stinkin manuals unless we have to!

 

If the 12V battery is dead, then there is no need to float the 12V bus while replacing the battery - your settings are already lost. Also, any current draw or arc would be no different than what you get when jumping a dead battery. That's what the jump points are for.

The point about shorting the +12V is important. Any time you work on a live system, you need to be very careful about shorts. There is no danger of shock from the 12V system (unless you stick your tongue on it, or attach it to your eyeball), but arcing, sparking, and heat can be dangerous to you and the car.

I wouldn't hesitate to float the bus when changing the battery, but then I work with live power systems that will do a lot more than arc if you short them. Be prepared, know what you are doing, and have an insulated spot for the +12V lead and you will be fine. If you are at all uncomfortable with the idea, take your car in and pay a professional to do the job.

Tom

 

Hello David, I don't understand how any mechanic thruout the world dares to replace a battery on a car. The danger factor is incredible. You left out a few safety procedures, a surgical mask, goggles, rubber mats everywhere, wipe up water spills, set up a fan system to insure H2 does not accumulate to excess, etc. The one thing that puzzles me, I wish to duplicate that 50AMP pulse that you mention while disconnecting the battery, but, I can't figure out how to press the brake pedal at the same time. Theory is interesting, but mechanics have to be practical. But I do enjoy your input. Incidently I have charged batteries and deliberately tried to collect hydrogen with the caps off so I could transfer it to a test tube and then burn it off, it was usually a dismal failure. The amount of H2 produced by a charging battery is miniscule. Now if you have a special Lab set-up just for that purpose, that's different.:cheer2: Andy.

 

gotchya!!!!

kk6pd

 

It doesn't take a lot. I have turned a battery into a good impersonation of a flower with pretty plastic petals surrounding a pile of lead by not doing it right. It also gave me a face full of acid and plastic debris resulting in several hospital visits.

 

Well, shorting the 12V battery can do that magic trick involving explosion. But it is not hydrogen related problem.

 

Absolutely correct. If one is so stupid to short out the terminals. Of course accidents can happen....One must be careful disconnecting, and then tape the cable ends, for absolute safety. Car batteries are elementary.

 

A hard, direct short across a fully charged battery will not lead to anything happening apart from the battery melting internally and a lot of heat followed by tremendous depression as you realise you buggered it up.