In READY mode, with the ICE start to charge the 12v battery?

What you are saying is correct.

The 12V battery only exists to "Power Up" the Prius. After the Prius is powered up the 12V battery is "floated" at a voltage above its nominal 12.7-12.8V. The float voltage varies between 13.4-14.4V, but is all handled automatically by the Prius.

When you draw current from the 12V battery (with Prius in Ready mode) the current is supplied by a DC-DC converter using the traction battery as its source & keeping the float voltage correct on the 12V battery.

IIRC the DC-DC converter is rated at around 100A. I would keep the 12V current drain below 50A at all times. The system probably can handle closer to 80A for short periods, but try to keep it less.

 

MOST of what you say is correct -EXCEPT- the engine doesn't have to start to charge the 12V battery. The hybrid battery charges the 12V system. When it is depleted enough -then- the engine starts to charge the hybrid battery. The engine doesn't generate 12V to charge the 12V battery, the inverter has a 12V charger built into it and that is what charges the 12V battery and supplies all 12V power when in "ready". There isn't actually a "charge mode" for the 12V system, it just runs at a "contant" voltage which keeps the battery sort of charged and the electronics happy. The actual voltage level generated does vary a bit, and it may be varied to charge the battery up faster, but its' variation is subtle.

 

no, not safe to say. the accessory drawing from the 12v cannot exceed the supply from the inverter/converter. i don't know the specs, but you would do well to find them. otherwise you can wind up with a depleted 12v.

 

i think you're onto something.

 

With one very important exception. If you are in "N" the traction battery won't charge. Don't use "N" for very long at a time (and just to underscore this the car will complain when you put it in "N" - at least Pearl S does ). If you have to use "N" in a car wash turn off the A/C!

 

I do not believe you can drain the 12V battery in your described scenario. The reason is that the 12V battery is protected by a fuse that I assume is no larger than the maximum designed current of the 12V system. I think the fuse is 100A on the 12V battery, it may be 125A. People have run their emergency house wiring by connecting 1 kW inverter to the 12V battery and keeping the car in READY mode. The ICE will come on when needed, but mostly it's off and the HV battery is supplying the power to the 12V battery via ds-dc converter. I believe 1 kW was pretty close to the max, but something like 800 W is safe and has been done.

 

I can think of at least four reasons:

1. To allow the 12-volt electrical system to meet brief demands for more current than the DC-DC converter can provide;
2. To power essential 12-volt loads, such as the four-way flashers and airbag system, even if the hybrid system is damaged or malfunctioning;
3. To power parasitic 12-volt loads, such as the smart key system, without the inefficiency of operating the DC-DC converter continuously; and
4. To allow the hybrid vehicle battery to be isolated for safety when the car is not in use.

 

I think I could add a 5th reason. If the HV battery was alway connected to the system and was the only battery, you could accidentally discharge it below the point of no return.

 

You've all missed the most important function of the 12V battery! Without it you couldn't put the car in "ready". As some on this forum have discovered, when their 12V battery went dead.
When you turn the car off, the traction battery is -completely- disconnected from the car. A relay in the battery "module" opens both the positive and negative leads, for safety. The 12V battery powers the computer in the battery "module", and when power up is "requested" by the user, checks if everything is ok and if so closes that relay. From then on the car runs from the traction battery in essence, keeping it at nominal charge level if possible with the engine from time to time, so it can provide power to maintain the 12V system, power the A/C cooling, control the HSD, etc.

As stated above, a healthy 12V battery is then needed to provide some "ballasting" of the 12V system, keeping the voltage semi-stable by absorbing large transients. The 12V system is needed because lights and the electronics in the car (radio, GPS, etc. etc.) run off 12V. And as already stated, it's needed to put the car in "ready" so the traction battery and MGs can do their thing.

Keep your 12V battery healthy!

 

Unknown how long a car battery will last, but with previous vehicles I only got 3 years out of them. Will the Prius be different? Or would it be wise to just go ahead and replace the 12V every 36 months. Call it preventitive maintenance. Dumb question, but do the Auto Club guys know how to properly jump a Prius battery? I've seen some horror stories.

And what is the best way to keep the Prius12V "healthy?"

 

It's easier if you just think of the whole 12 volt system as connected together. There is a 12 volt battery in it somewhere, and there is a DC/DC converter supplying power into it somewhere by stepping it down from the high-voltage system when the car is ready.

There are also things all over the car using power from that 12 volt system. Your accessory will be another thing. The DC/DC converter has a maximum output. If all those things using power add up to more than that, the 12 volt battery will be losing charge. If they don't, it won't. Doesn't matter where in the system any of them are connected.

From Bob Wilson (and probably others), we have studies of the output capacity of the DC/DC converter. If I remember right, Bob was seeing around 100 amps at the point where its output voltage sagged. (When the voltage back at the battery sags from above the battery's open-circuit voltage to below it, that's when the direction of flow switches from into the battery to out of it.)

There is an output signal from the DC/DC converter called IDH. The converter asserts this signal when it finds itself overloaded. As far as I know, the only use made of it in the stock car is by the HVAC controller: in cold weather, if it is running the supplemental electric heat elements, it will turn those off if IDH comes on. If you were adding a high-draw accessory, you might consider also monitoring that signal.

There are voltage-sensitive switches (like the InPower VCM-06) than can be used to enable a load when the supply goes above 13-ish (meaning the car is in READY and not overloaded), and shut it off when the voltage goes below. The typical use is to control a load that you only want to turn on when the car is READY, but you don't want to bother finding and tapping into a READY-switched circuit in the car to control it.

A VCM would also have the effect of switching your load off in the case of an overload causing the voltage to sag below 13-ish. That would be a less typical use, and you would probably want to add something else to get the load to stay off if that happened, otherwise the VCM would turn it right back on as the voltage recovered, and the resulting oscillation would probably be no good for anything.

-Chap

 

Quite true. But I think @Johnny Cakes Is asking if the relay and disconnection is needed at all. Why not just have the HV battery always on line? @Elektroingenieur explained it nicely, I think.

 

What was your previous car? I am no expert of auto mechanic, but my understanding is that 12 v battery lifespan is somewhat dependant on type of cars on it and more so on temperature and driving habits. For temperature of the battery the location where car is kept is going to affect the longevity of the battery. In colder climate like where I live, 5 years is average for most cars. I didn't keep my previous Prius long enough to be needing 12 v battery replaced, but for our Civic Hybrid which also use 12 v battery as auxiliary battery and not used for starting the engine, the OEM battery lasted 9 years before needing a replacement. As comparison my older gasser Civic went through 3 batteries during 15 years of ownership. I am attributing this long life of 12 v battery in hybrid car to the fact it is not being used as much for cold cranking during winter.

 

Some are asking why the HV Battery can't start the car.

When the Prius was new, Emergency Responders were VERY concerned they would be electrocuted using jaws of life.

Toyota produced Manuals explaining that if the car was not Ready, no high voltage would be present outside the HV battery.


Toyota Emergency Response Guides


Without this assurance, Prius accidents would be much more dangerous to the occupants.

 

12V battery is small price to pay for having 240V battery offline when the car is off. 240V DC is very dangerous. I wounder how Tesla deals with this. Does Tesla have a 12V battery?

 

If/When you connect external 12V to a Prius be VERY CAREFUL to connect with the proper polarity. Connecting backward WILL be VERY expensive!

To boost a GII or GIII there is a "boost point" under the hood (I've never played with a GI so I can't comment on it). Open the fuse box on the left side up by the firewall by pressing on the three lines on the latch on front and gently lifting the lid. It hinges on the back. Remove lid. Inside you will see fuses and relays. One device is red plastic and it just pivots up and over. Under this pivot protector is the "boost point". Connect your 12V source positive lead here. The negative lead goes to the chassis. Find a bolt on the firewall. Note that inexpensive boost cables often only connect on one side of the two clip "jaws". The "boost point" also has metal to connect to on only one side. Make sure you connect to that side with the side of your boost cable clip that has power. You don't need the "doner car" to be running. The Prius won't draw much from it (usually just a 1/2 sec. 50 Amp pulse or two). You can even boost a Prius from a gel cell "alarm battery" ($20 or less from alarm system suppliers). Or a boost pack. Try not to connect/disconnect in a jumbled fashion. One touch not many.

To take care of your Prius 12V battery DO NOT allow it to fully discharge, by, for example, parking using "aux. mode" listening to the radio. Use "ready" for this. The engine will run only when needed to keep the traction battery charged. If you have A/C cooling on it will be more often. Note that many Prius will keep the headlamps on until you open and close the drivers door, so do that when you stop and shut down the car but sit in it for awhile. If you leave the car unused for more than a few weeks, get a "battery tender" of some sort and use that to keep the battery charged. You can connect that device to the "boost point", which makes it easier to get to than digging into the hatch.

For long term "off" periods you can disconnect the battery negative terminal. If I were to do this (perhaps leaving the car at some airport for a month or longer) I would also leave an alarm battery or boost pack in the car on the drivers seat floor. I would then connect this device to the "boost point" when I returned so I could open the hatch gracefully to reconnect the 12V battery negative lead (no clambering over the rear seat fishing for the manual hatch release). Remember with no 12V battery the electric locks won't work. Only the drivers door has a key slot.
Once reconnected, you may have to "reprogram" the "auto up" windows (hold button in up position for a few seconds after the window closes) and you will probably loose all station presets on the radio.

Gen. IV doesn't have a "boost point" but the reason is the battery is right there under the hood. Boost on that using care not to cause a spark near it (connect the negative to the chassis a few feet from the battery - after- you have connected the positive cable to the battery positive terminal - most owners manuals tell you this). You can use an alarm battery or a boost pack on a GIV Prius. You won't need one though to reconnect a disconnected battery. The hood release is still mechanical. You just use the key to unlock the drivers door and open the hood.

Some have had the Prius battery last more than 5 years, some have to replace it after 3 or so. I suspect it has to do with the average temperature of your location (heat is the killer) and the length of time the car is "ready" on average. It takes some time to fully charge a lead acid battery when in "ready".

Sorry this post is so long. I wanted to be thorough.

 

Excellent. Thank you David. In my area, heat is the enemy. I'm just going to replace the 12V every 3 years; mark my calendar and just do it. Try and stay ahead of any trouble. I'm getting too old.