12-Volt Battery Questions

Please run the test at Weird stuff happening? MPGs dropping? Test The Battery | PriusChat after the car has sat MANY hours or overnight (or you can use a voltmeter). Please tell us the 3 values (not just 1). Don't go changing the battery first until you tell us the values...

If you have HIDs, you can avoid unnecessary cycles on the lights by toggling between off and parking light positions.

Weird stuff can start happening w/a low or bad 12 volt. If you're on the original 12 volt, it's likely near or at end of life now.

 

There are lots of threads on replacement batteries for the prius. Dealer battery will be expensive. Go aftermarket. You dont CCA, for the prius you need amp hours. (Prius battery only fires up the computers/the Traction Battery starts the engine.)

 

There are a couple of direct fit replacement batteries that fit the Prius. The one I would recommend is the Exide Edge FP-AGM51JIS. This battery is a 45 AH battery with a 4 year full replacement guarantee.

Your dealer has not got a clue about the current required to make ready the Prius. "the battery does not need high CCA". You will note I did not say start the Prius because the 12 volt battery does not start the Prius engine as in normal cars. It is the job of the high voltage battery to start the engine. The 12 volt battery only boots up the Prius computers and operates a couple of pumps for a few seconds. There are other options for 12 volt batteries "see my post fitting mobility batteries" but if your not into DIY on cars probably not for you.

John (Britprius)

 

I bought the Optima yellow top about 2 weeks ago.. no problems here.

 

As stated the Exide edge and Optima Yellow top are good replacement batteries sold in the US. (Any AGM battery with external venting and JIS posts is good, in case you are not in the US)


Exide AGM Car Battery (Group S46B24R)



Optima DS46B24R Optima battery direct fit replacement for Prius 2004 + & Plug-in Prius with Instructions (detailed instructions)

 

As stated in my previous post CCA has virtually no meaning as far as the 12 volt battery is concerned with the Prius. The 12 volt battery does not crank the engine as it does with the vast majority of other cars.

The initial AH capacity rating of the battery is set when it is built, but declines as it ages. This can be checked but is not a good indication of life left.

Battery life is governed by many factors, one of them being the batteries cycle life. If a battery is is discharged 50% and then charged that is normally considered one cycle. Deeper discharges decrease the number of cycles it is capable of shallower discharges increase it.

If you have a 30 AH battery and discharge it by 15 AH you have reached 50% discharge. If your battery is 60 AH capacity and you discharge it the same amount you will have only discharged it by 25% thus increasing the likely life span.

A poor but effective analogy, it's like your fuel tank the bigger it is, the more fuel it holds, the further you can go, and your less likely to run out of fuel.

John (Britprius)

 

This clicking noise is the three high voltage relays connecting as the systems power up.

John (Britprius)

 

That is normal the vent is there to allow air out of the passenger compartment when fresh air is entering through the heater/ac unit with the windows closed. There is a similar exhaust vent on the other side of the car in the same position.

John (Britprius)

 

Any truth to the rumor that a lawn mower battery can be used instead of automotive batteries?
If this is true, what kind of AH (or CCA, which is linear with AH) would be needed?

 

It is true that it is possible to make the Prius start by using a smaller battery than specified. The correct Prius AGM battery has a capacity of ~40 Ah. You probably could get the car to start if you used a fully-charged battery with 10 Ah capacity or less.

However there is more to consider. There are two reasons why installing a battery of the correct capacity is important.

- that gives you margin for a partially-discharged battery, as well as reduced battery capacity due to age
- that gives you emergency capacity if the DC/DC converter stops operating, for example because the inverter coolant pump has failed and the inverter overheats. That certainly is not a theoretical possibility.

If the DC/DC converter fails, the car is entirely dependent upon the 12V battery to power the vehicle electronics. The more capacity the battery has, the easier it will be for you to gracefully bring the car to a stop at the side of the road. This capacity is especially important when you have the vehicle exterior lights on, the cabin ventilation fan running, cabin PTC electric heaters running, the rear defroster operating, etc., all of which are draining 12V power.

If the 12V power is totally lost while the car is still moving, unpredictable things may happen. For example, will the parking pawl in the transaxle engage while the car is still moving, thus causing damage to the transaxle?

The transmission control ECU which controls the parking pawl may or may not know that the vehicle is still moving. It depends upon the combination meter ECU to provide vehicle speed info. Since the ECUs may not be able to gracefully shut down during a power loss, it is hard to say what will happen.

Hence, I recommend avoiding the temptation to cheap out on the 12V battery. If you must find an alternative to the correct Prius AGM battery, use a battery with similar Ah capacity in the ~40 Ah range.

 

While what you say is true.

-Smaller amp hour battery (if it’s good) would have enough capacity to get to somewhere to stop.

-Losing 12V while Prius is running would need the failure of both 12V battery and DC/DC converter at the same time which is highly unlikely. Or more than one cell full sort in other vice good battery also very unlikely. Or some other unlikely failure.

-Lots of peoples have used some very cheap 12v batteries in Prius without problems.

 

I have used the 12V battery from a DeWalt cordless drill to start without any other battery connected.

Once started no battery is required to stay running.

 

That is not necessarily good information to be throwing around.

The battery does a fairly good job of filtering the 12 V power. Without one in place, there may be spikes and other "noise" in the power.
Also the DC-DC converter might not perform properly without it in there.

If the converter shuts down, you're dead in the water.

 

"That is not necessarily good information to be throwing around."

But it is true.

Your comments are just unproven supposition and in reality do not happen.

 

It is true that once the Prius is READY, the 12V battery can be disconnected. However if the DC/DC converter fails then the Prius has zero emergency power available on the 12V bus. The bank of electrolytic capacitors located near the 12V battery provides emergency power for the skid control ECU and brake system. However without the other systems functioning, who knows what will happen.

So if you happen to be the type of person who feels that living with no margin is just fine, then you will find the tiniest battery that will start the car and install it. In that case you probably also would think it is OK to change engine oil and fill with only 2.3 US quarts on the basis that the oil level reaches the bottom dimple, hence why worry.