Prius 12 V Battery Drain

[jgodfrey]

Hi Guys

I was wondering if someone could shed any light on an issue we are having with our Prius.

Twice over the last 3 days it has failed to start due to a dead 12v Battery. Now before I go on I should add that the 12v Battery was replaced in October and I'm always paranoid about abusing it....

The first time I thought I must have accidentally left a door open, but on Sunday it had the same issue only this time the 12v was totally dead. The first time it happened I got an error message 'there is an error with the p lock mechanism, park the car on a flat surface and fully apply the parking break'. The interior lights were also dim so I figured the interior lights must have been on overnight (even though I thought this was unlikely) as I'd used it the previous evening without problem.

Got jump started by the AA on the both occasions. The second time there were definitely no lights/doors open and the 12v Battery wouldn't hold charge (sulfated). So something other than the interior lights is draining the battery overnight, any ideas what it could be? Its happened totally out of the blue which is strange. The AA read the battery drain with everything off as 0.1A, does anybody know what it should be?

Thanks in advance

[Joseph D]

Hi Guys

I was wondering if someone could shed any light on an issue we are having with our Prius.

Twice over the last 3 days it has failed to start due to a dead 12v battery. Now before I go on I should add that the 12v battery was replaced in October and I'm always paranoid about abusing it....

The first time I thought I must have accidentally left a door open, but on Sunday it had the same issue only this time the 12v was totally dead. The first time it happened I got an error message 'there is an error with the p lock mechanism, park the car on a flat surface and fully apply the parking break'. The interior lights were also dim so I figured the interior lights must have been on overnight (even though I thought this was unlikely) as I'd used it the previous evening without problem.

Got jump started by the AA on the both occasions. The second time there were definitely no lights/doors open and the 12v battery wouldn't hold charge (sulfated). So something other than the interior lights is draining the battery overnight, any ideas what it could be? Its happened totally out of the blue which is strange. The AA read the battery drain with everything off as 0.1A, does anybody know what it should be?

Thanks in advance

The quiescent current draw should be 20-50 mA, however you need to wait about 20 mins for the car to get in its fully quiescent state. The 100 mA the AA measured would be in the right ballpark, if the car was just closed up and locked.

When you say the the 12v Battery wouldn't hold charge, have you actually attempted to charge the Battery? Or did you just run the car for 20 min or so?

I doubt you have a phantom current draw problem. but rather your Battery is in poor condition.

If a discharged battery is not fully recharge ASAP after the discharge event there is the risk of irreversible damage.

Do you own a fairly recent model of "smart" battery charger that is capable of charging AGM batteries?

If you have multimeter I would test your battery by checking the voltages of your 12v battery and see what you get as below. Measure at the jumpstart point under the bonnet or even better directly across the battery:

• Voltage across 12V battery when car is IG-OFF (all doors closed and locked, Smart fob far away [more than 15 feet.] - if applicable) and has rested overnight (this will show approximate state-of-charge, 12.9V represents 100% charge of AGM battery, 11.9V represents approx. 0% charge.)
• Voltage across 12V battery when car is IG-ON and headlights on (the difference between the voltage measurements at 1 and 2 is is an indicator of the battery's capacity, the bigger the difference the weaker the battery is becoming)
• Voltage across 12V battery when car is READY (this will give an indication of how well the DC/DC converter is maintaining voltage on the 12V bus, and therefore, applying charge to the 12V battery)

To get into IG-OFF mode do not press any buttons.

To get into IG-ON mode press the START button and then press a second time both with your foot OFF the brake.

To get into READY mode press the START button once with your foot ON the brake.

If you can find the time to take those readings and post them it will give an idea if your 12v is in a good state.

HTH.

[Grumpy Cabbie]

To keep it simple, the Prius 12v is only ever so slightly up from a motorcycle Battery. It can not happily accept deep discharging such as leaving your interior lights on overnight, followed by many short runs. People also leave the car in Acc mode when listening to the radio whilst waiting for their OH to do the shopping, again running the 12v low. The system also takes a while to top up the 12v.

But in answer to your problem, chances are it's the boot light and unless you absolutely need it, you're best turning it off. The reason? It's been reported that on early gen3's the hatch doesn't always close correctly resulting in the boot light remaining on. The car may even lock the boot but it's just not quite clicked fully shut.

Give that a try and maybe charge your 12v up off the car. You really don't want to keep jump starting a Prius!

[jgodfrey]

Thanks for the replies GC and Jospeh. Its been recovered to Toyota now for tests, so I cant check the Battery with a multimeter as suggested. As I said the 12v Battery got replaced in October and hasn't been abused since then as I know how fallible they are, so really it should be in good condition. The car wouldn't start on Friday am having been used at 10pm on Thursday evening (it was jump started Fri am and driven 200 miles), then dead again by Sunday morning, so I would have thought it should have been fully charged. The car gets used for a 70 mile daily commute so lack of use shouldn't be the reason for 12v Battery damage I don't think....

All the interior lights were definitely off at the switches (including the boot one) on Sunday (I checked). My feeling is that there's a relay stuck open and the car isn't shutting down properly, or the 12v battery has failed prematurely?

I'll let you know what Toyota say once they've had a look at it.

Thanks again

[Joseph D]

... The car wouldn't start on Friday am having been used at 10pm on Thursday evening (it was jump started Fri am and driven 200 miles), then dead again by Sunday morning, so I would have thought it should have been fully charged. ...

... All the interior lights were definitely off at the switches (including the boot one) on Sunday (I checked). My feeling is that there's a relay stuck open and the car isn't shutting down properly, or the 12v battery has failed prematurely?

I'll let you know what Toyota say once they've had a look at it.

Thanks again

In light of this additional information I think you are on the right track, if not a stuck relay definitely something.

I presume your Battery was changed in October prematurely due to it being damaged.

Just to pick up your thought about the 200 mi journey, that would have been ~4 hours driving, so would have only charged the Battery halfway as it would take a minimum of 8 hours to fully charge the 12 V Battery from empty to full. The Prius does charge the 12 V rather gently. Also, your normal driving pattern should ensure the 12 V stays in a good state of charge,

Having said that, I think you would be correct in that it is a reasonable expectation that even a half charged battery should not be flat after sitting for 2 1/2 days. Definitely something amiss. I hope Toyota get to the bottom of it.

I'd be very interested in the outcome.

All the best.

[Grumpy Cabbie]

I'd be curious when you bought the car and where from - main dealer or back street jobby?

If you've had the car ages then it's just one of those things, but if you've picked it up in the last 6 months or so then hmmmm.

You haven't got any stupid additional amplifiers attached have you? Or do you keep the car parked near your house where the car could keep connecting/handshaking with the remote fob? Or have you started keeping the fob nearer the car at home? Or recently changed the fob Battery so it's now got a wider range and it's handshaking with the car suddenly? Do you keep a spare key in the car all the time or accidentally left it there?

Final long shot, have a phone mast near you recently upgraded to 4g? Maybe an O2 one?

[jgodfrey]

Curious isn't it?!

I had the 12v changed in October as there were some early signs of weakness (slow to start up). The Battery change cured this.

I've had the car a couple of years bought with low mileage and FSH from a main Toyota dealer so I don't know whether the original 12v was abused before I got it...

The key is kept as normal in the house about 10m away form the car and the spare is in the house too. The car is totally standard with no mods. No new phone masts nearby and the locations where the car died was different so I don't think it can be anything external.

Anyway, the dealer phoned this afternoon to say that there are no abnormal current drains on the vehicle and that the 12v Battery is fine but needed to be charged overnight, so they will check it again tomorrow. I have a feeling they just think the lights were left on or something and they'll just give it back to us with a charged Battery, but we'll see!

[kithmo]

Sounds very much like a duff Battery to me, get them to replace it under warranty if it happens again. Check the date on the Battery, it may have been new old stock.

[Grumpy Cabbie]

I'm leaning towards inadvertant 'abuse' of the 12v. Does anyone else drive the car? Do you use one of those 12v vacuum cleaners and operate it on Acc mode?

Just very unusual that a 12v was failing, replaced and then the new one quickly failed again. Perhaps there's a fault with the charging circuits? Better hope not as that'll be expensive, very expensive.

I wonder if the car has been serviced and/or repaired or valeted between the old Battery getting replaced and now? I've seen garages leave a car up on a ramp with all the doors, hatch and bonnet open for hours whilst they've worked on the car, then popped off for lunch etc. With the Prius 12v being so small it only takes one incident to ruin the 12v Battery for good.

Once it's ****** it's ******.

ps, was the replacement a Toyota 12v or an aftermarket jobby from Halfords or eBay?

[timberwolf]

Sounds very much like a duff battery to me, get them to replace it under warranty if it happens again. Check the date on the battery, it may have been new old stock.

Agreed.

In my experience, 12V batteries that discharge overnight, are not diagnosed correctly by dealers - the stock phrase, they must have all been taught on the same training course!?, is your Battery is fine but is just a little low...

[jgodfrey]

I'm thinking its the 12v Battery too; its not been abused by us although I guess it could have been during its servicing....

Anyway we've got the car back now. Jemca Reading cant find any faults with the Battery or the charging circuits and there are apparently no abnormal current drains when the car is off. I'll see how we get on but if it happens again I'll insist they change the 12v Battery under warranty. I've just bought a jump start pack so at least I can get the vehicle started if/when it happens again!

On an aside the Toyota breakdown cover we had was excellent, they even supplied us with a free hire car for two days whilst ours was being looked at.

[jgodfrey]

So my problem is back. Car would not start on Sunday, and again this morning.

On Sunday I drove an hour to Oxford did a 6hr cycle and by the time I got back to the car, the Battery didn't have enough power to get the car into ready mode. The car was locked and all interior lights switched off while I was out. Jump started it and it was fine.

Started and ran ok on Monday and again on Tuesday. Dead again this morning after being parked up overnight.

I'm going to get it back to the garage asap. I think I might insist they change the 12v Battery this time.

[Grumpy Cabbie]

You could check it yourself?

Leave car over night and use a volt meter on the Battery directly when not in Ready mode.

Alternatively put the car in Acc mode (not ready), wind all four windows down at one go and then all back up together. If they're smooth as normal then the 12v is fine, if they're slow and stutter and the interior lights almost go out then it's probably knackered.

I'm sure a new 12v is about £95 fitted at Toyota for the correct one. Be careful at Halfords etc as there is a vent tube that must be connected correctly.

http://www.toyota.co.uk/caring-for-your-toyota/service-and-maintenance/repairs.json

[timberwolf]

So my problem is back. Car would not start on Sunday, and again this morning.

On Sunday I drove an hour to Oxford did a 6hr cycle and by the time I got back to the car, the battery didn't have enough power to get the car into ready mode. The car was locked and all interior lights switched off while I was out. Jump started it and it was fine.

Started and ran ok on Monday and again on Tuesday. Dead again this morning after being parked up overnight.

I'm going to get it back to the garage asap. I think I might insist they change the 12v battery this time.

When you unlocked and started the car for the first time Sunday morning, you did not notice a problem?

[Duffryn]

When my Battery went flat overnight (due to interior light being left on) it emitted a few peculiar warning beeps when I first approached it the next morning, key in hand, warning me that not all was well with it, presumably that the 12v was low.

[jgodfrey]

No, no problems at all, it started fine straight away. Its strange that it would go dead in the space of 6hrs but not do so during the preceding night. My suspicion is the Battery but it also seems like something could be intermittently not shutting down properly on the car. I'm going to do the voltage checks on the 12v in the morning as Joseph and GC suggest and see what that turns up.

[timberwolf]

It is strange. A mechanical failure in the Battery maybe, but how does it fix itself when it is jump started? A break in the wiring, poor Battery earth-lead, faulty switch on a door or the hatch, cycle carrier and external lights?

You didn't do anything different with the car when driving or leaving it parked for six hours, portable sat-nav, mobile phone charger, a blue-tooth dongle in the diagnostic socket, ... ? Stood chatting with the hatch open for half-hour before setting off for 6 hours?

[jgodfrey]

No I didn't do anything different at all, nothing is plugged in to anything and the car is totally standard. I maybe had the doors open for 10mins max on Sunday. I guess it could be a door switch issue, but anything else would lead to a total non-starter and there is normally some juice left in the Battery just not enough to get it in to ready mode. Its like there is a gradual drawdown of power.

Just checked the 12v now (after 2hrs of driving today) and its reading 12.28v, a bit low I think. The car is locked with everything off, so I'll check it again in the morning and see what it says.

[jgodfrey]

The voltage this morning is 11.77v, so I don't think its going to start again. Either there is a current drain on the Battery or it cant hold charge anymore. Its going back in to Jemca Reading today for them to investigate, if I was them the first thing I would change is the Battery but we'll see what they say! Thanks for all the suggestion so far.

[PeteB]

That should be enough to start a Prius (just has to run the computers and power the breakers that connect the HV Battery to the system - plus a few seconds of pumps for hydraulic pressure etc).

As long as it can connect the HV Battery, that's what starts the car and feeds the 12V system, including recharging the 12V Battery.

Maybe the 12V battery has been damaged by being flattened on the first couple of occasions, whatever the underlying cause.

[jgodfrey]

It did start! I thought it might not because a post on Page 1 suggested that 11.9v represented 0% charge, so this might not be quite right? Anyway, its going and I measured the voltage across the Battery in Ready mode and it was reading 14.68v which is normal I think.

Do you think the overnight drop from 12.28v to 11.77v is an issue? I guess if left for a few days the drop would continue and it probably wouldn't start again. Does anyone know at what voltage the 12v Battery would struggle to get the car going?

[PeteB]

I've heard of people jump starting a Prius with 9-10V from a cordless drill Battery!

14½V or more in ready is normal. I thin mine is often under 12V when parked for a while, but mine runs a dash cam( Which isn't actually on the dash!) 24/7, with a protection in the circuit to shut it down at 10V. Car has started fine after being parked for about 10 days, the record on mine I think (just not had occasion to leave it unused for longer.

[Joseph D]

It did start! I thought it might not because a post on Page 1 suggested that 11.9v represented 0% charge, so this might not be quite right? Anyway, its going and I measured the voltage across the battery in Ready mode and it was reading 14.68v which is normal I think.

11.9 V does represent fully discharged, but that does not mean the the Battery will not perform at all, it depends on the current drain being requested. However, a fully discharged Battery should be charged ASAP in order for damage to be minimised. AGM batteries will tolerate a deep cycling better than a flooded acid, but deep cycling will take a toll in either.

You have already said that you have charged up this Battery, but it is apparent to me that it is failing to hold it's charge and (IMHO) is overdue for replacement.

Do you think the overnight drop from 12.28v to 11.77v is an issue? I guess if left for a few days the drop would continue and it probably wouldn't start again. Does anyone know at what voltage the 12v battery would struggle to get the car going?

Yes, in a word. It says to me that the battery is irreversible damaged. It possibly would drop further, but not necessarily. If it does, it just reinforces that the battery is damaged. 10.5 V is the supposed lower limit that triggers low voltage DTC's. The issue though, should not just be looked at whether the car can be made READY. Low voltage fluctuations on the 12 V bus can induce false DTC's in one or more of the ECU's - which in turn can confuse the car altogether and it may refuse to operate.

I've heard of people jump starting a Prius with 9-10V from a cordless drill battery!

14½V or more in ready is normal. I thin mine is often under 12V when parked for a while, but mine runs a dash cam( Which isn't actually on the dash!) 24/7, with a protection in the circuit to shut it down at 10V. Car has started fine after being parked for about 10 days, the record on mine I think (just not had occasion to leave it unused for longer.

Are you sure about that, I have heard of people using 12-14 V drill batteries, even 8-9 D cells in series.

14.4 V+ in READY mode on an ongoing basis is not normal. Let me explain a bit more. Even though you will see it, possibly often, if your 12 V battery is healthy you should to expect to see the voltage to drop to 13.8 V after a time. If it does not, it further points to the 12 V battery being in poor condition. At 13.8, the charge has moved to a float charge. After further time you should see the voltage drop again to around 13.5 V, which indicates the 12 V battery is now fully charged and on a trickle charge.

So to summarise:

14.4-14.7 V = Charging discharged to severely discharged battery

13.8 = Float charging nearly full battery

13.5 = Trickle charging fully charge battery.

PeteB, just curious, how often (if ever) has the voltage protection kicked in and shutdown your dash cam?

[jgodfrey]

Thanks for the info Joseph. The Battery was replaced in October (precautionary), but it wouldn't surprise me if it was irreparably damaged now due to the fact it has gone flat 4 times over the last few weeks.

It was charged up by the dealer a couple of weeks ago, but has since gone flat again twice despite daily use, so this suggests to me it is knackered.

I'm interested to hear how a fully discharged Battery can still start the car? If its fully discharged than surely it isn't good for anything?

[Joseph D]

... I'm interested to hear how a fully discharged battery can still start the car? If its fully discharged than surely it isn't good for anything?

I guess you have to look at in two ways. There is the chemistry of batteries and there is also the physics. The terms fully charged or fully discharged refer to the chemical state of the Battery. Physics is the Voltage and amperage.

A fully discharged Battery can still supply voltage and amperage, especially if the Battery is healthy. A healthy battery that has become discharged and then immediately fully recharged will remain healthy - although there are a finite number of recharge cycles. In real life batteries get discharged over time and tend to sit in lower charge states so damage occurs, then when they are finally are charged up, they no longer can achieve quite the same full charge. Over time with many repetitions of this cycle the battery eventually reaches a point where it will no longer hold its charge.

We have been conditioned to not maintain batteries (no maintenance required models) and with no volt meter on the dash we become blissfully unaware.

Anyway the shorter answer is that as long as the battery can supply the requested amperage and not dip below ~10.5 V at the critical moment, the car will go into READY. If it does go below ~10.5 V it will refuse.

The question is, do you want the car to start every time with confidence, or are you happy to (figuratively) play Russian roulette?

Me, I like my car to start every time with confidence, so I monitor my battery voltage and I charge externally as required. So far so good.