12v battery

[glynp01]

Just to let you CT200h owners know I have been having an issue with high fuel consumption I.e 33 mpg at best and being an ex mechanic I was looking to sort this problem out myself but alas after quite a few attempts I decided to go to my local Lexus dealer and speak to one of their mechanics, I was in fact quite shocked by what he told me, the first thing he told me was to check my 12v Battery was good, this I did and found it to be defective he then said because their was a cell or two down in the 12v Battery this would keep the engine running considerably longer in order to try to fully charge it without actually being able to. Anyway I have now replaced my 12v Battery and Abracadabra back up to 53 mpg around town so one happy chappie. One worth knowing for future reference for all Lexus owners.

[Herbie]

I'm sure there's some truth in this but I'm not quite sure it's wholly true.

When you disconnect the old Battery the ECU will forget your driving style and will have to relearn it, so the apparent increase in MPG may be a 'settling in' period until it gets used to you again.

Also, a conventional car will indeed run the engine more, and consequently the alternator, to try to charge the Battery. However, our hybrids don't have an alternator and the engine doesn't run all the time. As soon as the hybrid system is in READY mode, our 12V batteries receive approximately 14.5V via a DC/DC converter, which is our equivalent of the alternator (but obviously not dependent on rotational speed for electrical output), and is powered by the traction Battery.

In other words, it's not the engine that charges our 12V Battery but the traction Battery, and that get's its charge from regenerative braking (which there will probably be a lot of around town), although the engine will sometimes cut in to charge it up if not enough braking is taking place.

[Thackeray]

2 hours ago, Herbie said:

In other words, it's not the engine that charges our 12V battery but the traction battery,

As so often, John Kelly at WeberAuto youtube channel spells out the details. And Herbie, you'll be gratified to hear him back you up at 2:45 on the advice not to jump start a conventional car from a hybrid car, which in the case of the Prius has a maximum output of 100 amps to the 12v circuit. So you don't want another car drawing 300 amps for its starter motor.

 

[Thackeray]

On 7/23/2020 at 9:47 AM, glynp01 said:

he then said because their was a cell or two down in the 12v battery this would keep the engine running considerably longer in order to try to fully charge it without actually being able to

I've seen this idea before on another thread and it seemed to make perfect sense at first sight. But the more I think about it, the less sure I am that it can be correct. It would be good to see some figures to back this up but in the absence of actual figures, I've made some guesses about the numbers involved.

First, the mpg on the car went down from 53 to 33. I'm not certain how to calculate a percentage rise in fuel consumption from 53 to 33! So instead I looked at litres per 100 km. The mpg numbers would equate to around 5 l/100km rising to around 8. That's easy to calculate - it's a 60% increase in fuel consumption. So the question is, could constant charging of the 12 volt Battery (instead of the normal intermittent or trickle charging) increase fuel consumption by 60%.

Some more guesses are needed here, but the video above quotes Toyota as expecting the charge rate into the Battery to be no more than 10 amps. So 10 amps on a 12 volt circuit equals 120 watts. This is a number I can relate to - it's a bright old style light bulb but it's nowhere near enough to warm up the car interior on a cold day. So could a constant 120 watts be enough to raise petrol consumption by 60%? It would help to know how much the engine is producing - in particular how much motor generator 1 (MG1) is producing as most of the time this is where the DC/DC convertor gets its power from. (MG2 also supplies power when the car is braking.)

I don't know how powerful MG1 is on the CT200h but on the various generations of Prius it's been around 40 kw, though I think the more recent models are less. Most of the time MG1 won't be producing its maximum capacity but here's a video that gives an idea of power output. It's for a Toyota Auris but the numbers are probably fairly similar.

 

In this example, MG1 (on the right) isn't producing a lot of power. But as most of its power comes from the engine for much of the time, the engine power output could be looked at instead. The engine in this short clip tends to vary around 10kw while it's running (it's the figure in the centre). So the 60% increase in fuel consumption quoted originally might be expected to raise the power output from 10kw to 16 kw.

So taking these guessed and estimated numbers, the Lexus mechanic seems to be suggesting that a maximum of around 120 watts extra power requirement from a faulty Battery has caused an increased power demand on the engine of, say, 6,000 watts. Personally I'm not yet convinced.

Maybe a faulty Battery does cause increased fuel consumption but I'd like to see some figures that back this up.

[Richard Fixter]

A couple of things come to mind from when my 12v Battery went south, usually from cold when I move off the car moves off on electric and then starts the engine a little while later switches to petrol ,this didn't happen so I was revving a cold engine to get moving I assume running richer than normal. A lot of functional errors came up as well maybe the engine ecu could be affected by low voltage and cause the car to run in a richer mode.Seems like a lot of sensors are run by the 12v system such as the 02 sensors etc so could this cause the engine to run richer?

[Herbie]

A cold engine will always run richer than a warm one. Even though we don't have to manually 'pull the choke out' the car still does it, just automatically.

When the petrol engine needs to run, MG1 will spin it up to 1,000rpm before the system applies fuel and a spark to fire it.