Tim79

Rubbish mpg after 12v battery change

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So my ‘13 plate RX450h needed a new 12v battery the other day, the Ready light kept refusing to come on intermittently. Took it to my local dealer, new battery fitted, the old one wasn’t holding much charge apparently. All good. Except it’s not. My mpg is suddenly shocking. My mixed driving before saw a 33-34mpg average. Now it’s 23-24. I’m driving no differently, have checked all my car settIngs before the battery change are the same now (eg not in eco mode but normal), yet no matter how saintly I drive, I can’t get above 24! What’s happened?! I’ve noticed that I rarely get the car to stay in battery only mode when setting off - the slightest touch of the accelerator makes the power needle go straight through the blue zone of the dial into the economy and power sections. Something can’t be right can it? Any ideas folks?... thanks! 

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Hello and welcome to the LOC.

Generally when the 12 volt battery is replaced the engine ECU is reset and then it needs time to recalibrate itself, how many miles have you covered since the battery was replaced as it does take a little time.

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Thanks for the welcome! 

Ive done probably 250 miles since the change, maybe that’s the issue. I’m just a little surprised how sluggish everything is really and how little I seem to run on “batteries only”since. Im finding that it takes next to no pressure on the accelerator to go through the eco zone of the power dial and then straight into the power section since the change as well as I said, it just doesn’t feel quite right. I’ve read about an isc reset? Maybe that’s not been done properly? Anyway, it has to go back to the dealer (Guildford) in about a week as they accidentally drove into something in the workshop and dented the drivers side passenger door so if it hasn’t got better by then I’ll ask them to look deeper into it. Who would have thought a 12v battery could cause so many issues?!.... 

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You would think they would have used a plug in battery saver to keep all the settings, got mine changed and no difference ay all.

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New rx400h owner. Mpg not good. Cold weather?   Posted 

I seem to remember a way to reset Hybrid Battery Learning from a post by Assa1974 that I copied but have not yet tried. May be worth a try Here it is :-

"It's a little bit of a faff TBH. If you do a google search you should find plenty of advice and I'd suggest you do that even if just to verify what I've posted below. This is a procedure I picked up myself doing just that. I can't promise what results you will get but it seemed to give me an immediate improvement even if it was only a couple of MPG on a long drive.

1) Make sure the vehicle is fully warmed up. Do the procedure immediately following a long drive that runs the engine (highway speeds are best - not long gliding stealth runs at low speeds with the engine off). This will insure the coolant temp is at FULL operating temperature - it is not necessary to actually measure the coolant temperature if you warm up the engine sufficiently and don't let the engine shut down too long and cool off before doing the procedure.

2) With the vehicle stopped and in drive, press firmly on the brake to keep the vehicle from moving and then step on the accelerator while watching the power meter (to the left of the speedometer). Press down on the accelerator to keep the needle between 1/2 and 3/4 up the scale. You will find that the engine management system will automatically limit the engine load no matter how far you press down on the pedal so the amount you press is not critical as long as it is either 1/2 scale or to the engine's self-limiting load value. Keep the engine loaded for a full 30 seconds but not longer than 40 seconds (this procedure brings the hybrid battery up to a maximum state of charge - if you watch the battery symbol, you will see it tick up to a "full" display).

3) Shut off the engine and place it in park

4) Wait 5 seconds.

5) Turn the ignition key back on to display "ready"

6) make sure the A/C and heater system controls are set to off

7) Lightly step on the accelerator pedal until the engine starts -then immediately take your foot off the accelerator and let the engine run until it stops by itself.

This completes the procedure."

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I have been experiencing the same problem with my 2008 RX400H.

It all started after my kids left the interior lights on over the weekend about 2 weeks ago which resulted in the auxilliary battery being drained and I had to jump the car to get it going but since then the aux. battery has been fine but the consumption has dropped from around 31 mpg to around 26 mpg. I have noticed that the engine kicks in with the slightest touch to the accelerator pedal.

I have attempted the reset procedure successfully 3 times so far but after the car is switched off for a while, it returns to the previous state where once again the slightest touch gets the engine to kick in.

I have checked the aux. battery with a multimeter and it seems to be in a decent state of charge showing 12.5v in standby.

Any thoughts/ideas please!

Just to add that none of the settings of radio presets or other memory items such as seats and rear view mirrors were lost when the 12v battery had drained!

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