RX400h 12 v battery prone to discharge.

[welland]

Hi all, 

I have a little issue to resolve with the 12v Battery on my RX400h.  The thing is that the Battery keeps discharging  when the engine is not running.  It occurs when, for example, I am hoovering the inside

of the car and the doors are left open for a while.  It happened last time I went camping, during the load-the-car phase.  

I have taken care to ensure that everything inside the car is off (eg the radio and courtesy lights).  

I have had the Battery tested at Halfords and they said that it is fine. Can anybody tell me what circuits should be drawing power from the Battery when the engine is off?

[Zotto]

How old is your Battery? If it has the age of RX, may be it's time to change it. In hybrid cars 12V Battery is necessary only to start ECUs, so it lasts much time, but even if it's not stressed by great current requests it gets old like all Pb batteries. To avoid problems, if you need to keep on light or radio or any other service in your RX my suggestion is to put engine ON, so to use Hybrid Battery energy (and have your RX ready to go with engine warmed up), as after warming phase , in summer just few seconds, it will go off.

[Delphius1]

The 12v Battery isn't charged in the normal way by an alternator on the hybrids. It's charged from the hybrid Battery via the invertor module. It's possible the invertor isn't charging the 12v Battery fully or there is a drain in the module itself. 

Unfortunately the only way to find out is some thorough diagnosis.

The first step is to have the Battery properly tested. They get such light use in hybrids that it may be the original Battery and is past it's best and has a high self-discharge rate or very little capacity left. After that, if it is a current drain, then it's out with the meters to hunt it down.

 

[Britprius]

Plus 1 for the above. There is a date stamp on the Battery, and the strong probability is it will be 2006. 

John.

[Texas]

If it is circa 2005-6 then with regard the way the Battery is used/charged then I would say that was quite good life expectancy and you have had your monies worth! Not sure if the Battery you require is 'something special' like a Stop/Start for example which are heavy money compared to non SS.

paul m.

[ColinBarber]

Replace the Battery. You shouldn't really expect more that 6 years life out of it. Lexus improved this issue with the series III by using one that can handle being deeper discharged.

[welland]

Thanks for your advice. I haven't had chance to check the Battery for a date stamp yet.  The tests I had done were at Halfords,  who clipped on their tester to test first the Battery, then the charging circuit.  They said that both tests gave a good result. So, whilst i accept that batteries have a limited life, and it may be old (I'll report the age when I find it), if the tests came back "good", then how can the age be a factor? 

[Delphius1]

The test that Halfords does only superficially tests the Battery. One with very little capacity will come up as good on a basic test just after the car has rolled up on the forecourt and the Battery is fully charged. But that charge won't last on an old Battery that has little capacity. A deeper/longer Battery test would show the true state.

The good news is that it sounds like your charging system is working.

So it's either a duff Battery or something drawing current with the ignition off.

[sorcerer]

Quite simply, things age and the chemistry changes.

You obviously had to drive to Halfords so the charging circuit has been doing its job and the Battery was well charged at the time of the test. The age and altered chemistry mean that it no longer holds charge as well as it used to. Maybe if it had been stood outside Halfords for four hours before testing then it wouldn't have looked so good.

The trouble now is, do you just change the Battery anyway in the hope that it is the age that's causing the problem, or do you spend some money for an auto-electrician* to do some diagnostics first in case something else is causing the Battery drain?

If you've got a clamp ammeter and a decent multimeter and know how to use them properly you could do some initial diagnostics yourself.

* And I do mean a proper auto-electrician/auto-electrical specialist who has done a proper apprenticeship to the job, not a Halfords guy or a 'normal' garage.

EDIT:

Just to add to the above. When I was an apprentice electrician I worked in a factory that had a mechanic's workshop for maintenance and servicing of their own trucks and other vehicles. I don't know why but they decided it would be good for the apprentice electricians to spend a month in the garage learning about auto electrics, which I must admit, was quite interesting.

I remember using a vehicle Battery 'heavy discharge tester' and, although I'm going back to about 1976/77, a lead-acid Battery is still a lead-acid Battery so this should still hold good. My memory is a bit hazy but I seem to remember that the tester used a load to draw about 300A from the Battery and if left on for about 30 seconds, the Battery voltage shouldn't drop below 10V. If there were any duff cells, they would usually show themselves by bubbling and gassing quite rapidly whilst the good cells were 'quiet'.

Now, it's been a long time since those days so don't take that as gospel, but I'm just thinking that maybe Halfords didn't do such a test?

[Gliderpilot]

Just bought one for my 400h. Be careful what you buy as some that say they will fit wont because of the Battery base having a lip on it. got mine from these guys. https://advancedbatterysupplies.co.uk/ it's a Bosch silver s4021, £67 delivered, cheaper ones are available.

[julian1969]

Well known 400h issue, the Battery is minute, seriously, take a look at it, apparently it's the same size as for an MR2 roadster. If you are doing anything with the doors open or sitting waiting with the radio playing, etc. have it in 'ready' otherwise the tiny 12v Battery will die and you will get scary 'check hybrid / vsc' messages.

Personally when taking mine in for a service I turn off all interior lights and the auto headlights etc. as I've been handed the car back a couple of times with a severely depleted 12v Battery causing starting issues. Last time I had to replace the 12v Battery.