RX450h starter battery monitoring

[matd]

Hi, I'm new to Lexus and the forums - we've had our 2017 450h about 2 months now, and not many chances to drive it!

So of course the car was left for 2 weeks without use - and the starter Battery went totally dead. One AA visit later, we now know we have to start it regularly to top up the starter Battery.

I've fitted one of these Bluetooth Battery monitors, which seems to work pretty well.

When I first fitted the Battery monitor this evening (after 2 days idle), the monitoring app said the voltage was 12.05v, which it decided was *low*, and at '7%'.

But the car started OK, and we went for a 20 minute spin - charging at 14v apparently.

Back home, the monitor now reads '12.71v, 80%'

So my questions;

- does a voltage of 12.05v actually represent a very low (7%) Battery, or is it talking garbage? I would have thought that <10v would be low enough to start worrying..

- what Battery level *should* i use as a threshold to go and start it up for a while?

- any tips for reducing Battery drain? Disabling keyless-entry maybe?

 

Thanks for any thoughts. Kind regards

-Mat

 

 

 

[Herbie]

Hi Mat and welcome to the club.

You may find this chart helpful as a starting point:

Standing Battery voltage will be around 12.5V, increasing to about 14.5V when the car is in READY mode.

Because hybrids don't have a normal starter motor Toyota/Lexus decided they could save a bit of money by using small capacity 12V batteries - my RX has a 51Ah Battery whereas previous cars have been a minimum of 75Ah capacity. As you've already found out, one drawback to this is that you can't leave it for very long before it goes flat, so I, and a few others on here, keep a jump start Battery pack such as this one in the glovebox for just such eventualities.

It's a bit beefier than you need but at least it'll allow you to help someone else out if they ask you to provide a jump start for their car, because you should never use a hybrid to jump a normal car. This is because hybrids don't have traditional alternators either. In fact, because the petrol engine doesn't run all the time, anything that would normally be belt-driven, such as steering pump or aircon compressor for instance, is electrically-driven in hybrids.

A traditional starter motor will take 300A or more to crank the engine but a hybrid doesn't do that. The 12V Battery only has to boot the computers and one or two other things to get the car READY and draws well under 20A to do it, hence why they decided on smaller-capacity batteries.

On a normal car, once the engine has fired and the alternator is spinning then it's the alternator that provides all of the car's electrical needs. If a normal car needs to be jump started then the recipient car will take some current from the donor Battery but mainly relies on the donor alternator to provide the majority of current. Hybrids don't have alternators, but instead use a DC/DC convertor to take the 288V of the traction Battery and drop it down to 14.5V or thereabouts and these convertors can't supply anywhere near the amount of current a normal alternator would, so if you tried to do it there's likely to be a big bang and lots of smoke - hence why you never use a hybrid to jump start a conventional car.

Seeing as your Bluetooth monitor doesn't seem to be very accurate you could go for a CTEK one or if you didn't want to spend that much, another CTEK one here or even something like a USB socket version of a voltmeter that would replace one of the cig lighters

[Barry14UK]

I think you would have done better to have bought a reasonably priced multimeter and used with tables which would also have also served other modes.  Although providing temporary power will get you out of trouble, it is better to try to keep the charge in a Battery above the level that it won't start because the more often it needs emergency starts the more likely it is that the life of the Battery will be shortened. So a smart charger or similar source keeping the Battery charged to an appropriate level pro actively is advisable when car is not being used for a couple of weeks or more.  Of course as a Battery ages it becomes necessary to check more frequently if the charger is not left in place.  I am having to do this weekly now on my RX as it is needing replacement soon and my CTEK is also used on another car's Battery.

[matd]

Herbie, that's very useful info about the batteries, many thanks. I'll get a jump pack and keep it on charge in the boot or somewhere, for emergencies.

Barry, I can't use a trickle charger, as I have to keep the car on the street.. maybe a solar powered one would do the trick?

Does anyone have tips for reducing drain? Would disabling keyless-entry help?

 

[Herbie]

7 hours ago, matd said:

I'll get a jump pack and keep it on charge in the boot or somewhere, for emergencies.

I don't think you'll need to keep it on charge but you will need either a good memory or a diary or calendar to remind you when to top it up because they do hold their charge quite well. I've got a DB Power one and after five months in the car it still had 75% of its charge left. According to the instructions when it's fully charged you can use it about 10 times before it needs recharging, and remember that they're talking about it starting traditional cars, not hybrids, so 75% charge is still more than adequate for us.

[Barry14UK]

13 hours ago, matd said:

 

Barry, I can't use a trickle charger, as I have to keep the car on the street.. maybe a solar powered one would do the trick?

 

 

OK, understand your situation.  I don't think the solar charger referenced would help much at 3.3W Others have said they needed far more powerful ones than that.  Then unless sunny you won't get rated output anyway.  Unfortunately, anything readily on view can be tempting for thieves in road. too. I wonder why manufacturers don't fit a  thin smart solar panel beneath a fixed section of glass roof- hardly ever open my glass roof.  (A fixed one would obviate the need for sun roof drains and potential problems these cause) But perhaps this has been considered and rejected so academic).

[matd]

I've done a couple of tests, in 24 hours my voltage is dropping by around 0.45 volts. These 2 pics represent about 24h (between running the engine), the spikes are when the Battery is charging during a drive. @Herbie from the table you included, that looks like it's lost 25% of the charge! Surely this can't be normal?

[ColinBarber]

On 4/27/2020 at 7:23 PM, matd said:

When I first fitted the battery monitor this evening (after 2 days idle), the monitoring app said the voltage was 12.05v, which it decided was *low*, and at '7%'.

But the car started OK, and we went for a 20 minute spin - charging at 14v apparently.

Back home, the monitor now reads '12.71v, 80%'

The 20 mins drive would have needed to be at least a 6 hour drive to recharge a Battery from that low state - the charging that the vehicle does is about the same as a 5 amp Battery charger and really is just there to put back into the Battery what it may have lost through self-discharge since last driven, not to charge it from a discharged state.

The reading you get from a Battery that has just been charged is always higher than it truly is, it needs to rest for a while before checking. Also when batteries get damaged/sulfated it will appear to charge quickly and show a high voltage however it isn't really in good condition and has a very low capacity, which mean the voltage will drop away quickly as it is doing overnight in your case - this is probably due to it becoming discharged the first time as anything below a 70% discharge state will damage the Battery - and with every drop below 12.1 volts more and more damage is done.

You can try a Battery charger with a repair feature, e.g. a CTEK or Noco charger - these have been seen to restore some capacity to batteries however it doesn't always work. Otherwise you probably need a new Battery - it is unlikely you have an electrical fault that is excessively draining the Battery.

With a good Battery you shouldn't have lost more than 0.1v overnight.

[Herbie]

Sorry @matd, only just seen this, but Colin has answered your question far more eloquently than I could have done anyway