UX 250h 12 Volt Battery Reading Using Multi Meter

TRo · August 19, 2022

Hi

I am trying to monitor my new 12 volt Battery fitted by my local Lexus Dealership on my 2019 UX250h with a multi meter whilst connected in the car - aware that the 12 volt Battery is in demand immediately the driver opens the door/hatch (internal lights/security etc)

So I would be grateful for other UX 250h drivers experience :

After a short run ( under 5 miles ) the 12 volt Battery is showing = 12.45 VOLTS
Car not driven (96 hours ) the 12 volt Battery is showing = 12.27 VOLTS

Thanks

Tony

ColinBarber · August 20, 2022

12.45 V seems too low. Has the Battery been given the opportunity to fully charge?

TRo · August 20, 2022

5 hours ago, ColinBarber said:

12.45 V seems too low. Has the battery been given the opportunity to fully charge?

Hi

Firstly, thanks for your reply.

Yes after a longer run the new Battery showed 12.77 volts using a multi meter (again whilst connected in the car)

The difficulty is that I am concerned that the car could have been experiencing a "parasitic drain" but as this is the first hybrid I have owned I don't have any knowledge of what the normal car 12 volt Battery reading should be particularly as I drive the car infrequently. That is why I was seeking help on the forum.

I have contacted my local Lexus Dealership and was told that " Anything above 12 volts is fine particularly if the car is not driven for up to a week"

I have a solar charger I can use but until I know what a normal 12 volt Battery reading is in a UX 250h that is driven infrequently I am uncertain when it is necessary to recharge the Battery.

Thanks

Tony

Herbie · August 20, 2022

On 8/19/2022 at 10:27 AM, TRo said:

Hi

I am trying to monitor my new 12 volt battery fitted by my local Lexus Dealership on my 2019 UX250h with a multi meter whilst connected in the car - aware that the 12 volt battery is in demand immediately the driver opens the door/hatch (internal lights/security etc)

So I would be grateful for other UX 250h drivers experience :

After a short run ( under 5 miles ) the 12 volt battery is showing = 12.45 VOLTS
Car not driven (96 hours ) the 12 volt battery is showing = 12.27 VOLTS

Thanks

Tony

First of all, you may find this chart helpful, but be aware that this is only 'standing' voltage without any load applied:

Although I've never seen it in reality, each manufacturer should be able to supply the spec for an acceptable quiescent current drain when the car isn't being used. As a rule of thumb though, somewhere around 50mA (0.05A) is roughly considered the norm. If it reaches 100mA then I'd say there's definitely something wrong.

I don't know what size Battery is in the UX but we'll say 50Ah as an example. This means that the Battery can supply 50A for 1 hour or 25A for 2 hours and so on, so we can work out (very roughly, for reasons that will become apparent) how long the Battery charge should last if it isn't used.

NB - this assumes a brand new and fully charged Battery - something that yours almost certainly isn't.

So,

50Ah divided by 0.05A = 1,000 hours (or 41.66 days, or 5.95 weeks) from fully charged to fully discharged. However, for the purposes of starting the car, the Battery will become useless long before it's fully discharged (which is why this is only a very rough indication).

If you really think you have a parasitic drain then this is the best way to investigate it:

Las Palmas · August 20, 2022

While this is not answering your question, I wonder how a 2019 car can need a new Battery already. The Battery in hybrids are supposedly only there to start the computer so the hybrid Battery can deliver its power.

If the car has not been modified with extras ( not installed by Lexus ) I would think the 12V Battery should last more than 5 years. The 12V Battery in our car is now one month away from being 6 years and I believe it is still so good that it will last the next couple of years.

Parasitic drain should only come if something not original has been installed in the car. I believe.

Herbie · August 20, 2022

3 minutes ago, Las Palmas said:

I wonder how a 2019 car can need a new battery already. The battery in hybrids are supposedly only there to start the computer so the hybrid battery can deliver its power.

If the car has not been modified with extras ( not installed by Lexus ) I would think the 12V battery should last more than 5 years.

Sadly, car batteries just don't last as long as they used to. I remember the days when 8, 9 or even 10 years wasn't unusual and 5-year warranties were the norm - not these days though.

6 minutes ago, Las Palmas said:

Parasitic drain should only come if something not original has been installed in the car. I believe.

Not at all - almost always a fault on something and faults can occur in any wiring (original or not) and any location.

duotone · August 20, 2022

A new Battery direct from Lexus Parts costs £57.56p, so not a huge cost.

My previous Lexus IS 12v Battery needed replacing after 8 years mainly due to lack of use during lockdown.

ColinBarber · August 20, 2022

16 hours ago, TRo said:

The difficulty is that I am concerned that the car could have been experiencing a "parasitic drain" but as this is the first hybrid I have owned I don't have any knowledge of what the normal car 12 volt battery reading should be particularly as I drive the car infrequently. That is why I was seeking help on the forum.

I have contacted my local Lexus Dealership and was told that " Anything above 12 volts is fine particularly if the car is not driven for up to a week"

I would agree with the dealer. But if that continued for another week or two then you would run into problems without the Battery getting a good charge, and letting the Battery drop below 12 V would start to damage the Battery where its capacity would be affected, creating a vicious cycle where it takes less and less time to drop to a dangerously low voltage.

I've just checked my wife's UX. It had a very good run earlier in the week but has since sat there, or just done very short 5 mins journeys. It is now reading 12.2 V. Given she isn't going to use it much for the next few days I'll put it on charge tomorrow.

TRo · August 21, 2022

13 hours ago, ColinBarber said:

I would agree with the dealer. But if that continued for another week or two then you would run into problems without the battery getting a good charge, and letting the battery drop below 12 V would start to damage the battery where its capacity would be affected, creating a vicious cycle where it takes less and less time to drop to a dangerously low voltage.

I've just checked my wife's UX. It had a very good run earlier in the week but has since sat there, or just done very short 5 mins journeys. It is now reading 12.2 V. Given she isn't going to use it much for the next few days I'll put it on charge tomorrow.

Hi

Thanks again for your response particularly the information that your wife's UX 12 volt Battery is currently showing 12.2 volts after a good run earlier in the week followed by short runs and that you now plan put it on charge.

Since I had the new Battery, taking a reading first thing in the morning, the average drop in voltage compared to the previous day is between 0.04 and 0.07 volts. ( I assume subject to the outside temperature etc.)

Would you say in your experience that if the UX was not driven for say seven days an average daily drop in voltage would be around 0.05 volts so if the multi meter reading started at 12.6 volts after seven days you would expect the reading to be around 12.25 volts (12.6 minus 0.35 volts) ?

If not it looks more likely my UX has a "parasitic drain "

Thanks

Tony

Marlinleg · August 21, 2022

The link below is from Haynes how to test your battery

https://haynes.com/en-gb/tips-tutorials/how-test-car-battery

Bill Dawes · August 21, 2022

My opinion for what it's worth is that a 12v Battery that is reading close to 12v is pretty well dead so don't entirely agree with your dealer that anything above 12v is ok.

All cars these days as far as I am aware have a slow drain on the Battery for various electronics on moders cars.

Brings to mind many years ago someone I know had a problem with his Battery going flat, after many fruitless checks by the dealer it was eventually found by chance, touching the door mirror as he was getting in he realised it was warm.

Bill D.

Herbie · August 21, 2022

9 hours ago, Marlinleg said:

The link below is from Haynes how to test your battery

https://haynes.com/en-gb/tips-tutorials/how-test-car-battery

Hmm...

That article isn't bad but this last paragraph from it isn't exactly great:

"However knowing a cell is 'dead' is of no more use to you than knowing that the Battery won't hold a charge, so a test of the resting voltage is just as effective a diagnosis."

The problem is that using a standard multimeter to read 'resting' voltage can be misleading. It may show 12.6 or 13V, which according to the chart above is 100% charged but if there's a bad cell, when a load is applied to the Battery that voltage may drop like a stone.

Really testing a Battery needs something like this:

hieronymous · August 21, 2022

Using a multimeter is little better than guesswork.

I use the small BM2 12v Battery monitor, widely available, £25 or so. It is wired to your Battery terminals, gives you data as in @Herbie's table above, and transmits by Bluetooth to an app on your phone (android or iOS I think). Data is shown as realtime voltage, or in 1 day / 7 day charts on demand, up to 2 months data.

I have 3 of these, for my IS300h, my Kona EV, and my switchboard backup generator - they are invaluable. I also have 1A smart Battery maintainers on the IS and the generator, timed for 20min topups every 4 hours, same as the EV built-in schedule. All batteries stay at optimum voltage regardless of use.