Auxiliary battery failure

[JonWS]

My 2019 Hybrid RAV4’s auxiliary Battery has failed twice this year. I’ve found my vehicle completely dead on the drive. The Battery was completely dead even though I have made a number of long runs in the last week. I left the vehicle with our local Grimsby dealer & after two days could not find a fault, also they did not mention that there is a problem on this car which I have found out since by looking on the Internet.

[Avalon]

I’m general terms, batteries die because they are drained by something, you don’t say how long the car was stood for on the occasions it failed, or if anything else was left on, or if you checked for parasitic drain due to leaving something connected to a permanent 12v - dash and poorly installed for example.

[philip42h]

13 hours ago, JonWS said:

there is a problem on this car which I have found out since by looking on the Internet

So what is this problem John? Do tell ...

The auxiliary Battery is of pretty small capacity and runs the necessary car electronics when the car is switched-off so it needs to be charged regularly ... otherwise, as above.

[Cyker]

Do you park on a driveway and store your keys in a coat or shelf or something that's near the front of the house? That was a surprisingly common cause I've seen in this forum over the years.

Another common cause is not driving the car regularly enough.

Might also be worth checking the Battery - I remember the 3rd gen prius and auris originally just used normal car lead acid batteries, but like most automotive batteries if they get discharged more than 50% they're basically dead and will never recover.

Supposedly in newer model hybrids, they have finally switched to deep-cycle AGM batteries like we use in our data centre UPS racks, and these can tolerate being discharged lower without being fatally damaged, at the cost of lower amp/current draw (But since you don't need loads of amps from the 12v in a hybrid that's a perfect tradeoff!)

 

(NB: Just because a Battery says AGM does NOT mean it is deep-cycle; Deep-cycle batteries have thick lead plates for robustness, whereas in normal car batteries the lead plate is full of holes and looks like someone with a lace fetish made it, for maximum surface area to deliver the energy quickly)

[IanML]

3 hours ago, Cyker said:

....like most automotive batteries if they get discharged more than 50% they're basically dead and will never recover.

Not so.  I have had normal lead/acid batteries showing 20% SOC or less, and have recharged them with a reconditioning (gassing) charge, and they recovered to 75% SOH or better.  They are still in use.  Battery dealers will tell you they are finished, but, if you have a good, modern charger, its not the case.