RX400H

[Kennyboy]

Hello,

My Lexus RX won’t start unless I disconnect Battery and hook it back up.  Any suggestions?

[Herbie]

Most likely the 12V Battery dying.

When you disconnect it it just manages to build up enough charge to start it.

I would be tempted to get it on a charger overnight, possibly even a full 24 hours, then disconnect the charger and let the Battery stand for about 30 minutes to dissipate the surface charge. You can then use a multimeter to check it against this chart:

If it's well down then it needs replacing but if it looks alright you then need to get it load tested. The multimeter could show something like 12.6V but that may drop like a stone once a load is applied.

[Kennyboy]

Thanks for the tip, I’ll do some measurements.  It’s my understanding that the Battery only powers the electronics and doesn’t actually crank the engine.  The Battery I have in there is fairly new but not an AGM.  Should I have an AGM?

[Herbie]

9 hours ago, Kennyboy said:

It’s my understanding that the battery only powers the electronics and doesn’t actually crank the engine.

That's correct. The 12V Battery boots the computers and does a couple of other things to get the car to READY mode. When the petrol engine is needed the hybrid system energises MG1 and uses that to spin the engine to 1,000rpm before applying fuel and a spark to fire it.

9 hours ago, Kennyboy said:

Should I have an AGM?

You can if you want but there's no specific need for an AGM because your Battery is in the engine bay. The RX450h has the Battery under the floor of the boot, so if there's an accident and the car rolls, there's a slight chance of the acid leaking into the cabin which is why they are AGM and not wet.

If the Battery is fairly new, check the connections to the Battery posts and the earth straps, make sure they're clean and tight at both ends.

It might also be worth seeing if any OBD codes have been flagged.