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Installed higher capacity AGM battery in third gen RX450h

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As many are only too well aware, the 12v VRLA AGM original Panasonic Battery in the third generation is only rated at 51Ah and unless it is regularly charged by one of several ways, it can go flat quite soon, particularly as the Battery ages. So the risk of a flat Battery increases at

airport car parks for extended times for example. Certainly, there are ways to get the traction Battery to work in an emergency but the fact that the 12v Battery has been allowed to drop below a critical level means it's life will have been shortened. I therefore did a lot of research to find an AGM Battery to upgrade the original one. (There are a number of free flow lead acid batteries that could be accommodated within the Battery compartment under the rear hatch but I excluded these as I wanted to fit an AGM type like the original).

I wanted a Battery that met the following criteria:-

It would significantly increase the Ah over the original Panasonic one.

It would be less expensive than an original replacement, be widely available and have a long warranty.

It would not be necessary to cut the actual Battery compartment to install.

It would be easy to remove the Battery and fitted components so the next owner could return to stock if wanted.

All of the above was achieved by simple changes to fit the 027 60Ah AGM Battery, a size that is available from leading makes. I opted for the Halfords (Yuasa) one, using my Gold Membership 15% discount, as it provided a 5 year warranty. Furthermore, should the Battery fail prematurely, there are many branches where it could be exchanged. It was necessary to jack up this Battery to help clear the sloping sides of the Battery compartment and also to pack each side at the top where the angle of the retaining strap secures it each side. This is critical as there is a correlation between the top and bottom packing and each must be right. You then need a way to get the existing earth strap back over the hump of the Battery to its original fixing point. I achieved this by using an auxiliary terminal clamp onto which I screwed a brass post. (I could have used an after market earthing strap with a different end but wanted to retain the Lexus one).

So the above are the bits I needed. I tried to make the packing parts complement the original as far as possible by making them out of black polypropylene. I cut the Battery tray and let in 8mm of polypropylene to extend it. I must say that I have fused other plastics together fairly easily in the past but found polypropylene is a very difficult material to work with and I would not use it again! I would just omit the tray as it serves no useful purpose with an AGM Battery. (I note that on some USA forums members have just cut the ends of each propylene tray and extended the tray by flattening the lip). The advantage with my extended tray is that if an owner reverted to an original Battery, it would look very near stock. Even with the extended Battery tray I had to alter the angle of the projection at each end using heat in order accommodate the Battery clamping extensions. It would have made it easier to cut off these clamping extensions on the Battery as they are just in the way, but would doubtless have invalidated the warranty should I have to claim on it.

It would have been a lot easier to have made the two packing pieces that go beneath the Battery retainer at the top out of wood. The bottom packer could be made of plywood, sloped in at the bottom, best used without the original plastic tray or with the lip at both ends of the tray let down.

Cost of project in round figures.

Halfords (Yuasa ) 027 AGM Battery on offer with discount from local branch £103

Neg earth clamp and pair of brass terminal posts with allen screws , only neg used £ 20

Polypropylene for base and top packer £ 7

Milliput as core for top packers to prevent crushing £ 5

Vent elbow £ 6

Polypropylene DPC used 1 layer below tray and 1 above £ 0*

C.T.1 Adhesive/filler thin layer between DPC and tray to take out welding unevenness £ 0*

Swear box , should have been more!! £ 1


*Had some of this from another job and not necessary if wood is used instead of


Well here are some pics showing parts I made/purchased before and after assembly. NB. I did find I had to reduce the length of the top packer on one side as it came up to the Red cover of the positive terminal. I also had to replace the vent tube elbow with a more standard size as the Panasonic one has a larger port. Finally, I had to further relieve the cut away in the compartment cover slightly to allow for the longer Battery.

If anybody else wishes to do this, feel free to ask any questions including where I sourced the brass post and auxiliary negative earth clamp, the only one I could find that was threaded to take the 8mm Allen stainless screw for the brass post – they are usually 10mm. Hopefully, from the narration and pictures below this should made it clear.

1st Picture - with compartment cover in place

2nd Picture - Parts used less vent elbow

3rd Picture – Installed, and to borrow an Allen Millyard expression ,“It fitted perfect”


12v picture 3.jpg

12v Battery picture 1.jpg

12v picture 2.jpg

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