Converting Prius 3 lead-acid 12V battery to LiFePO ie LFP.

Hi all. Long time no post, but I read a fair few as research partly on this topic and for other points.

Everyone sensible says don’t replace the standard lead-acid 12V (14V+ as we know) battery with Lithium, of any sort, so let me back that up: THE SENSIBLE PATH IS TO SIMPLY PUT UP WITH A FAIRLY RAPID DISCHARGE WHEN INACTIVE/WINTER, AND STICK WITH A DIRECT REPLACEMENT LEAD-ACID 12V UNIT WHEN REPLACEMENT IS REQUIRED.

It’s a total disclaimer, don’t do what I am about to describe, even though the LiFePO or LFP batteries are much safer than the ‘normal’ Lithium type, and the assurances of the supplier seem wall-to-wall. That now said, here’s my take…

First, I did all that I could to minimise risk. This did not quite pan out, unexpected issue arose, see later. I researched and opted for the much safer LFP type (Lithium-Iron) or LiFePO as it’s been known, and chose to obtain a more modest capacity, that the supplier fitted cylinder cells to, and made safety claims about the Battery Management System.

So I bought 50Ah, on the face of it only 5Ah over the bog-standard lead-acid 12V battery, of course the LFP has two aspects that stand out: more actual energy than the lead-acid can possess; less self-drain of its energy; fast recharge rate.

I checked it ran static at first, ie turning the engine over and making all the hybrid recharge etc happen, and so on, and turning the car off and on during all parts of the cycle…and fitted a well-known battery monitor unit, for extra safety…this almost literally, backfired however.

All seemed well, until suddenly the battery monitor stopped sending to the iphone bluetooth app. In short, it’d blown internally. Nothing else happened. I disconnected it and removed it.

I replaced it with another of a different type that came with its own little monitor screen - this time it blew in dramatic style, taking the car down with it. Later after the removal of the device, the car got ‘ready’ and ran as normal. It refused to, until the device was detached, i believe it is fused together inside, This I take to be the large current spike a BMS of a modern type of battery can produce when shutting down. These things are not able to take the transients, but my car seems able to.

Thus, the only actual serious issues that arose, have been with the monitor devices that ought to have assisted in checking temperature of the battery, and voltage/energy level. I was lulled by the first breaking down innocuously. The second was a shock.

Moving past that, as the battery on its own is working well with the Prius, I have decided to permanently install it. There are literally no issues, other than the appalling battery monitor devices blowing.

The pics are of the physical mods to install. The base part of the battery bracket, I removed and drilled the three new mounting holes 1cm from the edge of the present ones, to accommodate the slightly ‘wider’ battery size. ie looked at from the top, with the two battery posts nearest you, the LFP battery is ok for ‘length’ across the way, but it stretches further back by maybe 1cm or so, and I am insulating it with wool sprayed with fire-proofing substance.

This permitted the base part of the bracket to be installed, and I used the same 3 small M6 (10mm spanner) bolts that the bracket originally was attached with.

I cut a slightly oversize piece of wool fireproofed material, and laid it flat then sat the battery in, obvs no terminals are attached at this time.

The top bracket is only slightly trickier, and all it needed was the hole that is furthest away as you install the battery, back in the rear recess there, to have a new hole drilled 1cm from the edge of the original hole, to suit the location of that strange vertical screwed rod that emerges up from the lower bracket.

The LFP/LiFePO battery is about 20mm taller than the original lead-acid. So when I sat the top bracket onto the battery surface, the lower end of it at the battery post side, inboard as it were, is 23mm or so higher off the car frame with the threaded hole for the M6 bolt that goes in there. I just got a 30mm stainless steel bolt and it fitted fine, albeit the bracket sits up with some air instead of clamping onto the car steel - I will probably find a suitable packing to fit there so it is solid, but ok for now.

That just leaves the issue of the vertical threaded rod at the back, being a couple mill short of this top bracket hole. I found a cheap solution, instead of the supplied original nut/washer combo used when a lead acid is installed (lower top remember), there’s a thing called a ‘screw nut’.

It’s basically a 17mm long thing with a cap about 13mm diameter with a allen hole of 4mm into it, ie it reaches down and screws over the slightly-short vertical rod from the lower bracket. This beautifully bridged the gap, and I just needed a couple of washers under the cap, to stop the rod from ‘bottoming out’ the thread the ‘screw nut’ has only so far inside it. It maybe is threaded for 13mm of the 17mm? Obviously to allow for the hex/allen hole to be there.

I tightened these up, on the top bracket, and the battery is fairly secure. I will pack the bolt-side of the top bracket however, to make it firmer still.

I have not done many miles in the car since but have run it and it’s fine, it charges and stops charging, and the setup works so well I really wonder if Toyota had this in mind all along. I still must caution against anybody else doing this…I have quite a lot of electronics and electrical training including test and repair work on power electronics, and STILL I can make mistakes and blow stuff up/take stuff down. My sole claim, is that on my Prius 2010, this particular mk3, all so far is well.

There’s none of the ‘fade’ of the lead-acid as a couple of weeks unused were occasioned for example. It was driving me mad, on principle, I admit. The lighting seems maybe a fraction brighter (interior diddy bulbs I mean), although I also replaced these with LED, a long time ago.

Ok, I will try and put the pics in - the rear quarter where the battery compartment is, will likely be familiar, and you’ll figure from there what’s what, it’s easier than I made it sound. Quick note, as it comes, the battery has two screws in the anode and cathode instead of posts. I had to get M6 posts, the correct Japanese sizes, I think from Ali Exp, and then I drilled the tops, to let me screw in the ill-fated battery monitors. Obvs, these holes now don’t have a purpose, but they could have if there was a need, maybe a rear 12V socket?!

Take care all, don’t take risks. I did this and don’t consider I took a risk - but I still had something blow on me and take me car down! It’s unpleasant, so many will be better sticking with trickle charge - I also tried a small solar panel charger before this, it was so complicated and did not work out in practice, I gave up on that!

My best, I will check back when more miles are covered.

Pics should be attached.