Winston 12V 40Ah LFP battery, good or bad idea???

in your circumstances, i would install the cheapest battery i could find

 

I just finished installing a 50Ah LiFePO4 that I bought on Amazon for $130 (I can't post links until I've made 4 posts, but search Amazon for LGECOLFP 12V 50Ah Lithium Battery). It was a tight squeeze, but I got it to fit with a longer bolt for the battery clamp. Here are the dimensions that I measured:
Toyota TrueStart B24 battery - 45Ah: 9 1/4" (234mm) L x 4 7/8" (124mm) W x 7 7/8" (200mm) H (not including posts)
LGECOLFP 12V 50 Ah LiFePO4 battery: 9" (229mm) L x 5 3/8" (137mm) W x 8 1/4" (211mm) H (not including posts)
The original battery weighed 28 lbs, while the new one is 11 lbs. And while the rated Ah is only slightly higher (50 vs 45), if you regularly drain a lead acid battery below 50% capacity, you dramatically reduce the lifespan. LiFePO4 can be drained to 10-20% SOC with no ill effects.
I started by loosening the 2 bolts on the black bracket and sliding it all the way outward to make sure I have every millimeter of width:

Even so, the battery got wedged between the body and this bracket, and the clamp had to pull it all the way down.


I put a longer M6 bolt on the left side of the clamp, with about 10mm of washers underneath to give the bolt something to tighten onto.

I also had to modify the battery terminal attachments of the car. The battery comes with M8 femaie-threaded holes, and I couldn't find anything on amazon that would adapt that to the tapered posts of the OEM battery. Instead, I removed the post clamp from the positive side of the fuse block thingy and just put an M8 bolt through it into the battery. That side was easy. On the negative side, I removed the bolt and nut from the clamp and enlarged the hole from the bolt to 5/16" so that it could fit an M8 bolt. I also had to do a little bending of the tabs that come out at right angles on the edges of that clamp to clear the battery post and bolt head, but it worked and I got about 4 1/2 threads of engagement. I'm confident these mods could be reversed if I go back to an OEM battery.

So far, so good! The car started up normally and drove fine for the first day. I'll post a follow-up after I've had it for a few months.

I'm not too worried about the low temperature charging limit where I live, as it rarely gets below freezing. I think charging below freezing once or twice won't kill a LiFePO4, but if it were a regular occurrence it might. I'm not sure you'd get much protection from it being inside the car, either, as it's in a pocket that's surrounded by the outside on 5 sides, with just think sheet metal separating it from the outside temps. a battery warming pad might be a good idea, but if you're powering it from the same battery, it could drain the battery if left for a few nights. What would be really slick is a controller that takes the charging current and feeds it to the battery if it's above 32°F, or to the warming pad if it's below 32°F.

One other thing to note: The battery I bought has a BMS (battery management system) that can only output 50A of current. The battery I took out is labeled 325 CCA (cold cranking amps). That had me worried, so I measured the current into and out of the battery at startup. I saw a peak of 40A to close the contactor for the traction battery, but just for a second, then a steady flow of around 20A back into the battery.

 

That looks good, all the best!