Upgrading the main battery

The standard Jimny battery is a Furukawa Batteries 55B24 using small JIS terminals; this is a 370 CCA and 45 Ah wet lead acid battery.

I’ve tried a couple of options for an upgraded battery, but with my current setup only using a lightweight winch and having a lithium 2nd battery, the best fit for me is an upgrade in the stock location without doing any further modifications.

Considerations before upgrading

There’s a few options to upgrade the main battery on the Jimny away from this stock size. To help make this choice it’s important to work out what you’re after:

  • Is it extra capacity to allow you to use it as a battery when camping for a night or two off grid?
  • Does your new battery need to run a winch? If so, how big is the winch?
  • How much desire do you have for further modifications or do you just want drop-in?
  • Are you looking to shave weight or are you capable of adding weight for the right battery?

Extra reserve capacity

The stock battery is only a 45 Ah total capacity. Being a starter battery it is not designed for deep cycle use, so it realistically only has a 33-40% total depth of discharge before it won’t start the car. This means you really have only about 14 Ah to 18 Ah, and doing this repeatedly will shorten the life of the battery. If you want to use your starter battery then you should look towards options that give you a larger capacity, and ideally something designed for more repeated discharges (e.g. an Optima yellowtop which is a sort of deep cycle/starter battery hybrid, although the red top ones work fine in this application).

Something like a DIN65LH battery will give you ~65 Ah total capacity, so usable 22-26 Ah instead of the factory battery of 14-18 Ah. If you’re camping only for a night that’s probably perfectly fine.

Large high capacity deep cycle batteries can be hard to fit into the stock location so you will need to find a specialised battery bracket and probably do things like relocate some factory wiring and the ECU. You definitely will also need different terminals on the battery leads since basically anything with a meaningfully higher capacity will use larger full-sized battery terminals..

Running a winch

Using a winch is a drain on the battery more like starting a car compared to the total reserve capacity. A rough rule of thumb on winch capacity versus required CCA is something like this:

Winch size (lb)Required cold cranking
amps (CCA, in amps)
4500450
6000600
9000800
120001000

You can help this out a little by minimising the length of the power cable to the winch and maximising its size so it has the least voltage drop, but ultimately big pulls require big batteries.

Drop in or further mods?

The stock battery location is not massive so a larger battery might require some modification or a custom battery tray. Given the ECU also mounts on a bracket just to the side of the battery tray this can also prove to be an impediment.

Although there’s a few options out there, they can be a bit hard to track down

I previously used (temporarily) an Optima 34R and an alloy battery tray to suit but I could never get it to work to a level I was happy with; swapping to a 4500 lb winch also means I can afford to use a smaller battery so I no longer use this option; if I ever get around to making it work well enough to recommend to other people I’ll document it

Lightweight?

Maybe you’re struggling up against the Jimny’s GVM and so a lighter but more powerful battery is attractive to you. Lithium batteries are way more powerful and lighter weight, though many do not work so well in underbonnet applications due to the temperatures involved.

An option previously used was the Megalife MV24L, however, this is out of stock and looking like they’re revising their product lineup.

Upgrade in the stock location

The official supported battery for an upgrade in the stock location is a Century NS60L MF (which I believe has JIS terminals) or a MF55B24L battery.. If you relax the requirement to go for a factory terminal size then the Century NS60LSX MF will give you 480 CCA, 50 Ah in the stock location and even using the stock plastic battery tray protector.

It is a fairly simple upgrade although you will need different (larger) battery terminals. I went for the UniLug 2 Up battery terminals as they give a good way to connect to the factory terminals and retain all of the stock fusing and the current sensor on the negative terminal.

You start off by removing the factory battery: take the negative terminal off first, then the positive, then fully undo the battery bracket & twist out the two threaded arms for the bracket.

If you’ve used your car offroad at all there’ll be a bunch of dirt in the bottom of the battery tray (more visible once you pull out the plastic battery tray).

Clean up the plastic tray…

and the metal battery tray using some all purpose cleaner and a detailing brush

Dry off both the metal battery tray and the plastic one and put the plastic one back in.

Now we should modify terminals to make them work with the new, larger terminals.

Starting off with the positive, undo the nut closest to the point where the terminal meets the battery.

This will let you separate the battery fitting part of the terminal away from the rest of the fuses etc. Because of the height of the main fuse, I used some M10 copper washers to hold the fuses away from the terminal body.

This ensures good conductivity even as things oxidise, and keeps everything sitting nicely.

Bolt it down and use the other stud to hold other positive wires that you want to access outside of the car’s battery fuses (e.g. winch or compressor power) and you’re all sorted for the positive side of things.

Onto the negative.

This will allow you to keep the tab for the current sensor and also the factory earth cable going through it, and works perfectly with the terminals I chose.

You do need to bend up the lugs that hold the current sensor from rotating on the factory terminal so it can work with the new terminal.

The round post on the other stud on the terminal is because you shouldn’t mount any earths to this connection; any current flowing there would end up not being detected by the current sensor and that’s not ideal with how the Jimny’s smart alternator works. If you want to have a fat earth point for things like some winches and stuff, then use some copper and extend out through the current sensor.

Once you’re happy with how the battery is going, put in the battery bracket and tighten it up to hold the battery in place.

Note that the UniLug terminals have lovely screw-on plastic covers to protect the terminals, put them on and you’re all done.

Doesn’t take long at all, and if you choose terminals to accommodate the positive fuses and the negative current sensor then it’s all awfully simple to do as a change.