Which battery?

My RV-10 starter is turning slower lately. I have a 4 year old Concorde RG 25XC battery. Battery technology is advancing at light speed and I am finding it overwhelming to make a good choice. My requirement is reliability since I commute to work with the plane.

Another Concorde, possibly 35A AGM?

lithium Ion?
If so, which make/model. (Would need to figure out an OV circuit and a fire containment box?)

Make it primary and the older 25 a backup? (How hard is this to wire up?)

Input appreciated.

More info needed.

Do you fly IFR?
Is the aircraft electrically dependant?
Do you have just a single alternator, or also backup alternator?
Do you have a separate essential bus?

The battery has two primary qualifications. Cranking power and alternator out endurance.

Bevan

Yes I fly IFR a lot. I practice with my G5 as it has a 4 hour backup battery. No dedicated essential bus and one alternator. No vacuum system.

I fitted the EarthX 1200 about a year ago. With an MT prop, my CG was further aft than ideal. So I saved 15lb and helped out the CG as well. The mod to the battery box was easy. The only issue I had was running a wire to the front for the warning light - thinks - should have run a couple of spares during the build .....

EarthX recommend that you fit their vented version in an inaccessible spot. However, it would take at least three separate failures/omissions to cause an over-voltage problem so I am quite happy. Given the cranking capability and capacity, you could probably even get away with a -900.

Price is a bit eye-watering but it probably lasts as least twice as long as a conventional battery so it gets close to evening out. Seeing the battery charging at 50A after start gets your attention at first but you get used to it!

ETX 900

I have an ETX 900 in my -10 and am very happy with it. Cranks the 540 with ease.