Battery Storage

Jeff
I would fit the new battery. If the old one is 40% lost already, then with any really cold start you may find cranking volts too low if heaters for cold start are also required. I use a digital voltmeter and see the lowest crank voltage at each start. When below a certain level AND you detect slightly slower cranking sound, it is time to replace the battery even though it sill works ok. A balance between reliable starting and cost I suppose.
50% loss of starting current is closer to what the starter and start aids draws, so it becomes a borderline issue.

I would fit the new battery and if you have a use for the old one, store it and charge it weekly. If it wont last a week between charges you might as well toss it.

Ahem, recycle it :-)

I have recently dealt with a similar issue. I have kept the old battery for light duty in the garage - pumping up bike tyres, running 12V appliances and chargers, etc

Jeff
I don't like storing any battery, new or old, for months at a time. I use a 240v mechanical timer set to charge for say 5 to 10 min a day so it always has an active electron pass through, on a regular basis. The last battery I changed for much the same reason as you are considering, didn't like being left for a while and then failed shortly after. Although an AGM my 10 year old AGM FullRiver is still going as it receives a regular charge albeit with less capacity now.

Years ago, our car was about 18 months old when the original battery died. I replaced it with a cheapie, because I was about to be sent overseas for 2 years on a job. The job thingy changed, and I didn't go. After about 2 years, the car needed new rings (a Mitsubishi Sigma). I fitted the new rings in the depth of winter. When I tried to start the car, the battery didn't have enough grunt to turn the engine over, so I had to buy a new battery. I emptied the acid out of the old "dead" battery into a bottle, and kept the battery as a doorstop for the garage door.

Fast forward a few years, and we had a Magna by then. Again, the OEM battery lasted just over the warranty period (1 year) and died. I bought a new Gel battery for the Magna and it worked for about 4 years before it too died. At this stage, as an experiment, I got the doorstop out, filed it with acid and installed it. To my amazement, it worked, and we used it for another 5 years before we sold the car to a young girl. All up, the Magna did 345000 km for us, so it was used almost every day, and I think that is the secret to getting a long life from your battery.

Certainly, if we had simply put the weak battery which couldn't turn the engine over with new rings on the shelf, it would have died within a few months, So if you are going to store a battery, drain the acid out from it and wash it out thoroughly. You have nothing to lose...

Just a thought but if you're happy with the old battery why not keep it, onsell the new to someone who actually needs it, and pocket the small profit?

Fit new toss old, doing anything else will be time consuming and will let you down. Michael

Sometimes the older battery can be used to run a fridge etc. Definitely put the new one in.

Just put the new battery in and donate the old one to simsmetal. You will just get caught using the old one, it has done it's time.