Combining new and nearly new batteries

[Rosscrumb]

How bad is it to combine a brand new battery with an identical, nearly new battery? The nearly new battery has had 1 months use, fully charged each day with a MPPT charge controller not run below 12.4v.

I'm aware of the problem of overcharging the new battery if the old battery is not at it's 100% capacity. I thought we could maybe run the new battery on its own for a month to simulate the use that the nearly new one has had before we connect them together.

We're running a budget operation here in the South east of Spain and bought a single 250ah FLA battery to go with our 420w of solar panels.

The plan was to get one of our two gas fridges up and running - it made sense to use what we had, rather than invest more money in expensive battery power. Now, neither of our gas fridges are working and we realise we can't run our 85w AC electric fridge overnight on a 245ah battery without shortening it's lifespan.

I understand the solar array is a little underpowered for the winter but we have generator backup to charge the batteries on a (rare) cloudy day through a victron multi compact plus, and can add more solar power at a later date.

Any help or advice would be gratefully received.

[Mike90250]

Simply adding another battery without increasing a way to charge both batteries, just means in 3 months, you will have 2 dead batteries. So keep an eye on the charge state, and I'd plan on running the generator often enough to keep the batteries healthy. Whats cheaper, fuel or batteries ?

[Rosscrumb]

Thanks for your reply Mike. Just now we have more than enough solar to charge both batteries but yes, I'll keep a close watch on the charge state especially in the coming autumn when the weather predictably turns a bit wet and grey.

Do you think the existing battery is new enough to be paired with a new battery without causing problems? And what do you think of the idea of "breaking in" the new one to be in a more similar state?

[Mike90250]

Best you can do is hook it up. As long as it's same brand & model. It will quickly age and effectively be as "old" as the oldest battery in the group.

[Rosscrumb]

Thanks Mike. I think we'll give it a try. The oldest battery is still in very good condition so that's not a great loss. Cheers.