... run a 5s 40c pack together with a 5s 50c pack as a 10s brick ? If so, what about charging ? can they be parallel charged together ?

Long story short, I have a buddy who was selling batteries but is now closing down shop - he's got these 2 packs that he's offering @ cost but I don't want to get them if I cant use them as outlined above.

Thanks ...

... (oopsie)

I mix batteries with slightly different C ratings but I don't charge as a 10S pack. Don't know.what proems it at cause but so far it works for me.

I've been doing it for a long time. I have a number of 6S 50C packs I also use in EDF models. I use them with a 4S 30C pack for the pattern models. I do charge them on separate chargers. I don't care for parallel charging and my chargers won't series charge a 6S and 4S as a 10S.

thanks for the replies ...

... charging would be as 5s but paralleling this new 5s 50C pack together with my other 5S 40c packs

...

For parallel charging, if you are using a low charge rate (1C or 2C), then it should not be an issue. As an aside, I always charge at low rates, just for battery pack longevity.

If you the packs have a substantially different charge rate capability (say for example 2C and 5C), then the pack with the lowest charge rating will determine the maximum parallel charging current (2C)
Same holds true for serial charging (if you're using a Cellpro 10XP for example). The pack with the lowest charge rating will determine the maximum charging current.