Replacement battery not taking a charge

The battery life on my Late 2010 MacAir 13" was poor so I decided to replace it.

I purchased the new battery from iFixit. The battery was for a 2011 model but claimed compatibility on the website.

The replacement went smoothly. However, the new battery would not take a charge.

I tried the SMC reset process a few times with no success.

I replaced with my old functioning battery and now that battery will no longer take a charge either.

The MagSafe charger is working on another MacBook of ours...so it's not the charger.

Thoughts? I feel I awakened some sleeping dog I shouldn't have bothered with.