I know it sounds bad right away, and it gets worse. I have a 7D Mark II, with three batteries (one is a Canon brand and two are off-brand), one off-brand charger, and one Canon charger. I took all my gear on a trip two weeks ago, charged all my batteries with the off brand charger, but only ended up using the camera once.
Back at home, I packed up to do a senior shoot after not having used the camera for those two weeks, and when I switched it on, it was dead. No big deal, I just figured the battery ran out of juice so I plugged two batteries in to my off brand charger for about a half hour. Came back, tried again, and still the camera wouldn't turn on. I remembered I had kept an extra Canon-brand charger in a go-bag in my car over the summer (where the weather gets easily over 100F), but I had brought it inside a couple weeks before summer ended so I didn't think anything of it and just tried plugging in one or two batteries to see if they'd charge. Left one in for 6 hours, came back, and the orange light was still blinking (had not turned green as it should). And still, that battery would not turn my camera on.
My mom happens to be a photographer as well and we both use EOS Canon cameras, so I ran to her house and the first battery of hers that I tried in my camera worked. No problems. Another piece to this puzzle is that when I was on the trip with her, I had accidentally swapped out one of my off-brand batteries for her Canon battery. The off-brand that she ended up with did work but the Canon one that I had brought home and tried to charge did not.
So, to summarize, a) it's not about the camera, since the 3 batteries belonging to my mom were able to power it up and I used it on my shoot today, b) the off-brand batteries work just fine, and the off-brand charger has worked for me for months now with no issues, c) I accidentally left a Canon charger in a 100+ degree F car for at least 2 months, and d) between myself and my mom, we have 6 batteries, I believe 4 of which are corrupted, but we have no idea if a charger can do that, or if it was even the charger's fault in the first place.
I am so sorry for the confusion (let me know if I should clarify anything), but I hugely appreciate any input anyone might have. My grandpa (my mom's dad) is also a photographer (3 generations already of using nothing but Canon gear) and none of us have run into this issue before.
Wow... that's a definite tale of woe there Katie Marie...
Right of the bat, you will get the inevitable response that you should use only Canon brand batteries and chargers if you want to minimize risk to your gear. That said, I have had to used 3rd party chargers and batteries, but I am extremely careful about which brands I use. Canon units have over charge protection built into their systems, which is one of the reasons they are on the expensive side. However, some 3rd party systems can prove that they too have these, but they certainly are not the cheapest. Honestly, I would recommend seeking out a genuine battery charger and trying all of your batteries with that. If you have someone technical available, they could use electrical meters to gauge some aspects of the battery performance from the batteries you have. I certainly would stop using the 3rd party charger at this point.
Your 7D Mark II uses the despicable LP-E6x series of battery packs and they are designed so that they can easily drop below the level where a charger will no longer recognize and charge them. I have a lot of Canon cameras (mostly 1 series) and as a ham radio operator a lot of devices that use portable battery packs and I have never had an issue with battery packs until I bought Canon 5DS and 5DS R bodies which use LP-E6x series battery packs.
I suspect that your charger is just fine, it is just the truly lousy design of the battery pack. And although my 1DX series Canon bodies will continue to get only genuine Canon LP-E19 which are a wonderful pack, all of my LP-E6NH future batteries will be Watson which at least are far less expensive than Canon packs and have the same odds of survival.
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