Hello. I have 4 AAA and 4 AA batteries. I would like more than 6V in voltage, so I was wondering if it's alright to connect AA batteries in series with AAA batteries. Thanks.
Not really! Similar size and capacity batteries should be used.
It is safe but the smaller cell may empty soonest and drag the 6V down.
It is possible to put batteries drained below 1 volt (called dead but not really yet) in series and achieve dangerous totals. Many demos use "dead" 9V cells.
With a .9V to 5V boost converter, a phone charger could finish off non-rechargeable "dead batteries" before recycling.
Could you explain why?
GoForSmoke already did but as far as I understand it, the gist is that the smaller battery will just drain sooner, and then the combined voltage of your two batteries will be far too small to actually drive your circuit, despite the fact that the larger battery is still half-full. So you're just wasting energy and throwing away AAs which still have plenty of life in them.
I have rechargeable ones, so I wouldn't throw anything away. Can it actually damage something?
Depending on the type of your batteries, you might have to deal with things like the memory effect, but to my knowledge that's mostly a problem with older batteries. In the long term, every turn of recharging does wear your batteries down anyway, but that probably isn't a huge concern financially, given how cheap they are these days.
Honestly I don't know a lot about batteries. I vaguely remember hearing that having batteries of different capacities will discharge asymetrically, and then the bigger one will try to "recharge" the larger one, which may lead to overheating, and overheating is never good. No idea how much truth there is to that.
Personally I'd just err on the side of safety and buy more batteries.
Other way round, NiCd are the old ones and have memory effect, NiMH are the modern ones and don't have the problem.
Correct, except it's reverse charge, not re-charge, plus other damage depending on the chemistry.
A general rule is every cell should be the same type, manufacturer and history.
It's even worse than you think - I wanted to say "modern rechargeables usually are Li-Ion" but then had a complete brainfart. TBH I generally use non-rechargeables these days.
Nothing wrong with reverse charging lithium batteries - if you want fireworks 
Like I said, I mostly use non-rechargeables, and my smartphone knows how to charge its Li-Ion.
And, to be fair, I did mention overheating, which "fireworks" is a subcategory of.
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Thanks
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For those who are interested this is a great resource for everything you need to know about batteries: https://batteryuniversity.com/
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Series strings should really be identical type and brand, bought at the same time. Otherwise the mismatched capacity will mean the weaker one's get back-driven by the stronger ones once they discharge, completely ruining them if they are rechargable.
For higher voltages its often more convenient to use cells with a higher individual voltage such as LiPo or LeFePO4, and to have proper BMS monitoring the individual cell voltages.
But your problem might simply be solved with a boost DC-DC converter.
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For what purpose? Perhaps to power an Arduino through its Vin pin or barrel socket?
Why do you want to use this mix of sizes? Why not use 8xAA or 8xAAA? Is it to fit them inside a strange shaped object with limited space?
my words: It is safe but the smaller cell may empty soonest and drag the 6V down.
please show where I wrote that such operation sustains, especially after that.
I don't assume that the battery power has to last forever. A full AAA on a full AA could run an Arduino data logger for a Very Long Time on such a cell pack clocking along at 1MHz and sleeping most of the time anyway.
Your assumptions waste my energy so I throw them away.
It's because those are what I have and I don't want to buy more. I ended up just using one 9V for three motors.
They meant you already explained the downside of using them together, not that it would sustain. They weren't accusing you of anything, just pointing out that you had already answered my question.
You mean a smoke alarm battery? No use for motors, cannot supply enough current and very low capacity.
They are small motors. I have two cordless drill motors as well, and I use 18V cordless drill batteries for them.