First post in the forum and picked the most general category to not potentially post in the incorrect subcategory.
Mech eng so my intuitive understanding of electricity isn't the strongest but I understand the voltages of batteries in parallel are consistently the same ideally and doing so is basically a method to increase the total mAh of the circuit. What I'm wondering, and was unable to find a solid answer for through google searches, is how unsafe is it to link two or three 9V batteries in parallel with different mAh remaining; considering that there are potentially different types of 9V batteries as well. From what I found 9Vs have a cutoff voltage of around 6.0-6.6V but wasn't able to really find anything on the safety of connecting two '9V's in parallel one discharged down to say 7V with a fresh 9V output battery.
Any help would be appreciated.
If you mean the sort of 9V battery used in fire alarms then they are best avoided as they have very little capacity anyway.
If you connect batteries in parallel they will cross charge and this is a very bad idea. Depending on the battery chemistry it can catch fire.
You can do this only if you put each positive connection through its own diode before you wire them together. This will balance the discharge and you do lose some voltage across the diodes.
To minimise this drop use a Schottky diode, not to be confused with a Shockley diode which is confusingly very different.
But 9V batteries are normally useless for anything but a very low current.
I call "XY Problem".
Incidentally - the forum software is totally flaky today!
Welcome to the forum.
Please read the post at the start of any forum , entitled "How to use this Forum".
What sort of 9V batteries, rechargeable or primary?
Lipo, NiCd, Zinc/Carbon, Alkaline.......
What is the application?
Identical new batteries can be paralleled, but with care if high-current units (by limiting the current until they
are matched). The problem is the word "identical", which may mean same production batch, not just same manufacturer and type, for certain cell chemistries (the flatter the discharge curve the more it matters).
Often there's no need to parallel as cells are available in a range of sizes, so you pick ones that are large
batteryuniversity.com is a useful resource
btw safety depends on the chemistry, most low current aqueous chemistries are inherently safe, the worst that happens is they get hot and lose much capacity, sometimes venting acidic steam. Lithium chemistries however pose a very real fire risk if abused.
As I discovered a few months ago, to be identical, your 9 volt batteries need to have the same manufactured date. I bought two new 9 volt batteries for a test instrument and discovered the voltages were quite different. Then I looked at the mfg. date and one was a year older than the other.
Thank you for the replies.
I was basically just trying to get a higher total mAh for my circuit to run not connected by USB/barrel jack. It sounds like I would need to make sure they are as ‘identical’ as possible to make sure it would work, same manufacture/manufacture date/chemical internals. I’ll probably end up not trying this then to err on the side of safety.
If you want more capacity than a PP3, use a PP9 perhaps?
Again, I call "XY Problem".
What batteries are you trying to parallel and for what purpose? You should not be powering an Arduino by batteries into "Vin" or the "barrel jack" as you are simply wasting power, you will obtain a third more capacity from whatever battery you have by using a proper switchmode "buck" converter to feed 5 V to the "5V" pin.