Right way to parallel batteries

[IanB]

Definitely a problem with the designer because he/she hasn't made a robust design that deals with real world use cases. Two years from now (two minutes in some cases) the instructions will have been lost and whatever is printed/molded on the product will be illegible. To many users, a battery is a battery if it still has some life left in it.

Playing devil's advocate here, but it is this attitude that makes products more expensive than they need to be. And then what happens is the bean counters force corners to be cut elsewhere to compensate for the extra complexity, and you get something less durable that won't last as long and will head into the waste stream anyway. So ultimately nothing was saved.

Frankly, I think that learning how to use batteries correctly is a standard life skill, like learning how to read and learning how to do arithmetic. You don't need to read the instructions to find out, because you were supposed to have learned it when you were growing up.

[Conrad Hoffman]

I think you're expectations of the typical person may be a bit high. As for the actual battery issue, in most cases I'd expect the batteries to equalize in a short time and carry on as needed. Life might be reduced, but I wouldn't expect explosions or anything, since the actual voltage difference is low if they're put in correctly. Most battery operated devices, save for drones and such, don't consume a lot of power, so a simple low value resistor between the paralleled batteries might be a good protective measure.

I have an old Keithley battery operated electrometer. I think the batteries were Eveready #266 type. I opened one up (simple cardboard box) and what was inside? Two (or three) standard 9V batteries plugged into a circuit card, in parallel, with two snap terminals on top. Easy way to get more capacity and seemed to work just fine. Also easy to put new batteries in, without paying the crazy price for a new #266, about $30 each, and you need three.

[IanB]

I think it has to be said though, that when you put cells in series as is most common, then mixing new and old is also likely to promote leakage of the old cells, and I can't think of a good technical way to defend against that.

[coppercone2]

what about lithium primary cells? NiMH? Mixes. You also have zinc carbon

Who knows what they will come up with in the future...