Right way to parallel batteries

[pacmann]

Hello. I want to connect 2 non rechargeable alkaline batteries together in parallel for more capacity. Im worried if one battery has more voltage than the other then it will try to "charge" the lower voltage battery. Is there a circuit to prevent that but without adding a voltage drop like a diode?


Thx!

[IanB]

There is no need to worry about any special circuits for this.

You simply must make sure to parallel identical batteries fresh out of the package. Do not mix new and old.

[pacmann]

There is no need to worry about any special circuits for this.

You simply must make sure to parallel identical batteries fresh out of the package. Do not mix new and old.

Thx. What happens if the user doesnt follow that rule and put one old and one new battery? Will current go into the old battery and cause wear on newer battery?

[coppercone2]

usually the commercial products that use parallel primary batteries have diodes or other circuits between the batteries

https://electronics.stackexchange.com/questions/535026/reverse-charge-protection-for-parallel-primary-lithium-cells-cr2450

scroll down

and yes, I have seen something *unknown* used with AA cells like that, but figuring out the ? is not my problem

[tooki]

There is no need to worry about any special circuits for this.

You simply must make sure to parallel identical batteries fresh out of the package. Do not mix new and old.

Thx. What happens if the user doesnt follow that rule and put one old and one new battery? Will current go into the old battery and cause wear on newer battery?

In parallel, it’ll basically charge the weaker battery.

[Bud]

And if the old battery has increased self-discharge, it will pull the new battery with it.

[Stray Electron]

usually the commercial products that use parallel primary batteries have diodes or other circuits between the batteries

https://electronics.stackexchange.com/questions/535026/reverse-charge-protection-for-parallel-primary-lithium-cells-cr2450

scroll down

and yes, I have seen something *unknown* used with AA cells like that, but figuring out the ? is not my problem

   Yes, but by using diodes the OP will lose ~ .6 volts. He didn't say what kind of batteries he's using but if he's using 1.5 volt or 2.0 volt cells then .6 is a very significant loss.  Correction: he did say alkaline batteries so he's only dealing with 1.5 volts to start with. Adding .6 volt drop is going to put him into the "almost dead battery" territory.

   Even with a cr2450 lithium cell (3.0 volts) the .6 volt loss is at least 20% of it's total capacity and probably almost 50% of it's usable capacity.

  The BEST solution for the OP would be to replace the original battery cell(s) with a single, higher capacity, battery cell(s). But paralleling two new, matched capacity & age, small cells would probably work fine.

[coppercone2]

you don't use a diode for low voltage batteries, scroll down on the link to see an example (not sure if its a viable circuit). I detected the existence of such circuits but never determined what they are/look like.

but I never saw it for a 2x 1.5V circuit, only for like multiple strings of series alkaline batteries that have a higher voltage (2x4.5). That circuit in the picture needs some higher rails I guess, since there is a comparator.

2x1.5V in parallel is kind of weird/unique, because normally... you just go up in cell size you know? 1xAAA -> 1x AA -> C -> D -> F. Then when you get parallel D or F cells for low voltage it gets funky I guess, but thats kinda strange !  A C cell can have 8500(like 7500 probobly)mAh, wheras 2AA = 5000?

IMO its kinda dodgy to see 2x aa in parallel unless there is a very good reason. And you must really not like power converters not to go 3V.....

[IanB]

IMO its kinda dodgy to see 2x aa in parallel unless there is a very good reason. And you must really not like power converters not to go 3V.....

I have a computer mouse that puts two AA cells in parallel. But it's the only example I have seen.

[bdunham7]

I have a computer mouse that puts two AA cells in parallel. But it's the only example I have seen.

Are you sure?  I have seen a very few odd devices where the cells are installed in the same direction and might appear to be parallel, but the contacts are actually wired so they are in series.  I've never seen a commercial product with AAA-D or 9V batteries actually wired in parallel.

[IanB]

Are you sure?  I have seen a very few odd devices where the cells are installed in the same direction and might appear to be parallel, but the contacts are actually wired so they are in series.  I've never seen a commercial product with AAA-D or 9V batteries actually wired in parallel.

Well, it works if I insert just one cell.

Most of the newer mice only take one AA cell, so maybe with this other mouse they were just aiming for a longer battery life on the same kind of 1.5 V circuitry?

[coppercone2]

measure the contacts with a ohm meter and see if you get short circuit between each pole.

[IanB]

measure the contacts with a ohm meter and see if you get short circuit between each pole.

Not only is there continuity + to + and − to −, but if I remove the cells and measure the voltages, they are balanced at 1.2888 V and 1.2889 V.

[Siwastaja]

Designers avoid this scenario because it's difficult.

You can't just let the user parallel cells; they would insert a fresh and dead cell in parallel, significant current flows, charging the dead cell, which is not desirable with non-rechargeable cells.

You can't use diodes because even schottky diodes have significant voltage drop.

So you need to come up with some kind of "ideal diode" orring circuit. Not impossible, but not worth it, when you can just specify a larger cell size, or use more cells in series and design the thing to work with higher input voltage (and lower current).

[coppercone2]

I think you might have a problem if you take a old battery out of a wall clock and mix it in there

[IanB]

I think you might have a problem if you take a old battery out of a wall clock and mix it in there

I don't intend to do that. Ever since I was old enough to read, I have read the instructions on product packaging to always replace batteries as a set, and always to use fresh batteries when installing them.

If someone does not follow these instructions and has problems as a result, the problem is with them, not with the product designer.

[IanB]

You can't just let the user parallel cells; they would insert a fresh and dead cell in parallel, significant current flows, charging the dead cell, which is not desirable with non-rechargeable cells.

True, but the consequence is that the charged alkaline cell might leak, damaging the product. So now the user has to buy a replacement. They have paid a price for doing something wrong, and the manufacturer has gained an extra sale. Cynical? Maybe. 

[bdunham7]

Well, it works if I insert just one cell.

OK, so that's a first.  It really isn't that terrible of an idea, certainly better than trying to cram a D cell into a mouse.  All this worrying about what happens when some idiot replaces only one battery seems to me to be much ado about nothing.  But I just haven't ever seen it in practice.

[IanB]

OK, so that's a first.  It really isn't that terrible of an idea, certainly better than trying to cram a D cell into a mouse.  All this worrying about what happens when some idiot replaces only one battery seems to me to be much ado about nothing.  But I just haven't ever seen it in practice.

It's a Microsoft Wireless Mouse 5000 if you are interested.

[drvtech]

Well, this topic got me thinking - I replaced the batteries in my Kensington wireless trackball the other day and noticed that the cells were the same way round but I just assumed they were wired in series. I've just had a look and sure enough, they are in parallel. Yuk! It scares me a bit to think what would happen if someone accidentally put a couple of fully charged NiMh cells in there in opposite orientations...

[IanB]

In some devices I have to get a magnifier out to see the cell orientation markings. But at least with this mouse the cell direction is marked in big, bold graphics so it's hard to miss.

[Nusa]

IMO its kinda dodgy to see 2x aa in parallel unless there is a very good reason. And you must really not like power converters not to go 3V.....

I have a computer mouse that puts two AA cells in parallel. But it's the only example I have seen.

Among my collection is a Logitech M705 mouse with two AA batteries in parallel. As I recall, I had to replace them every 3 months when I was actively using that one. It will work with just one (presumably for a shorter time), but then it's got a different weight feel. The obvious reason they didn't use a C cell was for space reasons...the mouse would have to be taller.

[Whales]

It's worth noting that alkaline cells are technically rechargeable.  I'm not sure of the parameters however, a fully fresh cell in parallel with a mostly flat cell is probably too much.  I know they can leak and vent gas, I don't think they explode?  Worth investigating

If you have a dual cell holder wired in parallel: eventually someone will install one of the batteries backwards.  This will give you about 3.2V across less than a few ohms.  The batteries will get very hot.

I presume your circuit uses the 1.5V directly and does not have any boost converters in it?  If you did have a higher voltage handy then you could use it to turn on a couple of N channel mosfets.  One mosfet between the negative side of each cell and ground.  When the mosfet is 'off' it will be a diode (crappy but still possibly usable to get your circuit started), then once the mosfet is turned 'on' this voltage drop will disappear.  EDIT: no this is useless!  Once the fets are on your cells are in parallel again, unless you have a micro to do sensing before making the decision. Doh.

[tooki]

. The obvious reason they didn't use a C cell was for space reasons...the mouse would have to be taller.

That’s a reason why they couldn’t use a C cell, but probably not the reason they chose AA: AA’s are by far the most popular battery size, with the end result being that they have the best cost-per-mAh ratio. Almost ALL the devices that used C and D cells in the past are now powered by LiIon cells: flashlights, portable music systems (boomboxes in the past, Bluetooth speakers today), etc. Consequently, C and D cells are made in much smaller volumes, and consequently are rather expensive.

[Conrad Hoffman]

I think you might have a problem if you take a old battery out of a wall clock and mix it in there

I don't intend to do that. Ever since I was old enough to read, I have read the instructions on product packaging to always replace batteries as a set, and always to use fresh batteries when installing them.

If someone does not follow these instructions and has problems as a result, the problem is with them, not with the product designer.

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.