I wanted to run 4 D alkaline batteries in series to get ~6V. Because alkaline batteries are not rechargeable, I was wondering if I put batteries in series with different voltages (some used and some new), won't the new batteries with a higher potential try to charge the used batteries and this will be a problem? I'm just curious if this is or isn't a problem but for testing I will be using batteries with the same voltages. Thanks in advance.
won't the new batteries with a higher potential try to charge the used batteries and this will be a problem?
No, it's not a problem. And because the batteries are in series, the higher-voltage battery won't "try" to charge the weaker ones.
Hi,
I put batteries in series with different voltages (some used and some new), won't the new batteries with a higher potential try to charge the used batteries and this will be a problem?
Yes, this will be a problem. The weaker cells will not be charged, they will be DIScharged.
It's not the voltage of individual cells that is the problem.
With the cells in series, the current will be the same in all of them. The older (more already discharged) cells will get to end-of-life at which time their voltage will go down rapidly and their internal resistance will increase rapidly. IF you allow that to continue, eventually the oldest cells will have their voltage go to zero and then actually reverse polarity. Many cells in that condition will quickly corrode and leak.
This is a well known situation and there are many products that specifically tell you not to mix types/ages of cells.
Adding to Terry's warning, this can be an issue with mixing fresh batteries from different brands as well.
It is very unlikely that a cheap, dollar store knock off will have the same capacity as a major brand like Duracell or Energizer. Even fresh, the difference in capacity can cause the knockoff the reach end-of-life before the quality battery and then all the bad things Terry listed will happen.
As another issue of practicality, batteries do not provide a constant voltage. They output a range of voltages as they discharge. An alkaline cell is only nominally 1.5V when fresh. Towards the end of it's capacity, it can be 1V or lower. Can your circuit work with 4V?
Cheap/unbranded non-rechargable batteries are never worth buying - non-rechargables don't have their
capacity marked so can be pathetically low(*) capacity in unbranded cheap batteries and you have
no redress in law. If possible stick to batteries and cells from reputable battery manufacturers who
publish a datasheet for their products.
(*) A few percent of what you might expect.