Beginner battery question (tying battery rails together)  - Page 1
Forum section: electronics / beginners

author: mortrek
author_rank: Contributor
timestamp: December 25, 2020, 05:31:30 pm
content: When using two different batteries in a single project, if you need to tie the grounds together, I'm curious how the return electrons get distributed. Due to their chemistry, are only the amount of electrons that have been pulled from one battery able to return to that battery?For example, if both batteries are used sometimes, but one of them has its positive side then disconnected, will its negative side of the disconnected battery sink ANY current from the circuit? Am I misunderstanding everything completely? I also get confused with how to think about the battery function in the context of electron flow because of the whole "electrons are positive"/"moving holes" thing.I have a low-power circuit using two different battery packs and they have to have their grounds tied together sometimes for an ADC. I want to minimize wasted current, so hopefully one ground does not sink ANY current from the other source, or it's in the low micro-amps.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: December 25, 2020, 05:36:38 pm
content: Put your multimeter in uA mode and test.

author: mortrek
author_rank: Contributor
timestamp: December 25, 2020, 06:04:29 pm
content: It's on a board, so I cannot at this time. I'd have to cut the traces or desolder parts. Or I'd have to build up a similar circuit, which I don't have time for right now, since I'm watching my kid most of the time. Otherwise, I would have. I was just hoping someone could let me know how it works.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: December 25, 2020, 06:34:45 pm
content: How is the battery attached? If you can take out, then you can put your meter in the circuit.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 25, 2020, 06:44:10 pm
content: If the positive is disconnected, no current can flow. No electrons are pulled or pushed through the single connected terminal.Beware of connecting batteries in parallel. They need to be at nearly same voltage before connecting, otherwise dangerously large current will flow.Always use fuses with batteries, as close to the battery as possible to reduce the unprotected area. Fuse can be in negative or positive terminal, whichever is easier.

author: mortrek
author_rank: Contributor
timestamp: December 25, 2020, 07:00:24 pm
content: Thank you. The positive sides are never linked, but sometimes the grounds are due to the ADC reading both rails/battery packs. I do not want any quiescent current due to the linked grounds at the voltage divider/ADC circuit. I realize that it likely does not allow any significant current to flow, but even insignificant current (sub-mA) is a problem for me if its happening 24/7.To be clear, the grounds are only linked some of the time. Sometimes the one of the grounds is disconnected while the other one is not, but the positive side is always connected. Thus, I do not want one battery pack to source current into the other battery pack's ground. I have a feeling that it does not, but without an easy way to get empirical data, I was hoping that someone could tell me how it works.So to simplify my question with a theoretical model:Say we have two real battery packs. One 5V, and one 9V. The positive sides are separate, but the grounds are connected together. It seems that electrons are sinked into the grounds proportionally, right?. Then, you disconnect the ground from the 9V battery, but leave everything else the same. The two positives are connected, and the 5V ground is connected. Does the 9V battery sink ANY current into the ground of the 5V battery?I would understand if, say, a tiny bit of current for a very short time sinks into it before saturating that side of the battery cells' electron capacity, but nothing will flow long-term, correct?I mean, it's almost more of a physics/chemistry question in the end than a pure electronics question, I guess.Even simpler question: Can a battery ground ever sink any current from another battery?My guess is no, but I want to be sure. If I get time, I'll try it myself, but for now I'd really like to know the answer as understood by science. I also understand that most people are busy on and around Christmas, so I don't expect an immediate or elaborate answer.

author: msat
author_rank: Regular Contributor
timestamp: December 25, 2020, 07:14:57 pm
content: Current is technically sourced from ground, but it makes little difference in this discussion. Any current that flows out from the battery must also flow in, otherwise a change in voltage potential between the terminals would build up, and that doesn't happen.So you can go ahead and disconnect one of your battery leads so that you don't deplete it. There may be negligible leakage currents, though. Probably not worth worrying about.