# Switching between two completely independent circuits

Background: I have built a 2S 7.4V battery out of two 18650 batteries. I have some TP4056 (5V 1A Micro-USB) charging modules lying around. I would like to use these the TP4056 modules (one per 18650) to charge the batteries but from the information I've found, having any connection between the two batteries is not a good idea while charging.

Therefore, I'm looking for a switch that would only switch on one circuit at a time. Either the serial connection b/w the batteries, or the charging input to the TP4056s.

Something like this:

What exactly is such a switch called (if it exists)? Is there a more elegant solution to this?

Excuse my noobness.

Update:

With a DPDT switch (thanks @Solar Mike!), does the following circuit make sense?

• Double pole double throw switch (dpdt) seems to be what you describe. – Solar Mike May 13 at 10:25
• Thanks! Seems like that's what i need! I've updated my question with a circuit that I think should work. Do you think so too? :) – PhoenixDev May 13 at 12:36
• You can connect eg top of one cell and bottom of other cell permanently to suppy + and - respectively and inly switch the opposite ends. – Russell McMahon May 14 at 7:27

simulate this circuit – Schematic created using CircuitLab

Figure 1. OP's proposal redrawn.

With SW1 in position shown BAT2+ will be shorted to U1's BAT- which according to the TP4056 datasheet is supposed to be connected to ground so the arrangement is likely to have a very unhappy ending.

simulate this circuit

Figure 2. A circuit with less potential problems. SW1 is a 3-pole switch.

• SW1b and 1c allows switching of BAT1 in series with BAT2 or in parallel with U1.
• SW1a prevents BAT2 receiving charge while BAT1 is disconnected.
• Thanks! Could you please guide me to a small form factor 3-pole switch that can handle upto 15A? – PhoenixDev May 14 at 15:41
• No, sorry. I would have to look it up for you. – Transistor May 14 at 16:59
• That would be great, thanks a lot! :) – PhoenixDev May 15 at 7:42
• I'm sure it would, but I'm not going to do it for you. That's your job! – Transistor May 15 at 8:18
• Hah! Sorry I misunderstood you. I've tried but haven't really found something small enough. This: for example would be an option. But I can't seem to find the exact dimensions for it. I would greatly appreciate some help if possible :) – PhoenixDev May 15 at 10:28