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New to designing circuits and PCBs. I'm working on a project and I can't seem to figure out how exactly how to do what I'm trying to do. I'm hoping one of you can give me a hand understanding the concept behind a few things I just can't seem to wrap my head around.

Basically, I'm designing a PCB for something that is battery powered but it's in a really tight space so it can't be removed for charging, it has to be charged in its spot. I have some 3.7v 18650 batteries that I am charging off of a TP4056. The batteries are constantly connected to their load and so I need to make a circuit that disconnects their load when the 5v is applied for charging. I'm told a MOSFET will do what I'm thinking but I can't seem to figure out exactly how.

The batteries are in groups of 3, 2 groups per each PCB. They're wired in series to get to 11.1v and in parallel with all the other groups to get me to the current I need. The plan is to charge each battery individually by being wired to the charger in parallel on a different PCB layer. I also threw in an LED as a charging indicator.

Ultimately this is a much larger design and this circuit has to be repeated 4 more times. (I need to fit this design in a pipe that is only 1.25" in diameter so it's about 7.5" total length after connecting (5) 18" PCBs.

Any advice is appreciated! Here's a picture of what I'm thinking will work:

enter image description here

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  • \$\begingroup\$ Use a micro-switch to capture the presence of the connector... \$\endgroup\$ – Solar Mike Feb 25 '18 at 5:37
  • \$\begingroup\$ I don't think I have space for one near the connector, it's going to be a very tight fit. \$\endgroup\$ – Cadmium685 Feb 25 '18 at 5:45
  • \$\begingroup\$ The circuit you show won't work, as it shorts all (most of?) the cells. You would need a large number of switches to change the cell connections from serial to parallel. \$\endgroup\$ – Peter Bennett Feb 25 '18 at 5:48
  • \$\begingroup\$ Normally, the longer lline in the cell symbol is the positive terminal - you've used it as negative, which will confuse the more experienced viewers. \$\endgroup\$ – Peter Bennett Feb 25 '18 at 5:50
  • \$\begingroup\$ Thanks for the heads up about the negative, i'll make that change. \$\endgroup\$ – Cadmium685 Feb 25 '18 at 5:53
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Wiring batteries in parallel for charging and series for discharging sounds OK if you say it quickly. However it's best to avoid it, and do things 'normally', which is to keep them wired in series and charge and discharge in series, with appropriate balancing, and a boost converter between charge source and batteries to make the required charge voltage.

Your drawing shows the batteries wired in series and parallel, which shorts most of them. The multi-way relay needed to switch them between series and parallel would take a lot of space, consume power, stop the ability to use while charging, limit the discharge current, and is an extra point of potential failure.

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  • \$\begingroup\$ What if I had a TP4056 per battery? The batteries would still be in (2) groups of (3) in series and those two groups would be in parallel but that should avoid shorting, right? \$\endgroup\$ – Cadmium685 Feb 25 '18 at 6:26
  • \$\begingroup\$ Something like this: imgur.com/a/3h8K9 \$\endgroup\$ – Cadmium685 Feb 25 '18 at 6:47
  • \$\begingroup\$ The problem with these 'easy' series parallel schemes is you tie yourself in knots. BT14 -ve is tied to BT13 and 15 +ve. BT14 +ve is tied to both BT13 and 15 -ve. See if you can figure out which way the current is going to flow. See if you can figure out why, in this commercial universe of smart people, you don't see parallel charge series discharge battery packs anywhere. \$\endgroup\$ – Neil_UK Feb 25 '18 at 6:52
  • \$\begingroup\$ Gotcha. Yea that won't work. What if every side of each battery had a mosfet acting as a switch that disconnects or connects it to the batteries and/or charger? Is that possible? \$\endgroup\$ – Cadmium685 Feb 25 '18 at 7:01
  • \$\begingroup\$ Keep digging! It's sort of possible, and honestly not worth it once you've done it. \$\endgroup\$ – Neil_UK Feb 25 '18 at 7:28

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