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On their documentation page, Adafruit offer a tutorial for using it with 2s li-ions. However that also required a 3pdt switch, which would have to be manually thrown. Since in my project I will not have the indicator lights visible, I can easily see my self or others forgetting to flip it and either wonder why it doesn't work, or wonder why it's not charging.

The only thing I can think of using is a 5V 3pdt relay, that would have the coil terminals connected to the DC-IN terminals of the charging board (the input power would come from the micro USB port) so that when the charging cable is plugged in, it would automatically switch to charging mode. Unfortunately a 3pdt relay is rather hard to find, especially in small sizes.

Is there another option I'm not thinking of? Or can someone point me in the direction of a small 5V 3pdt relay rated for at least 1 amp?

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  • \$\begingroup\$ Eh. Get a 2S charger and use balancing circuitry. \$\endgroup\$ – Ignacio Vazquez-Abrams Jul 3 '18 at 21:28
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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. 3-pole relay solutions.

You may find it easier to use readily available components. I haven't checked, but I suspect that the more poles are added to the relay then the stronger the actuator has to be and the more current required. This would have the effect of (kind-of) making the current required by the relay coil proportional to the number of contacts so there wouldn't be a hugh penalty for additional relays.

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  • \$\begingroup\$ well... yes but it would have a considerable size impact. Seeing as how the coil current would come from the charger, efficiency would not be a primary concern (within reason <100mah) Even fitting in one small SPST relay could be a problem. I have pretty good bit of space for flat thin stuff. Could I perhaps use SMD transistors to make a very thin "relay" board? I would have to use perfboard as I do not have proper pcb makeing equipment. I would have room for a 0.8 in X 2in X 0.25 in pcb. \$\endgroup\$ – 5hutd0wn R35tart Jul 4 '18 at 2:33

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