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We have a solid state relay controlled by a Raspberry Pi, The relay is interposed o the negative wire between a 11-12V battery and a motor.

If I measure the voltage before entering the relay, it is correct. If I measure it on the output side I get 0.44V and the motor won't start. However, while the Relay is ON if I interrupt the negative wire and I connect it back, the motor works(While the relay is still on).

I'm not sure if it counts, but I use a LiPo battery.

What is it happening? How can I solve this?

Edit: This is the SSR that I use: https://www.aliexpress.com/item/2-Kanal-OMRON-SSR-Relais-G3MB-202P-5V-DC-2-Channel-Solid-State-Relay-Board-module/32830801262.html?spm=a2g0s.9042311.0.0.27424c4d4M3JQz

And its datasheet: https://www.openhacks.com/uploadsproductos/g3mb-ssr-datasheet.pdf

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You've used an AC SSR on a DC motor.

enter image description here

Figure 1. The SSR photo in the ad. (1) The input is OK. (2) The output type is not.

It seems that the version you have used has zero-cross switching. That means that the output switches on at the next output power zero cross after the input turns on. Your tests confirm this.

I've written about these devices in Opto-triacs, solid-state relays (SSR), zero-cross and how they work.

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The problem is that this SSR is rated for switching AC voltage and also switch on zero-crossing.

You are trying to switch DC voltage. Since the DC never crosses zero volts unless you interrupt the power, it never turns on. Also, it will depend on the load supply voltage to go to zero to turn off, your motor will not turn off unless you again interrupt the supply voltage.

You need a DC-switching SSR.

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