Bi Stable Relay

Support for the UPS PIco HV3.0A Plus and UPS PIco HV3.0B Plus
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Graham S
Posts: 47
Joined: Fri Feb 07, 2014 12:07 am

Bi Stable Relay

Post by Graham S » Sat Oct 21, 2017 5:07 pm

Hi, I'm trying to figure out how to run the HV3.0A Plus from an external 12V lead acid battery, but add deep discharge/low voltage protection for this external battery. Note: the external 12V battery is changed from another source.

I can measure the external battery voltage using the aEXT1 A/D and a 24K scaling resistor (the 400uA current draw is acceptable given the large size of the external battery) and hence I can know if the external battery voltage is getting low. However, if I use file safe shutdown to shutdown the PI, the UPS Pico keeps powering the PI because there is still voltage from the external battery. This will further discharge the already low external 12V battery.

One idea is to use the bi stable relay to isolate the 12V battery from the external power input, but I'm not sure if both bi stable contacts wired in parallel are enough to handle the current draw of the UPS Pico and PI? You say that the charger never uses more that 800ma (4.2V?) and the PI's max current is 2600ma (5V). If so this would be about 1.4A at 12V which is within the capability of the relay. Is this correct?

If so, I would use the BI stable relay contacts (both of them) to isolate the UPS Pico's external power input from the 12V battery source and run on the internal battery. I'd then either shutdown once the internal battery is exhausted or resume using the external battery, by closing the bi stable relay contacts again, if the external battery change/voltage recovered.

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