ups

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yes the internal battery is to small but I wanted to run in in my truck The ups is 12v on the input but will not start on 12v I was hoping to use it to power the laptop and charger for the controls

About a year ago, I did a bit of reverse engineering on a small UPS and here is what I think I remember about how it worked:

The input line is connected to the output by a relay that is powered by the incoming AC. The inverter transformer secondary was connected to the output continuously. As long as the relay is on, it has a contact that keeps the inverter controls shut down. Since the relay connects the input AC to the output and the transformer high side is continuously connected to the output, the input AC feeds backwards through the transformer, charging the battery through the fet switch body diodes. The inverter controls are not powered directly from the battery, but from DC rectified from the transformer low side by another set of diodes, whether because the high side is connected to AC or because the inverter is running.

Once the input AC disappears, the relay drops out, first disconnecting the input line from the output and then telling the controls that the inverter is needed (break before make). The inverter controls have to get the inverter going before a small storage cap across the control supply bus fades out.

When input AC is restored, the relay first tells the inverter logic to turn the inverter off, and then connects the input AC to the output.

If your unit is something like the one I opened, you might be able to jump the battery to inverter logic (look for those two small diodes from the low voltage side of the transformer that connect to logic chip power pins or a low voltage regulator) through the on off switch, so that it will run the inverter from a cold start.

Remember that you are dealing with very lethal voltages once the inverter case is opened.

why not get an inverter? They aren't that expensive now.