The following circuit contains an Uninteruptable Backup System (UPS). So if the main battery fails, the UPS auto-switches on to power the essential instruments. (The UPS senses the voltage drop.)

The dual-pole switch connects the UPS to the main battery, and switches it on at the same time. When switched off, this dual-pole switch by-passes the UPS, via the bypass line, and powers the essential instruments from the main battery. This is in case of UPS failure, so the essential instruments can still get power.

The diode protects the UPS from reverse current, when the UPS is switched off.

This is all very logical (my circuit design).

However, what should I do with the negative cable on the bypass line? Will that need a diode in it too, or can I connect it direct to the main battery?

I would be grateful for any advice.

UPS circuit diagram

  • \$\begingroup\$ Does all the equipment run of 12V? \$\endgroup\$
    – HandyHowie
    Aug 29, 2019 at 14:26
  • 1
    \$\begingroup\$ It's kind of weird system. Much simpler is a battery and diodes, what actually is the UPS for 12V? No datasheet or explanations. Where is the charging supply? \$\endgroup\$ Aug 29, 2019 at 14:31
  • \$\begingroup\$ Mostly folks design redundant DC supplies as 'no-break'. With the switch in here you certainly don't have that. Try to explain what you have (instruments) and what you are trying to achieve as you diagram is quite confused. \$\endgroup\$ Aug 29, 2019 at 16:56
  • \$\begingroup\$ a. Yes, the system is 12v, except for the output from the UPS, which is 10.5v. \$\endgroup\$
    – rojellis
    Aug 30, 2019 at 13:35
  • \$\begingroup\$ b. A UPS, I thought they were common. It is a backup battery that switches in in microseconds, so none of the instruments have to reset themselves. \$\endgroup\$
    – rojellis
    Aug 30, 2019 at 13:37

1 Answer 1


If it's all DC as you have drawn it, then you can simply tie the negative cable from the ups battery to the main battery.

If you are using AC, then please ignore, and do not connect them.

  • \$\begingroup\$ Thanks. The UPS connections are a part of the UPS system. I was actually wondering more about the negative lead from the main battery, via the bypass line, to the essential instruments. \$\endgroup\$
    – rojellis
    Sep 5, 2019 at 9:51
  • \$\begingroup\$ All the negative lines can be connected together without diodes. \$\endgroup\$
    – Aaron
    Sep 5, 2019 at 14:25
  • \$\begingroup\$ Thanks. I guessed that, as you need a complete circuit for anything to flow. But was not certain. Thanks again. R \$\endgroup\$
    – rojellis
    Sep 6, 2019 at 15:51
  • \$\begingroup\$ You are welcome, don't forget to vote and mark answers that help you. \$\endgroup\$
    – Aaron
    Sep 6, 2019 at 15:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.