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I'm trying to power some scientific equipment that requires 24VDC/~4A on both mains and battery with solar eventually coming to the mix. In addition, I'm using a Raspberry Pi as the datalogger, which requires 5V/~1A. I'm trying to make sure that I don't start a fire when I do this. As will be evident below, I'm a scientist and not much of an electrical engineer.

This is my current plan: Mains (120V) -> some sort of 120V->24V converter, like this one ->two 12V batteries connected in series (I suppose I just connect the 24V to the + of one and the - of the other)-> split to a 24V->5V converter like this one and to the equipment.

The batteries will be expected to power the unit for ~4hr, so I am aiming for 18Ah batteries. The mains->24V will be expected to be inside, but we will disconnect the batteries et al and expose them to the elements from time to time. The current plan is to house them in a pelican case. That only needs to contain the Pi and everything from the battery onward.

No strong weight restrictions other than "not absurd". Aiming to have low electrical noise on at least part of the circuit- the instrument separates into a heater and instrument part, and the relatively low power instrument part (~500mA) should be pretty clean, but the heater can be pretty noisy. Appearance doesn't matter that much, but something that is clean/"looks safe" would be preferred as it's scientists working with this.

Three main questions here: will I start a fire/electrocute myself? Do I need a charge regulator for this setup or will that only need to come when I move to solar panels? Have I appropriately designed this setup to work?

Finally, one dumb question (although these may all be dumb): what kind of wires/terminals are safe to use to hook this all up? Should I get some kind of fused mains plug to go to the 24V power supply?

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    \$\begingroup\$ insufficient data. 25W=5W for ? hours without sun at 90% efficiency? DC current or pulsed? Load regulation voltage error or ripple? etc etc Better to shop for a solution with Mfg recommended power source \$\endgroup\$ – Sunnyskyguy EE75 Mar 1 '18 at 21:22
  • \$\begingroup\$ Yeah- the reason that I'm in this mess is that the manufacturer doesn't specifically recommend anything. I'm not concerned about the solar at this point, just want to leave my options open. Want to run for about 4h, so was aiming for two 12V 18A batteries. \$\endgroup\$ – Laxsnor Mar 1 '18 at 21:26
  • \$\begingroup\$ weight?? supply tolerance? noise tolerance? you need 30W*4=120Wh and car battery ~ 500Wh with a 24V boost convert or Lipo String? \$\endgroup\$ – Sunnyskyguy EE75 Mar 1 '18 at 21:30
  • \$\begingroup\$ We don't have severe weight restrictions- something like let's stay under 50 lbs. Supply tolerance is pretty robust, something like 10-28V. Noise tolerance is lower, although I don't have a number on it as the manufacturer doesn't give a number. We're powering a class 1 laser if that helps. \$\endgroup\$ – Laxsnor Mar 1 '18 at 21:33
  • \$\begingroup\$ I should clarify- 4A isn't just for the laser. Most of the power is needed for the heater, which doesn't care how noisy it is. They are powered on separate rails, but need the same voltage. \$\endgroup\$ – Laxsnor Mar 1 '18 at 21:34

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