# 12V DC power supply - how to make sure it's properly constructed

I am building a power source for my astro-imaging gear using a 12V marine deep-cycle battery (120 amp-hours) stored in a battery box. The battery is supposed to power the telescope mount (requires 12V DC, 3.5A) and my laptop (requires 19V, 2.4A). Later, I might also add dew heaters for which I would use this controller.

I want to add three 12V cigarette lighter sockets and a fuse.

My main questions are:

1. Where to add the fuse in the circuit and why?
2. What rating should the fuse have?
3. Should I also add a fuse in front of each cigarette lighter socket?

I read that the loads in a circuit will only take the amps the need.

1. Can a marine deep-cycle battery also actually "push" current into my devices and blow them? Or are voltage fluctuations more of a problem?

For the laptop, I am planning on using a power inverter. I know it's not as efficient as a DC to DC power adapter but I think the battery has enough power to handle this inefficiency no problem and it's also more convenient in case I want to add a different laptop with different power supply specs.

• For the laptop, use an inverter (for AC) and use the usual power supply brick with it or else use the power supply brick that is sold for it to use in a cigarette lighter socket. Either way, you are using the right power source for your laptop and this method also works for other laptops, too. A problem is that you are talking about $16\:\textrm{A}$. Given inefficiencies, I wouldn't expect more than 4 hours. And you still need to start the car again. Go here for some info on wiring your vehicle: k0bg.com/wiring.html
– jonk
Jan 10, 2017 at 6:32
• Thanks @jonk ! The battery will be separate from the car in a battery box only for powering my equipment. Jan 10, 2017 at 6:56
• Good that you are using a different battery, then. Read that site a bit and make sure you feel comfortable about the changes. Ham radio operators have been doing this kind of stuff for many decades and have a lot of experienced advice to offer.
– jonk
Jan 10, 2017 at 7:09
• Note that deep cycle batteries still have quite limited lifetimes if always deep cycled. (Say 500-1000 cycles - may be on the low side). That may be acceptable top you BUT check with the manufacturer for specs at % SOC discharge. LiFePO4 MAY prove better overall. Jan 10, 2017 at 11:35

## 2 Answers

A deep cycle battery is a good choice for this it will not "push" extra current to your loads. That does not happen unless the voltage is too high.

Each cigar lighter plug should already have its own fuse this will be rated to protect the wire to the device and the device.

For fuses in your box you are protecting the wiring in your box. Wires have an "ampacity" this can be found on the web. This is the amount of current it is safe to push through a wire of that size. In fault situations a lead acid battery can push very large currents and this is what you need to protect against. A single fuse at the battery rated at the ampacity of all your wire will adequately protect your box. If you need more current than this fuse will supply instead use a similarly sized fuse in each of the feeders to the cigar lighters again at the battery.

Why do we fuse at the power source, the battery. Taking a pessimistic view of the world a fault can develop anywhere along the wire. If this point is before the fuse it can result in a fire. From a safety point of view therefore it is good to get into the habit of fusing at the source.

Please do research before posting, Can a fuse be placed after a load? for the first fuse question.

This for the second question: Fuse rating and safety

Fuse each load you want to protect from shorting out. Do not fuse circuits that dont have a risk of shorting. If you think one circuit might short and you want the others to stay on, then fuse each one.

Study up on how voltage works, current flows from high voltage to low voltage.

• Don't you mean "current flows from high voltage to low voltage"? Jan 10, 2017 at 8:56
• And current can also flow from low voltage to high voltage, for example in a battery. Jan 10, 2017 at 10:29
• If by current you mean ion current. Current in circuits is from electrons. Jan 10, 2017 at 16:46