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Car power from car batteries while not in a car? Answered

I bought a car battery just so I could take it with me camping and charge my cell phone. Here is the problem: car powered devices are designed to be charged in a car while the car is running. The voltage of car power is 14.4 volts and the voltage of a fully charged car battery is 12.6 volts. The car battery will charge my cell phone because it is low amperage and does not drop the voltage very much (only if the car battery is mostly full), but if I try to connect my inverter and power higher-amp devices the inverter will beep and not work because it senses low voltage.

One way I can get my battery to work is if I had a DC step up transformer. A car battery ranges between 0 and 12.6 volts. I need something which can supply a constant 14.4 volts as the battery is drained. Another option is if I got two batteries and hooked them together in series for 24 volts. If I did that I would need a transformer which could convert 24 to 14.4 volts as the batteries drained.

So what is the best way to do this? This is for my camera drone: a DJI Mavic Air. If I could charge my drone batteries with a car battery I could fly a lot longer.





4 months ago

Jacob, You should be able to use an inverter to convert the nominal car battery voltage to 110v AC required by the MavicAir AC Power Adapter. Looking at the specs (https://www.dji.com/mavic-air/info) I believe the AC Power Adapter requires 60 watts continuous power so size your inverter to provide at least double this current (and size your cables from the battery to the inverter to take the current!). If we expect 80% efficiency that's over 6.25 amps from the battery nominal, not including inrush spikes. If your inverter is complaining then you may have too small/too long wires between battery and inverter and hence a voltage drop.
I also don't know if the AC Power Adapter is fine with a modified square wave or if it requires a pure sine wave inverter... that could also be your problem.
You could go with a 24 volt system as you suggest using two car batteries and a 24 volt inverter. Increasing voltage is much easier on the electronics since the resistive losses are proportional to the square of the current.
If I were you I'd stay away from any electronic project to make 14.4 volt boosters, etc. and stay with consumer products. Boosting the battery voltage artificially would probably only end up draining the car battery below its 50% line and significantly reducing its life.
Best Wishes


5 months ago

I need a battery supply that can output 120 volts because my drone battery charger runs off of house hold current. I thought I could plug the drone charger into an inverter which is plugged into the car battery, but I am having problems with the voltage dropping too far.


Reply 5 months ago

If you go that route then you might be wasting more energy than what is used to charge your battery.
For every half decent drone or battery pack you can get car chargers that run off 12V DC.


Reply 4 months ago

I just tested out a car drone charger. It is as I thought, it works in a car when the car is on but it will not work off of a car battery by itself, so I need a way to boost the battery voltage. A car battery ranges from between 0 and 12.6 volts. I need to a converter which will take car battery voltage and put out a voltage of between 13-16 volts. The car drone charger can take between 13-16 volts.

Jack A LopezNoblenutria

Reply 5 months ago

I mentioned external laptop battery, or "power bank", which is another common name for the same thing. These have stiff output voltage, and what I mean by "stiff", is "hard to pull down", by drawing current.

If you believe in "internal resistance", then "stiffness" is just the reciprocal of that resistance, or the slope of the battery's current vs voltage (I vs V) curve.

Also you say your drone's battery charger wants mains power, and that is why you are using an inverter.

However, if you look at the battery charger more closely, you might discover it is using some low voltage DC, internally.

Although, I do not blame you for not wanting to know, or mess with, what is happening in the internal parts of your gizmos.

However, it might be fun to know how much power, in watts, your charger gizmo uses, when it is charging a drone battery.

You know, then you could do some math, and compare the watt*seconds, or watt*hours, of energy per charge, and compare that to the size of the battery powering your charger, like to see if the battery is sized appropriately. Or to figure out what size battery, or power bank, would be appropriate, if you were going go shopping for a battery, or power bank.

Of course doing that would require you to actually measure how much power your charger gizmo, or charger gizmo plus inverter, is using.

Jack A Lopez

5 months ago

How about using an external laptop battery that comes with selectable output voltage?

Such things exist.


Basically it has a DC-to-DC converter in it, and a switch of some kind, for to allow the user to choose from a handful of different output voltages, e.g. {5V, 12V, 16V, 19V DC}

Also I want to warn you about using the word, "transformer, " also, "step up transformer," or "step down transformer." Strictly speaking, transformers are AC devices, and they are passive.

I realize the analogy could make sense, sort of like saying a DC-to-DC converter is like a transformer, but for DC, instead of AC. Why can't we just call it a "DC transformer"?

The answer is because that's not what it's called. It is called a DC-to-DC converter. Also a DC-to-DC converter with output voltage higher than its input voltage is often called, "boost converter." DC-to-DC converter with output voltage lower than its input voltage is often called, "buck converter." I guess those words, "boost" and "buck", are analogous to "step up" and "step down".

That concludes the vocabulary lesson.

Also if you want to buy some DC-to-DC converters, these are inexpensive when bought in module form, from the straight outta China, eBay sellers. Although, for the most part that is a kind of bare bones kind of thing, a circuit board with some tiny little potentiometers on it for adjusting the output voltage (or current, if it is capable of constant current) .

It seems like I have seen 'ibles here of different peoples putting those modules into a box, and replacing the tiny screwdriver pots, with pots with knobs big enough to be turned by hand, with the goal being a nice looking DC power supply, with output more easy to adjust.

Also there are inexpensive modules which measure (and display) voltage and current, and those can be seen in many of the same 'ibles here. I guess the category for this, if you need words to search for it, is "adjustable DC power supply," or "power bank," or "power converter," if it is the kind powered by batteries.

I am honestly thinking these things are here, in existing instructables. If you want me to point to some specific ones, let me know.