Hi all, this is my first ever post.
This is purely an academic question. I have no intention of doing it. However I have a "friend" who thinks it can work and I would like to know if they could be right.
I have a background in electronics so I do understand some of the answers that may pop up. eg No.
I want to get technical about it.
Lets say it is a 20Amp powerbank with the sole purpose when built to charge phones and possibly run low current devices.
Consider this unit as the example, 20000mAh Power Bank, ROMOSS Sense 6 Plus USB C Portable Charger with PD 18W QC 3.0
I'm sure there are many learned friends out there that will know the reason it can or can't be done.
I don't want the device to be modified but I guess it could be pulled apart to access the batteries.
Look forward to your feedback and help.
Cheers.
Hi there,
There are a couple different scenarios to look into here.
First, it will not start the car if there is no regular car battery in the car. The starter requires way too much current for those little power banks.
However it could start a car if the regular car battery is still in the car and the battery is not bad, just depleted somewhat. If the battery reads below 8 volts it is probably bad, but it might perk up with a charger. So we will have to assume the battery in the car is not bad.
In theory, if the car can start within 4 seconds and the starter draws 600 amps, then the maximum charge would be 2400 ampere seconds, which is only 2/3 ampere hours or in decimal about 0.67 ampere hours.
The power bank shown there is said to be 20000mAh which is 20 ampere hours, so at first it may sound like there is no doubt that it could start the car if the power bank is connected to the battery somehow. There is just a little more to it though, and that is the voltage. The voltage output of the power bank without modifications or an extra QC circuit 'fooler' will be 5 volts, and to charge the battery it will have to be stepped up to around 14 volts, and due to inefficiencies, we could say it is 15 volts for now. Since we have to step 5 volts up to 15 volts that means the current will step down from whatever the power bank can put out by a factor of 3, which means if it was rated for 4.5 amps out at 5 volts we could get roughly 1.5 amps at around 14 volts. It may be a little less, but from experience I know you can charge a car battery with 1 amp as long as the battery is not too old and is not sitting at 0 volts.
To get from 5v to 14 volts, we would use a boost converter. That would take an input of 5v and convert it to 14 volts. That would then start to charge the car battery. If the current is 1 amp and we know we need 0.67 ampere hours, that means it has to charge for at least two thirds of an hour, which is 40 minutes. If the current comes up higher then that's even better, but there are some power banks that only put out 1.5 amps at 5 volts. That would mean that we would only get about 1/2 ampere out of it, which is very small for a car battery although it may still charge if it is not too old. It will take at least twice as long though, like 80 minutes, in theory.
So that's about the size of it as theory would predict. The only other possibility then is to use a QC fooler circuit that allows you to set the output voltage higher, but you may not be able to get the right output voltage for charging without the power bank shutting down. That means it would depend highly on the power bank being used. Using a boost converter with adjustable output though means we can adjust the voltage (and thus the current) and that means the power bank should not shut down. It may be necessary to add a small series resistor though in series with the output of the boost converter (or power bank).
So theory says it can be done, all that would be needed to do is a few experiments. When I worked with a solar panel and car battery I found that it kept the battery charged very well even with a current of 1 amp for part of the day. In the case of charging with a power bank, I would opt for one that can allow us to put out at least that 1 amp at 14 volts using a boost circuit. To get that, it would have to put out 5 volts at 3 amps minimum, which means a charge time of about 40 minutes. If we do not let it go for the full time and try to start the car say after 20 minutes, the car may actually start, but if the car does not start then we have to start all over again and wait that 40 minutes anyway, and since the first try we stopped at 20 minutes, that would mean we would have spent an hour trying to start the car instead of 40 minutes.
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