Kip
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I'm not sure how many folks will actually benefit from this but I've wrote it all out now so here we go!
I've been trying to understand the difference between the way capacitors and batteries store electricity, and how to compare them to each other. I've learned quite a lot myself while writing all this so hopefully it is correct. The more I learned, the bigger this document got. I do have an actual conversion in mind myself, which is what started me off down this path.
Someone who is considering replacing a dead battery with a supercapacitor will want to know how many Farads their new cap needs to replace xxxmAh battery. It's not quite as straightforward as that. A better way of asking things is: what specification capacitor would allow my dashcam to work normally as it would when using a battery?
This thread will attempt to explain just that. In short and in theory, YES, you can replace a battery with a capacitor in a battery based dashcam. However, that is disregarding other things such as capacitor dimensions, available space inside the housing, how far you are prepared to go to mod the case if needed and how often the camera will be used. I'm not an electrical engineer of any sort so only attempt a conversion at your own risk. I'm just an enthusiast trying to solve a problem in front of me and sharing my workings.
Firstly, we need to understand the difference between how batteries and capacitors charge and discharge their energy.
A 12v car battery is considered fully charged at around 12.6 volts and considered flat at around 11.6 volts. In comparison, a 12v capacitor is fully charged at 12 volts and flat at 0 volts. A car battery at 50% charge (let's say 12.0v) might be able to start your car, but, a 12v capacitor at 50% charge would only be at 6.0v. No matter how much current might be available, nothing at 6v is ever going to run a 12v car starter motor well enough to start an engine (although, do check out YouTube for folks starting cars with supercaps). Also, a 12v car battery will easily tolerate being charged at 14v or more, a 12v capacitor could be permanently damaged if subjected to the same 14v.
Just like your car requires a certain voltage from its battery, a dashcam designed with a 3.7v lithium battery will expect to see a certain voltage too. This is between 4.2v (fully charged) and 3.0v (flat) in order to behave normally and most importantly, to save that crucial last file in an emergency. Therefore, we need capacitors that can safely tolerate and keep within these voltages in day to day use.
To tolerate the dashcams charging circuit, we need either a single 5.4v cap, or, two 2.7v caps wired in series to create a 5.4v capacitor bank. Since series wired caps are best when used with a balancing resistor or a balancing board, a single 5.4v cap is easier and might take up less space. Another reason to go parallel is that in series, one bad cap can slowly damage the other and it isn't always obvious. Caps in parallel always see the same voltage and don't have this problem. Remember, if you do buy two 2.7v caps and put them in series, you will halve the farad value of the finished bank. eg. Two 2.7v 10F caps in series will make a 5.4v 5F bank. Although it might first appear that a series wired capacitor bank will store less, when you work out the amount of energy it can store in either series or parallel, it is actually the same figure. An optional explanation of this can be found below.
Keeping the cap at or under 4.2v is simple, the dashcam will do that bit for us. The cap should be rated above any voltage it will ever see (I'd suggest 5.4v to eliminate any risk of overcharging). Once the cap is at 4.2v, the dashcam will assume the "battery" is full and should not charge the cap beyond 4.2v.
Keeping the cap over 3.0v is the complicated bit. We need the cap to provide enough energy for the cameras normal shutdown procedure to finish when the engine is switched off, plus, hold some leftover charge to keep the clock alive and retain user settings on some cameras. For this, we need to know how much energy our camera is using and for how long after the power is cut.
As an example, I will use an imaginary dashcam that takes 10 whole seconds to shut down and consumes a constant 1 Amp during this time.
1 Amp consumed over 1 hour is called 1 Amp hour
1 hour is 3600 seconds
1Ah (amp hour) is the same as 1000mAh (milliamp hours)
Now we can work out how much power our imaginary dashcam uses over time:
That's great, but caps are rated in farads. And what the hell are farads anyway?
This is where it starts to get a bit complicated.
One farad is defined as the capacitance across which, when charged with one coulomb, there is a potential difference of 1 volt. A coulomb is 1 amp over a 1 second time period (see table above). This means if we have a 1F capacitor charged to 1v, we have 1C, the battery equivalent of 0.27777mAh and we could power a 1A load for 1 second (assuming 100% efficiency and a load that could actually work all the way down to 0v).
Note 1: If you simply want to replace a defective capacitor in a capacitor based dashcam, simply buy something at the same voltage rating with equal or higher farad rating than the original. And make sure it will fit the housing!
Note 2: If you just want a quick step by step process and aren't interested in learning the why's and how's, jump to the bottom of post 3.
Note 2: If you just want a quick step by step process and aren't interested in learning the why's and how's, jump to the bottom of post 3.
I've been trying to understand the difference between the way capacitors and batteries store electricity, and how to compare them to each other. I've learned quite a lot myself while writing all this so hopefully it is correct. The more I learned, the bigger this document got. I do have an actual conversion in mind myself, which is what started me off down this path.
Someone who is considering replacing a dead battery with a supercapacitor will want to know how many Farads their new cap needs to replace xxxmAh battery. It's not quite as straightforward as that. A better way of asking things is: what specification capacitor would allow my dashcam to work normally as it would when using a battery?
This thread will attempt to explain just that. In short and in theory, YES, you can replace a battery with a capacitor in a battery based dashcam. However, that is disregarding other things such as capacitor dimensions, available space inside the housing, how far you are prepared to go to mod the case if needed and how often the camera will be used. I'm not an electrical engineer of any sort so only attempt a conversion at your own risk. I'm just an enthusiast trying to solve a problem in front of me and sharing my workings.
Firstly, we need to understand the difference between how batteries and capacitors charge and discharge their energy.
A 12v car battery is considered fully charged at around 12.6 volts and considered flat at around 11.6 volts. In comparison, a 12v capacitor is fully charged at 12 volts and flat at 0 volts. A car battery at 50% charge (let's say 12.0v) might be able to start your car, but, a 12v capacitor at 50% charge would only be at 6.0v. No matter how much current might be available, nothing at 6v is ever going to run a 12v car starter motor well enough to start an engine (although, do check out YouTube for folks starting cars with supercaps). Also, a 12v car battery will easily tolerate being charged at 14v or more, a 12v capacitor could be permanently damaged if subjected to the same 14v.
Just like your car requires a certain voltage from its battery, a dashcam designed with a 3.7v lithium battery will expect to see a certain voltage too. This is between 4.2v (fully charged) and 3.0v (flat) in order to behave normally and most importantly, to save that crucial last file in an emergency. Therefore, we need capacitors that can safely tolerate and keep within these voltages in day to day use.
To tolerate the dashcams charging circuit, we need either a single 5.4v cap, or, two 2.7v caps wired in series to create a 5.4v capacitor bank. Since series wired caps are best when used with a balancing resistor or a balancing board, a single 5.4v cap is easier and might take up less space. Another reason to go parallel is that in series, one bad cap can slowly damage the other and it isn't always obvious. Caps in parallel always see the same voltage and don't have this problem. Remember, if you do buy two 2.7v caps and put them in series, you will halve the farad value of the finished bank. eg. Two 2.7v 10F caps in series will make a 5.4v 5F bank. Although it might first appear that a series wired capacitor bank will store less, when you work out the amount of energy it can store in either series or parallel, it is actually the same figure. An optional explanation of this can be found below.
Let's look at two fully charged 2.7v 10F capacitors in both series and parallel configurations.
The figures for joules and mWh are the ones to take note of, both configurations hold the same amount of energy.
Code:
V = Volts
C = Coulombs
J = Joules
F = Farads
mWh = Milliwatt hours
mAh = Milliamp hours
F * V = C
V * C = J
C / V = F
J * 0.277778 = mWh
J / 3.6 = mWh (alternate method, same result)
C * 0.277778 = mAh
C / 3.6 = mAh (alternate method, same result)
V * mAh = mWh
mWh / V = mAh
mWh / mAh = V
Series | 5.4v | 5F | 27C | 145.8J | 40.5mWh | 7.5mAh
Parallel | 2.7v | 20F | 54C | 145.8J | 40.5mWh | 15mAh
The figures for joules and mWh are the ones to take note of, both configurations hold the same amount of energy.
Keeping the cap over 3.0v is the complicated bit. We need the cap to provide enough energy for the cameras normal shutdown procedure to finish when the engine is switched off, plus, hold some leftover charge to keep the clock alive and retain user settings on some cameras. For this, we need to know how much energy our camera is using and for how long after the power is cut.
As an example, I will use an imaginary dashcam that takes 10 whole seconds to shut down and consumes a constant 1 Amp during this time.
1 Amp consumed over 1 hour is called 1 Amp hour
1 hour is 3600 seconds
1Ah (amp hour) is the same as 1000mAh (milliamp hours)
Now we can work out how much power our imaginary dashcam uses over time:
Code:
1A over 3600s = 1000mAh
1A over 360s = 100mAh
1A over 36s = 10mAh
1A over 24s = 6.666mAh
1A over 18s = 5mAh
1A over 12s = 3.333mAh
1A over 10s = 2.777mAh
1A over 9s = 2.5mAh
1A over 8s = 2.22222mAh
1A over 7s = 1.94444mAh
1A over 6s = 1.666mAh
1A over 5s = 1.388mAh
1A over 4s = 1.1111mAh
1A over 3s = 0.833mAh
1A over 2s = 0.555mAh
1A over 1s = 0.27777mAh
That's great, but caps are rated in farads. And what the hell are farads anyway?
This is where it starts to get a bit complicated.
One farad is defined as the capacitance across which, when charged with one coulomb, there is a potential difference of 1 volt. A coulomb is 1 amp over a 1 second time period (see table above). This means if we have a 1F capacitor charged to 1v, we have 1C, the battery equivalent of 0.27777mAh and we could power a 1A load for 1 second (assuming 100% efficiency and a load that could actually work all the way down to 0v).