How many volts are in 2000 mAh

Compare battery capacity correctly

The alternative power supply for the camera is a big topic among video makers. You demand a lot of performance for as little money as possible. The power banks intended for smartphones and tablets come in handy here, as they promise "capacities" of 10,000 mAh with low weight, little space requirements and low acquisition costs. Who would come up with the idea of ​​buying expensive V-mount batteries when these power banks have a higher capacity and thus ensure longer runtimes?

This article is intended to explain why mAh is not the same as mAh and how to correctly compare the capacity of batteries.


How was that again in physics class?

This article was long overdue, at the latest when we came across the following post in a thread in the Slashcam forum:

I built a battery / rechargeable battery solution myself ...

Simply solder a battery holder (size 8 for normal batteries or size 10 for rechargeable batteries) with the appropriate connector and strands.
You can easily get 20000-30000mAh depending on the situation.

Costs approx. 5 € + normal AA rechargeable batteries / batteries.

Source: Slashcam forum


What's the problem here? Well, this topic is about how you can alternatively supply a BMCC (Blackmagic Cinema Camera) with power. You should be suspicious at the latest at the 20,000 to 30,000 mAh, which you can only get by using AA rechargeable batteries or AA batteries. This is such an unbelievable value (30 Ah!) That you could even start a small car with such a battery pack. Or not? Let's go back to physics class.

A simple AA (LR6) battery has a voltage of 1.5 volts. The capacity of the battery varies from make to make, but is usually in the range of 2300 mAh. With volts and amps you can calculate a third value that is very relevant in our case: watts (or Wh). The formula for this is volts x amperes = watts

With this formula one could calculate in school how much power a device needs if the voltage and current strength are known. A motor that had to run at 12 volts and required 1 ampere of electricity had a requirement of 12 watts. A motor that ran faster and required 2 amps consumed 24 watts.

If you operated one of these motors with a rechargeable battery, you could easily calculate how long the motor would run using a specific rechargeable battery, thanks to the known power consumption. If the battery used had a voltage of 12 volts, there were two options here. We calculate with a camera that needs 12V / 2A / 24W and a battery that delivers 12V / 5Ah / 60Wh (the 60Wh result from 12 x 5).

The mAh way: 5 Ah: 2 Ah = 2.5 hours

The Wh way: 60Wh: 24 W = 2.5 hours

Both calculation methods deliver identical results. This is also correct when the battery and the consumer work with the same voltage. If the consumer can work with a variable voltage (e.g. 12 to 30 volts like the BMCC), there is a problem with the mAh calculation method.


Variable voltage problems

The BMCC is able to convert the incoming voltage as long as it is in a range between 12 and 30 volts. This means that we can connect a 12V battery and could also work with a 24 volt battery. 18 volts would not be a problem either. Only at 36 volts does it "boom" - then we would need a new BMCC. Incidentally, the power requirement of the BMCC is 18 watts.

Suppose we have two batteries with which we can feed the BMCC.

Battery A: 12 volts / 10,000 mAh
Battery B: 18 volts / 8,000 mAh

Which of these two batteries will let the BMCC run longer? Anyone who stubbornly relies on the mAh value would now say: Battery A, because it has more mAh. But that would be wrong. To determine the true capacity of the battery, you have to multiply voltage and power. This gives the extremely practical value Wh. It looks like this (we converted the mAh into Ah beforehand):

Battery A: 12 volts x 10 Ah = 120 Wh
Battery B: 18 volts x 8 Ah = 144 Wh

The unit Wh stands for watt hours and means: the battery can deliver 120 watts for one hour or one watt for 120 hours. Or 30 watts for four hours (120: 30 = 4). Let us now remember a short sentence above: the BMCC has a power requirement of 18 watts. That would mean:

Battery A: 12 volts x 10 Ah = 120 Wh: 18 watts = 6 hours 40 minutes
Battery B: 18 volts x 8 Ah = 144 Wh: 18 watts = 8 hours

Oops! The battery works much shorter with supposedly more WUMMS.


Alarm in Absurdistan!

The example from the Slashcam forum naturally sounds all alarms in Absurdistan. The assumption that 20,000 to 30,000 mAh would be achieved with simple AA batteries is based on a logic error. If several batteries are connected in series, their voltage (volts) increases, but not the current they can deliver (mAh). Batteries can only "run longer" if they are connected in parallel - but then the voltage remains at 1.5 volts. There is only one of the two ways.

In fact, the self-made battery pack would deliver 12 volts at 2300 mAh (8 x 1.5V = 12V) despite eight LR6 batteries. Converted to Wh, this would mean a real possible power output of 27.6 Wh. The BMCC would be able to run for about 90 minutes with such a "power pack". In comparison, if you were to hang a small 12V / 30Ah car battery around your neck and connect it to the BMCC, the camera would run for 20 hours (because 12V x 30Ah = 360Wh: 18W = 20 hours).


Keep your eyes open when buying a battery

When looking for alternative electricity storage systems, one likes to run into a so-called Powerbank across the way. These small battery packs now not only have a USB output for the smartphone but also a 12 volt power output (often switchable between 9 and 12 volts). These power banks are often advertised with the addition of 10,000 mAh, without specifying the voltage. If you have paid attention to the above, you will see the problem: without voltage, it is not really clear which real capacity is available, because the mAh value is linked to the voltage.

The size of such a power bank alone makes it difficult to compare it with rechargeable batteries that actually have 10,000 mAh (e.g. V-mount rechargeable batteries), whereby the power bank and a V-mount rechargeable battery use power cells of the same type (lithium ion). If you search a little, you will find what you are looking for with reputable providers and you will see the surprise: the 10,000 mAh refer to the basic voltage of the built-in cells. Since this is normally 3.6 to 3.7 volts, the real available capacity is 36 to 37Wh (3.7V x 10Ah = 37Wh).

The professional V-Mount battery, on the other hand, offers a real capacity of 144 to 148 Wh at 10,000 mAh. If you were to calculate V-Mount batteries exactly as it is with Powebanks (i.e. with 3.7V), the battery would have almost 39,000 mAh. Wow! What a power monster!


Easier to compare with Wh

The mAh specification is important, but far too complicated for comparisons. This value can only be used as a reference when comparing two batteries that work with an identical voltage (e.g. 7.2 volts). Here, the battery with a higher mAh value has the lead. If, on the other hand, the voltage is different (e.g. 7.2 volts and 7.4 volts), the battery with a higher voltage but a lower mAh value can hold out longer. In this case the comparison of Wh is more reliable.

At the latest when the voltage of the batteries to be compared drifts apart, there is no way around Wh. Only by calculating this value will you find that a power bank with 10,000 mAh (at 3.7 volts) supplies power for exactly as long as a compact ROKO BP-U30 with 2600 mAh (at 14.4 volts).