Can someone explain mah to me?
So I'm in the process of my first light build. I'm doing a basic XM-L build that is based on Pucked-up's thread. Rather than just completely step on the shoulders of others I would really like to understand the power side of the equation. I get how series vs. parallel works, and how voltage goes up with series and mah goes up with parallel, what I'm not getting is how this relates to drivers and lamps. Ultimately I'm looking to do a 1s2p pack and trying to understand what mah I'm looking for in a driver and how I can get the highest lumen count with that.
I don't doubt there is already a thread out there with the information I would like, but I can't find it. If someone knows of it and can point me that way instead, that would work just as well. Honestly, I'd rather understand how to find what I'm looking for instead of having someone just tell me what driver to use. However, if the "perfect" driver exists for this project, pointing me in that direction would help since the supplier thing has my head spinning.
Think of mah as the amount of gas in the gas tank. It will determine how long your light is going to run.
So if mah is gas in the tank, the mah rating on the driver the rate of consumption? In other words, a 1s2p battery pack with a 3000 mah driver and 3000 mah batteries would run for two hours? So then does the rate of consumption correspond to brightness? In other words, a 3000 mah driver will burn the gas faster than a 1500 mah one would but give you more horse power?
Thanks for the gas in the tank analogy. I'm also a motorcycle guy so that makes it easy to process.
The driver will be rated in ma's not mah's. Think of ma's as the carburetor, the bigger the carb (ma's) the faster it will go (brighter lumen output) and the faster it will consume the gas (mah's) in the tank (batteries).
Here is a link to a runtime calculator - LED Runtime Calculator
Excellent, that helps. I think I get it now. Carb analogy makes sense.
We have these things:
1) The mA is the unit for current (equivalent sort of specking to flow of gasoline).
2) The mAh is the unit for charge (equivalent sort of speaking to volume of gasoline, but the analogy is not quite accurate).
The "h" stands for "hour", so "mAh" its mA multiplied with hour. A 1000mAh charge is the amount of charge flowing at 1000mA in an interval of one hour. Or 500mA flowing for two hours and so on.
So, if a battery has 6600 mAh for example, this means it can take theoretically 6600mA discharge current for an hour before it's depleted. Or it can take 3300mA for two hours.
- Things are a little more complicated, meaning the discharge current influences the actual charge the battery can deliver, higher current strains the battery a little and delivers less charge than a lower current. The battery capacity is rated for a specific discharge current. For simplicity, will consider that no matter the current, the charge delivered will be identical.
Now, the charge can be transformed into energy, this is what the battery delivers. That is E=V*Ic [mWh or Wh], (mili watt*hour or watt*hour) where Ic is the rated charge in mAh or Ah. Let's take two cells, say a Li-Ion with 2200mAh and a Ni-MH battery with 2200mAh. Because the mAh are identical, it will be logic to say that they both deliver the same thing, right? Well, no.
- The Li-Ion delivers E1=3.7V*2200mAh=8140mWh (mili watt hour)
- The Ni-MH delivers E1=1.25V*2200mAh=2750mWh (mili watt hour)
This is the energy they deliver, the real gas volume, quite different, because voltage is brought into equation.
When cells are connected together in series-parallel configurations, we have these rules:
- For series connection, the charge capacity of the resulting battery is the the same as the individual cell capacity in the chain and the voltage is the sum of cells voltage Requirement: all cells must have identical capacity, otherwise the cells with minimum capacity will be reverse biased at empty and destroyed.
- For parallel connection, the charge capacity is the sum of capacity in parallel.
- For combinations of multiple series-parallel, one must obey the previous two rules. That means, every "block" of cells connected in parallel acts as one cell, therefore must have the same capacity as the other "blocks" with which is placed in series. It's not recommended to have in parallel cells with different capacities.
Individual cell 3.7V 2200mAh, E=8140mWh
- A 2s1p battery (2 cells) will have 2200mAh capacity and 7.4V voltage. E=16280mWh
- A 2s2p battery (4 cells) will have 4400mAh capacity and 7.4V voltage, E=32560mWh
- A 3s2p battery (6 cells) will have 4400mAh capacity and 11.1V voltage. E=48840mWh
- A 4s3p battery (12 cells) will have 6600mAh capacity and 14.8V voltage. E=97680mWh
As a final check, the energy of the battery is the sum of energies of individual cells.
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