There is a lot of controversy about using resistors or diodes to knock down the voltage of two li-ions in series. We're dealing with 8.4v maximum cell output, and most modders want to knock that top voltage down to 5 or 6 volts. The following schematic shows three circuits that we will compare.
The SINGLE circuit is a normal one cell configuration. The RESIST and the DIODE circuits use values that knock down the voltage of a two cell arrangement to just a little above the typical SINGLE voltages. Altering the chosen dropping components can make those circuits tamer or more aggressive, as desired. But, the point here is to examine the difference between resistors and diodes as voltage dropping devices.
The atomizer coil is basically a fixed resistor (high resistance nichrome wire). When a dropping resistor is placed in series with the coil, it will always drop the same percentage of the total voltage. If both are the same resistance, then each drops half of the voltage. The diode is a different critter. It acts like a check valve, and once you push hard enough to open it, it will allow current to flow without further ado. In silicon diodes (1N4xxx, etc.), the opening force is .7 volts (topspice simulator uses .83 volts), so you have to string a few of them together to drop a couple volts or more. So in the following spice simulation, you can see that the dropping resistor always clips the voltage by a percentage, even down to zero volts. The diode string, however, maintains the same magnitude of voltage reduction down to 2.8 volts, where that voltage is too weak to keep the check valves open.
For a given wire diameter of the atomizer coil, the amperage through it uniquely determines its temperature. Sure, you can change the temperature of the coil by varying voltage and/or resistance, but for a given wire diameter, only amperage absolutely determines the temperature. In other words, for a given voltage, the temperature depends on the resistance, and for a given resistance, the temperature depends on the voltage. But at a given amperage, the temperature is nailed. So the value of the current is the most reliable parameter in determining the 'work' that the coil is performing, as well as at what point the coil melts and is destroyed. The following spice simulation demonstrates how the two dropping methods compare with each other, and to a single cell response, focused on just the operating ranges involved (6.4-8.4v for dropping circuits, 3.2-4.2v for the single circuit).
You can see that the resistor most closely matches the single cell response, but the diode has some possibly advantageous properties. Depends on what your goals are. You might like the faster drop off of the diodes if you have a large capacity battery and don't mind a relatively shorter period between charges, since the bigger cells last much longer, anyway. But if you're trying to squeeze out the longest charge life of tiny cells, the resistor network might be a better bet.
NOTE: The idea that there is no wasted power in dropping diodes is baloney. If you knock down the same voltage by same amount, the heat created in the resistor will be identical to the heat created in the diode. P = IE, period. If it takes four diodes, then that dissipated power can be spread over four devices, but the same could be achieved by using four resistors in series.
But, on to the next technology, Lithium Mangansese Oxide (Li-Mn) cells. These are probably known to you as CR2s, with a nominal 3.0 volt rating. The actual working range is 2.5 volts to 3.6 volts, with an average of 3.3 volts. Don't be fooled by the 3 volt moniker, stringing two of these together will initially output 7.2 volts, not 6 volts. So, you really have to knock them down by a couple volts, not just one volt (unless you have a super tough atomizer coil). Anyway, the following simulation plot might help you in a design for LiMns, I won't prattle anymore about the details, but there is a significant reduction in wasted dropping power, versus the 8.4 volts output by the Li-Ions.
The SINGLE circuit is a normal one cell configuration. The RESIST and the DIODE circuits use values that knock down the voltage of a two cell arrangement to just a little above the typical SINGLE voltages. Altering the chosen dropping components can make those circuits tamer or more aggressive, as desired. But, the point here is to examine the difference between resistors and diodes as voltage dropping devices.
The atomizer coil is basically a fixed resistor (high resistance nichrome wire). When a dropping resistor is placed in series with the coil, it will always drop the same percentage of the total voltage. If both are the same resistance, then each drops half of the voltage. The diode is a different critter. It acts like a check valve, and once you push hard enough to open it, it will allow current to flow without further ado. In silicon diodes (1N4xxx, etc.), the opening force is .7 volts (topspice simulator uses .83 volts), so you have to string a few of them together to drop a couple volts or more. So in the following spice simulation, you can see that the dropping resistor always clips the voltage by a percentage, even down to zero volts. The diode string, however, maintains the same magnitude of voltage reduction down to 2.8 volts, where that voltage is too weak to keep the check valves open.
For a given wire diameter of the atomizer coil, the amperage through it uniquely determines its temperature. Sure, you can change the temperature of the coil by varying voltage and/or resistance, but for a given wire diameter, only amperage absolutely determines the temperature. In other words, for a given voltage, the temperature depends on the resistance, and for a given resistance, the temperature depends on the voltage. But at a given amperage, the temperature is nailed. So the value of the current is the most reliable parameter in determining the 'work' that the coil is performing, as well as at what point the coil melts and is destroyed. The following spice simulation demonstrates how the two dropping methods compare with each other, and to a single cell response, focused on just the operating ranges involved (6.4-8.4v for dropping circuits, 3.2-4.2v for the single circuit).
You can see that the resistor most closely matches the single cell response, but the diode has some possibly advantageous properties. Depends on what your goals are. You might like the faster drop off of the diodes if you have a large capacity battery and don't mind a relatively shorter period between charges, since the bigger cells last much longer, anyway. But if you're trying to squeeze out the longest charge life of tiny cells, the resistor network might be a better bet.
NOTE: The idea that there is no wasted power in dropping diodes is baloney. If you knock down the same voltage by same amount, the heat created in the resistor will be identical to the heat created in the diode. P = IE, period. If it takes four diodes, then that dissipated power can be spread over four devices, but the same could be achieved by using four resistors in series.
But, on to the next technology, Lithium Mangansese Oxide (Li-Mn) cells. These are probably known to you as CR2s, with a nominal 3.0 volt rating. The actual working range is 2.5 volts to 3.6 volts, with an average of 3.3 volts. Don't be fooled by the 3 volt moniker, stringing two of these together will initially output 7.2 volts, not 6 volts. So, you really have to knock them down by a couple volts, not just one volt (unless you have a super tough atomizer coil). Anyway, the following simulation plot might help you in a design for LiMns, I won't prattle anymore about the details, but there is a significant reduction in wasted dropping power, versus the 8.4 volts output by the Li-Ions.
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