I'm asking this question because I recently had to upgrade to a 6v pack to solve a problem where my digital servos were pulling too much power and causing a voltage drop. It would drop to the point where the servos would stop working. This week, I ordered a 2700mah 4.8v pack for something else and it got me to thinking....

If you have a problem where the servos are pulling too much from your 4.8v pack (let's say 1000mah) would replacing it with a 2000mah pack solve that problem or would the voltage drop still occur? I know I could add a second battery etc, but was curious about the voltage to mah relationship.

Thanks.

If you have submarginal wiring (too small a gauge and/or very long leads) the voltage drop in the wiring is what gets you in trouble. Use heavy duty wire (at least 20 gauge) with gold plated connectors and heavy duty switches and you should be okay with either 4 cell or 5 cell batteries. Keep all leads as short as possible.

At the chance of looking really stupid I will give this analogy.

mAh is the size of tank of water. (capacity)
Volts is the pressure in tank.

The example assumes that the usage of the tank i.e. receiver and servos remains constant through out all the different combinations.


You can increase the tank to a bigger tank 1000 mAh to 2000 mAh, but if the pressure (Volts) remains the same, the output from the tank remains the same but the time of the output is increased.

Now if you increase the pressure in the tank from 4.8v to 6.0v, but leave the tank size at 1000 mAh, then the output from the tank increased but with a decreased output time.

If you increase the tank size (mAh) as well as increase the pressure (Volts) then you get an increased output as well as an increase output time. Changing both will cause the ouput time to be less than if you left the pressure (Volts) alone but increased the tank size (mAh).


I may be off on the exacts, but that is as simple as it gets for me at least.

Hope I did not confuse you worse.

Jason

Generally speaking, modern higher capacity cells have lower internal impedance when compared to the lower capacity family members. They will perform better at heavy loads and will have a smaller voltage drop at the higher currents.

BTW, with comparable mAH capacities, NiCD's nearly always have lower impedance than NiMH (can be more than several dozen milliohms). So, to avoid v-drop problems be sure to check the specs. I suggest sticking with high capacity NiCD's if you want minimal v-drop.

I agree with Rodney. Digital servo's are known for there high current draw. Especially under load. If your using standard servo leads there small guage diameter will resrtict the mah that the servos can draw. Length also will affect current draw when using standard servo leads.
But if your already using heavy duty leads and switches and still having a problem you may want to check the batteries carefully. One bad or marginal cell can cause similar problems. They may check good with a one of the voltmeters available. But the digital servos may put more of a load on the system, especially if your using six or more servos.
If that's all okay!!! You may have a short in your wiring. Check all leads for bare wires and anthing that doesn't look or feel right. A lot of work but, better safe than sorry !!!

The mah rating of the battery is just the capacity of the cells for a given current draw. [&:]

Pretty good analogy Jason. You might add that bigger wire is equivilant to a bigger pipe.

The Geezer

Mike

Theres "current capacity" and "power capacity"

A five cell battery has 25% more power capacity than a four cell battery of like cells. Your problem is directly related to current capacity and indirectly connected to power capacity.

Increasing the battery size/capacity will allow you to pull additional current from the battery with less voltage drop seen at the servo. This in of itself will not solve the voltage drop. Wiring, switches, mis-matched connectors, poor quality connectors excessive connector counts and wire lengths all lead to voltage degradation.

Some servos are more susceptable to this phenomena than others.

Larger batteries typically offer more reserve, so if you hammer them with a load the voltage degradation is minimal. Internal resistance is the key with all cells. Higher impedance numbers are undesirable. The lowest impedance cells are NiCDs, then NiMH and bringing up the rear is Lithium.

There is no direct connection to voltage and battery capacity.

mAh refers to milli-Amp-hours. A 2000mAh battery will deliver 2 amps for one hour. Voltage is not a factor with this rating. You'll not see the rated cell voltage delivered through the course of the discharge curve. BUT you will see the two amps for one hour.

Typically five or so digital servos flying will consume around 15mA per minute on average. A single digital can draw as much as 1 amp easily while under full load or stall, however this rarely happens in our models.

What it boils down to is power. It takes X amount of power to operate the servos. With the lower voltage seen while under load the current draw demand is elevated to provide the power required to move or hold the position. Watts is a common term that equates to power. Watts are calculated by multiplying, amps x voltage = watts

A typical 4.8V, 600mAh RX batteries Watt capacity is 600/1000 x 4.8 = 2.88 Watts nominal. Or lets look at it from another perspective 600/1000=0.6A. This means this battery is rated to deliver .6 amps continuous for one hour. Yes you can demand more of the battery. Another consideration is the numbers discussed herein are at rated voltage spec's, typically the voltage dips therefore skewing the numbers and drawing more current. Power requirements go up...

The operating current of a single 5645 digital is 350ma. Thats more than half the nominal rated capacity of the 600mAh pack described above, 350/1000 x 4.8 = 1.68 Watts

Five servos need 1750mA to operate under load, minimum. 1750/1000 x 8.4 Watts. So an 1800mAh battery would be ideal in this scenario.

OK are we confused yet? Lets try a little harder.

4-cell 1800mAH battery contains 8.64 watts
5-cell 1800mAh battery contains 10.8 watts.

The power required to operate the servos is relitively constant. So which pack will last longer?