Sunday, 1 January 2017

Controller Batteries (H901A)

The standard controller (H901A) is designed to run off EITHER 4xAA cells (NiMH or Alkaline) OR 2s LiPo.
Approximate voltage readings
The controller firmware is specifically set to recognise which battery pack you have installed. (more in a minute)
Secondly, ALL the controller electronics runs off 3.3v. The voltage regulation is done by the ubiquitous ACT4060A DC/DC converter.
 As long as you can provide above 4v at about 550mA it matters not a hoot what the battery voltage is. It makes no difference to the control, telemetry , or FPV reception, all which run on 3.3v


When you install a battery pack and power on, the controller decides which type of battery you have installed.

  • If it's between 8.4v and 6.5v it assumes it's a 2S LiPo
  • If it's between 6v and 4v it assumes it's 4xAA. 
Well because it calibrates the battery bar meter accordingly and,more importantly, sets the Controller failsafe voltage.

For a LiPo battery the bar indicates as follows: (all approx)

Segments   Voltage
     5              7.9
     4              7.8
     3              7.4
     2              7.1
     1              6.7
Failsafe is at 6.3v

For a 4xAA battery configuration, the bar indicates: (all approx)

Segments   Voltage
Battery Voltage
     5              6.0
     4              5.3
     3              4.8
     2              4.6
     1              4.2
Failsafe is at 4.0v

As part of your pre-flight checks, you can quickly read the actual battery voltage by simply powering on the controller whilst depressing the throttle stick.


Controller failsafe occurs to protect both the battery and allow you time to safely land your quad. Failsafe is indicated by a flashing red power lamp and the switching off of the VRX and the FPV monitor. The Vrx itself draws over 250mA and that with the display account for the majority of the controller power consumption. In failsafe the current drops to ~ 90mA.
The controller is still perfectly functionable, and with the low demand on the battery pack, a full 20 to 30 minutes of flight time is normally easily achieved after failsafe.

WARNING - Putting in or powering up with a depleted LiPo can make the controller think that it is a 4xAA pack. This can cause:

  • irreversible cell damage (FS is now 4v) 
  • probable loss of all control function at FS (because battery is really dead!) 

Other cell Chemistries 

A few people have tried 2S LiFe batteries. These unfortunately, when fully charged, fall in a no man's land between 4XAA and 2S Lipo being approx 6.6v. This means they normally immediately failsafe. If they are slightly depleted, they can appear as 4xAA but the absolute minimum voltage for 2S LiFe is 5v so well above the failsafe cutout. They are not recommended for those reasons.

Battery Types and Capacities 

For comparison, below is the power density (W/h) and approx runtime [0:00] for different battery setups from low to high, that people commonly use.

  • 2s x 500mAh (Lipo) =3.7Wh  [1:15]
  • 2s x 1000mAh (Lipo) = 7.4 Wh  [2:25]
  • 4s x 1300mAh (Alkaline) = 7.8Wh (Duracell or similar) [2:35]
  • 2s x 1500mAh (Lipo) = 11.1Wh [3:40]
  • 4s x 2500mAh (NiMH) = 12Wh (common AA NiMH set up) [4:00]
  • 2s x 2000mAh (Lipo) = 14.8Wh [4:55]
  • 2s x 2500mAh (Lipo) = 18.5Wh [6:10]
  • 2s x 2700mAh (Lipo) = 20Wh (Stock H501S Flight battery) [6:40]

I tested a set of 4x2150mAh batteries from full charge to the point of failsafe, then to the point of failure at which point the 501 would auto RTH because of loss of signal.
The cells I normally use are only 2150mAh low discharge. I do have other 2900mAh, but I don't have the patience to test them in the same way :-)
Anyway here's the figures:

  • From full charge to flashing warning icon = 3hrs 8mins 37secs
  • from flashing warning to LV failsafe = 8mins 22secs (3h16m59s)
  • from LV failsafe until loss of control = 36min 6 secs (3h53m05s)

So total run time of 3 hours 53mins and 6 secs.
That gives me 10 flights with capacity to spare.
The 2900s would increase that longer than I'm prepared to test.

As 4x2150mAh cells are approx 10Wh capacity, this equates to 1Wh giving about 20 minutes of run time. The table above indicates in minutes [0:00]  the approx run time using this calculation.

Handy Hint

To make replacing  AA cells super simple, just place a piece of tape around the two visible batteries.


There is no disadvantage in using good NiMh cells. Low discharge NiMh (like Eneloop) are excellent and will easily give a couple of hours flying. To gain any extra flight time you really need to go for a 2000 mAh Lipo or better.

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