Charging from Lipo Batteries

by: Bobby Barr

Believe it or not, you can use larger lipo batteries to power your chargers. Then use your chargers to charge smaller packs. This can be appealing for those flying ultra micros, park flyers and gliders.

Advantages:

  • Extremely portable
  • Reusable
  • Convenient
  • You may already have larger batteries

Disadvantages:

  • Must be careful not to over discharge your larger batteries
  • Limited charging source
  • Fire hazard

One of the really neat things about using your existing larger lipos to charge smaller ones is the portability. You can take them almost anywhere without worrying about the need of a generator or electricity. The ability to use the source lipos over and over is really appealing as well. This could be a downside if you are needing to purchase the larger “source” batteries.

Symbols used:

  • Watt (W)
  • Watt hour (Wh)
  • Volt (V)
  • milliampere hour (mAh)
  • Ampere hours (Ah)

If you have a charger that will accept a 12 V input, you could power it using a 3s lipo battery. For instance, lets say that you have a couple of 3s 5000 mAh batteries from your surface vehicles or multirotors. You can use that battery, or multiple batteries in parallel to power the charger. To simply the math, I am going to refer to the batteries in Ah rather than mAh.

5000 mAh  ÷ 1000 = 5 Ah (Converted battery capacity)

5 Ah x 80% = 4 Ah (Available for safe discharge)

4 Ah x 11.1 V = 44.4 Wh (Available for the charger)

If your charger can handle a 24 V input, then you could use a 6s pack or a couple of 3s packs in series for your source batteries. For some that may have a single 6s 5000 mAh pack for your 550 or 700 size helicopter, you would have around 88.8 Wh per battery.

5000 mAh ÷ 1000 = 5 Ah (Converted battery capacity)

5 Ah x 80% = 4 Ah (Available for safe discharge)

4 Ah x 22.2 V = 88.8 Wh (Available for the charger)

So how many batteries can you charge with that? If we figure that the charger is about 85% efficient at converting the the voltage, and that we only need to recharge the amount that the battery has been depleted. If the battery is 80% depleted then…

3s 1300 mAh

1300 mAh x 80% = 1040 mAh (% of battery is depleted)

1040 mAh ÷ 1000 = 1.04 Ah (mAh converted to Ah)

1. 04 Ah x 11.1 V = 11.6 Wh (Needed to recharge the battery)

88.8 Wh x 85% charge efficiency = 75.5 Wh (Available for recharging)

75.5 Wh ÷ 11.6 Wh = 6 charges on a single 6s 5000 mAh

1s 160 mAh

160 mAh x 80% = 128 mAh (% of battery is depleted)

128 mAh ÷1000 = .128 Ah (mAh converted to Ah)

.128 Ah x 3.7 V = .5 Wh (Needed to recharge the battery)

88.8 Wh x 85% charge efficiency = 75.5 Wh (Available for recharging)

75.5 ÷ .5 Wh = 151 charges on a single 6 5000 mAah 

A couple of things to keep in mind here. It may be a good idea to keep a battery cell voltage display plugged into your balance tap of the source battery to ensure that you don’t let it drop too far. There are also some chargers that let the user set the minimum input voltage. This would be nice because it would allow you to set the voltage of your source battery that you want the charge to stop at. For instance if you want to be certain that you 3s source lipo doesn’t drop below 3.3 V per cell, then just set your minimum input voltage at 10 V.