In a multiple-celled battery pack, it is possible for the individual cells to develop differences in their charge levels. Since Lithium Polymer (LiPo) batteries are very sensitive to overcharging, it’s important that their cells be kept at or very near equal levels when charging. A balance charger (or balancer) does this by monitoring the individual cell voltages in a pack through a connector on the pack (called a balance connector) and adjusting their rate of charge accordingly. When such a balancer is built into a charger, the charger is known as a Balance Charger or Balancing Charger.
Please beware that Lipos can catch fire or explode if not charged or handled correctly, so take necessary precautions and follow directions for the battery and the charger.
Watts, Amps and Important Electrical Stuff
When choosing a charger, you'll notice that the manufacturers focus a lot on the charging amps (A). There are a lot of cheap chargers out there claiming a 5A charge rate. What is also just as important is the total power output, measured in watts (W). Many of the 5A chargers only have a total power output of 50W. Lets calculate what this means using the basic electronic equation:
watts (W) = volts (V) x amps (A)
For a 3S battery pack which is a standard for most entry level 400 or 450 class, with a 2100 maH capacity, charging at 1C you have
12.6V x 2.1A = 26.46W
For most batteries this is ok, since its not safe to charge at more than 1C. If you pay a little extra to get premium batteries like Outrage or Hyperion that can charge at multiple C rates, this quickly becomes a problem. For this same example, a 2C charge rate would be
12.6V x 4.2A = 52.92W
Oops. This is more than your total output power, so it will drop the rate or some will shut off. The same will happen if you want to charge TWO batteries simultanously. So as you can see even though you haven't reached 5A draw you don't have enough total power.
If you upgrade your helicopter in the future to a higher end 450 class, or a 500 class these are using 4S and 6S packs. You are very limited in this case. A common 6S pack is 2300mah.
25.2V x 2.3A = 57.96W
As you can see it cannot charge a 6S pack at even 1C. You would be forced to drop the charge rate to less than 1C in order to charge the battery and it will end up taking much longer to charge than you want (especially once it starts balancing.)
If you are going to want to charge 4S packs or more, or going to want to charge multiple packs at once (you will) then its worth paying a little extra to get a charger with a higher total power output, like this one:
 Maxpro X6-plus from Helidirect. As a bonus it also includes a servo tester.
Notes on Brand Specific Chargers
This is a good cheap balancing charger from Esky that comes with the Belt CP or the Lama V3/V4 helicopters. Although it does not have the features of more expensive chargers, it still does a good job of charging a battery. Here are some highlights:
- It can charge a 2S (7.4V) and 3S (11.1 V) battery
- It charges each cell of the battery to the correct 4.20 volts
- Typical charging times: 1 hour for a 2S (7.4) 900 Mah battery and about 6 hours for a 3S (11.1V) 2200 Mah battery.
- Requires a 12V DC power supply which will sometimes be included with the product. It also comes with connectors that you can directly connect to a 12V car battery
- Charging output current is 800mA
- This charger charges each cell on a rotational basis, switching to a different cell every 1 minute. It cannot charge all cells at the same time and therefore takes longer time than other chargers.
- Average cost: $10
If you're looking for faster chargers, take a look at the Cellpro 4s
There are many other chargers on the market as well such as:
- Turnigy Accucel-6 50W 5A Balancer/Charger - Priced at just $38 and with excellent user reviews, this 6s charger can handle up to 5A charging current. It can also handle NiMH batteries. User manual
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