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LiPo care, chargers, brushless systems, BECs, anything electric-specific
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by: master_Rc
#2492
I am going to try and organize this best I can, it may be updated / modified down the line.



I realize this post calls for a little in-depth discussion of Lipo batteries, as they are the most common battery of choice among RC enthusiasts today, but I am not going to clutter this post with it, and just create another one for Lipo specifically sometimes in the near future.



NiMH/NiCD days are long gone, so lets not touch those individually. Chargers offered today support all types of batteries you may come across in this hobby(LiPo, LiFe, NiMh, NiCD) and then some, so lets dive in on their characteristics and major differences.



Lets go over few key specifications to look for when picking a charger first, and then go over what them are in detail. Some are going to be more self-explanatory than others:

1) Cell count charger supports ( Ex: 6S Lipo)

2) Power of the charger measured in Watts. (Very important! )

3) Charge rate output measured in Amps

4) Types of batteries charger supports

5) Internal vs External power supply

6) Features



1) Cell count. This is the self explanatory part of it. Chargers circuitry is rated to handle up to very specific amount of Volts ( or cells ). You can't charge an 8S battery on a charger rated for 6S. This is rather simple, so lets not go any further with this.



2) Power of the charger ( Watts). This is single, most important parameter in any charger you are looking to buy, as it determines the maximum output of it. I will go into further details on this one a little later, as one other variable needs introduction.



3) Charge Rate. This is the value we usually pay most attention to. It indicates how much current we can shove into our battery over time and is measured in Amperes or Amps. This is the value manufacturer will advertise most on their charger, but beware! Max Amps depend on the kind of P/S you are using, the Watt rating of your charger and the kind of battery you are using! I will provide formulas and more explanation a little later.



4) Type of battery. This is very straight forward - if you are looking to charge a Lipo battery, you have to get a charger that knows how to interact with lipo's chemistry without setting your house on fire.



5) Internal vs External P/S. This is going to go back to power rating in Watts. Some chargers come with P/S built in, some require an external 12/24V power source. Built in power supplies have to be small, and consequently are not as powerful as their large external brothers. Depending on how much Watts of output you need, you'll decide if you can settle for a little built in 50Watter or need to go with external 350+ Watt power supply. Fortunately, power supplies are made for variety of things, and can easily be acquired and modified for your hobby needs. Anybody who has an old computer, has at least a 250Watt power supply inside of it that can easily be modified to work with your charger.



6) Features. I can probably write an essay on this alone, but let's just touch on some basics here that you may want to have in a charger:

a) Charge/Discharge Cycles - almost all charger offer this simple feature. This is useful for conditioning those old NiMH batts into shape.

b) user preference setup. Time limit, temperature limit ( if you have temp sensor), back light settings, sound settigs etc.

c) Lipo storage function. This is VERY nice to have if you own lipo batteries, unfortunately not every charger offers it. Only one I had that had that feature was Bantam BC6.

d) Computer interface. Some of the fancier chargers can be connected to your PC and data about charger cycles, peak Mah and Voltages as well as time to charge and other info can be transferred, saved and reviewed on a computer. Why? Don't ask me... I've yet to figure it out myself, but it's out there available for you if you want it!





Now, lets go a little more in depth on Watts, Amps, Voltages and ofc something we are all interested in - charge times.

This is where a lot more explanation is due for the battery part of it, but for simplicity purposes we are going to assume that all of our batteries are going to charge at 1C, which for any given battery is going to be equal it's capacity / 1,000 ( Ex: a 5,000Mah Battery is going to be charged at 5Amps; 1200Mah battery is going to be charged at 1.2 Amps)



Here are some formulas to get started:

Amps = Watts / Volts and another one derived from the first one directly:

Watts = Amps * Volts



SO, how do I figure out how long it takes to charge my lipo you may ask. Once again, there's a little more to it, but charging any given battery at 1C ( see Ex: above to know what 1C means) will take exactly 1 Hour.



Now here comes the part you have to really pay attention to when picking your charger! Suppose a charger XYZ has following characteristics: 50Watts, 10Amps, 25Volts

We are going to take XYZ and charge 3 of our example batteries with it ( 2S 500Mah Lipo, 2S 5,000Mah Lipo, and 3S 5,000Mah Lipo. FYI: 1S Lipo = 3.7 Volts)

This is where the formulas I mentioned earlier come into play:



Battery 1 ( 2S 500Mah):

Watts needed to charge at 1C = 0.5A x 7.4Volts

0.5A x 7.4Volts = 3.7Watts. Our charger has 50Watts, so plenty of power for such tiny batt!



Battery 2 ( 2S 5,000Mah)

Watts needed to charge at 1C = 5A x 7.4Volts

5A x 7.4Volts = 37Watts. Still within limit of our 50Watt



Battery 3 ( 3S 6,000Mah)

Watts need to charge at 1C = 6A x 11.1Volts

6A x 11.1 = 66.6Watts. Ooops! Now, we are way outside of out 50 Watt limit, which means we have to use the invert of the formula to find out how much amps can this charger put out into this battery.

50Watts / 11.1Volts = 4.5Amps. This does not mean you can't still set your XYZ charger to 10Amps output, but it does mean that it won't be able to do any more than 4.5Amps in this instance because of insufficient power, so you'll have to charge at less than 1C. 1C in that instance was 6A.



What does it mean to charge at less than 1C rating? LONGER charge times! Nobody wants that right? So pick a charger appropriate for your batteries and pay attention to ALL of it's ratings, not just Amps, as you can clearly see, Amps are subject to available Watts.



Something else to keep in mind is that ALL power supplies operate at about 80-90% efficiency, so out of the max 50Watts listed on the web site of package, you will actually get roughly 45Watts and this number will keep deteriorating over time. It's just a property of P/S that you can't do anything about.



So now that you know how to calculate needed charger ratings for one battery, imagine for a second that you decided to charge 3 of your batteries in series! Apply the same formulas and you get the following for battery #2 (2S 5,000Mah)

(3 x 7.4V) x 5A = 22.2V x 5A = 110Watts



Now that you know that's possible, you can fully evaluate your charging needs and pick a charge with the right specs!



Hope this "little" write up helped some of you guys looking for a charger.



If you like it, I'll be doing another one on how to choose Lipos! So comment on whatever you feel appropriate and feel free to ask questions or correct me on anything I may have gotten wrong.

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