


Discussion
Relationship between power supply output and charge rate (?)
I have a power supply question. I’m asking for a friend, but I don’t know the answer either. I already searched through the first three pages of this forum and couldn’t really find anything addressing power supplies specifically:
What’s the relationship between the output capability of your power supply (Amps) and the rate you charge your batteries at? I see most of the common power supplies range from around 12A to 25 or maybe 30A. So then, one of the power supplies I use is 12A. I charge my LiPo flight packs through my Triton EQ around 1C—anywhere from about 2A to around 4A depending on which pack I am charging. Since the highest I charge at is 4A, a 12A power supply is more than enough, right? I could charge another pack (or two) at 4A and still a 12A supply would be enough, right? So then, as long as your charge rate is equal to or less than the power supply’s output, you should be good to go, right? My other power supply is 25A. But it’s just a spare and I don’t use it much. Then, it would be good to charge, for example, FIVE 5000mAh battery packs at 1C (5A) each, right? That should be WAY more power supply than most people would EVER need, right? Or, do you need your power supply to be some X% over your charge rate because of loss or something? Thanks if you can explain. Tim 




Think in term of watts.
A 12V 25A power supply is 300 watts. Most chargers are 80% plus effecient so 80% of 300 = 240 watts. A 3S LiPoly charges to 12.6V 240 / 12.6 = 19Amps. Thus if the charger is rated at up to 20Amps and the power supply is 12V and 25A then you could charge a 3S at up to 19Amps. Thread on power supplies http://www.rcgroups.com/forums/showthread.php?t=1082910 Charles 


Staffs, UK
Joined Nov 2003
12,508 Posts

As Charles say you need to think in power not just current and so you have to consider the voltage that is needed e.g.
Charging a 3S (12.6V fully charged) battery at 5A takes about 12.6 x 5 = 63W (plus a bit for inefficiency) so a 12V 12A (144W) power supply would be plenty. Charging a 10S battery at that same 5A would take 42 x 5 = 210W (plus a bit) and that same 12V 12A power supply wouldn't be nearly enough. Steve 



Thanks a log guys for taking the time. For some reason, I can't quite wrap my head around it, though I do understand and follow the math. I'll study up on the link to the other thread. Looks like that'll explain it all too (as you already have).
Tim 



As the previous posters point out, you must include voltage as well as current. Steve's second example of charging a 10S is a good illustration.
To charge a 10S you need 42 volts, but most power supplies deliver 12 volts or so. There is a circuit in the charger to boost the voltage, but it isn't free. It requires more current  Amperes. So, once again, it is power (Watts), not just Amps. Bill 



I think I've got it.
To find out "how much" power supply (and/or charger) you need, you need to know the capacity and Voltage of your batteries and what rate you want to charge them at (usually 1C). The higher the capacity (mAh) and/or the number of cells ("S", Voltage), the more Watts you will need in a charger (or power supply). Looks like the Watts limit on power supplies is much higher than the Watts limit on chargers (then, you can run multiple chargers off one power supply if it will handle it). For example, my power supply is rated to 12V, 25A. This works out to 300W available (call it 240W for a 20% loss as suggested earlier in this thread). More importantly though, my charger is a Triton EQ which is good only to 50 Watts. So then, given the 50W limit of my charger, I can charge a 1S battery up to 11,900mAh at 1C (50W / 4.2V = 11.9 x 1000  11,900mAh). Or, I an charge a 2S battery up to 5,950mah at 1C. Or, I can charge a 3S battery up to 3,970mah at 1C. Etc., etc. The larger the "S" of the battery (Voltage), the lower the cieling on the charge rate becomes (and the more impractical the charger becomes as you get larger batteries exceeding the Watts output of the charger). Most of my stuff is 2S, 3S and 4S from 1,800mAh to 2,200mAh, but I do have a 6S 3,300mAh battery that I cannot charge at 1C because of the 50W limit of my charger. The best it will do is about .6C (50W / 25.2V = 1,984mAh / 3,300mah = .6). Probably for the majority of pilots, a 50W limit on their charger is adequate, but I bet as electrics become more and more popular more and more people will need chargers with more power (Watts). IE the Hyperion stuff. Eureka. I've got it. I know it's simple math, but something I haven't really had to think about before (until I was asked by a local R/C club member). Thanks guys! 



Followup question:
When calculating the Voltage and current to determine the Watts necessary from your charger or power supply, do you use the nominal Voltage (3.7V/cell) or the fullycharged Voltage (4.2V/cell)? For example, does a 6S 3,300mAh battery require 73 Watts (3.7V x 6 x 3.3mAh), or 83 Watts (4.2V x 6 x 3.3)? Thanks again! Tim 



Quote:
Charles 



So. Cal.
Joined Oct 2004
9,732 Posts

Wattage consumption of a charger is always greater than wattage output. Most chargers are ~7090% efficient. To determine size of power supply needed, divide max wattage that you need on output of charger by ~.7. If you know the DCDC converter efficiency of your specific charger, use that number but allow for a little headroom.
I your specific case, 83 watts / .7 = ~120 watt power supply. As Charles indicates, power consumption will be less than max most of the time but power supply will need to accommodate maximum power output as this is the power required by the charger toward the end of CC mode when all cells approach 4.2 volts. Some chargers (iChargers, and perhaps others) require even more power as the output at the charge terminals is actually greater than 4.2 volts per cell to account for charge lead IR losses. When in doubt, go bigger. As power supplies in the form of harvested server supplies are silly cheap, there's really no reason to skimp. Mark 



Perhaps refer back to my first responce post #2
http://www.rcgroups.com/forums/showp...89&postcount=2 As Mark says server power supplies are cheap. My everyday ones are IBM 235s which I run in series for 24 volts but with a 12V tap also. I have been using them for years and they run many hours per week. I have HP server power supplies which are 55 amps. however I rarely need that many amps at home and at the field I use two 125Ah deep cycle batteries. Charles 



Allow plenty of room for growth when you pick a power supply. I bought a Pyramid 5A continuous/7A peak PS a few years ago when it had plenty of capacity to charge my few 3S 2100mah LiPos one at a time at 1C.
Now, I own more electric models, and with the high charge rates possible with some LiPos, the Pyramid I have isn't enough. It cuts off due to overheating even when charging several batteries in a row at 3.5 or 4A. I'm not much of an electronics guy, but I think with all the great info here, I'll be able to upgrade to a server type power supply soon. Good flying, desmobob 


So. Cal.
Joined Oct 2004
9,732 Posts

Quote:
Getting into an extremely powerful server supply is very easy. There are a few 'off the shelf' options that are already 'converted' for those that do not want to perform the conversion. IMO, here's the more rewarding (and far cheaper) way: 1) Peruse this thread and pick a few part numbers that match your current and possible future power requirements. 2) Go to eBay (or favorite search engine) and perform search on the various part numbers and procure one from a reputable seller. 3) Perform modification as detailed in the appropriate thread linked in the above thread and verify proper operation of power supply with a DMM after conversion. 4) Plug charger into your newly modified power supply and charge happily ever after. Mark 




Quote:
Thanks for the help.... you must be one of those resourceful and helpful folks here! I will take your advice. Good flying, desmobob 




Thanks again everybody.
I'm not shopping for a charger or power supply today. I was just trying to sort the math to answer a question for another member in my local R/C club. I've got it all figured out now. To that end, I wrote this...if nothing else just to help me retain the information better (I know it's extremely elementary, but writing it down helps me rememberand maybe somebody else!)... I know this has been covered time and again in other posts and forums (and links in other posts and forums), but just for fun I thought I’d take a stab at it my self—that is explaining how to figure out—“powerwise,” how much charger to purchase for charging LiPo batteries. Anyone out there feel free to correct me where necessary... (This does not take into account other factors such as number of charge ports, input capability (AC, DC), etc. I’m just talking about the critical numbers here.) You have to know three things: 1. The rate at which you want to charge the batteries. 2. Your battery pack’s Voltage when fully charged. 3. The power, or “Watts” required to charge your battery at the desired rate. 1. Charge Rate: The charge rate, or the rate at which electrical current flows into your battery is expressed in Amperes, or “Amps” (usually just “A”). So the rate you should use depends on two factors; 1) How long you want to wait for the battery to charge, and; 2) The size, or “capacity” of your battery. Concerning the time, this is already pretty much decided for you—or at least strongly recommended. It’s best for those of you new to LiPos to charge at a current flow rate (again, “Amperes,” “Amps” or just "A") that is equal to the battery’s capacity. Were the battery fully discharged, mathematically it would take one hour to charge. This charging rate equal to the battery’s capacity is called a “1C” charge rate because it’s one “times” the battery’s capacity. Simply, if you have a 1,800mAh battery, a 1C charge rate would be 1,800mA, or 1.8 Amps (1.8A). Note that we never fly our batteries until they are completely drained, so a 1C charge rate should actually take less than an hour to fully charge. And some LiPos can be charged as quickly as 30 minutes (2C) or even in fifteen minutes (4C)! But charging LiPo batteries this fast is best left to experts because it can be dangerous, not to mention potentially reduce the life of your battery. Most LiPo batteries will be “happiest” if charged at that 1C rate that will take about one hour. Okay, now we know that for our 1,800mAh battery we need a charger capable of charging at a rate of at least 1.8A. 2. Battery pack’s fullycharged Voltage: Battery packs have the “nominal,” or average Voltage printed on the label. But another factor we need to know to determine “how much” charger is required is the battery’s Voltage when it is fullycharged. To know this, first we have to know how many individual battery cells make up the battery pack. Sometimes the number of cells is also printed on the label—“2S,” “3S,” “4S,” etc. (By the way, “S” means that the separate cells making up a battery are wired together in “series,” adding the Voltage of the cells to each other. A 3S battery is 3 individual cells wired together to make a single battery pack.) But if the number of cells is not on the label, it’s easy to figure out. Divide the battery’s nominal Voltage (always printed on the label) by the nominal Voltage of a single LiPo cell which is 3.7V. So, if the label on your battery reads “11.1V,” divide 11.1V by 3.7V to end up with 3. Now you know you have a 3S battery. Don’t worry, if you’re new to this, eventually you’ll just know that a 7.4V battery is 2S, an 11.1V battery is 3S, a 14.8V battery is 4S, etc. Once you know the number of cells, multiply that by the Voltage of a fullycharged cell which is 4.2 Volts. Okay, for our 11.1V (3S) battery now we know we need a charger capable of charging at least 12.6V. (Sometimes though, the specifications for a charger might state only how many cells it is capable of charging—not necessarily the Voltage—“charges 1 – 6 cell LiPos”). So, now we know the charge rate required and we know the number of cells, or the Voltage required. But there is one more, easy, final calculation to determine the last of three criteria to look for in a charger: We need to find out how much “power” the charger must be capable of delivering. This “power” is expressed in Watts. Volts x Amps = Watts Let’s do an example: If we want to charge our 3S 1,800mAh battery @ 1C, how much Watts must our charger be capable of delivering? 12.6V x 1.8A = 22.7 Watts. We need a charger capable of charging 3S batteries (12.6V) @ 1.8A that can also deliver at least 22.7 Watts! Now, you might be wondering, if the specifications state that a charger can charge 3S battery and 1.8 Amps, then why do we need to confirm that it has enough Watts? Well, sometimes the specifications state “charges 3S, 2A,” but sometimes it cannot do both at the same time because it doesn’t provide enough Watts. It may be able to charge a 3S battery, but maybe only at 1 Amp (12.6 Watts). Or, maybe it can charge at 2 Amps, but only at 4.2 Volts (8.4 Watts). That’s why you also need to confirm the charger’s output power in Watts as illustrated above. Here are some of the specifications for a Great Planes Triton EQ charger: AC Input Voltage: 110V 60Hz  240V 50Hz (detachable AC cord) DC Input: 1115.0V DC, with large alligator clips Number of Outputs: One, banana jacks Battery Types, # cells: 114 NiCd, NiMH 16 LiPo, LiIon, or LiFe (3.6, 3.7 or 3.3V cells) 2, 4, 6, 8, 10, 12V Pb (2V per cell) Fast Charge Current: 0.15.0A (1C maximum for lithiums) 63W max DC, 50W max AC As noted in the specifications, the Triton EQ will charge a 1 to 6cell LiPo, and can charge up to 5 Amps. It can handle 63 Watts if connected to a DC power source or 50 Watts if charging from your AC wall outlet. Note again though, since its maximum Watts output (from an AC wall outlet) is limited to 50 Watts, the largest 6S battery it can charge at 1C is about 1,980mAh (50W / 25.2V (6S) = 1,980mAh). Put another way, even though the charger is specified to 6S, 5A, it will NOT charge a 6S 5,000mAh battery at 1C because 25.2V x 5 Amps = 126 Watts, but the charger is good only to 50W (if charging from a wall outlet). Again, this is why you need to calculate the Watts requirement for the battery you will be charging and compare it to the Watts output of the charger you are considering—don’t go by just the number of cells and the current. In the case of the 3S (11.V) 1,800mAh battery we were using as an example, the Triton EQ will certainly do the job. Given the 50W limit it could handle a 1C charge rate for a 3S (12.6V) battery up to 3,970mAh in size—anything larger than 3,970mAh will be less than a 1C charge rate. However, when shopping for chargers you might also consider any larger batteries you could be charging in the future should you become more involved in the hobby someday flying larger planes. This is just a guess on my part, but for most people (50%, 80% ???), a charger capable of 6S, 5A, 50W will be suitable for all of their batteries. On the other hand, the more and more threads and posts you read about batteries and chargers, the more demand you see for chargers and power supplies capable of 100 Watts or more. One last, very minor calculation: This is for modelers using a separate, DC power supply that they can plug into their AC wall outlet to power their battery charger. If using a separate, DC power supply, you have to figure in an amount of loss, or inefficiency when considering the Watts output required from a power supply. Most agree this factor is approximately 20% loss (or 80% efficiency). So, if we’re looking for a charger capable of 25 Watts (to round the 22.7 Watt figure required from the charger), you’ll need a power supply capable of at least roughly 31 Watts. Using our 80% efficiency factor, 25 Watts (required) / .8 = 31 Watts. This means we’ll need a 31 Watt power supply to get 25 Watts from our charger if using a separate DC power source. 
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