Battery packs are not all created equal. Even within a specific battery family, like the lithium-polymer (LiPo) family for example, there are internal design differences which allow some brands and grades of packs to be charged and discharged at higher rates than others. The maximum rate is almost always expressed as a C-rating and almost all modern hobby-grade battery packs will have two separate C-ratings listed on the label: One for discharging and one for charging. The discharge C-rating will usually be in bold numbers in a prominent place on the pack because manufacturers consider it a marketing tool. The charge C-rating is usually a lot more inconspicuous and may even be hard to find, but reputable manufacturers will include that info on their labels too.
A C-rating is a number followed by the letter C and the first thing you need to understand is it’s not a rate. It’s a mathematical formula that you use to calculate the rate. The number is actually a multiplier and C stands for the Capacity of the battery pack as expressed in milliamp hours (mAh). To use a C-rating to calculate a charge or discharge rate you replace the letter C with the pack capacity and multiply it by the number. See? No? Then keep reading:
Lets use a 2100mAh battery pack for an example: If it has a 25C discharge rating then you can calculate the maximum discharge rate with the formula 25 X 2100 = 52500. If it’s a 45C pack the formula would be 45 X 2100 = 94500. Calculating maximum charge rates works the same way except the multiplier will be a lot smaller (usually somewhere between 1 & 8): If that 2100mAh battery pack has a 4C maximum charge rating then the formula to calculate the rate would be 4 X 2100 = 8400 and if it’s a 70mAh pack with a 2C maximum charge rate the formula would be 2 X 75 = 150.
You may have noticed that the answers are huge numbers that won’t even fit in the displays on most chargers. That’s because the capacity of hobby-grade battery packs is almost always listed in milliamp hours (mAh) so when you multiply the capacity by the C rating the answer will be in milliamps (mA). But charge and discharge rates are usually expressed in amps (A) so it’s necessary to convert the answer of your calculation.
1 amp (A) = 1000 milliamps (mA) so you can convert mA to A by dividing by 1000. But there’s a much easier way which doesn’t involve any math: To convert mA to A simply move the decimal point 3 places to the left (add zeros if you have to). In our examples above, 52500mA becomes 52.500A, 94500mA becomes 94.500A, 8400mA becomes 8.400A, and 150mA becomes .150A and none of it required any math. Of course you can eliminate the insignificant zeros after you move the decimal which leaves you with 52.5A, 94.5A, 8.4A, and .15A. For rates greater than 10A or so the numbers to the right of the decimal point become insignificant so you can usually eliminate those too. Converting A to mA works the same way but you move the decimal point 3 places to the right and add zeros if you have too.
A 1C charge rate is the most commonly recommended “normal” charge rate but that’s really a throwback from early LiPo packs and chargers. A 1C rate will be safe for any LiPo pack and it’s what you should stick to if you’re unsure of the maximum, but most modern packs can be balance-charged with modern chargers at higher rates. 4C seems to be a fairly common maximum now and some packs can be charged at 6C or more. But understand that the C-ratings listed on battery pack labels are maximum “never exceed” ratings and anything less is almost always OK.
On a final note, you may have noticed that I haven’t even mentioned cell-count or pack voltage in this explanation and that’s because it doesn’t play any role whatsoever in the charge rate calculation. Charge rate is independent of pack voltage and while it’s important to set both the charge rate (current) and pack voltage (number of cells) correctly when you charge, the voltage doesn’t affect C-rating calculations. It’s also important to understand that packs with higher C-ratings don’t have higher capacity. For example, 25C and 45C 2100mAh packs both have the same capacity (2100mAh), but you can use the energy in the 45C pack at a higher rate (faster) than the 25C pack.
Yeah I used “final note” in the last paragraph, but I feel compelled to add one more thing: Math has rules, but there are no rules governing how manufacturers establish C-ratings and some of their discharge ratings aren’t very truthful; at least in real-world situations. As I already wrote, C-ratings are also a marketing tool and when the truth hurts it’s often bent. Thankfully, their charge C ratings are usually accurate so it’s reasonably safe to trust them. That said, I never charge at maximum rates unless I have a damn good reason to do so.