I have multiple LiPo batteries that I want to charge up so I can get out flying as fast as (safely) possible.
With my existing LiPo charger, how can I parallel charge multiple batteries at once?
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Only parallel-charge batteries with the same number of cells.
Safe parallel charging requires that the voltages on individual cells be close to each other. A difference of 0.1V per cell is considered to be the maximum safe difference (I found it online, and have used ever since, although I have no scientific proof of that).
One way to compare the packs before parallel-charging is to compare their total voltages and make sure that the maximum difference between any pair of packs does not exceed 0.1V times the number of cells. However, this may be unsafe due to the fact that some packs may have more unbalanced cells than others, so it is better to compare each pair of cells that will be charged in parallel.
With many packs of uneven charge, it may be possible to divide them into groups whose cell voltages are close enough to be safe to parallel-charge and then charge each group separately.
A parallel-charging board with enough protection is recommended, with fuses limiting the total current of each pack being the minimum. Some boards also have fuses or current indicators for each cell of each pack for more safety.
Choose an appropriate charge current, which equals the sum of charge currents of all parallel-charged packs. Charging at 1C times the number of packs seems to be reasonable and safe if there are no large differences in capacities and cell degradation between packs.
Regular safety rules of charging single LiPo packs apply, for example never leaving the batteries unattended while charging, keeping away from flammable materials, etc.
Use a dedicated parallel charging board, and charge only batteries with approximately the same voltage (difference less than 0.1V / cell).
It should be okay to charge batteries of different C-rating and capacity as long as the cell count is the same, but to be safe you might want to only charge batteries with the same capacity in parallel.
Parallel charging is awesome and can be done safely, but to do this it's important to understand what's going on.
First and foremost, of course, you can only parallel connect packs with the same number of cells! If you connect packs with differing numbers of cells, the higher cell count pack will immediately overcharge the smaller pack, and there will be fire, flames, choking smoke, alarms, and sadness.
Assuming the cell count is the same when you hook up packs in parallel, you basically have an equilibrium reaction happening. Energy will flow from the packs with higher voltage into the packs with lower voltage until they equal out and each pack has the same voltage. The greater the difference in voltages, the more energy transfers.
Because of this, people say you should only parallel charge packs with a similar state of charge, ie, it's bad to connect a fully charged pack up with a discharged one. Doing this won't actually end the universe though, it'll just mean the charged pack discharges into the flat-pack - very rapidly. Normally this just means you're charging the flat-pack at a rate far above the recommended rate, which will affect its life in the long term. In extreme cases, it can mean thermal runaway in the flat pack and a fire can result.
On the other hand, if you plug in a single charged 1300 with 7 other flat 1300s, they'll just get slightly charged up. It's still not good practice to do this of course, but it's just worth mentioning to illustrate the point that being super careful about differing pack voltages isn't the most important thing you need to worry about.
Far more important is the age of the packs, and their various internal resistances and relative capacities. It is not a good idea to parallel charge old packs with new ones. Similarly, be very careful about parallel charging any packs which have been crashed and may be freshly damaged.
On to the practicalities. Two types of parallel charge boards are commonly available, those with and without polyfuses on the balance connectors. If you make a mistake plugging in the balance leads, an unfused board will short and you'll burn the tracks off the board instantly (best case), so the use of the safer, fused boards is recommended.
The fused boards do have one dangerous gotcha though - if you trip the fuses by accident, you must wait for the fuses to reset before charging, or you risk unbalanced packs and fire. It's also possible to partially trip the fuses if you plug the balance leads in first, and too much current flows. So always connect the main pack leads first, wait for the voltages to stabilize (with similarly discharged packs 30 seconds is plenty) and only then should you connect the balance leads.
To determine the charge rate, simply sum up the capacities of all packs connected, ie for 3 x 1300s and an 1800 = 5700 = 5.7A for a 1C charge.
Lastly, and you should know this already, never ever charge unattended. Fires can and have happened. All it takes is a faulty or damaged cell, a bad balance connector... A smoke detector over your charge station doesn't go astray!
Remember that packs should not get warm while charging, if they do, or if one cell seems warm, discontinue the charge and quarantine that pack immediately until you find out what is going on.
If all this sounds scary, it's not. You just need to be aware of what you're doing and follow the rules. I've been parallel charging for more than 6 years and I've never had a fire (that I didn't intend to cause ;)).
In addition to the guidelines and precautions above - be careful when plugging in your batteries to the parallel board. Both leads have multiple conductors (wires) and the balance lead can be tricky! If you don't insert the connector straight down, you run the risk of getting some arc/spark action. I experienced this on my first attempt as I had the balance connector at a slight angle.
I highly recommend Joshua Bardwell's Video How To Parallel Charge LiPo Batteries Without Burning Down Your House