This information is what I have managed to dig up with considerable effort on the internet. I do not accept liability for its accuracy or for any damages from the use or misuse of this information. Bottom line: the
safest method of charging your 3.7V toy grade quad lipo battery packs is to charge them per the manufacturer's instructions which, in most cases, means using the provided USB charger dongle inserted into a 500mA USB 2 port.
First of all, what I'm about to discuss applies to 3.7V battery packs specifically designated by the manufacturer for use in toy grade quadcopters. You may find another lipo battery pack with the right connector, right voltage, right dimensions, and the right capacity, but if it isn't specifically identified for use in a toy grade quadcopter, this info about built-in battery protection circuitry probably doesn't apply and if you then charge this "looks right" battery with a toy grade quadcopter's USB charger dongle, you'll end up with a permanently damaged, probably physically swollen/puffed up, overcharged battery pack that may eventually at some random point in the future turn into a nasty incendiary device in your home or inside your quad now plummeting to earth 150 yards away from where you're at.
For general lipo charging safety, never leave a charging lipo unattended and charge it on a flame-proof surface well away from flammable items. I use a thick, clay ceramic cereal bowl for the small packs, a thick clay ceramic plate for larger packs. Fire resistant lipo charging bags can also be purchased. Don't place multiple charging packs in close proximity in order to prevent a failure cascade initiated by the catastrophic failure of one pack.
The circuitry in the 3.7V pack I opened and photographed was similar to, but not exactly like this one which is also intended for a single cell (3.7V) lipo pack:
Protection Circuit Module: 3.7V Li-Polymer (8A)
https://www.tenergy.com/32003
Here are its safety features. This is why a circuit like this is required in
consumer toy grade quad 3.7V lipo battery packs:
Over Charge Protection:
Over Charge Detection Voltage: 4.25±0.025V
Over Charge Release Voltage: 4.05±0.05V
Over Discharge Protection:
Over Discharge Detection Voltage: 2.5±0.062V
Over Discharge Release Voltage: 3.0±0.075V
Over Current Detection Current: 14.0～20.0A
Short Circuit Protection:
Short Protection: Exterior short circuit
Release Condition: Cut short circuit
The Chinese toy grade quad manufacturers are apparently using what is
supposed to be just a lipo battery (cell) protection circuit as a charge controller. Every USB dongle supplied with the six quadcopters I own from different manufacturers and ranging in original release dates of 2012 through 2015 contains within it just a very simple current detection circuit designed to turn an LED on or off when the 3.7V lipo protection circuit in the battery pack senses a 4.25V battery (cell) voltage and ends the current flow from the USB port
into the lipo cell. Whether or not the resulting charge follows an
ideal lipo charge curve is beyond me. The actual intent of that maximum cell voltage cutoff provided by the protection circuit is simply to act as protection against cell overvoltage and catastrophic failure (thermal runaway) due to an external lipo charger (for instance, a hobby grade microcontroller controlled charger) which might supply a too-high voltage while charging due to a malfunction or due to the incorrect setting of the battery pack's maximum voltage by the user of that hobby grade charger.
The over-discharge protection portion of the battery protection circuit is set to end all current flow
out of the lipo battery pack when the pack voltage drops to 2.5V and is
supposedly intended to prevent catastrophic failure of the battery pack
when it is later recharged after, for instance, having been left installed in an inaccessible crashed quad where the consumer is unable to remove the battery from the device which is discharging it. I said
"supposedly" because that info is from a hobbyist's post in an RC forum, so take that claim for what it might be worth and
understand that you recharge the battery pack of a recovered, previously lost toy grade quad at your own risk. (EDIT: If you do choose to take the
risk of recharging a battery from a lost quad,
definitely DO NOT recharge it using anything other than the manufacturer-supplied USB charging dongle plugged into a standard 500mA USB 2 port or, possibly better yet, using a hobby grade charger you are familiar with where you can set the maximum charge rate to be
even lower than 500mA; in other words,
DO NOT rapid charge the pack using a higher current USB port; the slower the charge rate, the better; also, follow all of the fire safety precautions mentioned above) The preferred minimum 3.7V lipo battery pack voltage when discharged is
3V, not the 2.5V trip point of the battery protection circuitry, and this criteria should be met if you remove the battery pack from the quad (or turn off the quad if it has a power switch) immediately after the quad gives its low battery indication (usually the flashing of the quad's LEDs).
The over-current protection provided by the protection circuit is intended to prevent catastrophic battery failure (thermal runaway) when an unacceptably low resistance load (ex., a dead short) is applied across its leads.
I've charged two different toy grade quad batteries using a bench DC power supply set at 5V and capable of delivering 3A. One pack was a 3.7V 380mAh lipo (Hubsan X4 clone battery), the other a 3.7V 650mAh lipo (Syma X5C clone battery). Both drew exactly 1A maximum when the charge voltage was first applied. The current draw on both went to 750mA within about 5 minutes, then dropped very slowly to about 450mA at which point the current instantly dropped to zero. Each battery pack was at 4.2V after the charge, right where the protection was supposed to cut off the charge. In both cases, the total charge time was around 40 minutes.
What that test means is that the charge times for these toy grade quad 3.7V lipo packs, at least those with a 650mAh capacity and below,
won't be significantly extended by a 900mA USB 3 port, but
will be by a 500mA USB 2 port. It also means that even if you're using a hobby grade microcontroller controlled charger to charge these protected packs, the maximum charge current they will accept is 1A (which is actually more than you'd normally chose when charging packs of these capacities; see next paragraph).
NOTE that the
generic recommended maximum charge rate for lipos is 1C meaning 1/h x the capacity rating of the battery. A 1C charge rate for the example 380mAh pack would be 380mA, for the 680mAh pack, 680mA. In both cases, that charge current limit is approximately provided by the 500mA current supply limit of a USB port. If you use a different USB charger capable of supplying more current than that, for instance, one of the chargers supplied with a USB port and meant for the charging of smart phones or tablets, the charge rate may be allowed at the 1A maximum charge rate allowed by the battery's protection circuit, in which case you'd be charging at a 2.6C rate for the 380mAh pack and 1.5C rate for the 650mAh pack. Whether those rates are truly safe long-term is unknown to me although they are obviously the maximum charge rates set by the
protection circuitry itself and I've charged these packs many times using the special 1.5A rated USB 2 and 3 ports on my PC motherboard with no noticeable negative effects. Lipo charging is endothermic meaning that the battery gets actually cold as it charges. It's an excessive discharge rate that can lead to heating and thermal runaway (and fire). However, be informed that charging these packs using anything other than a 500mA current limited USB 2 port will result in a considerably higher charge rate than the
generic recommended 1C rate for packs with a capacity lower than 500mAh.
If you're charging multiple packs using one of the cheap Chinese USB dongles capable of doing that, and if you accept the
possible risk of the high charge rate issue I've just raised, the USB charger used as the voltage and current source will be the cause of battery charge times beyond the minimum allowed by the battery protection circuit if it is not capable of supplying 1A times the number of batteries being charged simultaneously (eg., 2A for two, 4A for four, etc.).
ANY USB charger of any kind being used for such a purpose should be closely monitored for high temperature during battery charging, at the very least on its first use while under the maximum charging load it will ever experience.
Whose regulation(s) might require this sort in-battery protection is unknown to me, but I definitely agree with it. The 2011 meeting minutes of the ANSI CI8 Subcommittee on Portable Cells and Batteries found on the CPSC website suggests that standards for lipos used in consumer toys be included in ASTM F963-11 Standard Consumer Safety Specification for Toy Safety, a document which costs $74.00. Those meeting minutes don't indicate what those standards might be.
