Any thoughts on what can be improved??? Or what happened?
Are you really sure that is the 'whole setup' as in all the connections ?You certainly do need discharge protection that cuts of the batteries when they get to circa 3.0v, letting that number of batteries go down to 0V and then attempting a recharge is asking for trouble.
You can't check the resistance of a battery with a meter. A meter on resistance range puts a voltage across what it assumes is a passive resistance. But a battery is creating its own voltage so you get nonsense results and may damage either battery or meter.
Thank you Steve, this is exactly the kind of information I'm looking for. So this would mean that when the resistance is showed as 0 on the meter the batteries are in the state of rebalancing?
And from the minimal description of your powerbank thingy I can't tell anything about what it is doing. It looks like its main purpose is to provide a 5V 2.1A supply from a 3.7V battery. If it does anything else like charging and how good its charge algorithm is there's no information on.
Although it's normal for a lithium battery to recover some voltage when it has been on load and the load is removed it shouldn't ever go as high as 4.3V unless you have been overcharging it.
BTW for all practical purposes you can treat any number of Li-ion batteries connected in parallel as though they were one large battery.
Hmmm, the batteries were not being charged at that moment but they were fully loaded couple of days before. And once I used the sim800 to connect to the network the voltage started to increase slowly at the rate of about 0.1V/30sec.
So there are no issues like balancing when they are connected in parallel ? e.g. one has 4.1V while the other have 3.7 so the 4.1 one transfers energy to 3.7 ones
Charged - the voltage of a fully-charged LiPo cell is 4.20V, and charging above this will damage the cell. Nominal - this can be considered a sort of "half-charged" voltage, as it is 3.70V, in between charged and discharged. Nominal voltage is what manufacturers use when describing the voltage of their batteries.