I'm designing a custom battery pack for a computer with 2S1P alignment. However, I want to be able to add batteries on the fly, as the computer is designed to be working at all times without reboots.
Say, I have 3 (for two cells in series) pins to which I can connect an external battery pack for the computer, making the whole arrangement 2S2P. However, I know that voltage difference shouldn't be too low, else the batteries with highest voltage will charge those with lowest with no current limiting whatsoever. So how could I design such a battery pack? Is there something I can do, like circuit which connects the hotplugged batteries through a resistor for some time, or something MOSFET-controlled?
Ideally, my concept would be 4 sockets for removable 18650 cells (2S2P) which I can either remove or add on the fly without interrupting power supply, so I can replace them – or even add an additional battery pack in parallel through a connector – but I somehow have to take into account the possible problems of interconnecting a 3.5V discharged cell with a 4.2V charged one. Any guidelines on what's the simplest way?
Electronic – Hotplug li-ion in parallel for a custom battery pack
batteriesbattery-charginglithium ionprotection
Related Solutions
Can your 2s charger not charge in normal or 'unbalanced' mode with the 3 wire balance connector not connected? Almost all chargers do.
So why balance charge your 2s battery? The TP4056 chips should ensure that they each charge to the correct 4.2 volts and therefore will be balanced. Or will th TP4056 not work when charging in series?
Even if you connect the balance wires, it should work. All the balance wires do is measure the voltage and then at the end, or near the end of the charge, bleed small current from the higher voltage cell and then recharge the battery up to 8.4v. So can you discharge a small current (usually less than 300mA) from the TP4056 connector? If so, then I believe that it should work. If the TP4056 connector doesnt allow you to measure the cell voltage, then the charger won't even start.
However, I have no experience with the TP4056 so watch it closely if you decide to go ahead!
If you place multiple cells of different capacities in parallel, they act as a single cell with the combined capacity of the two. You should never put cells with significantly different capacities in series, since the lowest capacity cell would discharge before the others. This is also why you should (unlike what the picture you posted portrays) connect cells in parallel when they are not in series, as it helps even out the different cell capacities.
By connecting the 1152 mAh and 871 mAh cells in parallel you would get the equivalent of a 2023 mAh cell, while connecting the 1121 mAh and 931 mAh cells in parallel would yield the equivalent of a single 2052 mAh cell. You could then put these two parallel groups in series, to yield a slightly imbalanced 7.4V 2023 mAh pack.
You should also aquire a 2S balance plug extension wire, cut the male plug off and connect the wires to each parallel group of cells in the pack.
simulate this circuit – Schematic created using CircuitLab
That said, I wouldn't try to salvage old, end-of-life cells from some old laptop when 4 brand new 2000 mAh 18650s can be bought online for less than ten bucks. Just don't fall for the "6000 mAh UltraFire" scams, research online which cells are actually good.
Best Answer
The best scheme would be to not actually be trying to put the battery packs in direct parallel for the reasons that you have cited. A better approach would be to design it so that only one battery would drive the computer through a MOSFET switch. A second battery would be switched in automatically through a second MOSFET switch. A control circuit would manage which battery pack switch was active and auto switch to the alternate battery when the the current battery is depleted. The main point that only one battery MOSFET is ON at a time.
A sensor in the control circuit would provide the indication when the battery became depleted and a switch over was necessary. An LED would be placed by each of the battery packs with it being turned on to alert the user that the battery back that needs swapping out.
You would equip the front end power connection to the computer with some capacitors sized to keep the computer powered through the short time that the control circuit switches off one MOSFET and turns on the other MOSFET.