battery-charginglipo
I have a 3 cell lipo connected in series and I want to charge them without removing them from circuit.
I only have single cell chargers so I can only charge one at a time or charge them in parallel but if I don't remove them from the circuit, charging them in parallel will short the contacts. but what if I use 3 single cell lipo chargers? Is it possible for me to charge these lipos while they are still connected to the main circuit (while circuit is open)
The circuit you show is essentially OK in general operation but has the significant disadvantage that, as described, it deep cycles both batteries and reduces batterry life, even if solar energy available > required load energy. You can largely overcome this by cycling when Vbat falls only slightly. The changeover switch can be automated using 2 x MOSFETs and a comparator.
If switched manually you will get a changeover "blip" which the 50 uF will not do a lot to limit. 50 uF will drop 1 volt in 50 microseconds at 1A so in 25 uS at 2A. Your switch would need to be "rather fast" to achieve this. The comparator plus two MOSFETs as switches solution 'fixes' this with a suitably fast changeover. However, there are potentially better ways.
If you connect the battery to the load via an on off switch and the charger to the battery directly (assuming an internal diode or equivalent) the circuit will work OK in most cases. Problems might occur if you some advanced charging system (eg MPPT of some flavours) but in most cases there should be no problem. There may be some interesting "boundary conditions" (see below).
Consider:
Assume:
Charger sees load of semi-charged-battery + 2.5A = > 2.5A.
Charger "does what it can" and supplies 2.5A.
Battery charges in CC mode at 2.5-5 = 0.5A.
System voltage = battery voltage will rise as battery charges.
When /if battery reaches max voltage (ie Vbattery = say 4.2V) it will revert to CV tail (4.2V)
Boundary condition: As mentioned above, the presence of the load current hides the battery "tail current" state from the charger. As follows:
Charger now sees 2A load current + battery CV tail current.
As the load current swamps the battery tail current the charger will never "trip" the battery charging and charging will continue indefinitely as long as battery tail current + load current > 500 mA. If you charged the battery "all day long, everyday" in this mode and I load always > 500 mA then the battery would get damaged. But if load is occasionally removed or reduced to<< 500 mA the charger will trip off.
How much this matters depends on load and charging characteristics and a look at the typical load vs time situations will allow you to assess what should be done.
There are various "work arounds". One easy one is to stop charging at Vmax with bo CV current tail. This reduces available battery capacity t about 80%-90% of what you'd otherwise get - and usefully extends battery cycle life.
The switch changeover system is not without its bad effects on the batteries.
If you discharge to say 3.3V you are effectively doing multiple deep discharge cycles and battery cycle life will be low.
If Iload < Icharger_available then you COULD run the load on the solar charger with the bttery uninvolved. However, the changeover switch system does not account for this and cycles the battery unnecessarily and reduces its life.
The following is an "idea starter" only.
When Vp[v is high enough it will supply the load directly.
If Vpv is high enough and if the battery requires charging it will also charge.
Diode D1 is shown as an 1n5819 but ideally would be a MOSFet arranged as a "zero voltage drop" diode. If panel does not provide enough power for load battery will controbute.
This circuit "needs work" but has the makings of a useful system.
simulate this circuit – Schematic created using CircuitLab
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The cell balancing is performed by a specific circuit a BMS (battery management circuit). Balancing allow to protect a lowest cells from venting and thermal runway. Your configuration is 25S4P, so using a BMS to control current and voltage is the best solution. If you use 100 single cell charger, it will be difficult to control them, since they have different Stat of charge, so the weakest will be rapidly damaged.
Best Answer
With 3 x LiPo in series and no load connected, charging them one at a time is entirely acceptable.
The charger need to be truly floating with respect to the battery when disconnected so that any one cell can be accessed at a time without interaction.
You would want to avoid having to use the combination while batteries were in a different state of charge - although even this would do no harm as long an no cell was discharged fully. The danger in discharging a series LiIon battery combination with different states of charge is that exhaustion of any one cell is not readily detected without monitoring each cell individually. If you DO monitor each cell for minimum voltage on discharge and stop discharge when this occurs, then even discharging a differentially charged battery is acceptable.
As you are going to need to connect to the cells one at a time, you could consider making up a switchable connection to the charger, and then rotate between charging cells reqularly. The excessively enthused [tm] could could easily automate this with eg relays (or electronic switches if more venturesome.)
Swapping between cells every few minutes say should still produce an acceptable charging pattern due to the well behaved nature of Lithium chemistry cells when charging (as opposed to eg NimH where this would be a bad idea. For the first 70%-80% of charging from fully discharged the call is in constant current more and then changes to constant voltage, decreasing current when maximum voltage is reached. Both these stages would be well handled even if you swapped batteries every few minute.
Parallel charging with a single cell charger is not possible without isolating the cells electrically. If the charger is not capable of providing more than Imax for any one cell then there would be little advantage in doing so compared to the occasional rotation method mentioned above.
If the charger was capable of providing say 3 x Imax then parallel charging would be faster than one at a time BUT current balancing would be absolutely essential.