Pros & Cons of CV charging Li-Ion

What are the pros & cons of CV charging Li-Ion all the way?

What I have is a device that normally uses a 4.8V Ni-MH pack but it will accept 8.4V at a lower current. I'm going to use a 2S1P 18650 protected holder with raw cells. This is part of a mod where the charging circuit was modified as well but this might be beyond my old soldering skills. I was wondering the charging circuit puts out 7.5V which is a bit low but still with run down cells it might be enough to slow charge in CV, actually it will only charge partially.

The full mod raises the voltage to 8.4V and increases the current as well. The PCB in the holder cuts off at 8.4V and at 5V low end.

If you just plonk a fixed voltage across a discharged Li-Ion cell, you're taking the risk that the initial charging current will rise so high that it exceeds the safety limits specified by the cell manufacturer. The standard charging profile is CC/CV for a reason.

On the other hand, if you're just talking about supplying a fixed, but lower than rated, voltage to a charging circuit which then charges the cell, you may get away with it, depending on the exact details of the charging circuit. As you say, the cells won't get a full charge, however.

7.5V across 2 cells is 3.75V each, which might get you a 50% state of charge, if you're lucky. That doesn't take account of circuit losses and the need to balance the cells, either.

Some charging circuits don't start to balance the cells in a pack until the first cell gets close to full charge. With only 7.5V to work with, that might not happen, which could mean that the less well-charged cell would degrade under repeated cycles to minimum discharge voltage. In the worst case, the cells could get so far out of balance as to render the battery pack unusable.

Personally, I would do it the right way and be done with it.

They'll be in a holder that has a PCB so no danger there. Still waiting on the holder so I rigged up two 18650s in series which worked fine but I plugged in the wallwart/charger the load on the batteries increased by just under a 100mA which I wasn't expecting. The modder didn't explain in detail but this might be the reason that the charging circuit was modified. Running on battery only isn't a problem for me.

In "real" use with the PCB, it cuts out at 5V so if one were to plug in the charger then even if it tried to draw more current from the cells the PCB would stop it and it would just run off AC supply or at least it would seem so.

The right way is to use a 4.8V Ni-CD or Ni-MH pack. A OEM 3Ah Ni-MH using SC cells runs about $111 but I have been getting an RC pack builder to build a 5Ah Ni-MH pack for $25 and I solder in the wiring harness. This would have been my third pack.