Correctly manufactured lithium cells & battery packs should all have Charging Cutoff built into the battery. This requirement is due to the fact that Lithium chemistry batteries cannot accept overcharge or undercharge conditions. When a full charge is reached, the charging current must be cut off. A continuous trickele charge would cause plating of metallic lithium inside the cell & compromise safety.
All batteries should include protection circuitry to prevent occurence of the following conditions:
Lithium Ion chemistry batteries operate safely within their designed operating limits; however the cells become unstable if indavertently charges to a higher than specified voltage. Prolonged chaging above 4.3v on a cell designed for 4.2v will plate metallic Lithium on the anode. The cathode becomes an oxidizing agent, loses stability & produces Carbon Dioxide. The cell pressure rises & if the charge is permitted to continue, the internal cell pressure rises to the point that venting occurs – sometimes with flame.
Cell undercharge produces a similar instability in Lithium cells. Once the cell discharges below the minimum limit (around 2.7-3.0v for modern cells), the Copper current collector begins to corrode. Copper ions dissolved in the cell electrolyte can penetrate the cell separator causing internal short circuits. When the cell is recharged, these high-resistance paths through the separator cause high internal currents to flow, heating up the cell. A severe enough short circuit can cause rupture of the cell as for overcharging, as above.
Here at CASS Industries we have extensive experience in testing Lithium battery units for the required built in safety protections under the regulations.