Why cordless phones are made from NiMH batteries instead of lithium batteries?
Change, you must replace the protection of the circuit of lithium batteries (general mobile phones have been installed with protective circuit), find a large capacity mobile phone batteries, better transformation. 18650 or 14500, the battery is best not to use, in general, such batteries are not equipped with protective circuits.
The lithium battery is more fragile, more not overcharge protection circuit and the use of off the shelf is also too simple (just off protection). Even with the protection of overcharge protection, voltage is too high (usually 4.3V or higher, as the last protection measures to be used), and the protection, need to reset is available, it is not suitable for use in here.The solution is to make use of TL431 DIY, a precise high performance regulator tube. Among them, Vref=2.5V, select R1=68k, R2=100k, you can do VKA=4.2V, and the characteristics of very steep. Before reaching 4.2V, the current is very small. Once it reaches 4.2V, it is stable at 4.2V. The excess current is absorbed by TL431, and the maximum absorption capacity is 100mA. The regulator is connected to the battery in parallel to ensure that the battery voltage is no more than 4.2V. Before the battery voltage reaches 4.2V, the circuit has no current flow, and all the current flows into the battery.
Very simple to measure with a multimeter, my home phone, open circuit voltage reached 4.5V, while the battery is charged to 4V, the charging current is still 45mA. For Ni MH battery pack 300mAh, due to the use of the phone are regularly inserted into the power supply, which caused continuous charging, overcharge is so often, up to 0.15C, so the battery will soon die be not at all surprising.The solution is to replace it with a lithium battery. The rated voltage 3.6V is the same as that of the 3 Ni MH batteries.
