I have the 4Ah battery; you can always keep it permanently charged as I measured it and it charges each cell up to 4.05 volts. This means that the battery charges max up to 80%; you cannot charge it to 100%. They do this on purpose to prevent the battery from aging quickly, and they are right to do so (of course, we have already lost 20% from the battery). Using the company's vacuum cleaner, in 15 minutes, the battery dropped to 1 light (out of the three it has). We always measure the battery under load, meaning the device must be working, and that's how we measure; we stop at 1 light. If you test the battery without load, you get an incorrect reading of 2 lights (be careful with this). Always under load, and the device should have worked for at least 30 seconds before we test! So, at 1 light, the voltage in each cell is 3.6 volts, which is the nominal voltage, and it should never drop below this, and we put it to charge immediately. (At 3.6 volts, there is still another 20% of the battery left anyway; due to low volts, the vacuum's performance would be halved.) So overall, we use 60% of the battery, and in this way, the battery will last over 20 years. More specifically, from the 15 minutes I can operate the vacuum now, with this logic I mentioned earlier, as new as it is now with a new 4Ah battery, in 20 years it will work for 10 minutes and not 15. If you wait for it to turn off to charge it, in the 40 times you do this, your battery will have lost 30%+ permanent capacity!
