After prolonged use and heavy wear, four of our six 8 Ah smart batteries gradually stopped charging. After a quick search, we discovered that this is a known problem that can fortunately be remedied quickly.
To do this, you need to make two jumper cables, preferably with flat plugs at the ends. Then take a fully charged battery and the “problem battery” and jumper + to + and – to – and leave it like this for one to two minutes.
Then carefully disconnect to avoid causing a short circuit and put the problematic battery into the charger, which should now start charging again and, in some cases, already start with 2 out of 3 bars. The reason for this is that at least one of the cells inside has fallen below the minimum voltage limit accepted by the charger and the battery therefore refuses to be charged. Bridging briefly raises the voltage of all cells inside to exceed the charger’s limit voltage, allowing it to charge again.
Unfortunately, this is only a temporary solution. After repeated use with our four batteries, they ultimately failed completely.
In addition, here is the YouTube video that helped me understand the problem and make the batteries usable again: