Compatible batteries, otherwise known as replacement batteries, are sold in their thousands every day for such things as cordless power tools, mobile phones, laptops, etc.
Why do they sell so well? The answer is simple ... Price.
Why are they cheaper? Compatible batteries are mass produced in China and, in general, the quality of the manufacturing leaves something to be desired. This is reflected in the fact that most retailers will offer only a 1 month or 3 month warranty on the batteries. Compare this to the likes of a genuine battery from the likes of BOSCH, who offer an unrivalled 12 month warranty on their cordless batteries.
Why shorter warranties? Well, in our experience, compatible batteries suffer from problems which we normally only see on older, genuine batteries which have been worked hard and/or poorly maintained. In our experience the most common problem with compatible batteries is overheating ... and heat is THE big killer of a batteries cell chemistry. Overheating quickly leads to the battery being rendered useless because it won't hold a charge. Most of our customers who have purchased compatible batteries tell us that their batteries become hot whilst in use or whilst being recharged ... and a high percentage state that they purchased them just a few months ago!
It is also quite a common occurrence that compatible batteries simply will not fit without some sort of jiggery-pokery, if it fits at all. This in itself raises some serious questions about the safety of some compatible batteries.
Conclusion : It is our opinion, and many of our customers, that compatible batteries are only cheaper on the face of it. In the long run they are actually quite expensive.
To add some perspective ... Don't expect a Rolls-Royce when you purchase a Hyundai.
The old adage applies ... You get what you pay for.