I'm designing a product that will be powered by a single 18650 cell.
Specifically, I am basing my numbers off this cell in particular:
It has a typical full voltage of 4.2V and cut-off of 2.5V.
I've looked through TI, ABLIC, Diode Inc. and a few others for battery protection ICs for single cells and noticed that the OVP parameter for offerings that have a UVLO of 2.5V are typically higher than 4.2V by anywhere from 0.075V to 0.125V. Is this so the chip does not cut out too early when the battery reaches a full voltage of 4.2V?
If so, would an IC with a OVP of 4.325V be safe to use with a cell that has a full voltage of 4.2V?
Sorry if this question has a simple answer, this is my first time designing a full product, and want to understand fully before moving forward with my design.
I am also using the BQ24075 with the BQ27441 for charging and fuel gauge. This is from the reference design of the Battery Babysitter breakout from Sparkfun:
Best Answer
The purpose of a protection circuit in a single-cell pack is to provide backup protection in the case that the charge or discharge circuitry should have some kind of fault. This is why the protection circuit overcharge threshold is a bit higher than the recommended maximum charge voltage. This is normal. The idea is that we don't want the protection circuit to interfere with normal charge and discharge. We want it to operate only if the charge and discharge parameters are exceeded.
One thing that jumps out at me is that the cell you have chosen is a power tool type cell. In other words, it is specifically designed to deliver high power, and can be safely discharged at relatively high rates (e.g., 20A). Does your application require such high currents? If so, careful design is required for the power path. If not, you may consider looking at other cells (so-called laptop cells).
When you select a protection circuit, it is essential that you take the normal charge and discharge rates into account. Most single-cell protection IC's I have seen are not designed to support such high currents. But if you are using the BQ24075, then it seems that you are not intending to support high discharge rates. As a side note, the BQ24075 is NOT a charge protection IC. Just google "Seiko charge protection IC" to find examples of what a protection IC is.
I am not sure if it is still the case, but for a long time, Seiko basically owned the market for single-cell protection IC's. They have thousands of parts and you can fine tune over and under voltage cutoff points over a wide range. Someone commented that protection IC's are normally used by battery pack vendors, and this is true. But I guess there is no reason why you could not add the protection circuit yourself. But there is more to it than just the protection IC. It is a big topic.
Cell cycle life can be greatly extended if you keep your max charge voltage to 4.1V or so, in which case, you could possibly use a protection circuit with a 4.2V or 4.25V cutoff. Just make sure the protection IC cutoff is a bit higher than the charger voltage so that they play nice together. If you only charge to 4.1V, you will not get full rated amp-hours from the cell. So it is a trade off. I think you will get around 90% at 4.1V.