In the previous article we see some basic informations about lithium batteries.
Now we want to go more in details and explain my project of a socket plus a battery protection for a 18650 lithium battery: Guard18650.
The first thing I want to share with you is the schematic and then discuss every single component of it:
The most important component is the BQ29700, it manage all functions useful to protect the battery.
It comes with factory default voltage thresholds that determine different fault conditions and these are the most important parameters to consider when you choose this component.
As you can see in the page 4 of the datasheet, the part number BQ29700 has these thresholds:
what does this table mean?
The first important parameter we see is OVP: OVP is the over protection voltage and in this case it means that the battery cannot exceed 4.275 V .
if the battery voltage reach this value the battery will be disconnected from the powered circuit.
The second important parameter is UVP, this is the under voltage protection: battery cannot go down 2.8 V in this case.
the voltage reference for OVP and UVP is read from VSS and BAT pins.
Before proceding with OCC and OCD we must introduce the dual mosfet IC that I used.
Mosfet is useful to interrupt the circuit when a fault condition is detected by BQ29700.
The most important parameters to choose for the mosfet are two:
- Vgs : this must be lower as possible, however it must be low enough to ensure a very low Rds also in the worst case that is when the battery reach the lower possible value (in this case 2.8V).
- Rdson : this must be lower as possible and its value, multiply by the current accross the mosfet, decide the voltage useful to activate the OCC and OCD thresholds
so OCC (overcurrent charge threeshold), according with the table, is set to -100mV and this mean that as soon as this voltage drop will be reach across the path from V- to Vss, the battery will be disconnected from the load.
for ECH8693R(the dual N-channel mosfet chosen) the Rds on(at 2.8V that is the worst case) is about 7 mΩ,
mosfet are two in series so the total resistance beetwen them is 7mΩ + 7mΩ = 14 mΩ so theorically 100 mV / 14 mΩ allow to pass maximum 7.14 A of current before activate the charge current and discharge current protection , but for the parasitic resistance between mosfet and all the traces from V- to Vss, the overcurrent protection start at about 4A so the total resistance between V- to Vss is about 25 mΩ.
like OCC, OCD (overcurrent discharge threeshold) have the same threeshold value of 100mV (with inverted sign because in this case the battery source the current ) so also for discharging the protection start at about 4A.
The voltage reference for both OCC and OCD is read from VSS and V- pins as already mentioned.
Reverse polarity protection
In Guard18650 you can remove the battery every time you want trying a lot of different batteries that have the 18650 package, however, because that is the possibility to insert it uncorrectly, I added an high quality reverse polarity protection (over the basic reverse polarity protection made by R1) based on the P-mosfet DMP2006UFG.
In the configuration shown in the schematic, when the battery is insert correctly the mosfet allow current to flow and due to is very low resistence, about 5mΩ, the voltage drop accross the mosfet will be very low also at high current (at 4A for example it only has a drop of only 5mΩ x 4A = 20 mV)
When the battery is insert uncorrectly the P-mos is close and stop any current to pass through it, protecting battery and circuit from undesired currents.
The last components to analyze are the passive components like resistors and capacitors:
- R1 and C1 make an RC filter to reduce the BAT pin noise; R1 also limits the current during a reverse connection on the system.
- R2 is recommended for a correct voltage reading on V- pin.
- R3 and R4 deplete any charge on the gate-source capacitance.
- R5 and R6 are not populated but with these two footprint, a 0805 and a 1206, you can decide the resistance between Vss and V- and consequently the current when the OCC and OCD must starts (however the total resistance will be higher than 25 mΩ so OCC and OCD will start with current lower than 4A).
- R7 reduce the gate charge current of the mosfet.
In the next article we’ll see some oscilloscope screens for the faults explained in this post.