Designing and Making a 4S Battery Protection Board

Dodgy over discharge voltages

The auto activation version

I needed a battery protection board for my battery powered tube amplifier, I decided to buy several battery protection boards from Ebay, the reviews looked good and most of the boards seemed very popular.
Once I received and tested the boards I noticed a few problems:
All of them were using the ABLIC S-8254A IC, but the product code did not match the manufactureres datasheet, this led me to believe these ICs were fakes.

The over discharge cutoff voltages were very low, they were mostly 2.5v.
All apart from one board had to be activated by applying a charge current.
None of them had any mounting holes or headers.
Most were marked as battery management systems, they had no BMS function.

My biggest concern was the over discharge cutoff voltage of 2.5v, this is too low for most batteries and would damage the batteries. I was not planning on using the boards for charging and noticed the boards had a large tolerance on the over charge cutoff voltage, again this is not good.

I decided I did not want these pieces of crap in my project so I designed my own circuit with high quality components.

I designed two different version, the smaller circuit requires manual activation, the bigger circuit is auto activated by a relay when the batteries are connected.

The components

ABLICs part list

The IC used is the ABLIC S-8254A, the version I chose is AWFT, this has a over discharge cutoff voltage of 3v. This is slightly above the batteries I'm going to use which is perfect.

Capacitors are C0G where posible, otherwise they are all X7R, the reason for using C0G is so there is not too much value change, as the voltage detections are based on timings using the capacitors.
It's the same with resistors, I've used 1% metal film with a temperature coefficient of ±100ppm/℃.

A small Omron 12v signal relay is used to activate the circuit.

The schematic

Standard Version

Standard Version - No Auto Activation:

The schematic is based on ABLICs datasheet, all default component values are supplied, the explanation on changing these values to achieve different cutoff voltages is very poor. However the default schematic is more than adequate and as I chose a specific IC, I got my desired over discharge cutoff voltage.

Lower power mosfets can be used if high output current is not required.
A power indicator LED has been added, this is optional.

Auto Activation Version:

Auto Activation

This version has an additional circuit consisting of five transistors, these work with each other to delay activate the relay after the batteries are connected, two seconds laters the relay is turned off. The relay contacts when on short B+ with VMP.

The delay timing is done with capacitors C1 and C2, solid capacitors were chosen to ensure values don't change too much over time so delay timings remain unchanged.

Auto Activation Version

There is an addional LED that briefly turns on when the relay activates.

Both schematics have NETS for doing a manual auto activation using a switch.

The calculations

The only calculations made were for the LEDs and for the transistor base resistors.

IB = Base Current
IC = Load Current (the relay used here uses about 9mA, we'll use a value of 25mA)
RB = Base Resistor
VIN = VCC (16.8v Max, 12.8v Min)
VBE = Base Emitter Voltage
β = hFE

Q3 to Q6 - BC846

   β = from specsheet: 200 (this is the gain and is also known as hFE)
   VBE = 0.7v

Q7 - MPSA92

   β = from specsheet: 25
   VBE = 0.7v

The base resistors for Q3 to Q6 are not too important as their load current is very low and they have a very high gain, so we can just choose a 100kΩ resistor which will be more than enough to saturate them.
This value also determines the additional quiescent current of the relay circuit (16.8v / 100,000 = 0.00016A).

To calculate R10 for Q7 we first need to work out the required base current (IB), to guarantee full saturation I added an extra 30% by multiplying the result by 1.3:

                   IC                        0.025 mA
   IB = ______ x 1.3 = __________ x 1.3 = 0.0013 mA
                    β                                25

Now that we have the IB we now can work out RB (R10):

                    VIN - VBE              16.8 v - 0.7 v
   RB = ___________ = _____________ = 12.38 kΩ
                           IB                       0.0013 mA

A value of 10kΩ will be fine, the relay is only on for a couple of seconds.

High resistance values have been used for the LED resistors R12 and R13, as I did not want them to be too bright.

The PCB

Standard design, no auto activation, front

The first version is 58mm long by 20mm high. All components are one one side with the JST connectors on the rear.

The second board has some additional surface mount components on the rear along with the JST connectors.

Standard design, no auto activation, back

Right angle header pins are used for mounting both the boards.

With auto activation, front

With auto activation, back

Bill of Materials

Name	Designator	Qty	Manufacturer Part	Manufacturer	Supplier	Supplier Part
1M	RCOP	1	RC0805FR-071ML	YAGEO	LCSC	C107700
2.2uF	CVSS	1	CGA4J1X7R1V225KT0Y0E	TDK	LCSC	C342739
22K	R2	1	RC0805FR-0722KL	YAGEO	LCSC	C114565
17-21/S2C-FR1S2L/3T	POWER	1	17-21/S2C-FR1S2L/3T	Everlight Elec	LCSC	C264419
S-8254AAWFT-TB-G	U1	1	S-8254AAWFT-TB-G	ABLIC	Mouser	628-S-8254AAWFT-TB-G
1K	RVC2,RVC4,RVC3,RVC1	4	RC1206FR-071KL	YAGEO	LCSC	C131398
AOD403	Q1,Q2	2	AOD403	AOS	LCSC	C28969
100nF	CCCT,CCDT,CVC2 CVC1,CVC4,CVC3	6	CGA4J2X7R1H104KT0Y0N	TDK	LCSC	C415304
0.0005	RSENSE	1	MA251220F0m50SZ	Ever Ohms Tech	LCSC	C429364
XH-2AW	CN-,CN+	2	XH-2AW	BOOMELE	LCSC	C33132
51	RVSS	1	AC0805FR-0751RL	YAGEO	LCSC	C144556
PH-3A RHOS	LMH	1	PH-3A RHOS	BOOMELE	LCSC	C2320
1K	RVINI,RCTL,RSEL	3	RC0805FR-071KL	YAGEO	LCSC	C95781
MTP125-1102S1	H1,H2,RESET	3	MTP125-1102S1	MINTRON	LCSC	C358684
5.1K	RVMP,RDOP	2	RC0805FR-075K1L	YAGEO	LCSC	C84375