FAQ
TL;DR: Discharging a 3 S Li-ion pack below 7.5 V can cut capacity by 50 % [BatteryUniversity, 2023]. "Keep quiescent current under 10 µA to avoid self-drain" [TI, 2021]. RAF96’s discrete board costs ~PLN 60 overall [Elektroda, RAF96, post #19784046] but draws ~1 mA [Elektroda, krzysiek_krm, post #19784396]
Why it matters: Proper protection maximises runtime, cell lifespan and safety in DIY battery packs.
Quick Facts
- Board material: 2-layer FR-4, 1 oz copper, 100 × 100 mm max size [Elektroda, RAF96, post #19784046]
- PCB price (5 pcs, slow shipping): approx. PLN 30 ≈ US$7 [Elektroda, RAF96, post #19784046]
- Component cost (THT): approx. PLN 30 [Elektroda, RAF96, post #19784046]
- Quiescent current: ~1 mA with indicator LED, <0.3 mA without LED [Elektroda, RAF96, post #19787785]
- Ready-made 3 S BMS with balancer: PLN 7–15 on AliExpress [Elektroda, slawko_k, post #19791174]
What cutoff voltage should I set for a 3 S (12 V) Li-ion pack?
Set the trimmer so the MOSFET opens when pack voltage falls to 9.0–9.3 V (≈3.0–3.1 V per cell). This stays above the 2.5 V absolute minimum yet uses most capacity [BatteryUniversity, 2023].
How do I adjust RAF96’s potentiometer?
- Connect a bench supply set to 10 V.
- Turn the multi-turn screw until the board just disconnects the load.
- Lower supply 0.1 V, verify cutoff remains; raise to 9.5 V, confirm auto-reconnect [Elektroda, RAF96, post #19784046]
Why is the board’s standby current so high compared with dedicated ICs?
Discrete TL431 comparators and LED consume ~1 mA. Integrated protectors such as TI BQ297xx draw 2–6 µA, 99 % lower [TI, Datasheet 2021]. Power loss is negligible for packs recharged daily but can drain a fully discharged pack in ~10 days [Elektroda, krzysiek_krm, post #19784396]
Can I add over-temperature protection?
Yes. Insert a 10 k NTC thermistor in a divider feeding an extra TL431. When the NTC reaches ~60 °C, the TL431 pulls the MOSFET gate low, disconnecting the pack [Elektroda, LightOfWinter, post #19784016]
Is a balancer necessary?
Without cell balancing, one weak cell can hit 2.5 V first and reverse-charge, damaging it within tens of cycles [Elektroda, Anonymous, post #19786845] RAF96 plans to add an active balancer module, solving this [Elektroda, RAF96, post #19787785]
What protects against short-circuit or over-current?
A 10–15 A fast-blow fuse sits next to the cell holder; it disconnects the pack if current exceeds its rating [Elektroda, RAF96, post #19895711] Fuse response is milliseconds, limiting wire and cell heating.
Why use SMD parts instead of through-hole?
SMD shrinks area by 60–70 % and height by 80 %, letting the board fit inside most 18650 battery tubes [Elektroda, LightOfWinter, post #19784016] It also reduces trace inductance and cost in volume.
How much does it cost to build one board today?
PCB: PLN 6 per piece (5-up order at PLN 30) plus ~PLN 12 tracked shipping [Elektroda, zgierzman, post #19784886] Components: PLN 30. Total ≈ PLN 48 (≈ US$11).
Are there ready-made IC solutions?
Yes. TI BQ76920 or S-8261 handle UV, OV, and OC with <5 µA standby. They need only two FETs and three resistors, cutting BOM to seven parts [TI, AppNote 2021].
What happens if I leave the pack disconnected for months?
At 1 mA self-drain, a 2500 mAh pack discharges 1 % every 25 hours. After three months it would be empty and risk copper dissolution, an internal short-circuit edge case [BatteryUniversity, 2023].
Comments
Hello What immediately catches the eye - the use of through-hole components instead of SMD. It is a pity, the board could be smaller and much simpler. There are also a lot of tall elements, which also... [Read more]
Of course, the circuit could be on SMD elements, but for now I'm at the design level on THT elements. The thermistor could be added by adding one more TL431 chip with a few resistors to the project.... [Read more]
With all due respect, building such a system is actually pointless in my opinion. There are ready-made "hardware secondary protection" circuits, which are tiny, quite cheap, with a handful of small elements... [Read more]
PLN 30? As I checked a month ago, the delivery itself cost about PLN 45 - PLN 60. What shipping option have you chosen that the costs are so low? greetings AND. [Read more]
It has become a bit more expensive recently. I paid $ 12.66 for 10 100x100 tiles at the end of September, which is about PLN 50 at today's rate. Shipping $ 7.66, or about PLN 30. As of today, the cheapest... [Read more]
On the other hand, you can build a protection system from a handful of small elements (6-7), where the largest is the mosfet that cuts off the power supply. Somewhere after this forum there is a protection... [Read more]
Without a balancer? The cells in the package will be consumed quickly. [Read more]
@ RAF96 the plates were made very nicely and if you want to make them in THT technology, do them. I hope that someday you will start making projects in SMD technology. [Read more]
Active balancer will be added, it came yesterday, no problem to apply it. @krzysiekkrm If we give up the light that lights up when the protection is connected to the battery, this current will drop even... [Read more]
It's nice that someone else does something on their own. I used to like to design and do these things myself. Now, in the era of Chinese bms modules with a 3s balancer for PLN 7 on ali and PLN 15... [Read more]
Just here THT is very well designed. I don't see the need for SMD everywhere. If the author wanted to do a spider and pour resin over it, that's great. [Read more]
And do a security greater than the package itself, because there is no need to push smd? A completely pointless approach. If there were to be a bms and a charger there, maybe, but such a size is absolutely... [Read more]
Are we still talking about amateur projects? I don't think it matters here. [Read more]
Does amateur mean that it should have no utility and may not be thought out? A bit of logic in that. As an amateur, you can do something really cool that can be used normally. Reduce a bit, a little smd... [Read more]
The main purpose of the protection circuit is not to protect the battery from under-voltage (UV) conditions. Li-ion battery protection is mainly used for over-voltage (OV) protection during charge and... [Read more]
The fuse, located right next to the basket with cells, protects against short-circuit and the flow of too much current. [Read more]