FAQ
TL;DR: To stop backflow from a 18650 pack without cutting charge amps, use a low-RDS(on) P‑channel MOSFET configured as an “ideal diode.” Diode blockers waste 0.6–0.8 V as heat, while, as one expert notes, “Diodes are great for high‑voltage, low‑current applications.” [Elektroda, Anonymous, post #21684758]
Why it matters: You’ll protect the charger and keep full charging current with minimal voltage loss—ideal for Li‑ion packs used in tools, e‑bikes, and DIY power banks.
Quick Facts
- Typical silicon-diode drop: 0.6–0.8 V, which raises heat and lowers effective VCC. [Elektroda, Anonymous, post #21684758]
- P‑channel MOSFET with very low RDS(on) minimizes loss; check current, voltage, and max VGS. [Elektroda, Anonymous, post #21684752]
- “Ideal diode” behavior = forward conduction, reverse blocking, near-zero drop using a MOSFET. [Elektroda, Anonymous, post #21684753]
- Alternatives mentioned: diode, MOSFET, or load switch, chosen by voltage/current and efficiency needs. [Elektroda, Anonymous, post #21684758]
How do I stop reverse current from my 18650 battery pack to the charger without reducing amps?
Use a P‑channel MOSFET with very low RDS(on) as a high‑side ideal‑diode element. Select ratings for pack voltage and charge current, and verify the MOSFET’s maximum VGS limit. This approach blocks backflow while keeping voltage drop and heat low, so the charger still delivers full current. As one expert advises, “Use a device with a very low RDSon.” [Elektroda, Anonymous, post #21684752]
Can I just use a diode to block reverse current?
Yes, but expect a forward voltage drop that can upset charger behavior. The charger must overcome the diode drop to reach correct termination voltage, which can prevent full charge or alter cut‑off. This adds heat and wastes power during fast charging. An ideal‑diode MOSFET avoids this penalty for low‑voltage, higher‑current Li‑ion packs. [Elektroda, Anonymous, post #21684753]
What is an “ideal diode” MOSFET circuit?
It is a MOSFET-based arrangement that acts like a diode in forward direction and blocks in reverse, with much lower voltage drop. Designers often use a P‑channel device on the high side for simple chargers. “The circuit you are looking for will be one to behave like an ‘ideal diode’.” [Elektroda, Anonymous, post #21684753]
Which is better for low loss: diode or P‑channel MOSFET?
For low‑voltage Li‑ion packs, a P‑channel MOSFET wins. A silicon diode drops about 0.6–0.8 V, which wastes power and heats the system. That loss is significant at multi‑amp charge currents. A low‑RDS(on) MOSFET reduces drop to millivolts at moderate currents, preserving efficiency and charger accuracy. [Elektroda, Anonymous, post #21684758]
How do I choose a P‑channel MOSFET for a 1S (4.2 V max) pack?
Pick very low RDS(on) at your charge current, VDS and IDS ratings above worst case, and gate ratings within limits. “Watch out for maximum Vgs.” Ensure thermal performance suits continuous charge. Lower RDS(on) means less heat and less voltage error at the cell. [Elektroda, Anonymous, post #21684752]
Will a series diode affect Li‑ion charger accuracy?
Yes. The charger will see a lower battery voltage by the diode’s drop and may never reach target CV voltage. That can prevent proper termination or force higher charger voltage settings. This is why designers prefer MOSFET ideal‑diode solutions for Li‑ion. [Elektroda, Anonymous, post #21684753]
Are load switches a viable reverse‑current protection option?
Yes. The thread notes diodes, FETs, and load switches as common options. Load switches integrate control and protection, but check their on‑resistance and current rating. At higher currents, discrete MOSFET ideal‑diode solutions often achieve lower loss than many load switches. [Elektroda, Anonymous, post #21684758]
What does RDS(on) mean, and why does it matter here?
RDS(on) is the MOSFET’s on‑state resistance. Lower values reduce voltage drop and heat during charging. Choosing a device with very low RDS(on) keeps the charger’s current high and the cell voltage accurate. It is a key parameter for ideal‑diode performance. [Elektroda, Anonymous, post #21684752]
Quick how‑to: Wire a P‑channel MOSFET for reverse‑current blocking?
- Place a P‑channel MOSFET high‑side between charger and pack.
- Orient source to charger, drain to battery; tie gate to charger ground through a resistor.
- Ensure VGS never exceeds its maximum; add components as needed to control gate. [Elektroda, Anonymous, post #21684752]
Does a diode ever make more sense than a MOSFET here?
Yes, when voltage is high and current is low. The thread notes diodes suit high‑voltage, low‑current use, where their drop is tolerable and simplicity wins. For 1S–2S Li‑ion at several amps, MOSFETs are usually superior. [Elektroda, Anonymous, post #21684758]
What edge case should I watch for with MOSFET protection?
Exceeding the MOSFET’s maximum VGS can damage the device, defeating protection. Confirm gate‑to‑source voltage during transients and add clamps or dividers if needed. This is a common oversight when adapting chargers or packs. [Elektroda, Anonymous, post #21684752]
I saw “reverse charge” mentioned—does that apply to Li‑ion charging here?
One post describes “reverse charge” as a pulsed method with discharge intervals. This thread’s core solution targets reverse‑current blocking, not alternative charge waveforms. For Li‑ion safety, follow the protection approaches discussed and avoid backflow to the charger. [Elektroda, Anonymous, post #21684763]
