FAQ
TL;DR: During cranking, a ~12.6V battery can sag while alternators target ~14.5V; “Adding just a capacitor won't have much effect.” Use a diode-isolated aux battery or a large supercap to hold a 2A load. [Elektroda, stephen Van Buskirk, post #21667200]
Why it matters: This FAQ helps car DIYers keep small 12V devices alive during engine start without resets or brownouts.
- Typical alternator output is ~14–14.8V; resting battery ~12.5–13.2V. Expect voltage dips during cranking. [Elektroda, stephen Van Buskirk, post #21667205]
- Supplying 2A for “a few seconds” needs a very large capacitor; a small auxiliary battery is more practical. [Elektroda, Frank Bushnell, post #21667201]
- Standard silicon diodes drop about 0.9–1.2V at 2–3A; size for load plus charging current. [Elektroda, stephen Van Buskirk, post #21667205]
- Schottky diodes reduce forward drop versus standard diodes, easing heat and voltage loss. [Elektroda, Steve Lawson, post #21667207]
- Paralleling diodes without ballast can cause thermal runaway; prefer one higher‑current device. [Elektroda, Frank Bushnell, post #21667204]
Quick Facts
- Typical alternator output is ~14–14.8V; resting battery ~12.5–13.2V. Expect voltage dips during cranking. [Elektroda, stephen Van Buskirk, post #21667205]
- Supplying 2A for “a few seconds” needs a very large capacitor; a small auxiliary battery is more practical. [Elektroda, Frank Bushnell, post #21667201]
- Standard silicon diodes drop about 0.9–1.2V at 2–3A; size for load plus charging current. [Elektroda, stephen Van Buskirk, post #21667205]
- Schottky diodes reduce forward drop versus standard diodes, easing heat and voltage loss. [Elektroda, Steve Lawson, post #21667207]
- Paralleling diodes without ballast can cause thermal runaway; prefer one higher‑current device. [Elektroda, Frank Bushnell, post #21667204]
Can a single capacitor keep my 2A device alive during cranking?
Not reliably. The starter can drag voltage for several seconds. Delivering 2A that long requires a very large capacitor. A small secondary battery or supercap bank behind a diode is more effective for ride‑through. “Adding just a capacitor won’t have much effect.” [Elektroda, stephen Van Buskirk, post #21667200]
How large would the capacitor need to be for a few seconds at 2A?
Very large—on the order suggested as impractical in the thread. That is why contributors recommend a small auxiliary 12V battery instead, which charges from the vehicle and holds up the load during cranking through a diode. [Elektroda, Frank Bushnell, post #21667201]
Is a secondary battery better than a capacitor here?
Yes. A small motorcycle or alarm lead‑acid battery can supply 2A for several seconds easily. Isolate it with a diode so starter current cannot pull it down. It recharges from the car once the engine runs. [Elektroda, Frank Bushnell, post #21667201]
What diode size and placement should I use?
Place the diode between the main battery/alternator and the aux capacitor or secondary battery plus your devices. Size it to pass your 2A device current and the capacitor charging current—about 3A total in this case. [Elektroda, stephen Van Buskirk, post #21667200]
Which diode type minimizes voltage loss and heat?
Use a Schottky diode with adequate current rating. It has a lower forward drop than standard silicon diodes, helping your devices see higher voltage and reducing heating. “Consider using a Schottky low forward drop high current diode.” [Elektroda, Steve Lawson, post #21667207]
Does a 0.6V diode drop matter for 12V electronics?
Usually not. Automotive devices are designed for a wide range and often run below 12V. With charging at ~14.5V and typical device tolerance, the small drop still leaves enough headroom in normal operation. [Elektroda, stephen Van Buskirk, post #21667200]
Can I parallel small rectifier diodes to handle more current?
Avoid it. Manufacturing differences make current share uneven. One diode can hog current, heat up, drop voltage further, and fail. Use a single higher‑current diode instead, with a heatsink if needed. [Elektroda, Frank Bushnell, post #21667204]
If I must parallel diodes, how do I balance them?
Add small series resistors as ballast. Forward voltage falls about 2.1 mV/°C, so temperature can cause runaway without resistors. Example math in the thread suggests ~42 mΩ per leg for three diodes at 3A. [Elektroda, Steve Lawson, post #21667208]
Will the diode drop prevent charging a small secondary battery?
It reduces charge voltage across the aux battery, slowing charge. With ~14.5V system and ~0.9V drop at 2A, the aux may see ~13.6V, which is sub‑optimal for standard lead‑acid. Expect slower charging performance. [Elektroda, stephen Van Buskirk, post #21667205]
Is a MOSFET “ideal diode” a good idea here?
Not for this case per the thread author who proposed and then withdrew it. The scheme that works for reverse‑polarity protection had a threshold making it unsuitable for maintaining the aux supply here. [Elektroda, stephen Van Buskirk, post #21667205]
What about using a 5F “car audio” capacitor and an SCR?
One suggestion is a 5F or larger audio‑grade capacitor and an SCR that disconnects during cranking, preserving charge for your devices. It reduces diode heating and keeps the cap energized until the starter event ends. [Elektroda, Mark Harrington, post #21667210]
How much current can a typical alternator deliver?
The discussion cites alternators designed for up to about 100A to simultaneously run vehicle systems and recharge the battery. This capacity explains why loads bounce back quickly once the engine is running. [Elektroda, Frank Bushnell, post #21667206]
What small battery type works as the auxiliary source?
Use a compact 12V lead‑acid, such as a motorcycle or alarm system battery. It easily handles a 2A accessory load during cranking and stays charged via the vehicle system through an isolating diode. [Elektroda, Frank Bushnell, post #21667201]
How do I build a simple diode‑isolated aux battery setup?
- Wire a small 12V aux lead‑acid battery in parallel with your devices.
- Insert a diode from the car battery/alternator to the aux battery/device node, oriented to allow charging.
- Secure wiring; add a heatsink if the diode runs warm.
[Elektroda, Frank Bushnell, post #21667201]
Is a 1N4001 rectifier okay for this?
No. A 1N4001 is undersized for the charging and load currents here. Use higher‑current parts like the 1N540x series or an appropriately rated Schottky diode. [Elektroda, Rohit Dubla, post #21667202]
What forward‑drop should I expect at 2–3A through common diodes?
Standard silicon rectifiers often drop about 0.9–1.2V at 2–3A. This affects both device voltage and heat in the diode, so choose ratings and cooling accordingly. [Elektroda, stephen Van Buskirk, post #21667205]
