FAQ
TL;DR: 12 V gel batteries that drop below 10.8 V lose about 50 % usable capacity [Elektroda, Rzuuf, post #15581136]; "charge at ≤0.1 C or it dies" [Elektroda, rafbid, post #15582025] Revival is possible with ≤0.4 A and 14.4 V but success rate under 30 % [Battery University, 2023]. Expect hours of trickle charging; use a small 5 W bulb or current-limited PSU to stay safe.
Why it matters: Mis-charging can vent corrosive acid, fry electronics and waste time.
Quick Facts
• Safe charge current: 0.1 C = 0.4 A for a 4 Ah gel cell [Elektroda, rafbid, post #15582025]
• Float/stand-by voltage: 13.5–13.8 V at 20 °C [Battery University, 2023]
• Equalization/desulfation ceiling: 14.4–14.7 V, ≤0.05 C current [Gelco Services, 2013]
• Self-discharge: Approx. 3–20 % per month, temperature-dependent [Gelco Services, 2013]
• New 4 Ah scooter battery price: 30–50 PLN [Elektroda, luke666, #15581340; ArturAVS, #15583317]
Can a 4 Ah gel battery that reads 4 V be saved?
Only sometimes. Voltage that low indicates deep sulfation; less than one in three such batteries regain over 80 % capacity after careful desulfation [Battery University, 2023]. Several users called the pack “scrap” [Elektroda, Rzuuf, post #15580985], yet the thread starter revived it enough to crank a scooter [Elektroda, matusp37, post #15583791] Expect reduced life and sudden failure.
What current should I use to charge a small gel battery safely?
Use 0.1 C. For a 4 Ah battery that is 0.4 A maximum [Elektroda, rafbid, post #15582025] Lower current (0.05 C) is advisable once voltage falls below 10.5 V to limit plate damage [Elektroda, jdubowski, post #15581284]
Is it okay to apply a fixed 13–14 V from a bench supply?
Yes, if you first limit current. Without limitation the battery drew 2 A and overheated [Elektroda, matusp37, post #15580955] Set the LM350 PSU to 13.8 V and add a 0.4 A current-limit resistor or bulb. Over-voltage above 14.8 V can create gas pockets and rupture the gel separator [Battery University, 2023].
How can I limit charging current with household parts?
Insert a low-wattage automotive bulb in series. A 5 W/12 V lamp limits current to about 0.4 A [Elektroda, jdubowski, post #15581396] A 55 W headlamp allowed 2 A—too high [Elektroda, matusp37, post #15581392]
Does an LM350-based adjustable PSU make a good charger?
It can. The LM350 regulates up to 3 A and allows you to set both voltage and current. Wire a 1 Ω sensing resistor in series with its output to cap current at 0.4 A, then adjust voltage to 13.8 V for float or 14.4 V for desulfation. Provide heatsinking; the regulator will dissipate (Input–13.8 V) × 0.4 A.
What happens when a 12 V SLA drops below 10.8 V?
Lead sulfate crystals harden, capacity plunges, and internal resistance rises. Many manuals mark 10.8 V as the absolute discharge limit [Elektroda, Rzuuf, post #15581136] Repeated deep discharges can shorten life by 80 % [Battery University, 2023].
How fast do sealed lead-acid batteries self-discharge?
Typical self-discharge ranges from 3 % (cool storage) to 20 % (hot climates) per month [Gelco Services, 2013]. One forum test measured 0.4 % per day during peak summer heat [Elektroda, rafbid, post #15591657]
Is anecdotal recovery after ‘zero-volt’ discharge credible?
Rare, but possible. A user reported a car battery twice drained to 0 V yet still usable three years later [Elektroda, WędkarzStoLica, #15581151]. Such cases hinge on immediate re-charge and robust plate design; most gel cells lack that margin, so plan for earlier replacement.
What costs more: regeneration time or buying new?
A new 4 Ah scooter battery costs about 30–50 PLN [Elektroda, luke666, #15581340; ArturAVS, #15583317]. Regeneration demands hours of monitoring and electricity; labour value usually exceeds the purchase price, especially if the revived battery fails unexpectedly [Elektroda, ArturAVS, post #15583893]
Three-step DIY desulfation procedure
- Charge at 13.8 V, current-limited to 0.1 C until current falls below 0.02 C.
- Raise voltage to 14.4 V for 2 hours, still below 0.05 C, to dissolve sulfate.
- Rest 12 hours, measure open-circuit voltage; if <12.4 V repeat once. Stop if case warms >10 °C above ambient.
How do I know the battery is beyond recovery?
Fail signs: cannot hold above 12.4 V after rest, delivers <25 % rated capacity at 0.2 C load, or draws >0.2 A continuously during float. Swelling, bulging or acid smell also indicate scrap status.
Does cold storage really extend shelf life?
Yes. At 5 °C, SLA batteries lose only about 20 % charge after 18 months [Gelco Services, 2013]. Lower temperature slows chemistry but never freeze a discharged battery; frozen electrolyte can crack the case and short plates.
Why might a revived battery fail suddenly later?
Deep sulfation erodes plate material; initial capacity returns but micro-cracks remain. A vibration hit or high current draw can dislodge active mass, causing instant open circuit [Battery University, 2023]. "Temporary resurrection often masks irreversible damage" [Elektroda, ArturAVS, post #15583893]
