FAQ
TL;DR: Field tests show 80 % of lead-acid batteries left under 10.5 V for 24 h never recover [Battery University, 2023]. “Any battery discharged below 10 V is only suitable for scrap” [Elektroda, Anonymous, post #12013100]
Why it matters: Knowing when revival is futile saves time, money and avoids safety risks.
Quick Facts
• Absolute minimum voltage before sulfation becomes permanent: 10.5 V (6 × 1.75 V) [Battery University, 2023]
• Safe revival current: ≤ 0.1 C (e.g., 4 A for 40 Ah battery) [EnerSys Manual, 2022]
• Typical self-discharge of flooded lead-acid: 4–6 % per month at 20 °C [Battery Council, 2021]
• Internal resistance approaches ∞ Ω when a cell strap breaks, causing 0 A charge current [Elektroda, emeryt2, post #12018886]
• Recycling refund in EU: €5–€10 per car battery returned [EU BAT Reg., 2020]
Why does the charger show 15 V but 0 A?
The charger senses open circuit voltage; a broken inter-cell strap or dried-out cell creates infinite internal resistance, so voltage rises quickly while no current flows [Elektroda, ^ToM^, #12014769].
Can a 12 V battery survive a drop to 0 V?
Rarely. Tests show only about 20 % regain usable capacity after deep-zero discharge [Battery University, 2023]. Forum members who tried reported immediate scrap recommendations [Elektroda, Anonymous, post #12012254]
What causes an internal break between cells?
Plate corrosion or vibration cracks the lead strap that joins cells. Corrosion progresses even in storage, leading to a sudden open circuit without mechanical shock [Elektroda, ^ToM^, #12019171].
How can I check if the battery is really open-circuit?
- Connect a 21 W bulb across the terminals.
- Measure current; a healthy 12 V battery should deliver ~1.7 A.
- If current is near zero and voltage collapses, the internal circuit is broken [Elektroda, ^ToM^, #12019171].
Should I parallel-connect another battery to jump-start charging?
Parallel charging can trick simple chargers into starting, but it also forces the good battery to carry most of the current. Use only if you can monitor voltage and temperature every 15 min [Elektroda, matołek, #12012435].
What voltage and current limits are safe for revival attempts?
Set charger to 14.4 ± 0.1 V and limit current to 0.1 C. Stop if battery exceeds 50 °C or fails to take current after 6 h [EnerSys Manual, 2022].
Does sulfation really form in a few weeks?
Yes. Leaving a partially charged battery for four weeks can trigger hard sulfate crystals, especially below 12.4 V open-circuit [Battery Council, 2021].
Is chemical desulfation with perhydrol (H₂O₂) recommended?
No. Hydrogen peroxide reacts violently with sulfuric acid and lead compounds, risking heat and oxygen release. Manufacturers provide no data supporting performance gains [SAE Paper 2019-01-0064].
How long must I drive to recharge after short "burn-outs"?
With a 54 A alternator and 20 A accessory load, a 40 Ah battery needs approx. 40 min at 2 000 rpm to regain the 20 Ah lost during a start cycle [Bosch Tech, 2022].
What maintenance keeps vintage-car batteries healthy?
- Measure voltage monthly; recharge when it drops below 12.4 V.
- Use a 1 A maintenance charger during storage.
- Inspect electrolyte level every six months and top with distilled water [Battery University, 2023].
Do smart chargers fail on zero-volt batteries?
Smart chargers often require 2–3 V to detect polarity. Below that, they refuse to start, showing 0 A [Elektroda, 750kV, post #12014474] Simple linear chargers or bench supplies may work better.
When should I send the battery for recycling?
If after 6 h at 0.1 C the current stays below 0.2 A or a cell fails the bulb test, recycle it. EU schemes refund €5–€10 per unit [EU BAT Reg., 2020].
