FAQ
TL;DR: Discharging a 10 Ah lead-acid battery at 2 × the 10 h rate yields only about 6 Ah (−40 %) [BatteryUniversity, 2023]; “the more current we draw, the smaller the effective capacity” [Elektroda, electro, post #18844998] Ampere-hour shows charge at nominal voltage, whereas watt-hour lets you compare different voltages.
Why it matters: Choosing by Ah alone can halve runtime or damage the battery.
Quick Facts
• Lead-acid starter batteries are usually specified at a 10 h rate; small AGM/gel types at 20 h [Elektroda, sanfran, #18844886; Elektroda, kowal011, #18857847].
• Peukert exponent for fresh AGM ≈ 1.15; capacity drops ≈ 25 % when current doubles [BatteryUniversity, 2023].
• Safe cutoff for 12 V lead-acid is 10.5–10.8 V to limit sulfation [BatteryUniversity, 2023].
• Deep discharging beyond 80 % depth can shorten cycle life by 70 % [BatteryUniversity, 2023].
• Basic battery-protector modules cost €15–40 for 12 V systems [Amazon Price Range].
What exactly does an ampere-hour (Ah) measure?
Ampere-hour is the amount of charge a battery can deliver at its nominal voltage: 1 A for 1 h equals 1 Ah
[Elektroda, stabilizator, post #18844397] It indicates stored charge, not energy, so two batteries with equal Ah but different voltages hold different watt-hours.
How does watt-hour (Wh) differ from ampere-hour?
Watt-hour multiplies Ah by nominal voltage, giving energy. A 12 V 10 Ah battery stores about 120 Wh, while a 6 V 10 Ah one stores 60 Wh
[Elektroda, stabilizator, post #18844397] Wh lets you compare batteries with different voltages.
Why are capacities stated for 10 h or 20 h discharge times?
Manufacturers quote Ah at a chosen rate where chemistry behaves predictably. Car batteries use 10 h; small AGM use 20 h [Elektroda, sanfran, #18844886; Elektroda, kowal011, #18857847]. Faster discharge increases internal losses and lowers effective capacity according to Peukert’s law.
How do I estimate runtime for my 12 V 7 Ah gel battery?
Divide rated Ah by load current at the test rate, then reduce for Peukert. At 1 A you expect 7 h, but practical runtime is nearer 5 h because 1 A exceeds the 20 h rate [Elektroda, Tomek515, #18844654; BatteryUniversity, 2023].
What is Peukert’s law and why does it matter?
Peukert’s law relates available capacity to discharge current: Cₚ = Cᵣ×(Iᵣ/I)ᵏ. A Peukert exponent k≈1.15 means doubling current cuts capacity by ~25 % [BatteryUniversity, 2023]. "Higher current, smaller capacity" captures the idea
[Elektroda, electro, post #18844998]
Is it safe to drain a lead-acid battery below 10.7 V?
No. Voltage below 10.5–10.8 V signals ~100 % depth of discharge. Repeated deep cycles can slash cycle life by 70 % and cause sulfation [BatteryUniversity, 2023]. Use a low-voltage disconnect to protect it
[Elektroda, gold-game, post #18897684]
How do power banks label capacity, and why is it confusing?
Many state cell capacity (e.g., 5000 mAh at 3.7 V) but users draw 5 V. After boost-converter losses, delivered capacity may be only 3200 mAh at 5 V
[Elektroda, sanfran, post #18844450] Always check Wh or output spec sheets.
Why do electric-vehicle packs use kWh instead of Ah?
EV batteries operate at hundreds of volts; Ah alone hides energy content. Listing kWh (e.g., 60 kWh) standardises comparisons like fuel tank size
[Elektroda, sanfran, post #18844450]
How can I size a battery for my load?
- Calculate load power (V × I) or use rated watts. 2. Multiply by desired backup hours to get Wh. 3. Divide Wh by battery voltage, adjust for Peukert (×1.2) and add 20 % reserve. This ensures runtime and protects battery.
What is a battery protector and when should I fit one?
A protector disconnects the load when voltage falls to a preset threshold (≈10.8 V for 12 V lead-acid). It prevents harmful deep discharge, extending service life
[Elektroda, gold-game, post #18897684] Modules cost €15–40 and install in-line with the load [
Amazon Price Range].
Edge case: How does cold weather affect capacity?
At 0 °C, lead-acid capacity drops about 20 % and cranking amps fall 40 % [BatteryUniversity, 2023]. This compounds Peukert losses, so winter runtimes may halve compared with 25 °C.
3-step method: Calculate UPS battery bank for 30 min backup at 600 W
- Energy need = 600 W × 0.5 h = 300 Wh.
- For a 24 V bank: required Ah = 300 Wh / 24 V = 12.5 Ah.
- Apply 1.5× factor for Peukert and ageing → choose ≥20 Ah AGM batteries.
Generated by the language model.
Comments
Ampere-hour as a unit was (and is) used for families of fixed voltage batteries, e.g. on-board batteries in passenger cars, which was closely related to the current capacity of the device - including the... [Read more]
Thank you for developing the topic well and maintaining the description that will facilitate easy understanding of the issue for people who are just starting their adventure with electronics. :spoko: [Read more]
Recently, I was wondering what it is like when I have a 12V 7Ah battery. I could get 1A from it for 7 hours. But whether this capacity is given to the safe level of battery discharge, i.e. in the case... [Read more]
From what I can remember from the old days, the value was 10 hours for standard lead-acid batteries. So in the above case 700mA for 10 hours until full discharge - that is, voltage drop to a certain level.... [Read more]
For acid batteries, the so-called Peukert's law which determines the dependence of the available capacity on the current consumed from the cell. In short, the more current we draw, the smaller the... [Read more]
In small UPSs, the batteries have difficult operating conditions, the discharge currents are high, as you write, then the available capacity and back-up time are reduced. In larger UPS units, the batteries... [Read more]
For small AGM batteries, or the popular "gel" batteries from UPS, capacity is given as 20 hours. [Read more]
It is worth mentioning that the use of battery 'protectors' is becoming more and more popular. It is about preventing excessive discharge of the battery in a domestic installation, e.g. in a motorhome.... [Read more]