Choosing the Right Charger for Your Battery
When selecting a charger for your battery, the most important factor is matching the charger’s output (in amps) to your battery’s capacity (in amp-hours, or Ah).
Step 1: Find Your Battery’s Capacity
Look for the Amp-Hour (Ah) rating printed on the battery label.
Example: 35Ah means the battery can deliver 35 amps for one hour, or proportionally less current over a longer time.
Step 2: Choose the Right Charging Current
A good rule of thumb for most sealed lead-acid batteries (AGM and Gel) is to choose a charger with an output of 10% to 20% of the battery’s Ah rating.
Examples:
-
Small mobility scooter battery – 35Ah capacity → ideal charger output: 3.5A to 7A
-
Large RV house battery – 100Ah capacity → ideal charger output: 10A to 20A
This range balances:
-
Safety: Avoids overheating or damaging the battery
-
Speed: Provides a reasonable charging time
-
Battery life: Helps maximize the number of charge cycles
Step 3: Match to Battery Type
Even if the amperage is correct, make sure the charger is set for your battery type:
-
AGM setting for AGM batteries
-
Gel setting for Gel batteries (uses slightly lower voltage to avoid damage)
In most cases, a standard lead acid automative battery charger will work with AGM batteries, but not for Gel batteries.
Step 4: Consider Charge Time
You can roughly estimate charge time:
Charge Time (hours) ≈ Battery Ah ÷ Charger Amps × 1.2
(The ×1.2 accounts for charging inefficiencies.)
Example:
A 50Ah battery with a 10A charger: 50 ÷ 10 × 1.2 ≈ 6 hours for a full charge from empty.
|
Battery Capacity (Ah) |
Recommended Charger Output (A) |
Approx. Full Charge Time* |
|---|---|---|
|
1–10 Ah |
0.5–2 A |
5–15 hrs |
|
12–18 Ah |
1.2–3.6 A |
5–12 hrs |
|
20–30 Ah |
2–6 A |
5–10 hrs |
|
31–40 Ah |
3–8 A |
5–9 hrs |
|
41–60 Ah |
4–12 A |
5–9 hrs |
|
61–80 Ah |
6–16 A |
5–8 hrs |
|
81–100 Ah |
8–20 A |
5–8 hrs |
|
101–150 Ah |
10–30 A |
5–7 hrs |
|
151–200 Ah |
15–40 A |
5–7 hrs |