Should You Charge or Replace a Weak Car Battery?
3 lessons in this series, 6 min in total
- Signs Your Car Battery Is Dying 2 min read
- How to Test a Car Battery With a Multimeter 2 min read
- Should You Charge or Replace a Weak Car Battery? 2 min read
Sources: Optima, Interstate and AAA guides, NOCO’s GENIUS1 and GB40 user guides, and Battery Tender’s Junior page. More on how we research.
Charge it first when you know why it’s low
A battery drained by lights left on, many short trips or weeks of parking may be fine once it’s full again. Getting it full is the catch: Optima says alternators are designed to keep a nearly full battery topped off, not to recharge a deeply drained one; see does driving recharge a battery after a jump.
Optima advises charging a deeply discharged battery with a charger as soon as possible. Charge it fully, then rest it and measure again. If the reading holds, the drain you named was the likely cause.
For clamp order and safety, see charging a car battery without disconnecting it.
When to test or replace it
| Situation | Do this | Source |
|---|---|---|
| Low for a reason you know | Charge fully, rest, measure again | Optima |
| Low again soon after a full charge | Get a conductance or load test | Interstate |
| Smart charger shows a bad-battery error | Have it checked by a professional | NOCO GENIUS1 guide |
| Three or more years old, with signs | Test it; replace it if it fails | AAA, Interstate |
| Swollen, cracked or leaking | Replace it; don’t charge or jump it | AAA, NOCO guides |
| Won’t start, need to go now | Jump it; that doesn’t recharge or fix it | NOCO GB40 guide |
Interstate says a conductance test takes under a minute, and a load test checks performance under a power draw. It says to replace the battery if its measured cold cranking amps (CCA), its rated starting current in the cold, fall below what your engine needs; see what cold cranking amps measure. A battery can read full and still be bad, so the shop test settles it.
What charger it takes
A battery’s capacity is rated in amp-hours (Ah), and a charger’s rated battery size should cover yours. NOCO rates the 1-amp GENIUS1 to charge batteries only up to 30Ah, and its guide lists about 22.5 hours for a half-drained 30Ah battery.
Battery Tender’s Junior page puts typical car and truck batteries at 35-100Ah. So most drained car batteries need a larger charger than a 30Ah unit.
For the details, see is a 1-amp charger enough for a car battery and how long a NOCO Genius takes to charge.
If the test says replace, match the new battery’s CCA to what your engine needs; if it passes, keep it topped up over winter.
Sources
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- Optima Batteries: alternators are not designed to charge dead batteries
- Interstate Batteries: when your car battery is at death’s door
- AAA: know when to replace the car battery
- NOCO GENIUS1 user guide and warranty (PDF)
- NOCO Boost Plus GB40 user guide and warranty (PDF)
- Battery Tender Junior 750mA product page
