Is My SLA Battery Still Good? How to Test and When to Replace It
Is your battery still good? It's a straightforward question — but unfortunately, it doesn't have a straightforward answer.
The State of Health (SoH) of a sealed lead acid (SLA) battery is determined by three separate indicators, and no single test measures all three at once. Understanding what each indicator means — and how to evaluate it — is the key to knowing whether your battery is ready to replace or still has life left in it.
The Three Indicators of SLA Battery Health
1. Capacity — The Most Important Indicator
Capacity measures how much energy the battery can store. It's expressed in ampere-hours (Ah) and is the leading indicator of overall battery health.
When a battery is new, it should deliver 100% of its rated capacity. Here's a simple example:
- A 5Ah battery delivering 5 amps for a full hour = 100% capacity
- That same battery delivering 5 amps for only 30 minutes = 50% capacity
One important quirk of lead-acid chemistry: some lead-acid batteries ship at approximately 80–85% of rated capacity and reach full capacity only after several charge/discharge cycles. Once they peak, capacity begins a gradual, permanent decline.
Unfortunately, there's no quick field test for capacity. Accurately measuring it requires a full charge/discharge cycle — which is time-consuming and not always practical.
2. Internal Resistance — The Delivery Indicator
Internal resistance measures the battery's ability to deliver current when it's needed. As a battery ages, internal resistance rises, which limits how quickly it can release energy — even if the battery appears fully charged.
Testing internal resistance accurately requires a dedicated impedance tester (such as those made by Hioki or Midtronics). A standard multimeter cannot measure internal resistance in SLA batteries.
3. Self-Discharge — The Integrity Indicator
Self-discharge reflects the mechanical integrity of the battery and the cumulative stress it has experienced over its life. A battery that loses charge significantly faster than its rated self-discharge spec — even when sitting unused — may have internal damage or degraded plate condition.
Measuring self-discharge requires a rest-period comparison: fully charge the battery, let it sit disconnected for a set period, then measure voltage again. Faster-than-expected voltage drop indicates elevated self-discharge.
How to Test an SLA Battery
Voltage Test with a Multimeter
A multimeter measures open-circuit voltage, which gives a rough indication of state of charge. For a 12V SLA battery at room temperature, here's what the readings mean:
| Voltage | Approximate State of Charge |
|---|---|
| 12.7V or higher | 100% — Fully charged |
| 12.5V | ~75% |
| 12.4V | ~50% |
| 12.2V | ~25% |
| 12.0V or lower | Discharged — may indicate damage |
| Below 11.8V | Likely damaged or sulfated |
Important: Let the battery rest for at least 2 hours after charging before taking a voltage reading. Surface charge from recent charging will give falsely high readings.
Load Test — More Reliable Than Voltage Alone
A battery load tester is more accurate than a multimeter for SLA batteries. It applies a controlled electrical load — simulating real-world use — and measures how voltage holds up under that stress. A healthy battery maintains voltage under load; a weak one drops significantly.
Load testers are widely available at auto parts stores and online, and are worth the investment if you manage multiple batteries or depend on them for critical equipment.
When to Retire an SLA Battery
Because SLA batteries are sealed and maintenance-free, there is no single definitive test for State of Health. The decision of when to retire a battery depends largely on what the battery is powering.
For critical applications — medical equipment, emergency backup systems, essential military devices — batteries should be replaced conservatively, well before they approach end-of-life. The risk of equipment failure is not worth the savings from squeezing out extra cycles.
For non-critical applications — equipment that can tolerate reduced runtime or where failure is an inconvenience rather than a danger — batteries can be run closer to end of life.
Many organizations set retirement policies based on:
- Cycle count — replacing after a defined number of charge/discharge cycles
- Age — replacing on a calendar schedule (e.g., every 3–5 years for float applications)
- Capacity threshold — retiring at 80% of rated capacity, a common industry benchmark
The Fuel Gauge Problem
Some devices display a fuel gauge showing remaining charge as a percentage. This reads state of charge — not state of health. A fully charged battery that has degraded to 60% of its original capacity will still show "100%" on that gauge. The device will simply stop working sooner than expected. Don't rely on a fuel gauge to assess battery condition.
Give Old Batteries a Second Life
If a battery is no longer reliable enough for a critical application, consider whether it can still serve a non-critical one before sending it for recycling. A battery at 70% capacity may be perfectly adequate for a low-demand device.
SLA Battery Recycling
When a battery has truly reached end of life, recycle it — don't throw it in the trash. SLA batteries contain lead and sulfuric acid, both of which require proper disposal. Most auto parts stores, battery retailers, and municipal recycling programs accept them at no charge. The lead and other materials are recovered and used in the manufacture of new batteries, keeping hazardous materials out of landfills.
Frequently Asked Questions
How do I know if my SLA battery is bad?
The most reliable field test is a load test using a battery load tester. A voltage reading with a multimeter can indicate a deeply discharged or damaged battery (below 12.0V at rest), but won't catch a battery that has lost significant capacity.
Can a multimeter test a sealed lead acid battery?
A multimeter can measure open-circuit voltage, which indicates state of charge — but not state of health or capacity. For a more complete picture, use a dedicated load tester or impedance tester.
How long do SLA batteries last?
SLA battery lifespan varies by application. In float service (like a UPS), 3–5 years is typical. In cyclic service (regular charge/discharge), lifespan is typically rated in cycle count — often 200–500 cycles depending on depth of discharge.
What voltage should an SLA battery read when fully charged?
A healthy 12V SLA battery should read 12.7V or higher after resting for at least 2 hours following a full charge.