Test your batteries.
Understand your power.
Is your solar battery bank worn out, undercharged, or shutting down for another reason? Learn how to investigate with a real load test and useful measurements.
Put the battery to work.
For lead-acid batteries, I use a 500-amp carbon-pile load tester. For lithium batteries, I use a timed, controlled-load test. The important part is knowing what each test tells you—and recording enough information to make the result useful.
Can it carry the load?
A carbon-pile tester places an adjustable electrical load on a battery. Watch the voltage while the battery supplies current. This helps identify batteries that collapse under demand.
A short test does not measure overnight storage capacity.How much energy does it deliver?
A controlled load, a clock, and an energy meter reveal usable energy over the tested operating range. Record any alarms and the reason the test ended.
A shutdown is a clue to investigate, not an automatic verdict.Before you connect a tester
Identify your equipment
- Battery brand, exact model, chemistry, nominal voltage, rated Ah or kWh, and age.
- Number of batteries and their series/parallel arrangement.
- Charging settings, recent charge history, and the problem you are trying to reproduce.
- Battery manual and tester instructions, including voltage range, current limits, test duration, and cooling time.
Stop before testing if…
- A battery is leaking, swollen, cracked, unusually hot, smoking, or has damaged terminals.
- You cannot identify the bank wiring or isolate the battery safely.
- The tester’s voltage range does not include the battery being tested.
- You lack the manufacturer’s limits for the intended test.
For smoke or fire, move away and call emergency services. For damaged equipment or unclear wiring, arrange qualified assistance.
Meter setup: use the voltage input and DC-voltage range to measure voltage. Never place a multimeter in its current-input mode across battery terminals. Use an appropriately rated DC clamp meter or installed shunt for current measurements.
Your charge controller can tell a story.
Many Big Island off-grid homes still run lead-acid batteries with OutBack charge controllers. Those controllers can be instrumental in troubleshooting. I recently helped a customer by phone and diagnosed failing batteries using her charge-controller log. Reading the history together can be an effective first step before bringing out a tester.
Read the FM60 / FM80 daily history
On the FLEXmax 60/80 status screen, press the second button from the left for the summary. The third button steps back through earlier days. Photograph several complete days. Do not clear the history. Return with the first button when finished: the manual warns that leaving the summary displayed prevents the next morning’s counter reset.
| Label | Meaning |
|---|---|
| MAX / MIN | Highest / lowest battery voltage recorded. |
| ABS / FLT | Accumulated absorption / float time. |
| Ah / kWh | Controller output accumulated during the log period. |
| Vp / Ap / kWp | Peak PV input voltage / output current / output power. |
These instructions apply to the FM60/FM80 manual, Revision D, pages 20–21. Other models or revisions may differ. Logs cover up to 128 days; a new day starts at Wakeup, not midnight. OutBack FLEXmax 60/80 owner’s manual (PDF).
How I use the readings to narrow the problem
The following are diagnostic clues, not automatic pass/fail rules. Compare similar days and confirm the actual battery charging requirements.
| Pattern to investigate | What it can suggest | What to check next |
|---|---|---|
| On repeated days, charging voltage stays below the appropriate absorption target and absorption time is absent. | The bank may be chronically undercharged. | Weather, shading, array output, daytime loads, controller settings, and temperature compensation. |
| The bank reaches the charging target quickly, yet voltage falls rapidly under normal loads later. | Reduced usable capacity or poor charge acceptance may be involved. | Battery age, actual load, charge settings, connections, specific gravity if flooded, and individual load-test results. |
| Float is recorded, but evening runtime keeps shrinking. | Completing the programmed charging sequence does not establish healthy storage capacity. | Whether the settings fit the batteries, charging truly completed, and household consumption has changed. |
| A very low minimum appears on an otherwise normal day. | A sustained discharge or a short voltage dip may have occurred. | Appliance starts, inverter events, wiring voltage drop, and measurements during the symptom. A daily minimum alone does not show how long the dip lasted. |
| Daily solar kWh falls while nighttime performance worsens. | Reduced available charging, curtailed charging, or poor battery acceptance are possibilities. | Compare weather and loads; check array/controller operation and the rest of the log before deciding which component is responsible. |
What to have ready when you call me
- Clear photos of the status screen and about a week of daily summaries from each controller, identified by controller.
- Battery model, age, number of batteries, and 12-, 24-, or 48-volt bank configuration.
- Existing absorption voltage/time, float setting, and whether a battery temperature sensor is installed. Read settings without changing them.
- Cloudy versus sunny days, generator charging, unusual loads, and when the power fails.
- Any inverter low-battery messages and what was running at that moment.
With multiple controllers, match the actual days and record any resets or outages before comparing totals. If their present battery-voltage readings disagree, have the readings checked against a reliable meter at the appropriate points before changing calibration.
The next step depends on the pattern: correct a charging problem, investigate a connection, or confirm suspected battery weakness with the appropriate load or capacity test. You can gather the display information without opening the battery enclosure or disconnecting cables.
Lead-acid: the carbon-pile load test
The goal is a repeatable test of each individual battery under comparable conditions. A weak battery can be hidden by a bank’s total voltage. Comparing individual results helps narrow the problem.
This is a 500-amp carbon-pile load tester.
Photo: Associated Equipment model 6034, a 500-amp tester rated for 6- and 12-volt batteries. Shown to identify the type of tool; this is not a claim that Doug uses this exact model. Read its operator’s manual (PDF).
What you are looking at
- AMP gauge: shows the current the tester is drawing from the battery. Watch this gauge as you adjust the load.
- VOLT gauge: shows how well the battery holds voltage while supplying that current.
- Load knob: adjusts the carbon-pile resistance and therefore the load. The knob is not a battery charger control.
- Heavy red and black clamps: connect to the battery’s positive (+) and negative (−) terminals, respectively.
- Vented case: releases heat from the load. Keep its vents clear and follow the required cooling interval.
Many of the lead-acid banks I work with on the Big Island use L16 batteries or similar deep-cycle batteries. My usual L16 load-test current is 200 amps, using a compatible carbon-pile tester and watching the voltage under load. — Doug Mycko
This is Doug’s field practice, not a universal manufacturer specification for every L16-size battery. Confirm the exact model, permitted current, duration, temperature correction, and minimum test voltage. A 200-amp load alone does not define a complete pass/fail test.
One isolated battery. Two test leads.
Connection overview for a compatible tester and an individual 6-volt L16. Shut down and isolate the battery from the system first. Keep the load fully OFF while connecting or disconnecting.
Tools to have ready
A compatible carbon-pile tester, DC voltmeter, timer, battery thermometer, labels, and a worksheet. For flooded batteries, a suitable hydrometer adds cell-by-cell information. Use only the protective equipment and accessories specified for the job.
Label the batteries and inspect the bank
Photograph the wiring before changes and identify batteries as B1, B2, B3, and so on. Record any cable corrosion, terminal damage, or loose connections. Address defects safely before interpreting load-test readings.
Start with a properly charged battery
Complete the charging procedure for that exact battery type. “The charger reached its voltage setting” is not the same as proving the battery completed its charge. Record temperature and charge history. Follow the specified rest or surface-charge-removal procedure before testing; do not invent a heavy discharge to remove surface charge.
Shut down and isolate correctly
Follow the system manufacturer’s shutdown sequence for solar, chargers, inverter, and batteries. Opening one switch may leave other sources connected. For the individual carbon-pile test, have the battery isolated from the bank and all external circuits. Do not loosen or remove cables while current is flowing. If the isolation sequence is unfamiliar, have a qualified person perform it.
Choose the documented load and duration
Use the battery manufacturer’s procedure together with the tester’s ratings. Record the target amps, seconds, minimum permitted voltage, and temperature correction before beginning.
CCA-rated example: Crown describes a 15-second test at half the battery’s CCA rating, using the tester’s passing-voltage criteria. Where that procedure applies, a 600-CCA battery would be tested at 300 amps. This is an example for a battery with an applicable CCA rating, not an L16 prescription. Crown’s guidance.
For an L16: I usually use a 200-amp load, as explained above. Amp-hours are not cold-cranking amps; do not turn an Ah rating into a guessed CCA or automatically use the tester’s full 500 amps. Confirm the battery-specific duration and voltage limits before testing. If the test specification is unavailable, contact Doug or use a manufacturer-guided capacity test.
Connect with the tester load fully off
Follow the tester’s connection sequence and polarity instructions. Secure the clamps on suitable clean terminal surfaces, keep cables clear, and position the tester safely away from the battery. Never bridge the terminals with a tool or clamp. Confirm the starting voltage before applying a load.
Apply the load and record the response
Bring the tester to the prescribed current, start the timer, and monitor both current and voltage. Follow its adjustment instructions as the battery voltage changes. Record the current and voltage at the specified test time. Stop early for the specified voltage limit, abnormal heating, smoke, arcing, or any unsafe behavior. Never exceed the tester’s duty cycle.
Remove the load before disconnecting
Return the load control fully off, then disconnect according to the tester manual. Allow the required cooling time. Record recovery voltage at a consistent time if useful for comparison. Repeat on the other batteries using the same approved conditions; repeated tests without recharge or cooling are not comparable.
Interpret, restore, and recharge
Compare results with the model-specific limits and other comparable batteries. Recheck questionable clamp contact or incomplete charging before condemning a battery. Restore wiring, terminal torque, charging, and system operation using the equipment instructions. Recharge tested batteries promptly.
What the result tells you
| Observation | Possible explanation | Next step |
|---|---|---|
| One battery drops much faster than comparable batteries. | Weak battery, low charge, or a poor test connection. | Verify preparation and contact; repeat only as permitted. |
| Most batteries perform poorly. | Undercharging, age, inappropriate test conditions, or a bank-wide problem. | Check charging history and test specifications. |
| Short load test passes, but overnight runtime is poor. | Reduced storage capacity, increased household loads, or incomplete charging. | Measure energy use and perform an approved capacity test. |
| Battery-terminal voltage is acceptable but inverter voltage falls. | Possible voltage loss in cables, connections, or protective devices. | Have the DC path checked under load. |
Flooded batteries: add a hydrometer check
Hydrometer testing is for accessible flooded cells only. Never open AGM, gel, or lithium batteries for this test. Sample each cell carefully, record specific gravity and electrolyte temperature, and return electrolyte to the same cell. Apply the hydrometer/manufacturer’s temperature correction and compare with that battery’s specification.
Freshly added water can distort readings until properly mixed. If plates are exposed, follow the manufacturer’s watering-before-charging procedure. Do not add acid as a troubleshooting remedy. Equalization is only appropriate when specifically authorized for that battery; never carry a flooded-battery equalization procedure over to AGM, gel, or lithium.
Trojan specifies at least six hours idle, preferably 24, for its open-circuit voltage check. Resting voltage and specific gravity help assess charge condition; neither replaces a capacity measurement. Trojan battery maintenance and testing.
Lithium: a timed, controlled-load test
A battery can show a convincing voltage and still deliver less energy than expected. Test usable energy by starting from a verified charge condition, applying a measured load, and recording energy until the planned endpoint. This section concerns complete LiFePO₄ solar batteries, not loose cells or other lithium chemistries.
Define exactly what you are measuring
A DC shunt/energy meter can measure battery output. An AC kWh meter measures energy delivered to an appliance after inverter losses. These are different results. Testing a complete parallel bank measures the bank; it does not establish the capacity of each battery.
Record starting conditions and limits
Write down model, quantity, rated energy, temperature, charge settings, firmware where available, and active alarms. Determine the allowed continuous current, operating temperature, discharge endpoint, and restart procedure from the exact model manual. Choose a safe stopping point before starting.
Complete the prescribed charge
Use the manufacturer’s full-charge procedure. Check that every battery intended to participate is online and charging. Record SOC and available cell-voltage information, but do not assume a display reading of 100% proves capacity or balance. Follow manufacturer instructions for any balancing or SOC calibration.
Prevent charging from masking the result
Use approved controls and shutdown procedures to stop solar, generator, and utility charging for the test. Do not unplug PV connectors under load. Verify that the battery is actually supplying the measured load; utility bypass can make an apparent runtime result meaningless. Plan the test around essential household loads and backup power needs.
Use a suitable, stable load and energy meter
Keep the load within the battery, inverter, wiring, circuit, and meter ratings. A correctly rated load bank is ideal. With an AC appliance, measure actual energy: nameplate watts and thermostatic cycling are not a constant load. Keep heat-producing loads attended and clear of combustible materials. Do not use improvised high-current connections.
Start the clock and log the test
Reset or record the energy meter’s starting value. Record start time, watts, battery voltage/current, SOC, and temperature. As a practical logging schedule, check at the start, after a few minutes, then about every 15–30 minutes, with closer observation near the endpoint. Follow any more frequent manufacturer requirement. Record alarms immediately.
Stop at the first applicable limit
End the test at the planned manufacturer-approved endpoint, or earlier for an alarm, unexpected shutdown, temperature limit, abnormal behavior, or unsafe condition. Record the ending meter value and exact stop reason. Do not lower the cutoff to extract more energy. A trip caused by current or temperature is not a completed capacity measurement.
Calculate delivered energy and restore charging
Subtract the starting kWh reading from the ending reading. For a genuinely constant measured load, watts × hours ÷ 1,000 estimates kWh. Recharge according to the manual, restore normal charging sources and settings, and confirm normal operation. Save the log before resetting any diagnostic history.
Read the results in context
| Finding | What to investigate |
|---|---|
| Low AC energy compared with the battery’s nameplate kWh. | Inverter losses, idle consumption, reserve settings, start/end conditions, and whether the specification is nominal or usable energy. |
| One parallel battery stops while others continue. | Its alarms, SOC, current sharing, connections, and model-specific limits. Bank totals may conceal an individual problem. |
| Shutdown when a large appliance starts. | Load surge, BMS protection, inverter limits, or voltage loss—not necessarily exhausted storage. |
| SOC suddenly changes during the test. | SOC estimation/calibration and cell behavior. Compare with measured energy and available logs. |
| Repeatable low DC energy under the prescribed test conditions. | Request manufacturer evaluation with the complete test record. |
Discover’s AES manual illustrates how current, temperature, voltage, and BMS protection can affect operation, and recommends retaining diagnostic logs. Its numerical settings apply to that specific model—not every Discover or lithium battery. Discover AES example manual (PDF). For HELIOS, get its own documentation from the HELIOS product page.
Timed-load energy calculator
Use this for a constant measured load. For cycling appliances or varying loads, use the energy meter’s accumulated kWh instead. This calculator estimates delivered energy; it does not select a safe test load or diagnose battery health.
AC output excludes inverter losses and idle energy. A partial discharge measures only the energy delivered over that tested range. Do not divide by the displayed SOC change to claim a verified full capacity.
Keep a test record
Use the fields below and print or save this page as a PDF. Entries stay on this open page only; they are not sent to SolarMan or saved after you close or reload it.
| Battery / time | Volts | Amps / watts | kWh / SOC | Notes / alarms |
|---|---|---|---|---|
Common questions
My batteries show normal voltage. Can they still be weak?
Yes. Voltage without a meaningful load is only one observation. Record what happens under load, and use a controlled capacity test when the question is how much energy remains available.
Can I test all my lead-acid batteries together?
Only with equipment rated for the complete bank and an appropriate procedure. Doug’s individual carbon-pile approach is intended to help identify weak batteries separately. Do not put an automotive tester across a higher-voltage solar bank.
Can I use the carbon-pile tester on lithium?
This guide uses a timed controlled-load test for lithium. Do not transfer an automotive lead-acid procedure to a lithium solar battery; the BMS and model-specific current limits change the test requirements.
Does a lithium battery shutdown prove it is bad?
No. Capture the alarm, current, voltage, temperature, and operating conditions. Protection events and system settings need investigation before deciding whether the battery has lost capacity.
Can these results establish a warranty claim?
They can provide useful evidence. A manufacturer may require its own test procedure, operating limits, logs, serial numbers, and proof of purchase. Follow that procedure before conducting a formal warranty test.
Bring me the readings.
Let’s work through the problem.
I’m Doug Mycko, owner of SolarMan Consulting, LLC. I help customers with off-grid equipment selection, troubleshooting, system design, and battery upgrades.
Have your battery model, bank configuration, inverter and charger models, test results, and any alarm codes ready. Photos of labels and the overall system help explain what you have.
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