Start with the symptoms, not the instruments
Before picking up a meter, write down what the battery is actually doing. A battery that runs short on every shift has a different problem from one that dies suddenly, and the symptoms point the testing in the right direction.
- Short runtime. The truck used to run a full shift and now fades after five hours. That is capacity loss — usually even across the pack, or one bad cell dragging the rest down.
- Sudden death. The battery was fine last week and now will not hold a charge. That is usually a single failed cell or a shorted cell, and it shows up fast on a per-cell reading.
- Slow or weak lifting. The truck drives but the hydraulics are sluggish, especially under load. Voltage is sagging under demand — a load test will confirm it.
- Hot cells or a rotten-egg smell. A cell that runs hot, or a battery that smells of sulfur after charging, has a shorted or badly sulfated cell. Stop using it and get a dealer involved.
- Charger never finishes. The charger runs for hours past normal and never reaches its shut-off point. The battery is not accepting a full charge — sulfation or a dead cell is the usual cause.
All three tests below should be done after a full charge and a cool-down period, with the battery at rest. Readings taken on a hot or half-charged battery are misleading — specific gravity and voltage both change with charge state and temperature.
What a load test tells you
A load test is the one test that measures what you actually care about: how much work the battery can still do. A voltmeter can tell you a battery is charged. Only a load test tells you whether it can hold that charge under real demand.
The test works by discharging the battery at a controlled rate — usually over several hours — while the tester records voltage and runtime, then compares the result to the battery's rated capacity. A battery that delivers 80% or more of its rated capacity is generally still serviceable. One delivering well below that is on borrowed time, and the test result is the number you use to make a repair-versus-replace decision.
- Even capacity loss across all cells means the battery is simply aging. If it is near its expected cycle life, that is normal wear, and replacement is the honest answer.
- One or two cells collapsing under load while the rest hold up means the problem is isolated. A young battery with one bad cell is often a repair candidate — a dealer can replace individual cells.
- Voltage that recovers when the load is removedbut sags hard under load is a classic sign of sulfation: the plates can hold a surface charge but cannot deliver current.
Hydrometer readings: the cell-by-cell truth
A hydrometer measures the specific gravity of the electrolyte in each cell — how dense the acid is compared to water. Specific gravity is a direct measure of a cell's state of charge and health, and it is the most useful test an operator can do with cheap tools.
- Fully charged, healthy cell: about 1.265 to 1.285 specific gravity. This is the number you want to see after a full charge and cool-down.
- Below 1.225: the cell is sulfated or has lost significant capacity. A cell that will not come back above 1.225 even after an equalizing charge is a cell that is not coming back.
- Spread between cells: the highest and lowest cells in a pack should sit within about 0.030 of each other. A wider spread means the cells have drifted apart — the low ones are weak, and the pack is only as strong as its weakest cell.
How to take the reading: after a full charge and cool-down, remove the vent cap, draw electrolyte into the hydrometer with the bulb, and read the float at eye level. Test every cell, write every number down, and put the electrolyte back in the same cell it came from. Wear safety glasses and gloves — this is sulfuric acid. And correct for temperature if your hydrometer calls for it: specific gravity readings shift with temperature, and most hydrometers are calibrated for around 80°F.
One low reading is a data point, not a verdict. A cell that reads low once may recover after an equalizing charge. A cell that reads low month after month, or drops after every equalize, is a failing cell. The trend across your log is worth more than any single reading.
Voltage readings per cell: fast, useful, and easy to misread
A digital voltmeter is the fastest test in the kit. Measure each cell at rest — after a full charge and a cool-down, with no load on the battery — by placing the probes on the cell's positive and negative terminals.
- Healthy cell at rest: about 2.1V. A 48V battery has 24 cells, so a healthy pack reads about 50.4V at rest.
- Variance over 0.2V between cells is a red flag.If most cells read 2.10V and one reads 1.85V, that cell is weak or failing — and it is dragging the whole pack down, because the pack stops working when its weakest cell is empty.
- A cell reading near zero is shorted. It will not recover, and the battery should not be charged until a dealer has looked at it.
- Voltage under load tells a different story from voltage at rest. A battery can read a healthy voltage sitting still and sag badly the moment the truck lifts a load. If the at-rest readings look fine but the truck is weak, that is what a load test is for.
The common mistake with voltage testing is treating the pack voltage as the answer. A 48V pack reading 48V sounds fine — but if that 48V is twenty-three healthy cells plus one dead one, the battery is failing. Always test per cell. The pack number hides the problem; the cell numbers find it.
When a dealer test beats a DIY test
The hydrometer and the voltmeter belong in your battery room, and your operators should know how to use both. But there is a line where dealer equipment earns its keep:
- Load testing. Real capacity measurement needs a controlled discharge rig. There is no handheld shortcut.
- Conflicting readings. Voltage looks fine, gravity looks bad, and the truck still dies at noon. A dealer can run the full set of tests together and find the cause instead of guessing.
- Safety concerns. A swollen cell, a cracked case, a battery that runs hot, or a smell of sulfur — stop testing and call a dealer. A damaged battery can be dangerous.
- Warranty or insurance questions. A dealer's documented test result carries weight that a clipboard of operator readings does not.
- A repair-versus-replace decision. When the numbers are borderline, a dealer's load test and cell inspection turn a guess into a number you can budget against.
What the numbers mean for repair vs. replace
Test results point to one of three outcomes, and the pattern matters more than any single number:
- Keep running it. Capacity above 80% of rated, gravity readings in the 1.265 to 1.285 range, cell voltage spread under 0.2V. Stay on the maintenance schedule and retest monthly.
- Repair it. The battery is young, capacity is mostly there, and the problem is isolated to one or two cells — a cell replacement or a cable and connector fix restores it. See the battery repair page for what repair covers and what it does not.
- Replace it. Capacity has faded evenly across all cells, gravity readings sit below 1.225 across the pack, and the battery is near its expected cycle life. No repair fixes even wear — it is time for a new battery, and the load test result is your justification for the spend. The specs guide covers what to look for in the replacement, including voltage, capacity, and the lead-acid versus lithium decision.
Testing is not a one-time event. A battery tested once a year is a battery that fails without warning. Test monthly, log the numbers per cell, and watch the trend — the trend is the warning. A dealer can run the full test set on a regular visit, so if testing keeps sliding down the priority list, put it on a dealer service schedule and let the numbers come to you.