Short answer: to judge whether an NP-FZ100 replacement is worth buying, work through three steps — check that the rated energy and the weight agree with each other, run a ten-minute recording test in the camera, then squeeze the pack and lay it flat to look for swelling. The three steps map onto three different problems: inflated capacity claims, decoding compatibility and cell ageing. The first two can be finished before you order and on the day the parcel arrives; the third is a routine check for as long as you own the battery. Below are the pass criteria and the warning signs for each.
Step one: use Wh and weight to catch inflated capacity
Battery labels usually carry both mAh and Wh, and the two figures have to agree: Wh = mAh × voltage ÷ 1000. The original NP-FZ100 is 2280mAh / 7.2V / 16.4Wh, and all three numbers line up. If a pack claims 3000mAh while still listing 16.4Wh, the spec sheet has been copied from somewhere else, and the real capacity will not exceed the original.
The second check is weight. Energy density in lithium cells has a physical ceiling, so a larger capacity has to show up on the scale. Most NP-FZ100 packs sit around 80 grams. A product that claims 3000mAh at seventy-odd grams and feels hollow in the hand is hard to believe. Conversely, be wary of a pack that is noticeably heavier than the original — some products add ballast to create an impression of solid construction.
The third is price. Replacement batteries being cheaper than the original is normal; that is the whole reason they exist. But a price that is unreasonably low usually means the cell grade, the protection board or the quality control has been cut. A competent pack under a hundred yuan is plausible; below that, it is fair to ask where the cost was removed.
| Check | Pass | Warning sign |
|---|---|---|
| Wh against mAh | Converts correctly at 7.2V within a reasonable margin | Claims 3000mAh but Wh is still 16.4 |
| Weight | Around 80 grams, proportionate to capacity | High claimed capacity but noticeably light |
| Camera recognition | Percentage displays normally after power-on | Percentage jumps, frequent warnings |
| Discharge curve | Falls linearly to auto-shutdown | Shows 30% then shuts down |
| Physical condition | Sits flat, even thickness | Rocks on a flat surface, bulges at the sides |
Step two: a ten-minute recording test for decoding compatibility
“It powers on” is not the same as “it works”. Some packs start the camera normally but never update the percentage, or cut out halfway through a clip. Those faults only surface under actual recording. A ten-minute continuous take on the day of delivery costs almost nothing and screens out most of the risk.
Watch whether the percentage falls linearly
Record for ten minutes and watch whether the number moves smoothly. A healthy pack loses a few points over ten minutes of recording, and the curve is gentle. If the figure sits still and then drops in one jump, the fuel gauge is estimating badly, and usable capacity may be well below what is displayed.
Watch for early shutdown
The more serious case is the camera shutting down with a fifth of the charge still showing. That usually means the protection board is cutting output when a single cell’s voltage sags, which points to poorly matched cells. One occurrence is enough reason to ask for a replacement.
Watch how it behaves after a firmware update
Sony’s body firmware has changed battery recognition logic more than once, and a third-party pack that behaved perfectly can start throwing warnings after an update. Before buying, check whether your body model and firmware version have any publicly reported compatibility issues. If you hit one afterwards, update the firmware before concluding the battery is faulty.
Step three: swelling, judged by flat lie and thickness
Swelling is an inevitable part of lithium ageing; the question is when it starts and whether you notice it. The simplest check is to lay the pack flat on a table and press gently on opposite corners with a finger. A slightly swollen pack has already domed in the middle, so one corner will always lift.
The second move is to sight along the side edge against the light. A healthy pack has a straight side; a swollen cell pushes it into a curve. If you have calipers, measure the thickness and compare it with the day you bought it — anything past a millimetre of growth deserves attention.
The third signal is temperature while charging. Getting warm is normal. Feeling distinctly hot to the touch, or the shell staying much warmer than usual after a full charge, suggests internal resistance has risen. Stop using it at that point, and do not leave it in the camera overnight.
Cold weather magnifies all of this. Internal resistance rises below freezing anyway, and an aged pack degrades more sharply, so before a winter shoot it is worth running the checks in our sub-zero field test first.
How many to buy, and how to pair them
- Buy one first to test the water, confirm the percentage readout and shutdown point suit your body, then order more.
- Packs from the same batch behave more alike, and rotating them keeps the gauge estimates from drifting.
- Always keep at least one original or best-condition pack as insurance for the jobs that matter.
- Size the kit to the work: two packs covers a day of stills, and for video budget one per hour.
- A dual-slot case for charging and storage keeps the contacts from knocking against each other in a bag.
The point of this routine is not to find “the best battery” but to keep the risk inside a range you can live with — validate with one pack, then buy the rest. For more body compatibility and the wider product line, see the TNP-FZ100 range, or check the model list in our FZ100 battery section to see which group your camera falls into.
FAQ
If the rated capacity is higher than the original, will it last longer?
Not necessarily. The NP-FZ100 bay is a fixed size and the cell volume is limited, so a large jump over the original rating usually means a different rating method or an inflated figure. The more reliable approach is to check whether the Wh is self-consistent, and to measure how long it actually runs.
Can a third-party battery damage the camera?
A competent one will not. The pack delivers a standard voltage through its protection board and the body draws power from the contacts; there is no mechanism by which it “burns out the mainboard”. The risk comes from poor cells and failed protection boards, which is exactly why abnormally cheap products are worth avoiding.
It has swollen slightly — can I still use it?
Not advisable. Swelling means gas has formed inside the cell, and continued charging and discharging carries a risk of further expansion or leakage. Stop using it, discharge it to a low level and take it to a local collection point under your regional rules. Do not put it in household waste.
How can I tell whether the cells have been refurbished?
Three things: whether the shell seams are rough, whether the contacts show obvious insertion wear, and whether runtime after a full charge is clearly shorter than the rating. When all three appear together, the cells are almost certainly not new.