The "5 Ah" pack that doesn’t last
October, the woodpile to get done before the snow. The EGO chainsaw with its 5.0 Ah pack, fully charged the night before and taken out of the shed in the morning. Twenty-five minutes after the first cut, the pack shuts off. The neighbour says five amp-hours ought to last an hour, and that the pack is done. The voltmeter on the shut-off pack, ten minutes later, reads 50 V; back on the charger, it shows full after forty-five minutes.
Is the pack done? What quantities should be measured, and in what order, to find out whether the pack, the tool or the expectation is at fault?
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The answer
Nothing yet says the pack is done, and the arithmetic leans its way. The label says 280 Wh, counted at 56 V; in the cut, where the pack works closer to 48 V, it gives back 220 to 240. A 16-inch saw draws around 1,800 W in hardwood, less in spruce. Time is energy divided by power: 240 ÷ 1,800, eight minutes of chain in the wood. Twenty-five minutes at the woodpile, with the logs to turn, the restarts and the pauses, may well be only eight minutes of cutting. The neighbour confused a quantity with a duration: 5 Ah is 5 A for one hour, or 35 A for eight and a half minutes, a little less in practice because a pack gives less at high current. The saw draws 30 to 40 A.
The charger does not decide, and you have to read its label before reasoning. The fast charger fills a 5.0 Ah in forty minutes: with it, forty-five minutes say nothing. The standard charger pushes 3 A and takes a hundred minutes: forty-five minutes put back 2.25 Ah, less than half. Two readings then hold together: the pack shut off with a quarter or a third of its charge still in it, or it only holds half any more. The clue puts them level; it is the first measurement that separates them.
The voltmeter, on the other hand, does say something. The pack has fourteen cells in series; 50 V at rest is 3.57 V per cell, a quarter or a third of the charge still there. A truly empty pack rests around 45 to 48 V. So the protection did not cut for lack of charge: it cut on the voltage sagging under 35 A, and that points to the current, the chain, the wood, the cold of the shed, or to a cell whose resistance has risen.
Three quantities, which hold together more than they line up; start with the easiest.
- The actual cutting time, on a stopwatch, chain in the wood only. Free, and it can close the question: if it comes to eight minutes, everything is normal and it is the expectation that needs fixing. But "normal" assumes the saw’s power, hence measurement 2.
- The tool’s current in the cut, with a DC clamp meter on one of the two pack leads if you can reach them. Otherwise, the amp-hours the charger puts back (its current times its time), divided by the cutting time from measurement 1, give the average current. A dull or over-tight chain raises the current, say by a third for the same wood, that is an order of magnitude and the clamp will say by how much: 43 A instead of 32 A, and seven minutes instead of nine; beyond 40 A, the pack cuts on overcurrent before it is empty. That is the tool, not the pack.
- The amp-hours the pack gives back, into a known, steady load, away from the saw, starting from the pack that shut off. A 12 V 50 W bulb is no good at 56 V; a 150 W electronic load passes 2.5 A at that voltage and takes two hours, which is the point. A connector that holds, a fuse in the lead, never alligator clips across the pack terminals: 56 V does not hurt to the touch, a pack short melts lugs. A healthy pack gives back 4.5 Ah and more at that current; 80 %, 4 Ah, is the threshold where a pack is said to be at end of life. If it gives back 2.5 Ah, the pack is at fault, or one of its cells.
The instructor’s note
The question aims to knock down a confusion the label keeps alive: the amp-hour is a quantity, like the litres in a tank, and a tank has no duration until you know the flow. A pack’s "advertised time" does not exist; what exists is an energy, and the time comes out when you divide it by the tool’s power. And that energy depends on the voltage the maker counts at, like the mAh of the USB power bank.
The second aim is to show that a calculation cannot decide between two suspects, the pack and the tool, because both give the same symptom, a shutoff too soon; and that a clue, the charger, does not decide either when it fits both. Only a measurement that isolates one of them decides. Current in the cut isolates the tool; capacity returned into a known load isolates the pack, and it depends on the test current, which is exactly what we want: to see the pack without the saw. On a cold morning, below 10 °C, internal resistance rises and the protection cuts earlier, healthy pack or not; redo measurement 3 in the warm before condemning.
One real fault remains possible, and measurement 3 catches it: a weak cell that reaches the low threshold first and shuts off the whole pack while the others still hold half. That is the diagnosis of RAS 6.2, and it calls for other measurements, cell by cell.
What next?
- 1.2 The quantities: the Wh on the label, the W of the tool, the time.
- What the USB power bank really gives: the same misleading mAh, measured in ten minutes.
- 6.2 The pack: the weak cell that shuts everything off, and how to find it without taking the pack apart.