A tool battery often costs between a third and half the price of the bare tool. Three dead packs and you have paid for a second machine. Yet most of the packs that end up at the recycling point are not worn out in the strict sense: they were put on charge while still red hot, left flat for six months in an unheated garage, or charged at minus five degrees on a February morning. Lithium-ion chemistry is forgiving about how you use it and unforgiving about the conditions you use it in.
Understanding what happens inside the case changes the way you handle one, and it takes no equipment at all: three degradation mechanisms account for nearly all of the damage.
What is actually inside the case
A tool pack is an assembly of standardised cylindrical cells, usually in 18650 or 21700 format, plus a protection circuit known as the BMS. Each cell delivers roughly 3.6 V nominal. The rest is arithmetic: five cells in series give you the 18 V of a classic platform, ten in series give 36 V. When a pack claims a higher capacity at the same voltage, it is because there are several branches in parallel.
That architecture explains two things. First, a pack does not die evenly: the weakest cell in a series limits everything else, because the BMS cuts out as soon as any single cell reaches its low threshold. Second, a high-capacity pack suffers less per amp drawn, since the current is shared across more branches — a small pack on a hungry machine works permanently at its thermal limit. Incidentally, "20 V max" and "18 V" describe the same thing, the first measured with the cells full, the second as a nominal working figure: it is not a performance difference.
Heat, the number one killer
This is the leading factor, well ahead of the number of cycles. Lithium cell degradation follows a chemical-type law: it accelerates sharply with temperature. As an order of magnitude, ten degrees more roughly doubles the rate of ageing. A pack that lives at 40 °C loses its capacity far faster than the same pack at 20 °C, for identical use.
And we happily stack two heat sources on top of each other: heavy discharge on a chainsaw or an impact driver, then straight onto the fast charger. A charger that fills a pack in thirty minutes pushes a high current into it, and some of that energy is shed as heat.
- Let it cool before charging. Fifteen to twenty minutes after sustained work is enough. Plenty of chargers wait of their own accord for the internal sensor to come back down, but that time spent airing is a clear gain.
- Never charge in the boot of a vehicle, or against a south-facing wall. The pack does not see the ambient temperature, it sees its own temperature plus that of its surroundings.
- Alternate two packs rather than cycling one. For the same workload each one spends less time hot, and the standard charger is gentler than fast charging whenever the tool is not needed within the hour.
Deep discharge and cycles: the counter that lies
A lithium cell does not like going low. The BMS usually cuts out at around 2.5 V per cell to protect the pack, but two things carry on afterwards: the electronics draw a tiny but permanent standby current, and the cell self-discharges by a few per cent a month. A pack put away flat in November can therefore, by February, cross the threshold below which the chemistry degrades irreversibly. At that stage the charger often refuses to start — that is a protection, not a fault: recharging a cell that has gone too low can create internal metallic deposits, with a real risk of overheating. Never put a pack away empty.
As for cycles, the accepted order of magnitude for quality cells in domestic use is between 300 and 500 full cycles before dropping towards 80 % of the original capacity. Except that the cycle being counted is an equivalent full cycle: emptying a pack from 100 to 0 % once, or from 100 to 50 % twice, counts the same. And depth of discharge changes everything: a pack you never take below 20 or 30 % will take far more equivalent cycles. Hence a counter-intuitive conclusion: it is better to charge often and partially than rarely and to the bottom. The habit inherited from NiCd packs, running them flat before recharging, is exactly what you must not do here.
Cell imbalance
Over time, the cells in a series drift apart: slightly different capacities, internal resistances that diverge. The classic symptom is a pack that charges "full" in a few minutes and then empties in three cuts — one cell reaches the top, the BMS stops the charge, and the pack is nowhere near full.
Most BMS units do balance the cells, but only at the end of a charge. A pack you always unplug at 80 % never balances. Hence the compromise: partial charges for everyday use, and one full charge taken through to the end of the cycle every ten to fifteen uses. The warning signs:
- Runtime halved or worse while the other packs on the same platform hold up.
- An abrupt cut-out in the middle of a working load, with no gradual drop in speed.
- Unusual heat on one side of the case, or a gauge that goes from four LEDs to one in a few seconds.
