Makita BL Series Self-Discharge Rates After Long Storage
This article explains how Makita BL-series 18V batteries lose charge during long-term storage and why the observed “self-discharge” at pack level is higher than cell chemistry alone. It clarifies that apparent capacity loss includes both intrinsic cell self-discharge and continuous BMS quiescent currents, and shows how temperature, storage SOC, age, and handling conditions strongly affect the rate. The article outlines realistic monthly loss ranges under typical conditions, then provides reproducible field-to-bench measurement methods using controlled storage, OCV tracking, and low-rate capacity tests. It also defines practical warehouse thresholds, diagnostics, and best practices—such as storing at ~30–50% SOC and cool temperatures—to prevent deep discharge, BMS lockout, and unusable inventory after long storage.
After long storage, Makita BL-series 18V packs typically lose only a few percent of charge per month at room temperature, but apparent self-discharge rises sharply with heat, aging, and BMS quiescent draw, increasing the risk of deep discharge or BMS latch if unmanaged. This note explains the real pack-level mechanisms behind self-discharge, realistic loss rates to expect, how to measure storage loss reproducibly, and what fleets and warehouses should do to prevent dead or non-recoverable packs after extended layup.
Safety first
Quarantine swollen, warm, leaking, or odorous packs. Do not short, charge, or disassemble suspect packs—handle on a non-combustible surface and follow hazardous battery procedures. Use appropriate PPE and RCD/GFCI for bench wake/charge attempts. Do not bypass BMS.
What “self-discharge” means at pack level
Cell self-discharge = electrochemical loss of stored charge while idle. Pack-level apparent self-discharge also includes BMS quiescent/leakage currents, protection circuit behaviors (wake thresholds), and minor parasitic draws (ID pins, indicator LEDs). For practical storage planning, quantify capacity loss per time or OCV drift vs stored SOC.
Typical magnitudes & realistic ranges (practical guidance)
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Cells alone (modern Li-ion 18650/21700) at 20–25 °C: low self-discharge — commonly ~2–5% capacity/month (varies by cell chemistry, age, and state).
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Pack + BMS (real packs like BL series): add BMS/quiescent leakage — practical observed apparent loss often ~3–8%/month at room temp for healthy new packs; aged packs or poor storage can show higher drift.
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High temp or humidity: self-discharge roughly doubles for every ~10–15 °C increase in ambient; at 40 °C expect materially larger monthly loss and accelerated calendar fade.
Use these as ballpark planning numbers — measure your own inventory to set operational gates.
Factors that change apparent self-discharge
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Storage temperature: dominant factor; higher temps accelerate chemical loss and BMS leakage.
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Stored SOC: higher SOC increases long-term calendar aging; very low SOC risks BMS latch.
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Cell age & cycle history: older packs show higher baseline DCIR and faster self-discharge.
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BMS design & parasitic draws: indicator LEDs, ID/comm resistors, and BMS quiescent circuits add continuous microamps.
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Mechanical defects / contamination: electrolytic leakage, wet/dirty terminals, or internal shorts increase drain.
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Storage orientation & ventilation: trapped heat or direct sunlight increases effective temperature.
Reproducible measurement protocol (field → bench, safe)
Goal: quantify % capacity or % OCV loss per month under controlled conditions.
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Baseline conditioning: bring pack to target storage SOC (recommend ~40% for this test), charge with OEM charger, rest 30–60 min at test ambient (23±2 °C). Record serial/lot, initial OCV and initial reference capacity via a low-rate discharge (0.2C) for one sample per cohort.
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Controlled storage: place packs in a climate-controlled chamber or a monitored area; log ambient T and RH. Label with date-in.
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Periodic checks: at planned intervals (1 week, 1 month, 3 months) measure: OCV after 30 min rest, surface temp, and for sample units run a low-rate capacity check (0.2C discharge) to measure Ah remaining. For pack-level quick check you may convert OCV to SOC using the appropriate pack OCV→SOC curve (be aware of inaccuracy near extremes).
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Compute loss: percent capacity loss = (Ah_initial − Ah_now) / Ah_initial ×100; OCV drift gives quick flag but use capacity test for accuracy. Normalize results to monthly rate for comparison.
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Quarantine thresholds: if sample packs exceed your gate (example: >10% capacity loss in one month or OCV drop indicating SOC < ~20%), inspect cohort for handling issues and consider rebalance/top-up. (Set your own gates from baseline pilot data.)
Stop and quarantine any pack that shows swelling, unusual heating during test wake, or very low OCV (< ~10–12 V pack-level) — do not attempt aggressive recovery in warehouse.
Simple warehouse check (fast, safe)
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Visual inspect for damage/swelling.
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Measure OCV after 30 min rest. If OCV indicates SOC drift > ~10–15% from stored target, schedule sample capacity test and consider top-up to target SOC.
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Log results and move anomalous packs to QUARANTINE.
Engineering bench diagnostics for high accuracy
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Coulomb counting: fully charge → store → charge recovery and measure coulomb loss; accurate but requires controlled charge/discharge lab rigs.
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Low-rate capacity test (0.1–0.2C): gold standard for remaining Ah measurement.
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Leakage current test: with current meter across BMS/protection circuit to measure quiescent drain microamps (lab only).
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EIS / ICA: detect cell degradation patterns that correlate with high self-discharge (lab diagnostic).
Storage best practices to minimize self-discharge & BMS lock risk
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Target storage SOC: ~30–50% (40% recommended) for long-term storage.
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Temperature: store cool — ideal 15–25 °C; avoid >30 °C; for long term use climate control.
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Periodic maintenance: spot-check OCV monthly and run sample capacity test every 3 months; top up to target SOC if drifted.
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Physical handling: keep terminals clean, avoid metal contact, use non-conductive separators, label date-in and next-check.
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For very long storage (>6–12 months): recondition samples at 3-month intervals; consider partial recharge if drift exceeds gate.
Troubleshooting common storage failures
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Rapid unexpected drop (>10% in 1 month): check storage temp, contamination, and recent charging history; run bench leakage current test.
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Packs that refuse charger after storage: BMS latched or per-cell too low — attempt OEM charger wake briefly; if charger refuses, move to lab for controlled current-limited wake (≤ C/20) only. Quarantine if warm or behavior abnormal.
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Variable loss across same lot: inspect for mixing of older stock, prior deep discharge, or damaged terminals; perform sample EIS.
FAQ
Q: How fast will a BL pack go flat in storage?
A: At ~23 °C a healthy BL pack typically loses only a few percent capacity per month; hotter conditions or old packs can push that higher — measure your batches.
Q: Will the BMS prevent self-discharge?
A: BMS limits some parasitic drains but does not eliminate chemical self-discharge; BMS leakage and protection circuitry add small continuous draw that increases apparent pack loss.
Q: Can I top up stored packs to 100%?
A: No — storing at 100% accelerates calendar aging. Top up stored packs to ~30–50% SOC and only charge to 100% shortly before use.
Q: What if a pack refuses the OEM charger after storage?
A: It may be BMS-latched from deep discharge. Try OEM charger-wake per vendor guidance; if it refuses or the pack heats during wake, quarantine and send to lab/RMA.
Q: How often should I test my stored inventory?
A: Monthly visual + OCV spot checks, sample capacity tests every 2–3 months for medium-term storage; increase frequency at high ambient temps.
Summary — one-line takeaway + 3 immediate actions
Makita BL packs show low inherent cell self-discharge but pack-level apparent loss is increased by BMS quiescent draw, temperature, age and storage SOC — expect roughly a few percent per month at room temp for healthy packs and higher rates with heat or aging; measure your stock and set operational gates.
Immediate actions: 1) Bring spares to ~40% SOC now and label date-in; 2) place a temp/RH logger in your storage area and review the last 30 days; 3) run a sample low-rate capacity check on 2–3 packs from oldest inventory to baseline your facility’s loss rate.