Tool battery lasts only minutes or won't charge — rebuild it or buy new?

A fully charged pack that used to run half an hour now goes flat after a few driven screws. Or the pack won't charge at all, and the charger starts blinking red after a two-second pause. This is the exact problem behind most of the calls that reach our bench: an 18–20V tool pack has run its course. The real question is whether it's still worth rebuilding, or whether the money belongs in a new pack.

It isn't a simple choice, because the answer depends on two factors at once: how much the pack itself is worth (a professional platform pack versus a cheap no-name block) and what exactly has failed — the cells, the BMS, or the mechanics. Let's take each case on its own terms, from the bench.

Signs your pack really is done

First, confirm the fault is the pack and not the tool or the charger. The classic signs of a worn pack:

• Runtime has collapsed. A pack that ran 30 minutes now holds 5–10. That's the first and clearest signal that the cells' real capacity has dropped — often below half of the rated figure.
• The charger blinks red. You insert the pack, the charger lights up for a moment, then starts blinking after a two-second pause. That is almost always the pack's fault, not the charger's: either the BMS is locking the pack out, or one cell group is weak enough that the charger refuses to take it.
• The pack won't hold a charge. Charged to 100% in the evening, half empty by morning, even though the tool sat on the shelf the whole time. That's a sign of high self-discharge — a cell inside is "leaking".
• The tool loses power and cuts out suddenly. The drill turns slower and slower, then stops at once, even though the gauge just showed enough charge. That's the sign of one weak cell that sags under load and trips the protection.

Riga conditions speed this up. In the stairwells and garages of older blocks there's frost in winter and Baltic humidity with salt air in summer — cell contacts and the BMS board oxidise faster. A typical professional user wears a pack out in 4–6 years, but cold storage in a car or a damp cellar shortens that window noticeably.

If a simple charger-and-contact check doesn't fix it, the next step is the bench. Before that, look at the tool itself too — our page on power tool repair helps there.

What's really inside an 18–20V pack — cells, BMS and thermistor

Open a typical 18–20V pack and you'll find three parts, each of which can fail on its own:

• Lithium-ion cells. Usually 5 cylindrical cells in series (5S), and in larger packs two or three parallel rows (5S2P, 5S3P). These store the energy, and these wear out first.
• The BMS (Battery Management System) board. It watches the voltage of each cell group and locks the pack out on over-discharge, overheating or imbalance. It also "talks" to modern chargers (Makita Star Protection, the Makita XGT communication module) — if the BMS trips protection, the charger blinks even though the cells might still be alive.
• The thermistor. A small temperature sensor pressed against the cells. If it fails or its contact lifts, the charger "sees" the wrong temperature and refuses to charge — the same blinking again.

Why does one bad cell kill the whole pack? Because in a series string every cell carries the same current. If one cell has high internal resistance, it sags in voltage first under load and drags the whole string down. The BMS reads that as imbalance and locks the entire pack out — the other four cells may still be in excellent shape, but the pack feels "empty" in the tool. That's exactly why buying a ready-made pack at the shop is often money thrown away: you pay for a full set of new cells when really one weak cell group and the contacts needed sorting.

Rebuild or buy new — an honest decision matrix

The main question isn't "can it be fixed" but "is the pack worth it". The decision sits at the intersection of two factors: the pack's value and the type of damage.

The logic is simple: the more valuable and alive the platform, the more sense it makes to save the pack. For a professional pack a new original is worth noticeably more than a rebuild, and you already own a whole tool system on that platform — replacing everything makes no sense. For a cheap block it's the reverse: a set of original cells is worth nearly as much as the whole pack, so the repair often isn't proportionate.

We run a fast on-site diagnostic first, and once we see exactly what has failed in the pack we tell you honestly whether the rebuild is worth it or the money is better spent on a new pack.

Which packs are worth rebuilding and which aren't

In practice, from our experience at the bench:

Worth rebuilding:

• Makita LXT 18V (BL1830, BL1840, BL1850B, BL1860B) — the most common professional platform in Latvia, cells almost always in stock, a fast rebuild.
• Makita XGT 40V (BL40xx series) — high-voltage 21700 cells; BMS-part availability is narrower, but the packs are valuable, so a rebuild is still worthwhile.
• DeWalt 20V MAX and FlexVolt — popular, with good cell availability. FlexVolt packs are especially valuable as originals, so a rebuild is the logical move.
• Bosch ProCORE 18V — high-current cells, a platform worth keeping.
• Milwaukee M18 — likewise a professional system where a new original is valuable enough that the repair pays off.

Rarely worth rebuilding:

• Cheap no-name blocks from marketplaces, where a new block is worth about as much as a set of cells.
• Packs with swollen, melted or mechanically deformed cases whose seal can no longer be reliably restored.
• Packs whose platform tools you no longer use — there's no point saving a pack for a system you've abandoned.

Why a DIY rebuild so often goes wrong

The internet is full of clips where someone opens a pack, solders in new cells and celebrates. A couple of weeks later that pack is back with us — or, worst case, it's gone, because it caught fire. The three main mistakes:

1. Soldering instead of spot welding. Cells in a pack are joined with nickel strip and spot welding — a short current pulse that bonds the strip without overheating the cell. A soldering iron needs several seconds of high heat right at the cell terminal — that damages the cell's interior, raises its internal resistance and shortens its life. A soldered pack runs hot and lasts a short time.
2. The cells aren't matched. In a series string every cell must have the same capacity and internal resistance, and be balanced at the start. Throwing random cells together means instant imbalance — the BMS reads it and locks the pack out again.
3. The BMS and thermistor get overlooked. Even with perfect new cells the pack won't work if the old BMS has already failed or the thermistor contact has lifted. At home, nobody checks for that.

On top of that comes the safety margin. Lithium-ion cells can catch fire on a wrong connection, overheating or imbalance — that's not a theoretical risk. Cheap marketplace cells tend to claim inflated capacity and run unstable, which makes it worse. Cleaning the contacts and checking the charger you can safely try yourself. But if the pack won't charge, the tool cuts out under load, or the cells look swollen — the next step needs the bench. Bring the pack in for diagnostics; don't try to solder it at home.

What a proper bench rebuild looks like

So it's clear how a proper rebuild differs from "opened it and soldered":

1. Diagnostics. We measure the voltage and internal resistance of each cell group, and check the state of the BMS and the thermistor. This is where it becomes clear whether the fault is in the cells, the board or the contacts.
2. Cell selection and matching. We fit new high-current lithium-ion cells that suit this pack's exact discharge demands, chosen for matched capacity and resistance.
3. Spot welding. We join the cells with nickel strip and spot welding — never with a soldering iron.
4. BMS. We refurbish and test the old BMS, or replace it if it's the cause of the failure.
5. Balancing and a load test. We balance the pack and run it through a full charge-discharge cycle under load — the same way it will work in the tool. Only then do we hand it back.

We accept and return batteries in person only — no postal shipping. You don't have to bring the tool itself; the pack is enough for diagnostics and the rebuild, but if there's any doubt about compatibility, bring the tool too so we can check on-site.

More on the process by platform — on our page about battery repair in Riga (and separately on Makita battery rebuilds). If the problem turns out to be in the tool itself rather than the pack, the page on power tool repair will help.

Symptom → rebuild / replace / recycle

Diagnostics is the decision point: only after opening and measuring the pack can we tell you whether it's worth rebuilding or better to buy new. We identify the fault on-site.