Which battery lasts longer per charge?

The BL1850B. Makita rates it at up to 65% more runtime than the 3.0Ah BL1830, versus 35% for the BL1840B. Expect roughly 25 to 30% more work per charge from the 5.0Ah.

Is the BL1850B worth about $20 more?

Yes if you use high-draw tools like circular saws or grinders for long sessions, or want the LED charge gauge. For drills and light tasks, the BL1840B covers most jobs for less.

Do both fit the same tools?

Yes. Both are 18V LXT packs that work only in tools with the Star Symbol or a yellow battery receiver. Neither fits older non-Star Makita 18V tools.

Which charges faster?

The BL1840B, slightly. Makita lists 40 minutes or less for the 4.0Ah on the Rapid Optimum Charger and 45 minutes or less for the 5.0Ah.

Can I use them in cold weather?

Yes. Both deliver consistent power down to -4°F (-20°C), so winter job sites are fine for either.

Which is the better spare to keep in the bag?

The BL1840B. It is cheaper at $76.97 and lighter, which suits backup duty. Keep the 5.0Ah in the tool and the 4.0Ah as the spare.

Makita 18V LXT 4.0Ah Battery BL1840B vs Makita BL1850B 18V LXT 5.0Ah Battery

Both are genuine Makita 18V LXT batteries that fit the same Star Protection tools. The BL1850B gives about 30% more runtime per charge and adds a built-in LED charge gauge, but costs about $20 more. At the November 7, 2026 prices, the BL1840B is $76.97 at eBay and the BL1850B is $97.49 at Amazon. Buy the 5.0Ah if you run high-draw tools all day; buy the 4.0Ah if you want the cheapest spare.

Today the Makita 18V LXT 4.0Ah Battery BL1840B costs $20.52 less at its best price.

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