Doubling The Capacity of Power Tool Batteries

YouTube User [Vuaeco] has come up with a novel idea, combining power tool battery packs to double the capacity.

Starting off with two slimline 2.0Ah compact battery packs, [Vuaeco] wanted a larger 4.0Ah rebuilt drill battery pack. These battery packs are different in size so it wasn’t just a case of adding in more cells in empty slots, instead he goes on to show us how to connect the batteries in parallel using some thin nickel strips. Once completed he modifies the battery casing so it fits another stack of batteries. He does this by bolting the top and bottom together with long screws, and insulating the otherwise exposed battery terminals with insulating tape. The final product isn’t as aesthetically pleasing as a real battery pack, but it looks good enough.

There are a few things we might have done differently, for instance providing some hard plastic around the insulation so should the battery get knocked in an awkward position it would still have a hard shell protecting it. Also, instead of combining the batteries together fully charged as the video suggests, we might have done the opposite approach and fully drained them, avoiding unnecessary risks. If you try this, how about giving it a 3D printed case?

64 thoughts on “Doubling The Capacity of Power Tool Batteries

      1. Nothing special, found it in the trash. Threw a hub motor and old controller on it. Does ~35mph and maybe 10 miles on a charge depending on the batteries in it. Of course you could always pop on some bigger batteries or carry spares I guess. 6 Flex-volt size packs made of Tesla cells would be good for 40

  1. The 1.5Ah version of those Kobalt packs can be had NORMAL PRICE for $10. I don’t have any cordless Kobalt power tools, but I have several of the 1.5Ah packs. They each have 6 Samsung cells inside. Excellent value.

  2. Why combine them when you can charge one while using the other? If combined, you get 2x the runtime but have to stop and recharge still. Assuming you don’t own any other batteries. He shows a real 4Ah pack so maybe he wants 2x 4Ah packs so he can charge one and use the other, in which this hack makes sense.

    1. The performance of the tool is limited by the internal resistance and maximum discharge rate of the cells inside. The bigger packs not only provide longer runtime, they also give the tools more power. You won’t notice if you’re drilling small holes or driving short screws, but if you use big drill bits, or a power-hungry tool like a circular saw or impact wrench, the big packs can give you much better results.

      1. In this case, the electronics are from a single 2.0Ah pack. I believe these packs have some form of current limiting (polyfuses?) that disconnect the cells from the output when a short occurs or the tool draws too much current. That being said, with this modified pack, he may still be at the mercy of the current limit of the original, smaller pack.

        I’m unsure if higher capacity packs utilize different current limits as the lower capacity packs… I’ve always been too cheap to buy the bigger ones.

          1. They have undervoltage protection, as it’s possible to draw so much current from the pack that the voltage sags below the 2.5 V per cell safety treshold which would ruin the pack and possibly set it on fire.

            It may be just a simple cut-off so the power tool dies on you if you try to load it when the battery is nearly empty. It’s an annoying feature of lithium power tools.

    2. I think he maybe wanted two 4Ah packs, I see your point on the smaller packs but if he was working on a job it would be easier to just take one battery instead of two slim lines. Also sometimes (not always) two slimline batteries cost less than the higher capacity packs.

  3. Lithium Ion batteries in parallel tend to have a problem where the cell with the least capacity tends to control the performance of the whole pack. If you match the cells you can skirt around it a bit, but power tool cells are pretty cheaply made, and I doubt two packs are anywhere close enough to work well long term.

      1. Not quite. The parallel cells may have different ESR due to manufacturing tolerances and aging, which means some of the cells supply more current than the others and over time some cell goes empty before the others.

  4. “Also, instead of combining the batteries together fully charged as the video suggests, we might have done the opposite approach and fully drained them, avoiding unnecessary risks.”

    If you do that, you end up with a brick. Lithium batteries cannot be fully discharged without damage.

    1. You are correct, What was meant was discharged them to the power pack cut out point. Which would sitlll leave some charge in them but would still reduce risks..

  5. I’m just curious why he bothered, unless he already had the cells.

    The saving grace of the Kobalt batteries is that they are much more affordable than the other brands, with the 4Ah pack only running ~$50 new. The 2Ah packs are $20-25 if I remember correctly, almost exactly half the price for half the capacity.

    If he was buying the cells and then modifying them, it’s false economy, as well as increased risks with exposed cells and potential to damage the cells when wiring the interconnects.

        1. I am no chemist, but I’d bet you’re right. Copper would oxidize/corrode in general. Not to mention the different environments the tools/batteries have to be built for.

  6. If you need more oomph, use corded tools. The only battery powered tool I tolerate in my shed is a Makita impact driver. Quite useful because I have three battery packs so I’m never without a fully charged pack.

    1. These days, a good cordless drill can outpower most corded ones. Even my Ryobi 18V can mix mortar better than a corded drill that weighs twice as much. A DeWalt can easily hurt you from the torque.

      1. It sounds rather incredible. Though it’s true that a high current lithium cell can give you 35 Amps of current without the voltage sagging too much, that’s only 600 Watts or so. You got at least twice the power available continuously from the wall. The question of torque there is only a matter of gearing ratio.

        It might seem on the surface that 35 Amps would give you more torque than the 6-10 Amps you can safely pull from the socket, but there are fewer turns of copper in the motor for the lower voltage of the battery, and fewer amp-turns means weaker magnetic field, means weaker torque.

        The only advantage I can think of for the cordless drill is the fact that the universal motors used in corded power tools have slightly lower stall torque than permanent magnet DC motors of the same size and construction, but the corded power tool can easily have a motor twice the size because it has the power to run it.

    2. +1 Corded tools are wayyy better if you’re near a plug, and most people make their stuff in their garage or basement anyway. Cordless is only good if you have to do a shed far away from your house or something.

      1. Nah, cordless is less faff, and fewer cables trailing around the place is safer, especially with circular saws. And you’re never tempted to work at the limits of the cable length and end up pulling on the wire mid cut.

    3. Makita does make excellent impact drivers. A friend of mine bought a beat-up used display model from the hardware store. Was the best tool ever until the batteries finally stopped taking a charge. The 12V lithium batteries lasted 5 years, used weekly. Can’t remember what model it was, but I know they stopped making it.

      1. I’ve got an ancient one as well; it uses 14.4 V NiMh packs. Heavily used for years by a pro who then gifted it to me. It’s a bit clunky by modern standards but probably the best tool in my shop.

      2. Newer makita drills on the other hands are no longer of good quality. Bought one only to find unacceptable chuck run-out. I searched on the the internet and it turned out to be a common problem wih new makita drills. I ended up getting a small Bosch drill instead which rotates exceptionally true. I use it to drill PCBs.

  7. There’s a ton of Ryobi ONE+ things on Thingiverse to adapt those batteries for other uses or to other power tools.

    The neat thing about the ONE+ tools and batteries is Ryobi has been using the same physical shape and connections since they first went to 18 volts.

    Thus a 20 year old Ryobi ONE+ drill can use the latest Lithium-Ion ONE+ battery. And if you want, you can use the old NiCd batteries in brand new Ryobi tools.

    Curiously, Ryobi skipped NiMH for their tools. The LiIon chargers can charge NiCd ONE+ batteries, but not the other way around. The LiIon batteries have additional contacts at the base for the charge control. Discharge limiting is built into the battery to make them backwards compatible with every ONE+ tool.

    Now why can’t any other tool company make that kind of commitment? Design a thing and stay with it long term?

    Sanyo did with their TV remotes. Any Sanyo remote from 1995 through their last analog tuner only model works with all Sanyo TVs (except picture-in-picture models) made during that period. Samsung used the same 19 volt charger for all their laptops from the mid 1990’s through at least 2008.

    HaD could do an article on long term design stability, use these examples from Ryobi, Sanyo, and Samsung, and any others – if there are any others. If something works and fulfills its purpose, why change it for no damn reason other than changing it just to make it incompatible with what came before?

    The manufacturer benefits because they don’t have to waste time and money on pointless redesigns. The consumers benefit because it makes replacements of those items easier to obtain, and often at less cost, especially in secondary markets.

    It’s the same thing as when car companies went to putting air conditioning, power steering and power brakes and other options, like heaters, standard across the board. They saved tons of money NOT designing heating systems without AC, and brakes and steering systems without power assist. Worker training was also easier because they didn’t have to learn two versions of those systems.

    1. NiMH requires a special charger that pushes constant current through the cell and detects a slight voltage dip towards the end of the charge to tell when the battery is full, so you can avoid cooking the cells to death.

      Lithium and NiCD behave more like each other – they’re both charged up to a certain voltage with a simple current limiter on the way there. The reason why the NiCD chargers can’t charge lithiums is because the lithium cells are more strict about the voltage limits: the cells catch fire if you overcharge them, while the NiCDs don’t mind.

  8. AAA, AA, C, D, and 9v that’s as standard as it got. The 18650 Li cell seems to be somewhat of a standard though, just not as a consumer item. These Kolbalt batteries are lauded for price and availability on e-bike fora. Praise one tool mfr. that gets it right, but they are still proprietary. Every car in the world has 12volts on board and much will run on it. Yet there is no worthwhile connector standard. The power pack of tool size should be standardized too.

    1. 18650 is rapidly becoming a consumer standard, too. Lots of people are buying external chargers now and using 18650 in LED flashlights and vaporizers. As people get more comfortable with them, expect more products to get standardized.

  9. This is a nice hack. However I prefer two smaller packs over one larger pack. The reason is simple, one pack stays in the charger and one in the tool to do the job. The smaller packs also tend to charge very quickly. My own packs charge in about 30minutes or so, much faster than my burn rate through the impact driving 3inch screws into wood.

  10. I too was believing the charge time is also dependant on ensuring the batteries don’t cook – so smaller (std) packs = more surface area..
    I would imagine a good hack for using duplicate standard packs would be to have a voltage sensing switch which cycles through each pack in turn until it gets to the packs’ minimum voltage threshold – when it switches to the next pack in the sequnce. This would give best range as batteries not in use have a chance to stabilise before cycling through a second use.

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