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The other day I got tired of buying replacement 18 V battery packs for my power tools (drill, ripsaw, etc.) and I thought to myself, surely by this day and age the price of lithium-ion batteries can't be that high? I know people who have built a gnarly e-scooter with a smaller battery budget than mine, while having 5 times as many batteries!

My gut instinct as a tinkerer was to create my own 18 V battery pack and perhaps replicate the original battery's mounting bracket in a 3D printer, so long as I can make the battery terminals align with the contacts on the tools and charger it's as functional as you need a battery pack to be, right?

Apparently not; besides the essential Li-ion batteries, the pack contains a 4-inch circuit board, possibly a logic board or microcontroller. Obviously there's no mention of this in the manual and by opening the battery pack I have voided warranty, waived liability of the manufacturer, bla bla bla.

Another point I should make is pretty much all known brands for cordless tools have their unique kind of board.

Now it occurs to me, the boards most likely have some sort of firmware configuration to monitor what the battery connects with, ie. you need a Hilti-approved battery to power Hilti tools. A module like that would be no bigger than the one in a phone; these things are absolutely massive and the only function they serve (apart from powering tools) is an LED indicator for charge level.

The question I'm trying to ask is: are cordless tool battery packs indistinguishable from a makeshift DC battery pack of the same voltage? Are the battery contact points on tools always available to receive current?

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    \$\begingroup\$ Lithium-ion batteries are basically bombs under restraint. Be very careful. \$\endgroup\$ Commented Aug 22, 2022 at 9:11
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    \$\begingroup\$ Can you take some photos of this board so we can see? Could be a typical BMS, or could be "firmware" or some way to make the pack proprietary. \$\endgroup\$
    – rdtsc
    Commented Aug 22, 2022 at 12:20
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    \$\begingroup\$ This video shows a guy picking apart a (fairly dangerous) mini-chainsaw, but from 10:00 to 15:00 he looks at the battery pack and circuit board with explanations of what the components do. It is just one example of how you can do it, and it is pretty much what @Transistor says in his answer, but if you want it in video form then here you go: youtu.be/tgNGHehIsSw?t=600 \$\endgroup\$
    – Frodyne
    Commented Aug 23, 2022 at 7:02
  • \$\begingroup\$ Here is a BMS for one kind of battery pack ... befr.ebay.be/itm/… \$\endgroup\$
    – Antonio51
    Commented Aug 23, 2022 at 7:08
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    \$\begingroup\$ "by opening the battery pack I have voided warranty, waived liability of the manufacturer, bla bla bla." - If you live in the United States, I suggest you aquaint yourself with the Magnuson-Moss Warranty Act of 1975. Here's an article that briefly discusses it. ifixit.com/News/15464/… \$\endgroup\$
    – enhzflep
    Commented Aug 23, 2022 at 7:30

4 Answers 4

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The circuit board is, most likely, a battery management system to ensure that batteries are charged in a balanced fashion. When each cell reaches a predetermined voltage (indicating sufficient charge state) that cell is effectively bypassed for the rest of the charge cycle. This prevents over-charging and resultant damage or fire.

enter image description here

Figure 1. A BMS block diagram. Image source: AllAboutCircuits.

A good BMS might also disconnect the battery when any cell reaches the lowest recommended state of charge. The linked article may help you further.

You might also find that the "battery charger" is actually just a fairly constant voltage power supply and that the actual charger (current and voltage regulator) is in the battery pack. This is how mobile phones are done; the charger is in the phone. The wall-wart is just a power-supply.

You need to be careful!

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    \$\begingroup\$ Some "management" is also made in the wall-wart ... I think it is a voltage/"current" source because of the "many" voltage batteries it can handle. \$\endgroup\$
    – Antonio51
    Commented Aug 21, 2022 at 14:43
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    \$\begingroup\$ Most (TM) wall warts are just voltage sources with an Overcurrent protection. Having anything more advanced in the wall wart is uncommon - unless we're talking USB PD, which needs books to explain all the features. \$\endgroup\$
    – Christian
    Commented Aug 22, 2022 at 12:45
  • \$\begingroup\$ and USB PD is also a negotiable voltage source. \$\endgroup\$ Commented Aug 22, 2022 at 22:06
  • \$\begingroup\$ For power tool batteries, I believe it is quite common for the solid-state switches to only be present in the tool. The very high current drawn by such tools means that the switches end up being large/expensive, dissipate a bunch of heat, and cause volt drop. The tool needs switches anyway, so essentially the BMS is more giving the tool 'permission' to start, and usually acting as a cycle counter and maybe balancer. sevarg.net/2016/09/11/dewalt-20v-max-30ah-battery-pack \$\endgroup\$ Commented Aug 23, 2022 at 9:23
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Basically a Lithium battery requires a BMS providing:

  1. Overcharge protection with cell balancing
  2. Overdischarge protection
  3. Overcurrent protection
  4. Temperature protection

Without 1) you can't do fast charge properly, as the cell with the lowest capacity will overcharge and either explode or have a short life, depending on chemistry. This requires monitoring voltage on each cell.

Without 2) the pack would be destroyed by overdischarge. This also requires measuring voltage on each cell to cut off current when the lowest capacity cell reaches the lowest allowed voltage.

Both 1 and 2 have to be done in the pack (although the MOSFET switch may not be in the pack, just the sense circuit and logic), otherwise the connector would need many more pins which are points of failure. This is practical for RC Lipo packs where the priority is lowest weight. It is not practical for power tools where the priority is toughness (no flimsy multi pin connector).

3 has to be done in the battery pack because the tool doesn't know the maximum current the pack can deliver, but the pack does.

4 also has to be done in the battery pack because that's where the temperature probe is. Charging Li batteries is dangerous if they're frozen or too hot.

The question I'm trying to ask is are cordless tool battery packs indistinguishable from a makeshift DC battery pack of the same voltage? Are the battery contact points on tools always available to receive current?

I will only answer for the brands for which I know the information.

Makita batteries don't check if the tool is Makita. You can use them on anything that has the proper connector. There are several tools and adapters online for these batteries. This is quite safe because the battery has its own protection, so if whatever is plugged into it fails and shorts, it will disconnect itself. The batteries have an extra connector to talk to the charger. The charger allows LiIon and NiMH from 7.2V to 18V, so the battery has to tell it what voltage and current to use. The onboard micro also counts charge cycles, which they use for warranty.

I changed the cells in LIDL/Parkside batteries. So for this brand (and probably many others) it is possible to replace worn cells and get a "new" battery. You need tabs and a spot welder. For this brand, the 14.4V battery fits on the 18V charger. I have not tried to charge the 14.4V battery on the 18V charger though, for obvious reasons. Please do not stick the hot end of a soldering iron on a Lithium battery. It's a bad idea:

@TooTea: soldering cells directly will likely partially melt the (plastic) internals of the battery keeping the electrodes apart, so the cell could short out internally at any point. In the case of lithiums this is likely going to make the battery go up in flames (perhaps while you're soldering it, perhaps two years later), but it's just as much of a bad idea for nickel batteries

It is very likely that many other brands will let you change the cells in the pack. If this works, the microcontroller on the pcb will probably use its previous settings, including max charge/discharge current, so the new cells must be chosen to be compatible with the old ones (same chemistry). I wouldn't be surprised if some brands keep the setting in RAM, to make sure the micro forgets them if it is unpowered during cell replacement, which would screw the pack and make it useless.

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    \$\begingroup\$ It should be noted that a lot of the more modern & high power offerings don't do the over-discharge and over-charge protection within the battery (if you inspect the board carefully you will be able to see the main power terminals are linked directly to the cells). The pack does manage cell balance though. The charger is then responsible for over charge protection, and the tool for over-discharge. I believe they have switched to this to reduces losses in adding additional semi-conductors into the current path (modern tools pull a lot of juice), it does make it less hacker friendly though. \$\endgroup\$
    – Hugoagogo
    Commented Aug 22, 2022 at 4:09
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    \$\begingroup\$ Perhaps worth pointing out that soldering cells directly is not just a "bad idea" in the sense of "could somewhat shorten the lifespan, no big deal". It will likely partially melt the (plastic) internals of the battery keeping the electrodes apart, so the cell could short out internally at any point. In the case of lithiums this is likely going to make the battery go up in flames (perhaps while you're soldering it, perhaps two years later), but it's just as much of a bad idea for nickel batteries. \$\endgroup\$
    – TooTea
    Commented Aug 22, 2022 at 8:32
  • \$\begingroup\$ @TooTea Thanks, added to the answer \$\endgroup\$
    – bobflux
    Commented Aug 22, 2022 at 9:16
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    \$\begingroup\$ Great explanation of the "why" as well as the "how" \$\endgroup\$
    – iono
    Commented Aug 22, 2022 at 17:00
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    \$\begingroup\$ Instructions clear: only solder with the cold end of the soldering iron. \$\endgroup\$ Commented Aug 22, 2022 at 22:07
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Because they need PCB to mount components. Battery packs for any device, including laptops and drills, needs components for battery pack protection and management circuitry, including cell balancing, to have a safe pack with long useful life span.

Sometimes they are also used for fancy features like displaying battery charge level when you push a button.

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A nickel-cadmium or nickel-metal-hydride charger can be pretty simple. If one doesn't need to charge a battery very fast, one can simply feed a constant amount of current into a NiCd or NiMH battery without regard for its present state. Although the battery will start converting all energy supplied into heat, this will be relatively harmless if energy is being put in too slowly to heat the battery much above ambient. Fast-charging circuitry needs to be more complicated because batteries will be damaged if they are overcharged with enough current to overheat the insides. Even if such batteries are grossly overcharged, however, the consequences will be limited to the degradation (possibly to the point of uselessness) of the batteries involved.

Lithium-ion cells, however, are another story. Their electrical characteristics throughout their charge/discharge cycle are harder to judge externally than those of NiCd or NiMH cells, they are more readily and severely damaged by overcharging or over-discharging, and if they aren't treated properly they are prone to enter a thermal runaway state which results in them emitting streams of flaming gases. This can have consequences far more severe than the destruction of the batteries in question. Further, lithium ion cells will last longest if the last bit of charging is done far more "gently" than the bulk of the charging, but the electrical characteristics of the battery won't noticeably change at the point where charging should be throttled back. A monitoring circuit that's permanently mounted to a battery, however, can keep a count of how many electrons have been externally pushed from positive to negative, and how many have been allowed to externally flow from negative to positive, how many hours the battery has sat idle, etc. If a charge monitor has seen 3600 coulombs (1 amp hour, or about 22.5E+21 electrons) flow from negative to positive without any having been pushed the other way, it may advise the charging controller that it may safely push about that number of electrons through the battery at a fairly high current without risk of over-charge, since it would take at least that many electrons to recharge the battery fully.

Even though the power-control circuitry need not be part of the battery pack, the charge-monitoring circuitry needs to be kept assembled with the pack in order to ensure that the battery doesn't get charged without its "knowledge". If one were to drain 3600 coulombs from a battery, disconnect it from a charge monitor while keeping the monitor powered, put 3000 coulombs into the battery externally, reconnect it to the charge monitor, and then plug that assembly into its normal charger, the charger would likely overcharge the battery significantly before the battery's electrical characteristics changed enough for it to notice. Even those circumstances shouldn't create a risk of thermal runaway, but would be likely to severely degrade the useful capacity of the battery.

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  • \$\begingroup\$ Hmm I’ve never seen a coulomb counter used for charging; only for measuring state of charge. Most chargers just apply a constant 4.2V for a constant time in the final phase. \$\endgroup\$
    – Navin
    Commented Aug 22, 2022 at 22:34
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    \$\begingroup\$ @Navin: I was under the impression that the better charging designs would use coulomb counters to turn off fast charging before end-of-charge. I've also seen laptops with a configuration option to either charge fully, or charge to about 95%--a feature that would require a coulomb counter. \$\endgroup\$
    – supercat
    Commented Aug 22, 2022 at 22:37
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    \$\begingroup\$ I'm no expert, but the li-ion an li-po chargers I have inspected just used CC CV charging, without energy monitoring. Implemented as just constant voltage with current limit, not some staged construction. \$\endgroup\$
    – Sebi
    Commented Aug 23, 2022 at 8:31

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