Introduction: Lithium Upgrade to 18v Black and Decker Single Source Battery Pack

Picture of Lithium Upgrade to 18v Black and Decker Single Source Battery Pack

I have 2 Black and Decker line trimmers that uses their 18v nicad "Single Source" HPB18 battery pack. Inside are 15 sub-c nicads tab welded in series.

The orange battery packs are the high capacity version, black is standard. Stay away from the hi-cap packs. They burn out easily. To get the higher capacity in the same size cell, the internal separators are thinner and prone to burn through.

After both hi-cap packs went bad I opened them both up and salvaged enough cells to build one and that one worked once and it died too.

Black and Decker prices are not rational. I bought a rebuilt line trimmer, hi-cap battery and charger for $28 with a 2 year guarantee. A single battery new is $26. There are lithium versions for sale but a single lithium pack is at least $26 and the charger used is over $32.

I managed to get 3 normal capacity battery packs and three chargers for eBay for a good price, but there are large areas of my lawn that can be line trimmed much easier than mowing, so I needed more packs.

I disassembled the 2 dead packs and removed the nicad clusters.

Then I built a pack of six 18650 sized lithium-ion 3.7v cells in series. The cells are held in two spring-loaded plastic 3-packs that are connected in series with one wire soldered between them [the red wire visible in the second picture]. 6 cells in series at 3.7 is 22.2v The nicad pack when fully charged is 21.75v The lithium cells are protected with a small chipset under the negative end. As such they can be carefully soldered, but not tab welded. The spring-loaded packs eliminate the need to solder any cell and if a cell goes bad makes it very easy to replace the bad cell.

The lithium packs are stuck together with 2-sided tape and then shimmed with foam tape so that it doesn't rattle when the shell is reassembled. Two wires are soldered to the pack and then to the charge terminals inside the pack. The screws holding the shell together are T-10 torx. That size is common on auto headlamp adjustments so you can buy the screwdriver at an auto parts store for cheap if you don't already have one.

Step 1: Fixing the Original Charger for Lithium Cells

Picture of Fixing the Original Charger for Lithium Cells

Even with the protection circuit, the charger supplied for nicad use will quickly overcharge and destroy the lithium pack. If a 3.7v lithium cell is charged above 4.2v and left that way, metallic lithium will plate out inside the cell and ruin it.

I opened the charger head [also T-10 torx screw] and added cheap 50v rectifier diodes in series with the positive terminal until the finish voltage did not go above 25. Six times 4.2v = 25.2v

It does not harm a lithium cell to undercharge it, it just shortens the runtime. 4.1v for each cell is much better than even one reaching 4.5v. Even with the diodes and protection disk on the cells, it's still a good idea to only charge the pack for an hour or so and then check the voltage. These lithium packs charge much faster than the original nicads did.

the lithium cells on eBay and

the plastic holders.


russ_hensel (author)2017-03-20

Just a note to let you know I have added this to the collection: Cordless-Drill-Battery-Maintenance ! ( even though you do not show a drill )


Take a look at a bunch of different/similar approaches to this project.

DavidN71 (author)russ_hensel2017-03-20

Thanks. I avoid cordless tools except where I don't have an easy choice. It seems batteries are expensive and don't last very long. My parents used to live on a boat, so there were many times safety came first even though they had more onboard 120v power away from the dock than most houses. I put a vacuum cleaner cord on a Makita drill once and ran it from my car battery. The on-board battery could not be swapped out. When it ran dead I had to wait for a recharge.

r2musiczone (author)2016-08-10

I just made one of these. Used good, high quality battery holders and ensured the wire gauge was more than adequate. I bought batteries similar to what you described (over voltage/current protection) With no load, voltage is right at 24VDC. When I tested on the trimmer, as soon as the load increased (like when trimming heavy grass), the power dropped considerably, to where it was inadequate for trimming. I could pull the battery out and see the voltage coming back up (from around 22VDC by the time I got it out). The batteries were warm, but not hot. Anybody had any issues using this with your trimmer?

I did try replacing 4 of the cells with standard (no protection) 18650 batteries, but same result. Unfortunately, I don't have 6 to try. Could this be an issue with current protection kicking in on the batteries?


DavidN71 (author)r2musiczone2016-08-10

For high current draw you need cells rated with 25C. A weed wacker is pretty much a dead short when trimming grass. That's one of the reasons the original nicads don't last long. A cordless drill doesn't pull anywhere near the current and therefore is much gentler on the battery pack.

r2musiczone (author)DavidN712016-08-10

Thanks for the info. I have the UltraFire BRC 18650 4500mAh, looks like the same you linked to. From what I can find on the net, they are definitely a lower current battery. I am having trouble finding "C" ratings for specific batteries, but can back it our based on the formula (C = mAh*MaxCurrent). I am seeing some higher end LG and Sony 18650 batteries with ~30A max draw. Would these be adequate? What do you use for your trimmer, and how does it compare with original nicd?


DavidN71 (author)r2musiczone2016-08-11

The cheap cells I bought don't do well on a weed wacker, but they aren't soldered in, so they can be swapped out for better if I wanted. I still have a couple of cheap nicad packs and between all 5 I can do what needs to be done. The cheap cells were like $1 each so I didn't expect miracles. I have bought completed packs from HobbyKing and they are careful to grade their packs/cells and charge accordingly. I built 2 packs from 3.2v lithium iron phosphate cells for a cordless drill at work and those work well. Very fast recharge times.

But even those had issues with the spring loaded battery holders softening from heat when improperly recharged. The lithium iron phosphate cells got hot enough to melt off the plastic label when overcharged. I was using the wrong charger. They also quit working, but at least didn't catch fire.

TES86 (author)2016-07-01

So, the red wire is soldered between the 3-packs. What happens to the black wire from the same pack? Is it soldered along with the red?

"The cells are held in two spring-loaded plastic 3-packs that are connected in series with one wire soldered between them [the red wire visible in the second picture]."

DavidN71 (author)TES862016-07-01

The six cells are in series, however your pack is wired. So the red (pos) lead of the top battery pack is soldered to the pos output tab on the completed unit, which is also connected to the pos lead of the charging circuit.

The short red lead soldered between the two battery packs connects the neg of the top pack to the pos of the bottom pack. The bottom pack neg lead goes to the neg completed unit output tab and the neg charge connection

Bob Barnes (author)2016-06-14

dang... I guess I need to buy 4 more plastic holders then... Looks like the seller on the posted amazon link is out of them for now.



-one more thing... I'm having a difficult time with the factory solder on the 2 metal blades (plus/minus)... why won't it melt? I even tried a blow torch... :)

xbeerd (author)2016-06-12

Ok. My last question, I hope. Hah

So I got a stronger 24v power supply. (2Amp out). It meters out at 24.2 or so, which I'm good with, But so.... I need to bypass the circuit board in the original charger right? And solder the power leads to the battery connectors directly? I just thought I'd try using the board and fried a resistor on it. Doh.

DavidN71 (author)xbeerd2016-06-12

24.2 is excellent. Wire directly to the pack from your supply. The cells themselves have current limiters in the base end and 2 amps is the max your power supply will put out, so you should be safe. I still would not leave lithium packs plugged in unattended for long.

xbeerd (author)DavidN712016-06-13

Cool did that and it charged up to 24v in about an hour or soft on 21. That being said the pack was plenty cool as far as i could tell but on the other hand my power supply was a tad warm haha. Maybe I'll add a cooling fan to it LOL!

Bob Barnes (author)2016-06-09

OK... I just got everything wired and as soon as I put the first 3 batteries in... it started smoking! I immediately got them out and noticed that it was the 2 wires(red and black) that created a series...? They look like tiny 22g or 18g at best..?

xbeerd (author)Bob Barnes2016-06-12

My battery holders with the "coil spring" type negative cooked too. I found the holders where the cells "snap" in more robust (link in my comment below)

xbeerd (author)xbeerd2016-06-12

This was due to the amount of current, not a short or anything

Bob Barnes (author)2016-06-08

yes, I did have to remove both center screw posts to make it fit.


xbeerd made it! (author)2016-06-07

just wanted to add, the battery holders i mentioned below in my comment are smaller in width than the plastic holders noted in the instructable. the plastic is a little thinner, but the contacts are larger, the wire coils on my other holders fried when i tried to use this in my weed whacker.

here are some pics, i also drilled a couple small holes in my charger to insert my DMM probes in to check voltage. and you can see my ghetto fabulous diode boards, once i get this all firmed up i'll be 3d printing a small addition tot he top of the charger to conceal.

xbeerd (author)xbeerd2016-06-08

just ran the pack in my blower, it works. LOL. ran it until shut off and now going to fully charge the pack, however, with 210mA output of the charger, i suspect this will take quite some time, as the total capacity of all the cells is 2600x6
on my charger, there is a Green LED, i seen yours has a red. well, after about 2 hours now my voltage has not changed, it was 21.8-9 on my voltmeter after running till it stopped on the blower when i put it on, and still sits there now. am i missing something? or is this 210mA power supply just to much a sally for it? the charger still says its puttin out 24.5v when i test it without the pack on.

Bob Barnes (author)2016-06-02

Great write up... I just got all of the parts you listed and notice that the plastic battery holder is a little too wide for the original shell? I 'm hoping that I can make it fit even if I have to remove some plastic somewhere..?


xbeerd (author)Bob Barnes2016-06-03

to get my packs to fit, i just had to snip off the center screw holes in the original battery casing.

Bob Barnes (author)xbeerd2016-06-03

thanks...that's what I thought I would do as well. I'm thinking just removing one of the center screw posts will make it fit.

xbeerd (author)Bob Barnes2016-06-07

if your packs are like mine, you'll need to remove both. or maybe one you could shave down with a knife, but honestly. i just used a side cutter/wire cutter and snipped it off in the span of a few mins, i dont see much reason to try to save one of them.

DavidN71 (author)2016-05-20

Xbeerd: There isn't a lot of room inside the battery chargers, so the 1 amp 50v diodes would fit well. Radio Shack sells 6a 50v diodes but they are probably too fat to fit. These cost more but would fit and the amp rating is much better.

xbeerd (author)DavidN712016-06-03

so, i was looking at my single source charger, the 'wall wart' says its 24v output. and with just a few resistors inside and a diode or 2., in theory, cant i just bypass those resistors and connect the wart directly to the battery pack without modification? (i wanna put a meter on the output to verifiy voltage yet, but theres nothing i see in there thats going to UP the voltage)

im going to run the charger off a timer to make it turn off after an hour so i dont forget to unplug and burn down my shed, LOL.

xbeerd (author)xbeerd2016-06-06

i needed 12 of the diodes i got to drop the voltage down to 25v, actually its 24.7v when charging, however, seems as the battery holder isnt robust enough to hand the current output, coils on the grounds cooked and barely makes good contact because of how tight the cells fit, my pack voltage reads 0v after being on the charger for an hour, but each cell reads 3.7v so im leaning at the battery holders as being the issue here.

i went and ordered this style of cell holder...

DavidN71 (author)xbeerd2016-06-06

The wall warts that come with the original batteries are kind of harsh and cheap. Sometimes it's easier to just buy a switching power supply and use that.

Even with nicads you are supposed to allow the pack to cool after charging and cool before recharging, otherwise you can easily damage the pack. The 24v 3 amp version is $11.30 Switching power supplies usually have tight voltage regulation. 24v is 4.0 per cell, which is 90% and safe to leave plugged in for a long time.

xbeerd (author)xbeerd2016-06-03

finally got a meter on it, the output was 30v. glad i didnt just give er and go

xbeerd (author)2016-06-03

im curious, what is the amp output on your charging power supply (wall wart)

mines 210mA@24VDC.

Richard Young made it! (author)2016-05-22

Great instructions for this awesome upgrade. I found the diodes and battery holders on Amazon. I made the first battery pack without the holders but will change it over later. I just finished adding the diodes to my second charger. Everything is working as described. Thanks again.

xbeerd (author)2016-05-20

i was looking at 50v 1A Rectifier diodes, are those sufficient?

Why (author)2015-07-03

This is the way to go. Have eBay items saved but I'm a little uncertain on the charger mod. Two questions: 1) "added cheap 50v rectifier diodes in series with the positive terminal until the finish voltage did not go above 25. Six times 4.2v = 25.2v'. How many rectifiers did you have to add? 2) Does the amp rating of the diode matter? Thanks!

DavidN71 (author)Why2015-07-05

The original chargers varied a lot on output voltage. I picked 2 that had the lowest voltage (no-load) and added diodes until the output was correct. Charging current is low, it's the 50v rating that matters. Most of the chargers put out close to 30v no load. It took 5 or 6 diodes dropping .6v each to get the final voltage close. I still don't leave the charger plugged in for long without pulling the battery to check it.

Why (author)DavidN712015-07-08

OK, got it! Thanks for the clarification. Off to get 'er done as the various B&D tools are hungry!

Why (author)Why2015-08-20

Well, got one pack done. However, an interesting problem. The pack works just fine in a grass trimmer but fails totally in anything else, i.e. B+D hedge trimmer or pole saw. These tools just drain the pack while running at a very slow speed very quickly dying away. Something is different here than is with the trimmer: amps maybe? Anyway, be advised: LI pack works great with grass trimmer not so with other tried tools.

DavidN71 (author)Why2015-08-21


The output of lithium cells varies a lot. They are graded at manufacture according to internal cell resistance, which is the best indication of dead short amps. The ratings are multiples of C, the hourly rate needed to charge the cell. A 10C lithium cell can deliver 10 times C (amps) of current. A 25C cell can deliver 2.5 times as much as a 10C. Higher C values command higher retail prices.

The reason NiCads are still used is that they can deliver very high currents without self-destructing, but that also means if you discharge a series pack of them too far the high current across a weak cell will cause a voltage drop high enough to reverse that cell and ruin the pack.

Bottom line is battery powered tools are hard on batteries, but sometimes you need the portablility or safety from not being plugged into 120vac.

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