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





Introduction: 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

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.

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2 Questions

Any reason not to replace the 6 ni-cads with just 5 lithium cells which would equate to 18.5V when at design voltage or 20.5V max?

No reason, except that lithium cells are cheap and with 6 cells you get longer run time and don't need to change the charger except to add a couple of diodes. If the cost of the cell matters that much, then you could certainly do that. The tool won't care much.

Thanks David. I’m not concerned by cost (just ordered a box of 10 cells) more about protecting the motor - is it an issue putting potentially 28V through an 18V motor? - and keeping things simple by using cell cases within the body and charging the individual cells using a 4.5V charger. It only takes a few seconds to get inside the main case to access them. Just thinking “out loud” really. Cheers!

I'm just wondering where does the rectifier go in this project? A pic or two would be nice if available.


It's in one of the pictures. I cropped it below. The rectifiers are under the black shrink wrap.


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.

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?


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.

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?


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.

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]."

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

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... :)