It's a familiar story. Later a few years of use, the cordless screwdriver needs charging more than often, and the cordless vacuum cleaner doesn't have the energy to collect your crumbs. In my case, it was the robot lawnmower giving upwardly the ghost mid-mow, stubbornly refusing to render to its charging station. The simplest and nearly costly solution is to social club a replacement battery pack. But have y'all considered just replacing the cells in the battery pack? This approach saves money and reduces waste. Furthermore, you can select replacement cells with a larger chapters than the originals. This isn't just a repair; information technology'due south an upgrade!

It's All Gone Quiet…

Battery-power ed equipment running on Li-ion cells certainly retains its functioning much longer compared to the NiMH jail cell-based ability tools of the by. However, after many charge/discharge cycles, there comes a time when the energy storage capacity of even the best lithium bombardment drops so low that the bombardment pack needs to be replaced. I take already seen this with many of my own devices, and friends and colleagues oft turn to me for advice on this topic.

The simplest solution is to visit the equipment manufacturer's website to see if a replacement battery pack is available. Sometimes there isn't and, when in that location is, the prices tin can come every bit a shock. In my example, the equipment worked only fine and looked to have a skilful few more than years of life in it — a new bombardment would suffice. In such cases, it can exist worthwhile hacking the battery pack and replacing the individual cells when the fourth dimension comes, which is oftentimes cheaper overall. We can fifty-fifty consider improving the performance by replacing the original cells with some of a higher specification. If you besides choose this road, you'll demand to fire upwardly the soldering atomic number 26 in addition to breaking out the screwdrivers.

In my case, I noticed that my Robbi lawnmower only wanted to mow for half an hour before it beetled off to its charging station for a i.five-hour recharge.[i] Previously, the usual pattern had been an hour of mowing followed by a one-60 minutes recharge. Was this change in routine a sign? Having had Robbi for four years and knowing it is powered by lithium cells, it was clear that they would likely demand replacing.

Later that afternoon, I realized it had been tranquillity outside for some time. There, in the middle of the lawn, Robbi had shut down and couldn't exist woken by pressing its buttons. I lugged it over to the charging station and put it on charge. Robbi eventually sprang into life, and I read from a bill of fare option that its elapsed operating time was ii,938 hours. Corresponding to about i,500 accuse cycles, it was clear that I should consider sourcing a new battery soon.

'Before long' came earlier than expected. After its full charge, Robbi finished its work, but it didn't wake up the side by side forenoon. Putting it on accuse didn't assist either. I had recently treated the lawn with some fertilizer, and we have had quite a lot of rain, so I needed to act quickly. I could virtually hear the grass growing…

Hack a Battery Pack?

The manufacturer'due south replacement battery pack was priced at around €100, and a replacement from a 3rd-political party supplier was bachelor for around half that price, which is not that bad. From its specification, I was looking for an xviii V replacement pack with a capacity of two.1 Ah. That meant five cells, probably in the standard 18650 outline. To confirm my suspicions, I set about removing the lid of the battery compartment.

Unsurprisingly, this was easier said than washed. Iii of the screws came out easily, only the fourth was completely jammed. Somewhen, it came out, but the captive mounting nut was ripped out in the process, jammed on the spiral thread. It looked similar it has been cross-threaded during assembly at the manufacturing plant. Effigy1a shows the convict nut recess in the mower casing after removal. Some debris remained. I eventually separated the spiral and nut and, despite part of the captive brass nut breaking off (Effigy1b), it looked repairable.

captive nut recess in the mower casing after removal and the nut
Figure 1: Ane of the screws had been cross-threaded during assembly. It eventually came out but took the captive nut with it. It was repaired using epoxy.


The battery compartment is designed to exist waterproof, so the captive nut required remounting to ensure all four screws could be tightened for a good seal. I used an epoxy adhesive to secure the nut, and yous tin see the consequence in Effigy2b. The brass nut at the elevation left looks as if information technology's been at that place forever. With the mechanical role stock-still, attention turned to the battery pack. Figure2b shows the bombardment in its compartment. The outline of five cells is clearly visible, and a ruler confirmed that they were 18650 cells. In that location is also a lot of complimentary space (Figure2c), which got me thinking: could I meliorate employ the space by using more than or larger replacement cells? My mind was now made upwardly: purchasing an off-the-shelf replacement battery was no longer an option.

The battery compartment
Figure 2: Battery compartment hat (2a), with the former battery in situ (2b) and the empty compartment (2c).

Battery Cell Swap

Figure2a shows that ii recesses in the battery chapeau encroach into the available bombardment space, ruling out the fitting of ii rows of five cells to double chapters. In that location are, however, more than expensive cells in the 18650 format with higher capacity. Some of the better-known brands have cells in this format with ratings of up to three,500 mAh. Such high-capacity branded cells cost a proficient ten € each. You tin also find cells with much college ratings on eBay, AliExpress, and similar sites, but you should take such claims with a compression of common salt.

I almost placed an society for five cells until I noticed some others in the somewhat more than unusual 21700 format. Although only slightly larger, they offered significantly more chapters for the coin. I was certain they would fit in somehow (I was thinking of a row of three and a row of two to form a Westward profile). With a quantity discount and postage stamp, the five × 4,000 mAh cells came to €26 in total. They dropped through the letterbox two days later, and it wasn't a day too soon — the grass was now definitely in need of a trim.

Considerations

Building a bombardment pack from individual cells generally requires a caste of dexterity, electrical expertise, and a spot welder. Equally you can see from the quondam unwrapped bombardment pack in Effigy3, the five dark-green cells are neatly connected at the positive and negative contacts via thin, spot-welded nickel strips. This blazon of permanent connection method does not impairment the jail cell. Although the estrus generated during spot welding is intense, it is swift and localized, so the cells are barely warmed. It'due south well known you can never have too many tools, merely, in this case, I can't imagine where else I would apply a spot welder, and so I can't justify the outlay. Instead, I chose to solder the wires directly to the cell contacts. This is not recommended unless you lot follow some rules and are aware of the dangers.

The old pack, the cells and BMS
Figure 3: The old battery pack is wrapped in blackness gaffer tape (3a). The cells and BMS board are visible with the top layer of tape removed (3b). A boxed and unboxed 21700 cell in comparison with the old 5-cell 18650 pack (3c).


The battery pack used in Figure 3 is typical of that found in many other bombardment-operated devices. It consists of several bombardment cells connected in series plus a Battery Management System (BMS) PCB. This is the circuit lath shown in Figures 3b and3c. The latter image also shows a size comparison between the new cells and those in the old battery pack. The BMS performs three bones functions:

  1. Information technology balances the cells (i.eastward., keeps them all at the same voltage or charge state).
  2. It prevents the cells from being overcharged.
  3. Information technology disconnects the load in the outcome of undervoltage to avoid deep discharge.

The bit on the BMS board with the most legs manages all these tasks. Information technology is a specialized microcontroller that monitors the cell voltages (via the connector on the left). In the event of an overvoltage or undervoltage status, it disconnects the cells using 2 × ii MOSFETs (at the bottom). Further reading on balancing lithium batteries is available under [2] and [iii].

The BMS is included if you buy a whole new battery pack assembly, and so the BMS lath (which withal works) from the old pack will be redundant. On the other hand, if you merely swap the cells, you can reuse the existing BMS board. An important feature of the BMS that you demand to be aware of is described in the BMS Flip-Flop Function section beneath.

BMS Flip-Bomb Office

When I removed the bombardment shown in Figure 3c, I was surprised to mensurate 19.2 5 directly on the plus and minus contacts of the 5-cell battery pack. Had I been too hasty in ordering replacements? On the other paw, I could only mensurate nigh 18.5 Five at the battery pack connector where information technology connects to the mower motor (to the right of the BMS board), which had a very high source impedance. Using just my fingers, I could discharge this to ground, causing the voltage to drop to just a few volts. Was the BMS broken? I continued a 24 Ω load resistor directly to the cell contacts on the battery pack and measured a current of 0.75 A and a bombardment voltage of 18.2 V. I then asunder the load and put the pack onto charge. After just a few seconds of charging at 0.v A, the voltage output from the BMS switched to low impedance fashion to draw some current from the battery via the BMS. It looked similar the BMS had detected the battery voltage falling below the undervoltage threshold when the mower was last used and had turned off the FET to disconnect the battery. This 'off' state had been latched in the BMS. To examination information technology, I connected a 12 Ω load to the battery. After v minutes, the BMS switched off at just over xiii V and switched on again after another charge. Phew… With the BMS skilful to go, I proceeded with the jail cell swap.

Go on Soldering On

With no spot welder to hand, I decided to solder stranded wire direct to the bombardment terminals. As long as you are careful, this tin be done without harming the batteries. Whatever thermal harm inflicted on the constituent materials of the cells is roughly proportional to the integral of time and temperature. In other words, yous need to exist quick! Three things are important here. Firstly, the soldering iron must exist powerful plenty for the tip to maintain its temperature during soldering. This ensures that the end of the cell quickly achieves soldering temperature. A solder that melts at a lower temperature is as well appropriate. I used a xc W iron equally it has a regulator that allows the flake temperature to exist set at over 400°C. It is all-time to avert pb-free solder for this job as it melts at a higher temperature and does not wet the surface as nicely every bit the good one-time SnPb 60/40 that I prefer. In my experience, the metal contact surfaces of the lithium cells take the solder easily. With the tip temperature set to 385°C and using 1 mm diameter, flux-cored solder, I completed each joint in around one 2d — fast plenty to avert dissentious the cell.

If you demand to keep the soldering tip on the jail cell contacts for much longer than this (considering the soldering fe is underpowered, the temperature is too low, or through the use of lead-costless solder), you risk overheating and damaging the cell. This will impact the jail cell's electrical capacity and mayhap reduce the number of accuse/belch cycles. As long as the fe is only in contact with the surface for around a second, information technology shouldn't cause any harm. Alternatively, y'all could buy some slightly more expensive tagged cells that come with a short nickel strip already spot welded onto the cell contacts. The strips can and then be soldered together to make upwardly the pack without any hazard of overheating the cell contents. In principle, this is safer, but you must insulate the bare strips to prevent whatsoever short circuit. A curt excursion during normal operation volition create a significantly more dangerous status than could occur by applying a short nail of heat to solder wires to the cell contacts.

A close-up of my soldering method can be seen in Figure4. The positive contact consists of a metallic cap attached to the bombardment electrode at iii points in this cell. This arrangement increases the thermal resistance between the outside contact surface and the cell's internal structure, making it more tolerant of the soldering process. Start by applying small drops of solder to tin all the positive contacts (Effigy 4a) quickly. Next, cut 1.five mmii stranded wire to a length of 3 cm, which allows some leeway for later positioning of the cells in the pack. These can at present be soldered direct onto the negative contacts (Figure4b) in ii stages. Firstly, employ the soldering iron and solder them quickly to constitute a small tinned area in the center of the contacts. Next, strip and tin can the ends of the connecting wires. One time the cells have cooled, chop-chop solder a wire to each of the negative contacts. In Figure4c, yous tin see the other finish of a wire now soldered to the positive contact of the next cell to brand the series connection. The thinner ruby-red wire is the jail cell voltage sense connection to the BMS lath.

Battery contacts and solder
Figure 4: The positive contact is more tolerant of the soldering procedure (4a). Brusk lengths of wire are soldered to the negative contact (4b). The cells are finally connected in series, and the thinner voltage sense wires are connected to the BMS (4c).

Assemble and Test

As already mentioned, the battery compartment cannot adapt the five cells arranged in rows of two and three to form a Due west configuration, and then I had to find a dissimilar pack construction. In Effigy5, yous can come across that iv of the cells are placed next to one other and fixed in place with hot glue. The fifth jail cell is glued across the other four. The hot gum makes the finished assembly very stable. You could alternatively use silicone sealant here.

Finished battery pack and BMS board
Figure 5: Underside of the finished pack (5a) and top view with the BMS board (5b).

The double-sided tape on the back of the BMS board did not demand to exist replaced as information technology held firmly to the new bombardment pack. All that remained was to connect the vi wires from the white connector (Figure5b) to the corresponding cell contacts, along with the plus and minus connections of the entire pack, to the BMS lath. This must exist completed before nosotros tin test the new bombardment. Double-check all the wiring to ensure you oasis't made any mistakes. Taking a photo of the old battery pack volition assist at this later stage when checking everything. The battery pack functioned equally expected and could be charged and discharged without a problem.

The finished battery pack was wrapped in gaffer tape to make the assembly more robust and provide insulation and moisture ingress protection (Figure6) before fitting into the mower's battery compartment. After screwing downward the lid and switching on the mower, information technology wanted to calibrate itself with the signal from the guide cable and to starting time mowing immediately. I canceled the latter functioning and put it into its charging station. It needed a full three hours to charge, indicating that the battery now has virtually twice the chapters of the original.

New wrapped battery pack
Figure 6: The new pack wrapped in orange gaffer tape (6a) fits neatly in the housing (6b).

The robot mows as well as it e'er has — an hour of mowing followed past an hour of charging. The battery is only partially discharged during these cycles, then I presume that these larger capacity cells will tolerate significantly more charging cycles before they need replacing again. With whatsoever luck, I reckon the new pack should terminal twice equally long as the original. If that is the case, the effort involved in iii hours of tinkering was probably worthwhile. Working out the hourly wage charge per unit for the fourth dimension taken, the fiscal aspect looks poor, but, on the plus side, I at present take a solution that can't exist bought off the shelf.

This method of battery cell replacement isn't limited to lawnmowers. You lot can use the aforementioned approach to extend the life of cordless vacuum cleaners and other devices, even if the available battery space is less generous. The last vacuum cleaner I repaired using the method (using high-capacity 18650 cells) has been in use for three years without any performance complaints.

Questions or Comments?

Do you lot have whatsoever technical questions or comments about this article? Email the Elektor team at editor@elektor.com.