20400maH Battery Pack


To power the mower, a 20400maH 12.6V pack needs to be built.  This is done using individual 18650 cells.

3S Protected Circuit

To create the battery pack we ned to buy individual 18650 LiIon cells.  

Please use good quality Panasonic 18650B LiIon Cells.  Its worth investing in good quality batteries once.

These single cells can be purchased online (e.g. Ebay).  Dont be fooled by manufacturers claiming enourmous cell capacities.  3400mah is the standard cell capacity.  I always use Panasonic NCR18650B cells.


Before staring to build the pack, ensure that the individual cells have almost the same voltage.  Be especially careful if you are using a mix of old and new cells.


(For another battery design for spot welding or soldering see below)

1. Solder connection spots on the positive (+ve) and negative (-ve) side of all the 18650 cells.   The +ve side will solder easily, the -ve side needs extra time to get hot as the -ve is the complete case of the battery. (do this before installing them into the frame)

2. Press the cells into the custom battery holder as shown here. The STL's fpor the battery holder can be downloaded here.

Battery Holder STL Files.zip

you may need to remove 10mm of the green packaging on the negative side so the cell fits in.  Tap them in home gently with a rubber hammer if needed.


2. Now connect the cells on the top and bottom side as shown here.  You can use electrical wire 1.5 - 2mm thick


3. Now join the seperate packs together (purple wire) and add a longer wire to go to the battery protection board (BMS).



4. Connect the wires to the BMS in the positons labelled,  Connect a battery lead with an EC5 connecter to the P+



5. Using a multimeter you should measure the following DC voltages

  • Between B- and B1 3.2V - 4.2V
  • Between B- and B2 6.4V - 8.4V 
  • Between B- and B+ 9.6V - 12.6V

If all this is correct you should measure:

  • Between P- and P + 9.6V - 12.6V

If the first 3 voltages are correct and the last voltage is not correct it could mean

A. (MOST LIKELY) The cells have too much voltage difference (mix of old and new cells) and the BMS is cutting the power.  Check the voltage using a voltmeter across these points on the BMS.  The voltage at 1,2 & 3 should be almost the same (within 0.1V).

If they are not within 0.05 - 0.1V the BMS is probably cutting the power due to the imbalance. The cells voltage need to be balanced so the 1,2 &3 have roughly the same voltage.


B. A faulty BMS board)


6. If everything measure correct the battery is ready to be charged and installed.




This design has a lot more connections and takes advantage of the ease of spot welding

STL Files Here

Insert the batteries into the holder. Pay attention to the positive and negative positions.


Spot weld the connectors as shown.

Spot weld the connectors as shown.





 Connect the BMS as shown.



Tutorial on Battery Packs:

I think its important to understand how to work with batteries.  Batteries can cause fire if not used correctly.  They are the part of the project that needs care and attention to ensure safe operation.

The following information is a good starting point if you are working with LiIon or LiPo batteries for the first time.


1S / 2S / 3S?

When we talk about 'S' we talk about how many cells a battery has in series.  A series connection connects the +ve side of the battery to the negative of the next battery and so on and so on.

Each Lithium Ion (LiIon) cell has a maximum fully charged voltage of 4.2V.  

1S = 4.2V

If we put 2 cells in series we create a max 4.2V x 2 = 8.2V or 2S battery

if we put 3 cells in series we create a max 4.2V x 3 = 12.6V or 3S battery.




In a series connection the Amps produced by the cells remains constant.  This means the voltage is increased, but the storage capacity of the battery remains the same.  The Amps or mAH tell us how long the battery can last in that situation.

To have the battery pack produce more amperage, we need to connect batteries in parallel. A parallel connection connects all the +ve terminals together and all the negative terminals together. As shown here.


In the circuit above we have added 3400mAH of capacity for every battery we added to the pack.  The pack voltage remains though at 4.2V maximum.  We therefore created a 1S (4.2V max) 20,400mAh pack

20,400mAh = 20.4 Amps.



if we connect batteries in series we increase the voltage

If we connect batteries in parallel we increase the amperage


Creating a pack for our needs.

To create a 3S 20.400mAh pack we therefore need to create a combination of series and parallel packs.

6 x 3400mAh batteries in parallel creates a 20,400mAh 1S pack.

3 of these packs in series will increase the voltage to 3 x 1S = 3S.  The amps in series will remain the same.  We therefore create a 3S 20,400mAh pack.


After creating 3x 1S parallel packs of 6 cells giving 20,400mAh, we now connect the positive of the first pack to the negative of the next pack and so on until the 3 packs are connected in series.

This pack meets our needs but the pack is not protected.

Protected packs

When we connect packs in parallel and series, we need to be sure that the batteries are charged and discharged evenly.  We want the individual cells voltage in the whole pack to have more or less the same voltage at any point in the use of the pack.  So to say +-0.1V max difference.  

We also don't want the pack to become too drained (damages the cells) or overcharged (potential hazard).

To keep all these factors under control we need to use a protection circuit which will monitor the cells voltages.

Protection boards come in different specs to protect different battery packs.  1S / 2S / 3S etc...   and the maximum amps it can handle.   The board above is a 3S max 25Amps board.

Just like in the previous example as we connected the parallel packs in series to increase the voltage, the protection circuit is used to connect these parallel packs.  The protection board monitors the voltage of the parallel packs and at the same time connects them in series to the next pack to create the 3S pack.

During charging the circuit also distributes the charge applied to the different parallel packs and ensures a maximum 4.2V is charged to each parallel pack.  This keeps the voltage difference between the individual cells in check and ensure safe use of the pack.

See the top diagram for the 3S 20400maH circuit, which hopefully now is clear why we come to this particular setup for the mower.



Charging a Pack.

We need to charge the packs safely.  To charge the pack we need to use a 3S charger.  (3S = max 12.6V charger output).  

The Amps the charger produces gives us the time the packs require to charge.  To safely charge a pack it is best to charge at 1S or less.  What does this mean.



Battery pack is a 3S (12.6V) 5000mAh

Charger is 12.6V max 5Amps.

The charger at 5Amps is the same as the pack mAh of 5000mAh or 5Amps.

5 / 5 = 1S

The 3S 5 Amp charger will charge the pack @ 1S 


Example 2

Battery pack is a 3S (12.6V) 10'000mAh

Charger is 12.6V max 5Amps.

The 5Amps has half the amperage of the 10'000mAh or 10Amps.

5 / 10 = 0.5

The 3S 5 Amp charger will charge the pack @ 0.5S 


Example 3

Battery pack is a 3S (12.6V) 2500mAh

Charger is 12.6V max 5Amps.

The charger at 5Amps has double the amperage of the 2500mAh or 2.5Amps.

5 / 2.5 = 2

The 3S 5 Amp charger will charge the pack @ 2S

a 2S charge could be damaging for the pack depending on the type of battery


So how is this for our 3S 20'400 mAh pack??

Mower Pack

Battery pack is a 3S (12.6V) 20'400 mAh

Charger is 12.6V max 5Amps

5 / 20.4 = 0.25

The 3S 5Amp charger will charge the pack @ 0.25S


We could safely use a 20Amp charger (20/20.4 = 0.98S) , but this would be a very expensive charger and we have time between mow s for the batteries to charge at a slower rate...



Never use a 4S charger (16.8V) or a 2S (8.4V) charger on a 3S pack.  The charger needs to have the same max voltage as the pack, not more and not less.  Using the wrong charge is very dangerous!!



When you attach a charger to the batteries there is a natural cut off of the charging action when the batteries reach there full charge.  A charger only works when there is a voltage difference between the charger and the pack.  As the pack cells reach its 4.2V maximum voltage, The amps pushed by the charger will naturally decrease.  At some point the charger will be delivering 0.1AMps at 12.6V or "trickle charging".

To be 100% sure that a charger failure is not providing too many volts the protection circuit is there to protect the batteries.  It is always advisable to use a protection circuit to ensure safe operation.