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The safety key that comes with the CM/SPCM1936 installs in the safety key/charging port near the top of the handle. The key works by making/breaking the power connection between the top and bottom connectors. The third connector in the middle is for charging.

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If the safety key is lost, it is possible to make a shorting lead from some 30A Anderson PowerPole connectors and a short piece of 12-14AWG wire if you have the right crimp tools or soldering skills. However, you are much better off getting another key as this fix DOES NOT STOP YOU FROM ACCIDENTALLY CONNECTING THE WRONG TWO CONNECTORS AND SHORTING THE BATTERY. I'm not sure what happens next if the wrong connection is made - maybe a breaker trips or maybe some wires melt - not good things! But this can get you going in a pinch if you are stuck and only costs a couple of bucks to implement.

Battery

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Total size of the three batteries together

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Configuration

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12S4P (36 V)

 26650 batteries sizeis standardized and it is:

• 26.5 mm (1.04 inches) in diameter, or ~26 mm.
• 65.4 mm (2.57 inches) in length, or ~65 mm.

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This configuration would fit the existing battery footprint (slightly smaller) but may grow a bit in size when we add cell spacers and bms. 

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Another Configuration

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Battery Management System

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The polymer is wired in the same way as lithium iron phosphate, supporting both the same port and split port.
1; B-line to the negative of the lithium battery pack
2; connect the cable (make sure the cable is unplugged from the protection board, start wiring from B-, 8-connect the total negative pole of the battery pack, the second line B1 of the cable is connected to the first string of the positive battery of the lithium battery pack) The third line B2 is connected to the positive pole of the second series of batteries of the lithium battery pack, followed by
3: Insert the cable after checking the error.
4: Take out the C line
5: Whether the total voltage of the lithium battery is equal to the voltage of C-to the positive pole of the battery (the equivalent board is working normally)
Precautions
The B-, C-, and P- lines should be connected with thick wires and ensure that the pads are soldered correctly to the positive and negative terminals of the corresponding device.
Before installing the protection board, the battery must be matched well (the voltage difference between each battery is not higher than 0.05v, the internal temperature difference is not more than 5mΩ, the difference is less than 30mAh), the protection board is to protect each string of batteries, as long as there is a string of batteries If the voltage is too high or too low, it will be protected. The better the battery consistency, the longer the battery life, and the best performance of the protection board.
A single battery needs to be connected in parallel to the required number of battery packs, and then the battery packs are used in series.
When connecting the battery packs in series, please ensure that the voltage of each battery pack is the same. If it is different, please fill each battery pack separately and use it in series. In the discharge test, the circuit with the fastest voltage drop is the differential power.

Parts

https://www.ereplacementparts.com/black-and-decker-cm1836-type-36v-18in-mower-parts-c-4167_9514_173258.html

Alterations

Added:

• a voltage meter
• a charging port.

The charging port allows me to charge it with an alternative charger since it appears that my original charger no longer works.

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References