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I have made a 3S5P (18650 Sanyo NCR18650GA 3500mAh with 10A discharge rate) battery pack (about 220 watt hours when fully charged) for a robot.

I'm using a BMS with balance (supports up to 60A continuous discharge) for the pack.

Battery Protection BMS PCB w/Balance 3S

Sense its a self-charging robot the docking station will also have a smart charger that disconnects when the battery is fully charged. The docking station does not have a balance connector because the BMS w/Balance should take care of that.

Is it safe to use the discharging cathode while the robot is charging so I can keep the robot awake?

There will be two cables coming out of the BMS. One connected to the charging cathode that connects to the smart charger, and the other is the discharging cathode that powers the robot. Unfortunately, there is no datasheet for the BMS except these information and the diagram pictured above.

Dimensions : 96 * 45MM
Application: 3 series 12v lithium battery protection board
Type of batteries : lithium cobalt / manganese lithium / ternary materials
Single overcharge protection voltage 4.25V ± 0.05V (4.20-4.35V / / 0.05V per upgrade )
Single overcharge recovery voltage : 4.10-4 .00 V
Single Over-discharge protection voltage : 2.50V ± 0.1V (2.50-3.0V / / every .05 V into the class )
Single Over-discharge recovery voltage : 2.80V ± 0.1V
Protection Current consumption : ≤ 300UA
Short circuit protection current : 120A
Short circuit protection time : 500MS
Temperature protection : 55 / 65 / / 75 degrees
Continuous operating current : 60A
Single balanced voltage : 4.19V ± 0.02V
Single balanced current : ≤ 55MA

The robot also needs a way to determine when it is fully charged so it can continue exploring. Which voltage source should I use to monitor it while its docked?

For added safety, I used a temp sensor to monitor the battery while charging if it overheats the robot will disconnect itself.

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  • \$\begingroup\$ For 18650GA cells, you don't disconnect at a given voltage, you disconnect when the charge current is below 700ma for your 5P pack. This assumes a 4.2v/cell CC/CV charge at 7A. You'll have to account for balance discharging during this though. \$\endgroup\$
    – Bryan Boettcher
    Dec 7, 2017 at 17:00
  • \$\begingroup\$ @BryanBoettcher I will try to do it with a current sensor. I'm using a 12.6V 5A CC/CV smart charger for the docking station. Will there be any problems when the charger is connected to the battery and powering the robot at the same time? Would that confuse the charger in any way with regards to detecting when the battery is full ... etc? \$\endgroup\$
    – someone
    Dec 7, 2017 at 19:39
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    \$\begingroup\$ It doesn't matter how much power the electronics are drawing -- shut it down when the current into the battery pack drops below 700mA @ 12.6v :). The current shunt should be on the path into the battery pack, and the electronics shouldn't draw so much power that the power supply comes out of CV mode. \$\endgroup\$
    – Bryan Boettcher
    Dec 7, 2017 at 20:02

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