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BMS with over-voltage protection and CC / CV charger, charging without disconnecting load.

Background -

I've designed and certified a product with the intension that the user NEVER charges the device whilst it's on, eliminating the need for pass-through charging circuitry (this was primarily down to component availability and design time requirements - I did design a board to handle pass through charging but was never able to source the components to test it, never mind manufacture it).

This is fine for the standard use case, however, a situation has now arisen where we need to completely remotely operate the system (with robot manipulation to place the device on a charger).

Adding pass-through charging is not possible, due to time constraints and component availability.

Situation specifics -

We now need the devices to be deployed in situ via a mobile robotics platform with manipulator, in an environment where it cannot be retrieved. The device was intended to be manually deployed and retrieved for recharging - that is not possible here.

I'm hoping to be able to manipulate the device on/off switch before charging it with a 6 DOF arm, but anyone that's ever operated remote robotics will now how difficult this is to do.

Question -

The device battery (4S2P Li-ion) has a BMS with over-charge protection, so my question is, assuming a ~5 W load which cannot be removed: will the BMS prevent over-charge when using a CC/CV charger?

I.e. will the BMS disconnect the battery if the charger does not detect the CC / CV switching point due to the load, and prevent an overcharge situation?

I don't care too much about battery longevity. The device needs to work for about 6 weeks before it's going to be crushed, mixed with molten glass and buried forever.

Another solution to this is to rely on the BMS and use a constant voltage setup, but this will need to be built, and will need to be able to cope with potentially 10 A of current at the start of the charging procedure, which I'd rather avoid.

Charger - https://docs.rs-online.com/0c03/A700000008699400.pdf

Same charger but 1.2 A version and marginally better data sheet - https://docs.rs-online.com/0515/0900766b816d1506.pdf

Battery with BMS - https://docs.rs-online.com/7c4d/A700000008880235.pdf

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    \$\begingroup\$ Ben - Hi, (a) Can you add the detailed specification / datasheet of the BMS? Not all BMS are the same. I assume it was bought in and not designed by you - correct? (b) Same question for the charger. (c) Can you add relevant schematics? (d) I didn't see mentioned the 3rd part of the typical Li-ion battery charging algorithm - the charging supply cut-off when Icharge drops below the battery manufacturer's specified value. In your design, which part of the battery charging subsystem is doing that? || Can you please edit the question & add those details? Or state that you can't do that. Thanks \$\endgroup\$
    – SamGibson ♦
    Commented Sep 10, 2022 at 19:00
  • \$\begingroup\$ Hi Sam, Edited with info you requested. Unfortunately I don't know the specific algorithm, other than it is some variant of CC/CV. \$\endgroup\$
    – Ben Bird
    Commented Sep 10, 2022 at 19:31
  • \$\begingroup\$ @SamGibson A better solution maybe - amazon.co.uk/Adjustable-1-2-36V-Regulator-Converter-Constant/dp/… CV charging with limited current of say <5.2A (1C) and utilising the BMS? As I said, I really don't care about longevity past 6 weeks of use which would be say 12 hours on charge, 12 hours discharging. \$\endgroup\$
    – Ben Bird
    Commented Sep 10, 2022 at 19:52
  • \$\begingroup\$ Indeed the charger will never stop and the BMS will not detect this. Can you estimate the charge time and stop after a safety margin of +1h? In your use case this should be viable. \$\endgroup\$
    – Jens
    Commented Sep 11, 2022 at 1:11
  • \$\begingroup\$ Hi @Jens, That was an idea, but isn't really feasible (or rather is no more feasible than using the manipulator to switch the device off before charging) as it would have to be remotely done and we have no control over the AC power, or at least not easily. Everything has to be operated remotely, for fear of growing a second head. \$\endgroup\$
    – Ben Bird
    Commented Sep 12, 2022 at 0:37

1 Answer 1

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A better, and now tested solution, is to use a 24V power brick, with a DC/DC converter that has a variable voltage and variable current limit, i.e https://www.amazon.co.uk/Adjustable-1-2-36V-Regulator-Converter-Constant/dp/B097XNNQL2/ref=sr_1_20?crid=233P83FV12HFK&keywords=variable%2BDC-DC%2Bconverter%2B10A&qid=1662839454&sprefix=variable%2Bdc-dc%2Bconverter%2B10a%2Caps%2C72&sr=8-20&th=1

Set current limit to 2A and voltage to 16.7. Battery charges quite happily and doesn't heat up. This is probably not that far off what the charger would be doing if you plotted the voltage / current for at least part of the charge cycle, and then current drops off. So really, it's "safer" charging, just a bit slower. So long as I can charge a 5.2 Ah battery in 12 hours, it's fine. Can always up the current to 0.75 C without going out of the battery specification.

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  • \$\begingroup\$ "and now tested"; could you quote a source that has tested and proven that this is an acceptable and safe way of charging a Li-ion battery pack? \$\endgroup\$
    – ocrdu
    Commented Sep 12, 2022 at 6:47
  • \$\begingroup\$ Hi @ocrdu, I am the source, as I tested it with the linked hardware. \$\endgroup\$
    – Ben Bird
    Commented Sep 12, 2022 at 11:16
  • \$\begingroup\$ How does this set-up stop the charging? Or does it float the battery indefinitely at 16.7 V? \$\endgroup\$
    – ocrdu
    Commented Sep 20, 2022 at 7:55
  • \$\begingroup\$ @ocrdu just floats at 16.7 V. I wouldn’t say “indefinitely” as the charge cycle will stop every morning by virtue of deploying the hardware. But yes, you are correct. \$\endgroup\$
    – Ben Bird
    Commented Sep 21, 2022 at 8:12
  • \$\begingroup\$ @ocrdu I can also confirm that this is an acceptable way of charging. I do this often. Like Ben Bird said, It's not the fastest way to charge, but it's the easiest and probably the safest way to charge a pack. \$\endgroup\$
    – bunker89320
    Commented Dec 11, 2023 at 20:45

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