I converted two old 20 volt power drill tools batteries from Nicad to lithium with bms boards. The two battery packs now consist of 5 number 18650 - 2000mAh - 3.7v discharge - 2000mAh - 7.4Wh cells providing for a total output of 21.0 volts.

Can I use a 3amp charge supply to charge these battery packs?

 

5 cells is 18.5v at nominal voltage. 21v required for charging, but you shouldn't charge them at higher than, typically, 0.5 Capacity, which is 1A. Normally the BMS will control the charge rate and balancing of the cells, so as long as your charging source is at least 21v and over 1A output it will suffice. What BMS did you use?

 

What BMS did you use?

The cheap AliExpress type described as follows: 3S/4S/5S BMS 12v 16.8v 21v 3.7v 100a Li-ion Lmo Ternary Lithium Battery Protection Circuit Board Li-polymer Balance Charging

 

Those boards do not control the full charge cycle, they rely on the external charger to control the CC/CV cycle - what charger do you have?

Also that board doesn't provide discharge current limiting, at least not below 100A, so it might protect against a short circuit on the output, but won't stop you pulling, say, 50A, destroying the cells and setting fire to the wiring! From some online info its possible to modify the BMS to restrict output current protection to a lower level by changing/removing some or all the R004 resistors on the RH side of the board, eg replacing all four with R04 would limit discharge to 10A before it cut off.

Are there any markings on the big IC, top left?

 

Are there any markings on the big IC, top left?

The item in the top left hand corner is marked (best as I can ascertain) as follows:
BM3 451
TNDC - T28A
WED
........ the brand seems to read something like BYD or BVD, it's all very small but that is my best reading of it.

I'm guilty of watching Youtube videos where digital creators demonstrate the conversion process from nicad to lithium but they never go into the charging process.

My charging approach at the moment is to use a Power Adaptor as per the back and front images attached.

 

That chip is indeed a BYD BM3451-TNDC battery manager. BYD are best known in the West as an EV car manufacturer, but way back they started life designing and building battery packs for other EV manufacturers and have their own range of dedicated chips in that field. The important info is the 'DC' in the product code - this tells us that the chip over-current setting for charge and discharge is ±0.1v across the 1mOhm sense resistor (the 4 x R004) = 100A, which is far too much for your application - basically no protection at all. It wasn't needed with NiCADs, they would self-limit, but those 18650s will happily discharge at 50A+ for a few seconds, enough to do some serious damage to themselves and/or whatever they are plugged in to, but possibly not trigger the protection. I would advise replacing the R004 with something like R04 (10A).

Now that doesn't control the charge rate - that will be determined by the external charger. The unit you have will produce 3A allegedly, and the batteries will happily charge at that rate which isn't ideal - can you measure what it actually is? As I said before, 1A is a safe charge rate for a 2Ah 18650. You should only charge at a higher rate if you have some way to check the battery temperature and stop charging if it goes above 37degC or so. See that gap on the LH side of the big chip - that's the connection for a temperature sensor, and the resistor immediately below it determines the temperature that will stop the charge if it goes too high. Can you read the marking on it?

 

can you measure what it actually is?

I have measured and assuming I am doing it correctly it is reading over 5.5A. Will have a search for an old charger delivering something closer to the safe requirement.

 

Sounds high, but not impossible. Multimeter on 10A range should be inserted between power supply + output and the BMS + input.

 

According to sellers on AliExpress the ZY-009 Charger is rated for 5A output, not sure why yours says 3A.

What you need is a 21v 1A or 1.5A '5S non-balancing lipo charger', not just a 21v 1A power supply which will shutdown if the batteries try to draw too much current.

 

Sounds high, but not impossible. Multimeter on 10A range should be inserted between power supply + output and the BMS + input.

The 5.5amp result came from multimeter on 10 amp range testing the unit in isolation on its own.

 

According to sellers on AliExpress the ZY-009 Charger is rated for 5A output, not sure why yours says 3A.

What you need is a 21v 1A or 1.5A '5S non-balancing lipo charger', not just a 21v 1A power supply which will shutdown if the batteries try to draw too much current.

I suppose we cannot believe all we see on AliExpress or might it be my testing capability. I have seen a 21v 1A 5S charging unit on Ali Express so back to the long distance supply line. 21V 1A Battery Charger Power Adapter For 5S 18650 Li-ion Lithium Battery Pack Inside Positive And Outside Negative - AliExpress 44

 

What is the difference between 'Synopline' and 'Contact' as outlined in the attached S5 wiring diagrams and what circumstances determines which to use?

 

What is the difference between 'Synopline' and 'Contact' as outlined in the attached S5 wiring diagrams and what circumstances determines which to use?

Not entirely sure. The first arrangement allows the charger to power the load directly so might be more appropriate for UPS type applications. The second can isolate the load completely from both battery and charger. See the data sheet attached, page 13.

The 5.5amp result came from multimeter on 10 amp range testing the unit in isolation on its own.

That's the short circuit current of the charger at 0v, not what it will supply to a battery on charge.

 

See the data sheet attached, page 13.

Thanks for the detail. For the situation I described which is more suitable. When putting it together I did use the Synopline method as outlined in the image.

 

For the situation I described which is more suitable. When putting it together I did use the Synopline method as outlined in the image.

Assuming you never run a power tool while its battery is being charged. then #2 is better option - you're either charging or operating, never both at the same time.

 

The AliExpress BMS I used was described as a 21v 3.7v 100A BMS. I note that some of these BMS's advertised are described as being 20, 25, 40 and 60A. What determines which Amp one should be used on a project like I just worked on.

 

Assuming your charger limits the charging current, then its the maximum average discharge current plus a working margin to allow for peak currents plus a safety margin. A 2Ah LiPo should not be discharged harder than , say, 5C, eg 10A so a 15A or maybe a 20A would be suitable. A 60A wouldn't provide much protection under fault conditions if they only drew say 50A, yet wiring sized for 10A peak would be glowing red hot!