0
\$\begingroup\$

I am making a 3s2p 18650 battery to make a 12v battery pack for a project I am working on.

When I test the various battery cell pads on the BMS I get the expected voltages, 3.7, 7.4, and 12. However, when I test the output from the "P" terminal I was only getting 10.6v. I assume that one or more of my cells are bad so the BMS is throttling the good cells since I can get 12v from the B3 pad.

I am disassembling and testing the pack now, one of my sets was reporting 3.97v while the others are reporting 4.02v. I can only assume that one of these two cells is bad or was not fully charged when I created the pack. Once I have the cells apart again, I will test/charge them all before reassembling the pack.

I guess there are a few parts to my question / assumptions here.

  1. I am assuming that my beliefs are correct here, that since the P terminals are outputting 10.6v there must be a problem with my pack that is making the BMS assume that one or more of the cells is low, so the BMS is balancing the pack.

  2. I am assuming that by using the BMS, I can forgo the balancing terminals, and just have a single DC I/O that I use to power my circuit, and also to charge the pack.

  3. I am curious about the various Bn pads. Right now I am attaching the output form the P terminals of the BMS to an L298N. This powers a motor, and provides the 5v output to an Arduino board for the logic. Could I use the B2 pad to output 7.4v directly to the Arduino (it has it's own power management so it can operate as high as 9v) and thus forgo using the L298N for logic power?

  4. I know that technically to charge the pack I will need to provide it 12.6v (4.2*n). I have an actual RC battery pack charger, but it is not immediately available. In the interim could I use a 12.3v wall wart to charge the pack? (I know this is not good for the longevity of the pack, and won't be able to fully charge it, but would only be a few charges before I can get the better charging unit to my location)

Cheers!

\$\endgroup\$

1 Answer 1

1
\$\begingroup\$

When I test the various battery cell pads on the BMS I get the expected voltages, 3.7, 7.4, and 12.

Those are not the voltages you should be expecting. They indicate the first and second cells are 3.7 V each, but the third cell is 12 - 7.4 = 4.6 V, which is extremely unlikely (above 4.3 V there is a high probability of the cell blowing up).

However, when I test the output from the "P" terminal I was only getting 10.6v. I assume that one or more of my cells are bad so the BMS is throttling the good cells since I can get 12v from the B3 pad.

BMS doesn't 'throttle the good cells'. If it has a balancer it will gradually discharge any cell that goes above ~4.3 V.

I am disassembling and testing the pack now, one of my sets was reporting 3.97v while the others are reporting 4.02v.

Up to 0.05 V difference is not unusual.

The total voltage must be close to 12.01 V, which matches your first measurement of all cells but doesn't explain the other voltages. Perhaps you were measuring from the negative BMS terminal while it was shut down? (most BMS circuits switch the negative battery terminal).

Your cell voltages look good, but the BMS output voltage is low. This may mean the BMS has to be 'woken up' by applying charging current or shorting 2 terminals on the BMS (read BMS instructions to find out what to do), or perhaps it is just faulty. Also, make sure the cell taps are connected to the correct cells!

Could I use the B2 pad to output 7.4v directly to the Arduino (it has it's own power management so it can operate as high as 9v) and thus forgo using the L298N for logic power?

No, this will unbalance the pack.

In the interim could I use a 12.3v wall wart to charge the pack?

Only if you limit current by eg. putting a resistor in series, and only if the voltage never exceeds 12.6 V. If the open circuit power supply voltage is higher than this, or might go higher due to eg. higher mains voltage, it could blow up the pack and set fire to everything around it (the BMS should cut off if any cell voltage exceeds ~4.3 V, but can you guarantee it?). I wouldn't risk it.

\$\endgroup\$
6
  • \$\begingroup\$ The "expected voltage" is based on what the back of the PCB has printed on it. If I measure from the "B-" terminal to the various battery connections, I get the appropriate corresponding voltage on those terminals. That's what I meant by that. If I measured B- & B1 I got 3.7v, B- B2 7.4v and B- B3 12v. \$\endgroup\$ Mar 1, 2022 at 4:08
  • \$\begingroup\$ Good to know about unbalancing the pack by tapping B2 and so on. It makes sense, but I was not sure, since the voltages were printed on the PCB I wasn't sure if that was a means to make testing easier, or to let you know what terminals to tap for various voltages. 0.05v sounds reasonable as a variable for cells, seeing as they are technically 3.7v. (Just some paranoia from other posts I've read about these cells and their voltages). \$\endgroup\$ Mar 1, 2022 at 4:08
  • \$\begingroup\$ I'm going to reassemble the pack in a bit, and try testing it again. I have taken the lower reporting cells and charged them individually to ensure that I am not mixing full and empty cells (I assume that the BMS would handle that, by discharging the live cells to match the dead ones, but BMS's are new to me so I am treading on new territory here). \$\endgroup\$ Mar 1, 2022 at 4:09
  • \$\begingroup\$ My testing of the pack initially was fairly anecdotal, I tried attaching a laser LED that is rated for that voltage and it would barely light up, then tried applying it to a small DC analog (doesn't require PWM) motor, and got no response. Using an alkaline 9v battery, both of those devices responded as I would have expected. I then tested the terminals with the volt meter and saw the 10.6v. \$\endgroup\$ Mar 1, 2022 at 4:09
  • \$\begingroup\$ As for the charging thing, I absolutely understand the concern, I'd rather just wait and be safe to make sure the pack doesn't catch fire. Li is a fun element* (similar to how C4 is a fun chemical compound). The charger I have that I was thinking of using was from a 12v 18650 pack (It is one of those nifty cell packs that you can use to jump a car battery with) since it was 12v 2A I figured it would be the safest one to use (effectively the same battery pack but different BMS so that's the source of the concern for it there). In the end, I'll probably just wait till I have the right one. \$\endgroup\$ Mar 1, 2022 at 4:10

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.