DIY 18650 power pack, two problems: voltage drop at P+/P-, Charging drains it.

Thread Starter

ju1234

Joined Sep 15, 2018
7
I made a power pack using 18650 batteries from an old laptop. All batteries tested individually are good. 4 batteries in 2S format. I used a cheap 2S 3Amp rated protection board I got on Ebay from China. I correctly connected all the terminals and properly soldered, B+/B- to the +/- of the final ends of batteries and BM to the middle.

The voltage measured directly at the battery terminals (at B+/B-) is 8.34 volts but at P+/P- (the input/output terminals of the board) it measures 7.7V.

So, I tried charging. I used a Sony camera charger, 110volt input, DC output 8.4V 1.7amp and measures at 8.45V. I have successfully used this charger to charge the camera batteries that are rated 7.4V but actually measure at 8.3V when fully charged. So I know the charger is working. But charging this DIY with that charger actually drained the pack. After 15 minutes of this charger the pack had drained to 5 Volt. Is there an explanation?

Then I used 12V lead-acid battery, reduced to 8.4V with a step down, to charge this pack. Within 30 minutes, it was charged to 7.74 volt. I continued to charge to see if it would charge to 8.4. After another hour instead of going up it dropped to 7.5. I continued to charge it further, and it topped out at 7.74. The voltage at B+/B- (+/- of the battery) terminals at this point was 8.2. Any Explanation and solutions?

Does this mean this protection board is not working correctly or is there some thing else wrong?

Thanks for the help.
 

Yaakov

Joined Jan 27, 2019
1,602
7.74V is 3.87V/cell which is a good open terminal voltage for a settled 18650 cell. The self discharge makes e suspect that you have a cell with a high ESR (Equivalent Series Resistance). To determine is the cells are actually good, you need to measure the ESR if each one.

It one is bad, when the BMS stops the charging, the others could discharge into the bad one, lowering the voltage of the pack to the expected settled OCV (Open Circuit Voltage).

Testing the ESR is a very good way to check the condition of the battery. The easiest way is with an ESR meter, but a differential test of the OCV and the current across a load resistor cab be done.

This looks like a decent explanation: https://www.instructables.com/id/How-to-measure-the-internal-resistance-of-a-batter/ so I won’t try to reproduce it here.

It all the batteries test OK, then you will need to look elsewhere, but give the salvaged cells and the behavior, this is my first guess.
 

oz93666

Joined Sep 7, 2010
672
....Then I used 12V lead-acid battery, reduced to 8.4V with a step down, to charge this pack. Within 30 minutes, it was charged to 7.74 volt.....
There is no need to step down , the circuit can take care of that , usually anything up to 20V for charging and the protection circuit will allow through the correct current .. this is probably your problem too low a charging voltage....

I've built many packs from old laptops ... first step is to make sure the cells don't self discharge too much , then measure capacity and keep the best cells .... Int resistance is never an issue , if they have over 500mAhr capacity , the int resistance will be acceptable .....

But Match the cells together
so that one parallel pack of 2 cells has about the same capacity as the other.
 

Yaakov

Joined Jan 27, 2019
1,602
ESR increases as the cells fail even when the terminal voltages at full charge seem OK.

Cells with higher than normal ESR shouldn't be used even if they appear to charge.
 

oz93666

Joined Sep 7, 2010
672
I've recycled over 1000 18650s ... even cells with 100mAHr capacity can still discharge at over 2A ... (most people who recycle throw away cells with capacity below 1,000mAHr )....

Cells with higher than normal ESR shouldn't be used even if they appear to charge.
That's what the manufacturer says ! Experience of recyclers is otherwise over many years of use.

ALL these cells will have "higher than normal ESR".... they have been scrapped ... none the less they still have half their usable life left , despite having internal resistance of 100-200 mOhm ... (new cell int resistance is 50mOhm)
 

Yaakov

Joined Jan 27, 2019
1,602
It is my direct experience. I am not sure why yours appears different. It has nothing to do with manufacturers, for me.
 

oz93666

Joined Sep 7, 2010
672
It is my direct experience. I am not sure why yours appears different. It has nothing to do with manufacturers, for me.
All I can do is link you to this forum .... https://secondlifestorage.com .... it deals almost exclusively with reusing 18650s .... I've been a member there for over 2 year , and the procedures I outlined in the above posts are followed by the 100's of members there with no problems.
 

Yaakov

Joined Jan 27, 2019
1,602
The TS describes apparent nominal charging along with mysterious discharge. Not cells he’s cycled repeatedly with good results.
 
Top