I got this 2S BMS charger and Im trying to charge 2x18650's which started out at 4.00V (and 8V together) today at 11am. After a sunny day at around 5pm I took them back inside and measured their voltages and found that one battery is at 3.91 and the other at 3.8 and together they give me 7.7-7.8V.

What could be happening?

 

I assume those cells are scavenged from packs. First thing, do they charge using a known good charger?

 

Another possibility is that as you are using a battery holder to connect to the cells the contact resistance may be higher on one cell than the other. So when the cells are being charged the voltage seen at the charger is slightly higher than the voltage across the cell due to the voltage developed across the contact resistance due to the charging current.

Les.

 

Im not really sure I see the problem... from the look of the solar cell its not a very big one anyhow so even in good conditions it probable does not produce that much power (you should check the output voltage and current into your charger). Your batteries appear to be about 60-70% charged anyhow:

 

Yes they are scavenged and no I haven't tried them on a good charger so I'll do that today. As for the resistance, I don't follow. But the issue is that I put the batteries in the charger and measured the voltage of 4V before charging at the charger terminals and at the bms board. After a few hours of charging that same voltage was less (4->3.7).

 

Yes they are scavenged and no I haven't tried them on a good charger so I'll do that today. As for the resistance, I don't follow. But the issue is that I put the batteries in the charger and measured the voltage of 4V before charging at the charger terminals and at the bms board. After a few hours of charging that same voltage was less (4->3.7).

Do try charging them, they may be fine but if you don't know then you might be chasing nothing.

 

OK I charged them yesterday on a wall charger and they are at 4.18 and 4.21V. So it must be a problem with the charger. The solar panel terminals give 5v or so in the sun so it should be enough to charge them no?

So it has another terminal that yields around 9v, I could try tapping that one.

 

OK I charged them yesterday on a wall charger and they are at 4.18 and 4.21V. So it must be a problem with the charger. The solar panel terminals give 5v or so in the sun so it should be enough to charge them no?

So it has another terminal that yields around 9v, I could try tapping that one.

Measure the output of the panels when they are connected to the charger.

 

The panel measures 5.24V at 2 terminals and almost 12v at two other terminals.

I've marked the green terminals which is where I have the charger connected to. Both at the panel and at the charger terminals the voltage is 5.24V because there its cloudy today. But it rounds about 6V in the sunlight.

The red terminals which are actually marked + and - on the panel itself measure between 12-14V right now.

 

Do you have any specs for your 2S BMS board?
Where did you buy it?
What Cell Voltage was the 2S BMS designed for?

 

The description reads :

2S 20A 7.4V 8.4V Li-ion Lithium Battery 18650 Charger PCB BMS Protection Board Voltage Regulator

Oh OK I just read that the charging voltage is 8.4-9.4V, so that might have something to do with it.

So if my panel puts out 12-14V, I would have to bring it down to 9v, how?

 

Ok the sun came out. So the green terminals produce 5.24V thereabouts. The red terminals produce up to 17V in the sun. Both of these measurements are directly on the panel itself with nothing attached.

Then I measured the panel with the charger connected to it, without the batteries as it appears in Post#9 and the voltage is 9V across the red terminals (the ones that give 17V without anything connected to them).

 

OK it worked like a charm. I plugged it into the right terminals at 9v and I got the batteries to go from 4.13/4.20 to 4.23/4.24 instead of down to 3.7.

So I'll just leave them in the holder connected to the charger and panel inside the house just to make sure they don't drain for some reason.

One last question, so I made this to power project that needs 5v for a Nano and 6v for a vibrating dc motor. This is fine running off of 2x18650's which in series would give me 7.4-8V which are in turn charged by a 9V panel feeding a 9V charger.

But if I needed to power something 12V or 24v, I would need 4x18650's for 14.3V. For this I would need a 4S bms which has a charging voltage of 12.6-13.6V. So I would need a different solar panel obviously, more like a regular solar panel which nominally puts out 12-14V. For anything above that it's just not microelectrónica anymore I guess huh?

 

One last question...where is the best place to take power from the batteries in this setup? From what I've seen its taken from the same two pads where power comes in from the solar panel.