[NOTE] I understand that the adapter that I bought for my battery pack is not actually a charger. I also understand that it could be dangerous what I'm doing. I still wonder why there is no current flowing from the adapter towards the battery pack though. If anybody could help me find an answer to that question I'd be very grateful. [/NOTE]
I'm building an autonomous boat, and for this I was using this 12V Li-ion battery pack which came with a very simple charger:
This works great, but since I'll need a very specific form of battery (long and thin to fit in my boat) I decided I need to built my own battery pack. From this other project I got the idea that if I use an 18V solar panel I can build a 5S LiFePo4 battery pack which means I will not need a solar charge controller. His reasoning is as follows:
The behavior of a solar panel connected directly to a battery is similar to your typical CC-CV lithium battery charger: a lot of current until the battery gets close to fully charged, and then the voltage rises while the current decreases (the solar panel will put out less current as the voltage rises). TO put numbers to this, the Renogy solar panel's max power voltage is 17.7 volts (depending on temperature) and the 5-cell LiFePo4 battery's nominal voltage is 16 volts and its fully-charged voltage is 18 volts.
By not needing a charge controller I would save on complexity, money and most importantly: space in the boat. So if possible I would like to avoid using a charge controller.
Since I don't have the solar panels yet I bought this simple 18V 2A charger:
The batteries came all charged at exactly 3.33V. So after soldering together my battery pack I attached it to my boat. It worked and I left it on for a while. When I came back to it after about an hour the boat shut down. I checked the battery cells and their voltage was 2.8V (way too low). I then attached it to the charger and let it charge for a day or so. When I came back to test it, the battery didn't seem to have charged at all though. It was only charged up to 2.85V per cell.
I then attached a watt meter between the charger and the battery pack, and that showed me an input voltage from the charger of 18.3V, but no current flowing:
This explains why the battery isn't charged, but I wonder why this is happening? The only reason I can think of is that the BMS is blocking any incoming current, either because the voltage supplied by the charger is too high OR because the battery voltage is too low. I checked out the specs of the BMS (pasted below) and in there it says the "Single Overcharge Protection Voltage" is 3.75V, which means the supplied 3.66V (18.3V / 5) from the charger is within limits, but very close.
Am I right in my thinking that the voltage of the charger is too high? And if the voltage is too high, would it work if I simply reduce it by for example 1V? Or can I built in some kind of thing that caps the voltage at 18V or 17.5V or something?
Again; I understand I need a proper charger instead of this adapter. But I would like to understand the reason why there is no current flowing. Would anybody know how I can test/debug this? All tips are welcome!