Open amcewen opened 1 year ago
The data sheet has a formula for the 'Full scale charge current' as a function of Rcs (R5)
Ich = 120mV / Rcs
Charge termination is determined by: "In constant voltage mode, the charge current decreases gradually. When the charge current decreases to 16% of the full-scale current, the charging is terminated"
Under PCB layout considerations it recommends; "(6) Place the charge current sense resistor RCS right next to the inductor output but oriented such that the IC’s CSP and BAT traces going to RCS are not long. The 2 traces need to be routed together as a single pair on the same layer at any given time with smallest trace spacing possible."
So it's possible that a modified trace, or different value for R5 could resolve this.
The original boards had Rcs as 0.068 ohm Giving a charging current of 1760mA
https://www.digikey.co.uk/en/maker/blogs/charging-lithium-ion-batteries suggests a charge current of about half the capacity of the battery. So for the 13400mAh packs we get ~6700mA
So (I think) we should be aiming for Rcs of ~0.018 Ohm
I've made up a board with the closest we currently have to that. Two 0.068 stacked in parallel giving 0.034 ohms It's charging so I'll see if this triggers the 'Done' light
Based on what was discovered in #12 the next step is to setup board C with a bench supply and measure current with one of the INA219 sensors.
Running an 8 hour test with board A gives this data for the changing current. The Bus voltage was a fairly constant 3.11V for the whole period.
First thoughts are generally the values are much lower than I was expecting, and I can't really explain why the current is negative after 7hrs.
Test two with board A makes even less sense. This is a five hour series.
The starting battery voltage was 3.87V, bus voltage remains constant throughout.
I am starting to think the chip on this board is no longer working. The next test will be with board D
I've been wondering about this some more today. No real progress, but...
Beyond that, maybe a different charger chip would be a solution (or the next step to try)? Presumably something which can cope with an input voltage of ~12V (to allow 12V solar panels, which are quite common), but it might not need the 28V and up to 4A ratings of the existing chip - maybe the fact we're often at the lower-end of its design range is what's causing some of the trouble?
A couple more thoughts (courtesy of Paul H):
I've bought a few of the CN3781 chips to try sometime too.
I've been using it as a USB charger for a few months now, and while it works well, it never reports that charging is done.
I've had a 10500mAh Li-Ion battery on charge for five days solid, and it's still showing the "charging" LED. I don't think the battery was especially discharged (might even have been fully charged) before I put it on to charge, and its sitting at 4.22V now so I think it is charged.
Obviously it would be better if the "done" LED on the charging circuit worked :grin: