I could not find ref. to OC V of 13.5; from cell count V = about 16 V, Nothing beats real measurements.

The link to the battery in the 1st post shows only 12V, 3000Ah and nothing else. It looks like a Li-PO.
It says it is 100% safe then it might have a protection circuit in it.
It has only 2 wires then it might have a balancing circuit in it.

If you go on Aliexpress or Alibaba and search you will find both products and if you look at enough of them you will find the detail...

Link to solar panel https://www.aliexpress.com/item/32970372014.html (One of many)

Annoyingly I can't find link to battery I had earlier, but I seriously doubt its 3000mAh, its too light at 156g and 14mm thick rules out most cylindrical cells (14650 wont give 3000mah and anyway won't fit) so must be <4mm pouch but even that would struggle to fit those dimensions,,,

 

I see no reason that a 5 W , "12V " SP should not be used to charge 3 18650 cells in series, with a simple on- off controller.
.

Because at 13.5v OC and 12v/0.4A working point, even with a very low drop-out regulator it can never charge them to more than 85- odd %, you need to get them to 12.6v. If you direct connect the panel it might improve slightly but not while battery is on-load which is what TS wants...

A boost controller/charger *might* work, depending on TS load requirements, if we knew what the 'load' is.

 

The capacity of each cell in a multi-cell battery are different.
They say a Lithium multi-cell battery should have a balance charger (sensing the voltage of each cell and limiting the charging current on the weakest cell that reaches a full charge voltage before the other cells. If balance charging is not used then there could be a spectacular explosion and fire when the weakest cell has over-voltage.

You reduced the charging time a lot which reduces the battery capacity.

A charger circuit with voltage and current regulation should be used because a 5W, 12V SP might produce 18V at the low charging current when the battery is being over-charged.

 

The capacity of each cell in a multi-cell battery are different.
They say a Lithium multi-cell battery should have a balance charger (sensing the voltage of each cell and limiting the charging current on the weakest cell that reaches a full charge voltage before the other cells. If balance charging is not used then there could be a spectacular explosion and fire when the weakest cell has over-voltage.

You reduced the charging time a lot which reduces the battery capacity.

A charger circuit with voltage and current regulation should be used because a 5W, 12V SP might produce 18V at the low charging current when the battery is being over-charged.

Agreed you should always balance a multicell li-ion pack but the right time to do that is during the CV phase, not the CC phase. The changeover point from CC to CV is when the first cell reaches 4.2v but an external charger doesn't know when that is. The internal balancer bypasses the cell with a low ohm resistor to maintain it at 4.2v but each cell is bypassed individually, theres no feedback to the charger. Basically, any charging regime where the charger isn't directly connected to each cell individually is flawed and will never charge a pack to full capacity - protection boards are NOT chargers nor are dumb balancers.

I've designed and built several MCU controlled 40A onboard multi-cell charger/monitor for large 5kW LiFePO4 packs (6-7 hours charge time) for wheelchairs to replace the useless 1.44kW SLA batteries and the weedy 8A (12-16 hour) charger. They use active balancing rather than a passive resistor so they balance much faster, typically at 4A/cell rather than the 0.5A of a passive balancer. The latest charger doesn't need to balance because it has a separate secondary/mini-charger off the flyback transformer for each cell - only practical for 8S or smaller though.

Proper charging doesn't reduce capacity, it maintains it, its excessive DoD by having insufficient capacity that damages cells. A 10 - 15y lifetime over 4000 charge cycles is perfectly achievable at 60% DoD - my earliest pack is getting on for 8y old now and still has >90% of original capacity - that was only 2kW so has had a harder life than more recent packs (and cost ~£2k). A 5kW pack (now <£1k) gives me 60+ miles range, which means I only need to charge it 2 or 3 times a week instead of every night with a pair of SLAs (which would be dead within a year at £400 a set).

 

Does it seem reasonable that my SP will put out a measured 5W & is 3.8 X
the area of TS's advertised 5W panel ?

 

Does it seem reasonable that my SP will put out a measured 5W & is 3.8 X
the area of TS's advertised 5W panel ?

Not surprising at all. Your panel is probaby much more likely to output 5W under a much wider range of conditions (radiance & temperature) than the TS's one, which probably needs a pretty sunny day to get close. It doesn't state the radiance level its output is measured at. It claims 18-20% efficiency and peak output of 12v @ 433mA (5.2W) but with a SC current of 650mA and OC Volts of 13.5v I seriously doubt that, its power output curve would be a very strange shape - normally the peak power point is around 65-70% of OC volts, rarely above 80%, yet they claim 88%. Since there are no published curves I can find its anyone's guess.

 

Peak output V = OC V ? Maybe power = about 3.6W ??
It is all guess work with out a true data sheet or measurements.

 

Peak output V = OC V ? Maybe power = about 3.6W ??
It is all guess work with out a true data sheet or measurements.

Power output at OC v = 0 by definition, its open circuit so I = 0A

You need a good flat light source, a light meter, and a calibrated electronic load (or a bunch of low-ohm power resistors for a small panel and 2 multimeters to measure I and V simultaneously)