Solar battery charger based on LT3652

My original answer (below was based on my incorrect assumption that Arik was suggesting using the wiring inductance in a controller to replace formally provided inductance. In fact, he is saying that in the controllers of interest there is NO formal inductance, and he was wondering if the wiring inductance served a useful role overall.

Simple PWM can be used to vary the current which a PV panel will deliver and to control bttery voltage. It can act as current limiter, constant current controller or voltage controller.

A PV panel used without an energy converting controller, suh a an MPPT controller, usually acts much like a CC (constant current) source. This is because Vmp (Voltage at max power) is > Vbattery under most sun conditions and the panel is loaded with a lower effective resistance load than is optimum. An MPPT controller increases the effective load resistance so the supply voltage can rise to the optimum value.

If you connect a PV panel to a battery then current flow will redcue as PWM duty cycle redcues. It will not be a linear reduction as vpanel will rise as load is reduced, tending to act against the current reduction from the PWM. However, in practice you can set current to any value equal to or lower than what ypu'd get with a hard connection.

If you want to limit battery voltage to a certain value then simply reducing or stopping current flow when the voltage is high enough, will work as a "constant voltage' source. current

Simple MPPT can be little more than the buck converter I outlined plus a controller. By doing no more than holding panel voltage at about 80% - 85% of Voc_panel_full sun you will get very close to true MPPT performance.

Second addition

As the question evolves, so can the answer :-).
There is no doubt that simple bang/bang on/off control is undesirable and causes undesirable battery current and voltage variations. My comments about the controller being able to control voltage are true over a long time period relative to a PWM cycle but all sorts of interesting stuff may happen over a single cycle or a small number of cycles.

Adding an inductor allows energy storage and smoothing - an existing controller MAY be able to be "improved" by just adding an inductor and flyback diode and maybe one or 2 reservoir caps depending what is there now BUT the existing control circuitry may have a fit (or not) due to the changed response. It would probably in many cases to use the existing power level hardware with L,C,D as requisite plus either new software or (possibly more easily) a new control core. An MPPT controller needs cost little more than what is there now. Pricing is often controlled by "because we can" and "because they can't" factors.

Having the series switch (probably MOSFET) in linear or resistive more would help make behaviour nicer at the expense of power dissipation in the switch. The heatsink size is uncertain as can only be seen end on but it looks substantial. If the switch is run as a resistor then it COULD be setup to operate a setady current feed to battery. If desired this could be only done in holding mode where current is low. eg in Panle V_light)load is say 17V and Vbat hold is say 12.6V and Itrickle is say 100 mA then dissipation in a FET in this mode is P = V x I = (17 - 12.6) * 0.1 = 0.44 Watts = minimal. If you could sink say 5 Watts and needed to provide current from 18V to 12V you could have I = W/V = 5/(18-12) =~~ 800 mA.

Using on/off PWM is non ideal and will lead to waveforms something similar to those shown by Arik in his second edit. The magnitude of the spikes will depend on how much capacitance is present BEFORE the switch, to a lesser extent how much capacitance after the switch, wiring resistance (and to some extent inductance) and battery characteristics and state of charge and, importantly, PWM frequency. Arik has shown the signals as step changes at switching boundaries followed by linear ramps. I would expect the step changes to be modified by effect of capacitances and linear ramps to tail off into more or less steady state flat spots as PWM on or off time became long relative to battery & PV panel settling down to steady state under the given conditions.

I do not show a capacitor on the PV panel in my outline schematic below but if there is one then the PV panel will slew more slowly and can be held near a constant voltage more closely. This would limit the more objectionable spikes and excursions shown by Arik.

Also, an ideal battery may exhibit step change and "instantaneous steady state" conditions as suggested but it is likely that in real life you get more complex responses- an oscilloscope would be your friend here.

Original answer - useful but not what was asked for.

It is extremely likely that you circuit diagram is incorrect and that the simple PWM controllers are Buck converters as shown below.

Wiring inductance could notionally be used but in practice is too small to use in sensibly practical converters. Resonant frequency is not the critical factor. At resonance Vcap would swing 'widely'. What is required is an inductor such that delta V is small during the on cycle - perhaps 1V p-p and ideally quite a lot less. Using wiring inductance would probably require MHz range switching and would be likely to produce ill defined low efficiency high RFI situations.

With a suitable controller such a circuit can provide constant current or current limiting or voltage controlled output.

D1 is usually either a Schottky diode or a synchronously controlled FET switch.

A solar panel is not a constant voltage, or constant current source. It can be thought of as a constant power source with maximum rated voltage and maximum rated current. The power is relative to the light hitting the panel, the voltage is maximum with no current, and drops as current is drawn from the panel.

If you are using a 10W panel, and it's in its full rated sun exposure, you'll get 10W out.

If you draw 1A in that situation, the voltage will be about 10V. If you draw two amps, the voltage will be about 5V.

If your battery is full, you probably aren't going to draw much current, so the voltage is higher.

If the battery is nearly empty, it will draw a lot of current, and it will cause the panel's voltage to drop.

In your specific case, what you're finding is that the panel can't provide full charging current all the time - whether that's due to less than full sun exposure, or a low-charge battery depends on the situation.

However, you can still use this system, even though the voltage is low. If you disconnect the battery and measure its voltage, then connect it to the charging system and measure the voltage at the battery, you'll find that the attached voltage is higher - the battery is accepting current from the system, and is charging. It isn't charging as fast as it could be, but that's due to the panel's limitations.

If you want to learn more about this, and what professional solar charging systems do in order to handle this effect, do a search for MPPT circuits - maximum power point tracking. The solar panel is most efficient at a certain voltage and current for a given sunlight input, and these circuits attempt to track that maximum point so you get as much power from the panel as possible.

Also, note that SLA batteries are very forgiving. It may be that you can eliminate the voltage regulator, and just use the diode in the circuit. This will increase the voltage at the battery since the regulator drops 1.5V-3V depending on load, and thus charging efficiency. Given that you're having a hard time keeping it charged, I'd expect the solar panel is unlikely to damage the battery, but check the panel's maximum current at 7.2V and see if the battery can accept a constant trickle charge of that rate.