So wow! I missed a ton here. I didn't get any notifications to my email that there were responses. I've been away gathering some parts and finding time to try a few scenarios out. Lots to unpack here in all the responses!
A power supply doesn't have the same characteristics as a solar panel.
These are just hasty example ideal sources, both being theoretical 85W sources. The power supply in this case would be rated 14V@6A. The solar panel would be Isc=6.5A, Voc=17.1V and MPP=13.91V@6.2A.
View attachment 649948
Notice how a power supply reacts, it's always got 85W (or 14V and 6A) available until it doesn't. If you ask it for more than 6A the voltage drops and you can't ever get more than 14V but is constant under 6A. The PV power will be a function of voltage and current and neither of them are fully constant (current is practically up to MPP at least). In particular notice how the PV power forms a distinct peak that falls off both sides.
In the design of power supplies and solar controllers assumptions are made about the nature of their upstream input sources and downstream loads. In this case substituting a power supply for a solar panel into a controller might demonstrate where assumptions made are not capable. Sometimes it works just fine, too.
It's usually MPPT where you'll find the limits of doing this. If you assume there's not a cliff to walk off, which is a safe assumption with a solar panel, you don't need nearly as robust of an MPPT design. Better MPPT controllers will measure current in or out to know they've hit set points or limits. Those could run happily at their limit as long as the power is stable.
Others will walk to the edge and go right over to find the fold back current of the supply or exceed their own power limits.
PWM can work if the power supply and battery are compatible but will run into issues if you ask the PWM to match them beyond it's limits, which means a max voltage difference and current limit.
So I understand that the PWM solar charge controllers are designed with an input source from solar. If I'm reading your reply correctly, (mainly the last sentance) in the end, a PWM controller could work ok w a constant power source as long as the power source fits within the PWM controllers specs, correct?
So a power source of 19v at 5amps (using a booster from 12v to 19v/5A) into a PWM Solar Charge Controller (those $20 blue ones on amazon for example) that can handle :
20A PWM controller
*Rated Charge Current: 20A
*Rated Load Current: 20A
*Over charge Protection: 14.4V/28.8V
*Over charge Floating charge: 13.7V/27.4V
*Charge recover voltage: 12.6V/25.2V
*Over discharge Protection: 10.7V/21.4V
*Over discharge Recover: 12.6V/25.2V
It would seem that the 19V/5A would be ok. Minus the characteristics of solar, 19v at 5A would be like a large 100W solar panel connected as a source during a super bright clear and sunny day, would it not?
100W PV panel
This is correct. A PWM controller will NOT work in this situation. Remember how a PWM controller works, for the bulk part of the charge the PWM controller doesn't do anything - it is fully on all the time. So in this situation it would be the same as connecting the DC-DC converter directly to the battery - the DC-DC will try to provide 19V and the battery will try to draw as much current as it can, leading to either the DC-DC going into an overload protection and shutting down (if it is a decent one) or going up in smoke and shutting down for good (if it isn't protected). In either case your battery doesn't get charged.
A solar panel is electrically very different to a DC-DC converter. A solar panel is a current source, whereas a DC-DC converter is a (typically) a voltage source.
I would think the PWM controller would regulate how much voltage and amps it would put out to the battery for charging. My current mini solar panel is 18V/18W/1A so it would take that 18V and charge accordingly. So if you used a larger solar panel like a 100W one like linked above, the PWM controller would reduce that in a way and charge the battery according to how it is programed....?
I actually connected my booster (19V/5A out) to my PWM controller to test and took some readings. This is what I found:
Output from the booster: 18V/4.5A
Output from the PWM controller into the battery : 13V at about 4.5A or so. So it would seem that the PWM controller is lowering the voltage and it is ok because the input voltage and current is within mfg specs...?
Is this not OK or expected?
This assumes the solar controller doesn't regulate voltage at all during bulk. That might or might not be true. It's a likely assumption that most PWM solar controllers will try full duty cycle to get Isc, though there's also reason to assume a designer might implement an max voltage, perhaps 15V or 16V. This would have a resultant loss of average current but that's acceptable to prevent damage or over volt at light load. This would be have to be an assumption made about the battery impedance, range of types and sizes.
I believe this is supporting my response above...?
Unfortunately the controller wouldn't see 15 or 16V until your battery is boiling as the battery is setting the voltage in this circuit. The controller would also reach the absorb voltage of 14.X before it reached any voltage limit, so the max voltage limit wouldn't actually do anything. Some higher end PWM controllers could have current sense and control, but we are talking about doing things on the cheap here.
I still don't see how the PWM controller would see the 19V/5A (besides power characteristics as outline by Dave in the graph) differently between it coming from a PV or constant power supply source.
So as I alluded to above, I purchased the booster and connected it to a 12V source (cig lighter rated for 120W) in my car and connected the booster's output to the PV port on the PWM controller. The PWM controller is connected to a small 18ah SLA battery. I turned on my car and it seemed to work fine. My battery was topped off from the PV panel earlier in the week so the PWM controller showed it was charging at 1A. I then connected my laptop for a few hours to run down the battery. I connected it back up to the booster and the PWM controller was showing it was charging at 4.5A. I didn't do any longer testing as I didn't have time to leave it connected and watch it.
Seems like it is working as expected? Again, I'm not an electrical wiz, I'm just doing some reading, researching and common sense deduction. Anyone see any flaws with my line of thought?
