I have a van which has two 12V solar panels on the roof. The charger is connected to the batteries and seems to constantly be outputting about 13v. I've had it installed for a couple of days and I've noticed that my USB cigarette lighter now no longer works and the voltage throughout the vehicle is reading as 13V (I only have a low-res digital voltmeter). And the LED on the cigarette lighter charger is pulsing suggesting it's receiving a fluctuating voltage, roughly in time with the charge LED on the MPPT charger.
The charge LED on the MPPT charger flashes around 3 times a second, but irregularly. I think it is putting charge into the battery at 13.5V when the LED is illuminated. This is causing the voltage throughout the van to vary between 12V and 13.5V multiple times each second.
Does this seem plausible?
Most importantly, how can I regulate this voltage to a steady 12V throughout the van? I am drawing up to 20A from the battery to run lights, an amplifier, a fridge etc.
Am I doing the (Lead Acid) batteries any harm by having this constant charging happening?
Best Answer
It sounds like your batteries aren't properly buffering the MPPT output. With the MPPT off, do your accessories still work off the batteries (even momentarily)? If not, you may have a bad cell or open battery fuse.
With healthy batteries connected in parallel to your MPPT, the voltage shouldn't vary more than a few tenths of a volt with a constant load.
You don't want to regulate the MPPT output to 12V, as it would not be able to float charge your batteries at that level. It is generally safe to float charge (constant voltage) a 12V monoblock at 13.5V indefinitely. Specs on your specific battery should identify the ideal float charge voltage as as function of its specific gravity.
If your batteries turn out to be failing, make sure that you're using deep-discharged rated batteries and not batteries intended for engine starting applications. So-called starter batteries are designed for very high instantaneous current, but relatively low discharge depth (you start your car, then they recharge immediately). Your application will likely pull your batteries down to a much higher depth of discharge on a regular basis, so you'll need a battery specifically designed for this use case if you expect a decent working life from the batteries.
EDIT:
Most MPPTs have a relatively low current output at 13.5V - usually less than a couple of amps. The reason that batteries are used in parallel is to allow loads that exceed the capacity of the MPPT. For this to work continuously, the total energy supplied by the MPPT per day/week must exceed the total energy removed from the system per day/week, as the battery just stores/buffers energy; it doesn't create it.
Consider this example (done in amps rather than watts for clarity, even though watts would be more accurate) - Assume an MPPT with a ~1A output is connected to a fully charged battery. Now assume a 10A load is connected to the system. Roughly 1A of that 10A will be supplied by the MPPT and the remaining 9A will be supplied by the battery. The battery's terminal voltage under discharge will define the voltage at the load and the MPPT will adjust its output to match (being a constant power device). This load can't be sustained, as the battery will eventually be depleted. When the load is removed, the MPPT's output will float charge the battery until it is returned to 100% SOC or until a load is re-applied.
Rapid load voltage fluctuations between 12.0 and 13.5 strongly suggest that the MPPT (or the load, if electronic) is being forced into a fold-back self protection mode as a result of a load that exceeds the MPPT output without adequate battery buffering.
This thread may also be helpful.