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?


2 Answers 2


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.

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.

  • \$\begingroup\$ The batteries, the load and the MPPT are all connected in parallel. So the MPPT pulses are passing directly through to the load. \$\endgroup\$
    – Moyersy
    Commented Jul 29, 2012 at 19:11
  • \$\begingroup\$ @Moyersy, some voltage variations are expected in this configuration, but if the voltage really varies between 12.0 and 13.5V, your batteries are pretty much dead. Under normal operation, but batteries will absorb extra output from the MPPT when there is a surplus and supplement the MPPT output when load exceeds its output. What you describe indicates that the batteries aren't doing their job as a buffer. \$\endgroup\$
    – HikeOnPast
    Commented Jul 29, 2012 at 19:25
  • \$\begingroup\$ I'm not sure how the batteries can act as a buffer in this situation? If all three (charger, load, batteries) are in parallel, is the load not effectively connected directly to the charger output and hence exposed to the PWM fluctuations? The batteries have worked fine for a month or so, trickle charging off a mains battery charger. \$\endgroup\$
    – Moyersy
    Commented Jul 29, 2012 at 19:36
  • \$\begingroup\$ @Moyersy: If you see PWM output (5-50KHz) on your DC link, your MPPT is broken and should be replaced. I've edited my post to better explain the role of batteries in such a system. Put your mains-based trickle charger back on. My guess is that things will work fine for another month. \$\endgroup\$
    – HikeOnPast
    Commented Jul 29, 2012 at 19:52

As far as Lead Acid batteries concern, Constant current charging may cause damage to battery, These Lead Acid batteries are safely charged at voltage around 13V to 13.5V, But after complete charging charger must be use Trickle Charging, Which gives pulses of voltage at constant interval to maintain its voltage levels.

For voltage regulation a simple technique to may apply Large vale capacitor after battery connection. No further regulation is required.

  • \$\begingroup\$ It looks like it's doing this trickle charging, and that is passing through to all my circuits after the battery. I think you mean a large valUe capacitor, and I will read up on that. \$\endgroup\$
    – Moyersy
    Commented Jul 29, 2012 at 19:06
  • \$\begingroup\$ Provided that the MPPT has adequate output filtering for its own switching stability, there's nothing that a large external capacitor could help with that wouldn't be addressed by a healthy buffer battery in parallel. (Assuming that @Moysery doesn't have loads that spike to the high hundreds of amps instantaneously, which would be the only plausible scenario I can think of that would benefit from a large buffer cap in parallel to the batteries and MPPT.) \$\endgroup\$
    – HikeOnPast
    Commented Jul 29, 2012 at 19:10

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