What is a solar regulator and what is its job?

Most people who have anything to do with solar power know that a solar regulator is necessary. Not too many of these people understand the exact function of a solar regulator or how it does what it does.

For the purpose of this discussion and unless I state otherwise, I will be talking about the excellent PL series regulators from Plasmatronics. These are some of the most sophisticated, flexible and full featured regulators currently on the market. However, if you strip away the fancy features and displays, the PL series regulators carry out the exact same basic task as almost all of their cheaper and less sophisticated cousins.

So what does a regulator do?

If we were to look at the job description of any solar regulator, the first item (and in some cases the only function) is to prevent the battery bank from being over charged. The regulator does this by simply disconnecting the solar panels from the battery. So at a really basic level, the regulator is just a switch.

Plasmatronics PL40

Plasmatronics PL40

It is however a switch with a brain. Clearly we want the regulator to disconnect the solar panels only when the batteries are fully charged. To do this it must employ some intelligence and have some understanding of battery technology. The dumbest of regulators simply watch the battery voltage until it reaches the regulation point (battery maximum voltage) it then switches the panels off.

Most regulators have a bit more smarts than this. With some understanding of how batteries like to be charged, it is more reasonable to allow the battery voltage to rise to the regulation point, then carefully regulate the charge from the panels to hold the voltage at this point for a time (normally 1 – 2 hours). This is referred to as the absorption phase. This absorption phase typically takes the batteries from 92% up to fully charged.

After this absorption phase is complete, a smart regulator will drop down into float mode. In this float mode the regulator is letting just enough charge from the panels through to the batteries to keep the battery voltage up at its float point.

So this is the three stages that we hear about when the sales people are trying to convince us to pay for a smart or “three stage” solar regulator (or battery charger).

  1. Bulk – push as much power into the batteries as possible.
  2. Absorption – control the charge to hold the battery voltage at a point for an hour or two.
  3. Float – maintain the battery at close to full charge by controlling the charge and holding the battery voltage at a preset point.        

Other features and functions

So we have talked about the basic function of a regulator, but what other features and fancy bits?


Most regulators present some kind of information display. At the most basic level this is simply a couple of colored lights that give you some idea of the stage that the regulator is currently in. The PL series (and others) have LCD displays that provide a huge array of information about what is happening and what has happened. The PL series holds basic data for 30 days. This means you can step back through 30 days of historic data and see what has been going on with your system during this time.


By adding a shunt to your system, the regulator can “watch and record” the power that you use from your batteries. This allows the regulator to have a complete picture of the solar/battery system. If it can “see” all the power that is going into the battery AND all the power that is coming out of the battery, it is not difficult to calculate how much usable power remains in the battery (assuming you have told it how big the battery is). The PL series display this information on an often misunderstood screen called the SOC% (state of charge).

 Here are some other features of the PL series of regulators that I think makes them worth the extra few dollars.

  • Fully programmable regulation set points – you can set the regulator to the exact absorption and float voltages recommended by the battery manufacturer (and who better to recommend these values?)
  • 30 day history stored within the regulator.
  • Ability to track power used by high power appliances (such as microwaves etc) by using one or more shunts.
  • Ability to track charge provided to the batteries from the solar panels separately from charge supplied from other sources (eg generator or wind turbine).
  • Can be programmed to automatically start a generator or sound an alarm when the batteries are critically low.
  • Add on devices available to allow connection to a computer (for configuration or data download), modem (for remote monitoring).
  • Can disconnect the load (things using power) when the battery voltage is critically low.



A simple solar regulator can be purchased for about $25. It will tell you nothing about what it or your batteries are doing. It will have just three simple terminals to connect and will function perfectly for years. Alternately, you can spend $300 – $400 on an “all singing, all dancing” regulator that will tell you everything about the status of your electrical system and do everything except cook muffins.

I think the important thing is to know what your regulator should be doing!   

Footnote – Plasmatronics has recently announced their new regulator, the Dingo. This shares many of the advanced features of the PL series and has the advantage of regulating on the positive side and having a nice fully enclosed case.

Plasmatronics Dingo
Plasmatronics Dingo





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4 Responses to “What is a solar regulator and what is its job?”

  1. Leon Murray Says:

    may friend had a china made battery charger and it overheated after a week;'”

  2. john Says:

    hi again
    hows the travels??
    few qiuck questions

    got my pl 40 today.. $630
    but worth the money . this included the shunt (200 amp) which they recomended and the cable($160)

    first i didnt realize how small this unit is..lot of bucks for littlle toy

    ok first q

    if i want to run my 24v charger can i put it in line with the solar imputs without damaging the solar panels
    its 20 amp charger for deep cycle batteries,,,, boost float etc

    it would only be used when sun has gone
    , this way the shunt will pick up the imput and the pl 40 will still see a charge????

    q 2

    i have been using 2 truck battires till now,, for testing reasons
    with the 24v charger on they seemed to stay charged fine, but noticed the charger going up and down from 27v to flicking up to 30v for a second now and then

    now ive installed 4x 6v bateries 225ah in series =24v (deep cycle)

    the charger seems more even float is about 27.7 v to 29v

    but i have noticed by chance
    the new batteries ( when i went to back of bus)
    i could hear a slight bubbling from them??? is this normal

    they were not hot or even warm…. was first thing i checked

    and voltage at batteries confirm same reading as what is on charger out put reading
    when i put the metre on them

    oh and the manual for the pl 40 is bigger than the unit((( maybe thats where the cost is lol )))


    i was going to just run my lights off the output off the pl 40( i have 10 leds lights in total in bus)

    total is 2 amps

    this should be ok because they all not going at any one time

    i have a 100amp breaker from the batteries
    is that enough or should i have fuse as well??

    plus i have isolation switch

    ok thats enough
    hope u well

    cheers john

    ps i pucked up pannels today

    6 x 80 w to start with

    worked out good price $1500

    i get more later

    see ya

  3. Hobo Says:

    Gidday John, All good here. You made a good choice with the PL40 – great little device and best of bred (IMHO)

    Q1 – NO, do not connect the charger to the solar input. You should connect it to the battery via the shunt. See the PL40 install diagram for full details.

    Q2 – without knowing the specifics of the charger – I can’t say. Slight gassing is ok but heavy bubbling on float is not. I would check with the battery manufacturer to see what their recommended float voltage is and make sure it is not being exceeded.

    Q3 – that is fine. The load terminal of the PL40 will handle 5amps. I run my fridge from this terminal, it has the advantage of providing separate daily data for this terminal – so I can see how much power my fridge is using each day.

    Q4 – Circuit breaker is fine (in fact much better than a fuse – large fuses are expensive and hard to find in the middle of the Simpson Desert ).

    That is a very good price for those – just over $3/w.

    Thanks for the Questions John!

  4. Motorhome and Caravan Info Australia » Blog Archive » MPPT Solar Regulators for Motorhomes and Caravans Says:

    […] you have read my article that discusses the function of a solar regulator, you will know that a standard solar regulator is a fairly simple device with a fairly simple job […]

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