12V Guru – can I mix solar and mains to charge my battery?

Question to 12V Guru: Can I mix solar and mains?

Firstly, thanks for all the great suggestions/answers r.e. 12V systems.

Secondly, I will appreciate it if you could address this question with regards to a mix of solar charging and mains charging.  The situation is this:  I have a mains 3 stage charger in my caravan.  I want to add a solar panel and a 3 stage solar regulator.  Can these two outputs be connected directly to the battery?  Reason for asking is, that if I do connect them to the battery, will they “fight” each other?  In other words, will they feed into each other (the mains output trying to feed into the solar regulator output, and vice versa)?  Is there a “magic box” that needs to be inserted so that they do not feed into each other?

Eddie Roux, Sinagra, WA

12V Guru answers:

Hi Eddie,

A great question as I would be very confident that there is a myriad of different combinations installed over the ‘000s of RV’s that are roaming the country.  Now a vast majority of these RV’s sit idle in a caravan park and are never really using the benefits of multiple sources of available energy – especially the one that is for free – the Sun! (well after you’ve paid for the panels that is).  As power management systems become smarter and more complex there are options for maximizing the use of available energy.

Age and type of solar regulator and battery charger

The problem with answering your question exactly is there would need to be a whole range more information provided such as the age and type and the solar regulator and 3 stage battery charger.  The fact that you mention it is a 3-stage charger certainly is a give-away that it is only a basic charger and not overly complex.  Probably not a bad thing in this scenario as the more complex (smart) a charger gets such as the newer switch mode power supplies the less likely it is to appreciate an additional power source trying to charge alongside.  Sometimes electronics get too smart for their own good.

Switch mode power supplies use what is called “mosfets” which are highly sensitive devices that continually turn on and off at high frequency and when combined in circuitry with inductors and capacitors they create power supplies that are far more efficient that the traditional transformer/rectifier style chargers.  Also, this style of electronic design allows for a lot more flexibility in design. 

If the solar regulator is a standard Pulse Width Modulating (PWM), it, like the battery charger, will be looking at the voltage at the terminals of the battery and with this input will be pulsing (opening and closing a switch) matched to the rising voltage required by the battery as it charges. This pulsing occurs with a very high frequency as a method of reducing solar panel voltage to a voltage usable by the battery.  

Isolation switch or a power management system?

Enough of the geek talk! In simple speak – smart electronics don’t like talking to other smart electronic devices and equally similar in our day-to-day worlds both smart and simple devices don’t communicate well and could well get confused when charging a lead acid battery.

In the real world you will probably be fine, but it is not recommended. A simple isolation switch between the two chargers is probably the simplest and safest way of managing the two devices. Whilst both rely upon the terminal voltage of the battery, the solar regulator and the battery charger are both operating internally in different ways and will both have different charging set points from their manufacturer depending upon battery chemistry.  No two are ever the same. The battery may get close to being fully charged but as the battery charger current tapers off as the voltage rises it could take a long time to truly be fully charged.

Our recommendation would be to seriously consider doing it properly. First option is to install an isolation switch to charge from one or the other device as required or available. Alternatively, install a Power Management System such as the BMPRO BP35 range where the unit blends the power from various sources, including solar but then manages the battery charging requirements with one source of power and provides output power to the loads as required.  Whichever path you choose – be safe!  Both AC and DC power are to be taken seriously as they can cause serious injury, including death.  On that note …

Safe travels – 12V Guru

The article is written for Caravan World Magazine.