How to Choose the Amperage of the Charge Regulator

In addition to being divided into the PWM and MPPT categories, the charge controllers differ in amperage (10,20,30A, etc.). To understand the one suitable for your system, you have to calculate the current it must withstand, always starting from the same formula Current (A) = Power (W) / Voltage (V) where the power is that of the panel and the voltage is the 12 Volt of the battery.

When you choose the size of the regulator, however, calculate an additional 25% of the current calculated with the formula above to prevent it from being damaged in the event of voltage surges. You are now really ready to buy your regulator!

That said, we traveled for almost a year in New Zealand with a PWM regulator (because it was included in the panel, and we didn’t want to invest in a van that we would have to resell after a few months), a 110W monocrystalline panel and an 85Ah and we always got away with charging PCs and using the fridge.

However, we had to pay some attention not to waste energy unnecessarily (in fact, we tried to charge the various devices while we were traveling), and sometimes we had to turn off the fridge during the night, also because the sky was often cloudy.

How else can I charge the battery?


Charging the marine battery with the solar panel is not the only solution. We can also use the vehicle’s alternator (the one that normally charges the BM engine battery) to charge the service battery.

What is the alternator?

The alternator is a dynamo that converts the mechanical energy supplied by the engine into electrical energy.

It is a component that is already part of every engine, so breathe a sigh of relief because no calculations are needed to choose the one that best suits your use. It is already there to help you!

It would help if you connected the latter to the engine battery to use the alternator to charge the BS.

Since the BM and the BS are physically different and intended for different uses (and to avoid being left with the BM on the ground), we cannot simply connect them with two cables. Still, we need to insert another device in the middle of this connection.

Again, we have two possibilities:

  • Relay
  • Parallelator


The relay is an electrical switch that opens and closes the electrical circuit.

The installation is quite simple in the case of the relay we installed (this one). Here’s how to proceed:

4 contacts are coming out of the relay (4 metal tabs), each marked with a number:

  • The positive cable from the engine battery must be connected to the tab with the number 30,
  • To the one with number 87, the positive cable of the service battery
  • he 86 instead must be connected to the positive “subkey” or to the D +, which is the positive of the alternator
  • 85 is the negative of the subkey so that you can connect it to any negative (BM or BS or to any ground)
  • Sub-key means that it must be connected to the positive cable of something that turns on only when the vehicle key is turned to start it, such as the car radio or cigarette lighter.

This connection causes the circuit to close, thus letting the current pass only when we turn the key and start and then open, and stop the current flow when we remove it.

In this way, with the engine running, the alternator charges both the engine and service batteries. In contrast, once the engine is switched off, the circuit is interrupted, and no risk of using the van’s electrical system will also discharge the battery. Motor.

It is not pleasant to wake up in the morning with the battery on the ground and not be able to leave, as one who has experienced this tells you!

The expense is minimal, 10 euros for relays and a few euros for the cables you need to connect the batteries (take them thick, 16-25mmq given the voltage that crosses them), and one more up for the keypad.


Another possibility is to put a parallel tour.

This device does the same job as the relay I was talking about. Still, it ensures that a part of the energy produced by the small solar panels in addition to charging the BS, also loads that of the motor and other functions, such as controlling the charging voltage.

This device can be especially useful for campers that often remain parked for long periods in garages without being switched on because it keeps both batteries charged and prevents them from being damaged.

The cost of a parallel tour is a little higher than the relay. There are, in fact ranging from 30 euros to those 80 like this one to others even more expensive based on the functions they perform.

The more expensive ones normally also give you the possibility, in the case of a grounded engine battery, to use the energy present in the service battery to charge that engine. In practice, they save you from having to connect with cables to another machine if you are left with the battery on the ground or have to use an external starter.

Before buying it, remember to check the technical specifications of each device to know what it can or cannot do since there are all types and prices.


The third and final way to charge the battery is also the simplest: a battery charger. The only difference between the two methods listed above is that, in this case, you need to connect to an external source.

In our case, having motorhome an Opel Vivaro with mobile outfitting, we wanted to avoid drilling the bodywork to install the external socket (as is normally found in all standard motorhomes). We, therefore, purchased a battery charger (Ctek MXS 10) whose cables (included in the package) always remain connected to the service battery. We connect them to the current through a normal extension cable. By doing so, we can continue to use all devices as normal.

The model we took works very well, charges at 10Ah, and checks the battery’s health in various cycles (on the site, you can check all the technical specifications).

In this way, when connected to the external current, the 12-volt equipment normally works thanks to the battery. If we need the 220v current instead of using the inverter (which I’ll tell you about in a second), we connect a power strip to the extension, and that’s it.

How do I connect my 220v appliances to the battery? The battery supplies current at a voltage of 12v, so to use the equipment with a 220v socket (like the one at home), you need another device: the inverter.

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