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Solmax-4 Smart Tracking Regulator


(c) 2004-2008 D Kerr
What is Solmax?

The Solmax can be likened to the gearbox in a car. Engines do not connect directly to the wheels of a car because the engine performance is best at a specific speed and this needs to be matched to the varying requirements at the wheels (accelerating from rest, climbing hills, cruising at speed etc.) Likewise, solar panels produce their maximum power at only one specific voltage, which then varies depending upon the temperature of the panel and the insolation level. (Insolation is the term used to describe the sun shining on a surface). The battery voltage also varies depending upon the charge level, type of battery, age, temperature and several other parameters. Solar panels, when directly connected to a battery bank, rarely put out their maximum power because the battery voltage seldom matches the point where the panel delivers maximum power. Solmax solves this problem by ensuring that the solar panel always operates at the point where the panel produces maximum power and delivers the maximum current to the battery, depending upon its state. Thus, Solmax is an electronic gearbox; unlike the gearbox in a car, it has an infinitely variable ratio depending upon the conditions. It samples those conditions (depending upon what parameter is being measured) between three and 1,000 times per second.

Benefits of Solmax:

Solar panels are built so that on average, a reasonable power is produced across a range of insolation and temperature conditions. Solmax will increase the amp hours delivered to a battery bank (compared with traditional solar regulators), with the biggest improvements in cooler conditions and/or when batteries are not fully charged or their voltage is down because they are supplying a load. On a cool day, Solmax-4 will deliver 46% more charge to a flat battery than if the solar panel was directly connected to that battery. On hot, clear days with nearly full batteries, Solmax will deliver little or no gain unless you are using Unisolar solar panels. On cloudy days with patches of sun, Solmax ensures there is higher output than would otherwise be possible each time the sun emerges from behind the clouds (because the panel is cooler from its time partially shaded). Solmax will also deliver more benefit in winter than in summer because of lower panel temperatures. Solmax is extremely efficient (close to 100%), so almost no power is lost in the “electronic gearbox”. A working average  is 10-15% more amp hours delivered to the batteries in summer and 15-20% in winter. Solmax with Unisolar panels will approximately double these percentage improvements.

Solar Panel characteristics:

Let’s use a commonly available good quality 50watt panel (eg BP Solarex) as an example. The warranted minimum power from this panel is 45watts. The 50 (or 45) watts are only produced at a panel temperature of 25deg Celsius, whereas such a panel will normally be at about 50deg C when the ambient temperature is 20deg C! The output of panels falls 12% to 14% with a rise in temperature of 25deg, so our nominal 50watt panel may  produce less than 39 watts on a summer day. The actual power obtained is further reduced because the battery connected to the panel is usually below the maximum power point of the panel. For instance, at 25deg C, the panel will produce its 50watts at a voltage of 16.8 volts, whereas at 12.8volts (for a fully charged battery “at rest”), the panel only delivers 43watts. A discharged battery will receive a paltry 34watts. At 50deg C, maximum power (about 44watts) occurs at 15.2 volts. Less power is delivered at lower voltages, although the difference is less pronounced than at lower panel temperatures. The Unisolar range of panels is far superior to many other panels for two reasons. Firstly, the incorporation of diodes on each cell ensures that shading reduces their power output roughly proportional to the amount shaded. So, 10% shade means a drop in output of about 10%. Conversely, 10% shading of a Solarex panel will cut power output by a massive 90%! Unisolar panels have far better performance than many other panels in hot temperatures. For instance, my tests have shown them producing their full rated power in ambient temperatures of 35deg C where other panels are down to half power or less.


Solmax-4 tracks the maximum power point of the panel (with changes in panel temperature and insolation level) and ensures that the panel is always operated at the optimum voltage. It then very efficiently  matches the voltage of the battery. Any difference in voltage between the input and output of the Solmax is converted into extra current which is put into the battery bank.

Solmax uses state of the art microprocessor controlled electronics to track the maximum power point and convert all available power into useable current for the battery. You will obtain more power from your solar panels and the cost of the Solmax is less than half the cost of adding the equivalent extra solar panel area to gain the same benefit. Operating the panels at their maximum power point actually reduces the panel temperature, which helps the power output and longevity of the panels. Connection is very simple- there are just two input wires (to the panel/s) and two output wires (to the battery bank).

Solmax is available in two models, to suit solar panel arrays of 10-45watts (cheap model with no display)  and 25-110watts (upmarket model with display screen).


Remove any “blocking diodes” from  solar panels and replace them with a piece of thick wire (eg 3mm diameter) or a bridge; blocking diodes will reduce the available power from the panel if left in place. The positive and negative leads from the panel or panels must not be connected to anything else on the boat- they must be completely isolated other than the connections to the panels and Solmax. Connect the wires from the solar panels to the input terminals of Solmax. Connect the positive (red) and negative (black) output of Solmax where the panel/s used to be connected.  The Solmax box should be mounted with a strap in a dry, shaded area, preferably within the boat and ideally nearer to the batteries than the panels (because the wiring from the Solmax will often carry more current than the wires to the Solmax. It is important that the wiring be suitably thick so that the benefits of Solmax are not lost in the wiring. Note that the case of Solmax-4 is connected to the -ve lead from the solar panels. DO NOT connect either of the Solar Panel leads or the metal box to the boat ground. Minimum wiring specifications are shown at the end of this document.

Switch press definitions- Solmax 4
There are two switches, the left one is the “Display” switch and the right hand one is the “Mode” switch.

Normally, each press of the Display switch will cycle through display screens. The measurement is taken when the switch is pressed (i.e. the display does not update when it is “just sitting there”).

Microprocessor. The microprocessor is equipped with a Watchdog Timer which will reset Solmax if something goes wrong with the software. In 7 unit years of operation, this has never happened. If you see "Solmax 4 "and the type of panel displayed on the screen, there has either been a watchdog restart or the power was removed from the unit for more than two minutes.

Solmax will go into a special “Sleep Mode” to save power if solar level drops very low AND the battery voltage drops below about 12.8V. The blinking of the LED will slow right down and the buttons will effectively be de-activated. The display will read the last screen that was showing.

The Display switch cycles through the following:

A. Panel voltage- current operating point for the panel. Will vary with Insolation & temperature.
Open Circuit Panel Volts measured by temporarily disconnecting panel for 1/1000th of a second, three times per second.

B. Battery Volts
Battery Temperature

C. Operating Mode. Mode 0 is “normal” operation when the Solmax is "Bulk Charging" the battery. Mode 1 starts when the battery reaches its [temperature compensated] "Absorption Voltage". Mode 2 is entered after 4.5hrs of Mode 1 or when the unit is placed into “Unattended mode”. Solmax will not exceed the temperature controlled trickle-charge voltage for the battery in Mode 2. Note that maximum amps will still be delivered below this voltage. This reduces water usage in the battery, but the battery will not be given a maximum charge.
Kilowatt Hours since Solmax manufactured. Can only be reset in the factory.
PWM Percentage. This is the Pulse Width Modulation percentage

D. Output current in amps
Output Power in watts

E. Amp Hours delivered to battery since last reset and
T1 time which is the time in hours since the absorption mode voltage was reached. Once the T1 time reaches 4.5hrs (currently), the Solmax will change to trickle charge mode unless the battery voltage is pulled below 12.8V which will reset the T1 timer.

T1 time will be replaced by  "Switched Off" or "Unattended Mode" if these modes are selected If Equalization is in progress, the second line of the display will show "Equalization" and the time remaining to the nearest 0.01 of an hour. The counter starts at 8.00hrs and only decrements when the Solmax is in Mode 1.

Mode Switch.

If you wait until the LED has flashed (it normally flashes about once per three seconds), and press this button once momentarily, Solmax will go into Unattended Mode (described above) and reset the T1 timer. You can tell because the LED will double flash and the Display will indicate “Unattended”  when you cycle through the display screens. Pressing the button again will revert you to Mode 1 which is the normal mode.

If you should experience interference with your HF, you can temporarily put Solmax into “Switched Off Mode” (which is really “direct connect mode”) with a double press of the Mode button. This will remove any interference and when you cycle through, you will see “Switched Off” in the display. Remember to revert to normal mode later with a double press of the mode switch or you could cook your batteries ultimately with the panels effectively directly connected to the batteries!

The following are other features:

Three press the Mode switch to enter Equalization Mode. Equalization Mode is also automatically selected when Solmax has recorded 6,000watt hours of output since the last automatic equalization. It provides eight hours of deliberate overcharging [by 0.7volts]. These eight hours are only counted when Solmax is is Mode 1. In practice, if your boat is unattended, there is no load on the batteries and sunshine is bright, Solmax will only go into Mode 1 once per day [for 4.5hrs]. So, if equalization is set, it will occur for 4.5hrs the first day and 3.5hrs the second day. If a cloudy day intervenes, equalization will be deferred until there is enough insolation.

Four press Mode resets the mode and T1 time plus the Amp Hour memory (but not KWH) and equalization Mode if it was either manually or automatically selected.

Hold Mode and press Display enters special calibration mode

Calibration (Experts Only):
This is usually utilized in conjunction with laboratory power supplies and accurate measuring equipment. Don’t do it on your boat!

Hold Display switch and 1 press Mode = Calibrate 0 Amps
Hold Read switch and 2 press Mode = Calibrate 4 Amps

When it ends, use Display to cycle results and Mode to choose max power



Warranty    1year against defects in manufacture or components.
Type    High speed microprocessor
Input voltage    25V max
Continuous output current    10A max
Maximum output power at 14.5V    110W
Ambient operating temperature    10C to +50C
Panel type    Programmed at factory for Unisolar (preferred) or Solarex panels
Output regulator settings    Multistage, temperature compensated
Minimum operating point    0.5 watts
Power consumption when battery “full”    <0.04watts
Sampling rate    Up to 1,000 per second
“Dark” current    Depends upon panel. Approx 0.09A for 100W panel.
Efficiency    99.5% at 100W out. Over 90% at 5W. >95% at 12W. Increases with power.
Output fuse    10A ceramic fast blow M205 (110W)
5A glass M205 (45W)
Weight    550 gram
Box    Solid Zinc diecast box for interference suppression and solidity.
Tracking accuracy    Within + or - 0.15% with specified cables

Distance of cable run from panels to Solmax to battery bank versus MINIMUM cable size:

Distance Min diameter of each cable core Cross sectional area of core
2metres 2mm 3.2sq mm
4 metres   2.9mm 6.4sq mm
6 metres 3.5mm 9.4sq mm
#8 metres 4mm 12.8sq mm
#12 metres 4.9mm 18.9sq mm
#14 metres 5.3mm 22.4sq mm
Note 1: Cables meeting the above specification will mean a loss of 1.5% of power at 7 amps output from Solmax-4 and should therefore be considered as the minimum. Solmax will still work with thinner cables, but some of the benefit will be lost and the maximum power tracking point will be incorrect at high powers. Likewise, Equalization effectiveness will be reduced by inadequate cabling.

#Note 2: The terminal block is 4mm, so cables of greater thickness will need to be “thinned” and preferably tinned for secure connection to the terminal block.
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