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Review by John Clarke Solar Charger Controller from Oatley Electronics Oatley Electronics (www.oatleyelectronics.com) has two new solar packages suitable for charging 12V and 24V lead-acid batteries. These can be used to maintain battery charge where mains power is not available, or as the basis of a solar power supply system for lighting or other low-voltage loads. F or the 12V package, you get a single 16W solar panel, while for the 24V package, two 16W solar panels are included. These two panels, connected in series, form a 24V, 32W equivalent panel. The same solar charge controller is included in either package, and it operates with either 12V or 24V batteries and matching solar panels. Both packages include 5m of 15A-rated figure-8 wire to connect the battery to the load and/or extend the solar panel wiring. The pricing of both kits is very reasonable, as detailed at the end of this article. The charger itself is only suitable for lead-acid batteries such as flooded cell (standard liquid acid), absorbed glass mat (AGM) or gelled acid (gel cell/SLA) types. However, note that some lithium-based rechargeable batteries claim to be directly compatible with lead-acid chargers. Possibly the most pressing need for a solar charger is to maintain the 68 Silicon Chip charge in batteries that see infrequent use. If a lead-acid battery is left to self-discharge over time, its life will be reduced, and it can be permanently damaged. Where available, you can use a mains-powered trickle charger to maintain the charge, although it will draw power from the mains all the time. In more remote places, using mains power is either inconvenient, dangerous or non-existent. Solar charging is more practical there, especially on boats, in sheds, on farms, and at campsites. Even for locations where mains power is available, the long-term cost of using a solar charger is likely lower than paying for mains power. This system doesn’t cost much more than a mains-powered trickle charger, but there is no ongoing cost. Each 1W of continuous power drawn from the mains adds up to 8.76kWh per year or around $3 at current prices. One practical use for the solar Australia's electronics magazine charger is to maintain the charge in a vehicle battery when it is not used often or stored for an extended period. That includes classic and vintage vehicles, farm tractors, ride-on petrol mowers (especially when unused during a drought) or spare vehicles. When used as a solar lighting system or for any other application where power is being drawn from the battery, the solar charger includes features to prevent discharging the battery to the point where its life will be reduced. This includes dusk-to-dawn operation (suiting outdoor lighting) with a timer and an adjustable switch-off voltage when the battery is deemed discharged. The battery capacity used with this system (measured in amp-hours [Ah]) needs to be considered based on the power that will be drawn from it over the day and night, and the number of days in a row that available solar power might be insufficient to recharge the battery. siliconchip.com.au The solar panel(s) The supplied solar panel consists of an aluminium frame surrounding amorphous poly-crystalline solar cells sealed within a clear glass cover. These TUV NORD BL16P-12 panels measure 355 × 355 × 25mm and weigh 1544g. The maximum power output is 16W in full sunlight at 1000W/m2. At least with these solar panels, you know you are getting what you expected; we’ve published multiple letters from readers who purchased a panel rated at a particular power level when they could never achieve that! We’ve tested these, and they actually exceed their specifications. Their electrical specifications are an open-circuit voltage of 21.6V and a short circuit current of 0.97A. These two parameters are easily measured using a multimeter. The short-circuit current is measured by setting the multimeter to measure current and connecting the probes to the solar panel leads. The open-circuit voltage is a simple voltage measurement between the wires. Both are measured in full sunlight. Maximum power from the panel is specified as 18V and 0.89A (18V × 0.89A = 16.02W). The solar panel is supplied with a 650mm length of dual-core cable attached. Fig.1 shows the power curve for the solar panel. The red curve is the quoted specifications, while the blue curve is what we measured at midday in early April. The sample panel produced 17.6W, 10% higher than the specified 16W. It could produce a bit less on a sweltering day, so that is likely why the rating is conservative. For the type of solar charger supplied, the usable power region of the panel is shown shaded. This covers the region where the panel is used to charge a battery from near-flat to full charge. So with these packages, the maximum power available from the panel is between 10.1W (at 11V) and 13.5W (at 15V), assuming the panel follows the specified curve. Power and voltage is doubled for two series-­ connected panels for 24V use. Note that if a (presumably more expensive) maximum power point tracking (MPPT) charger were used, it would maintain the operating point at 18V/36V to take full advantage of the available power from the panel(s). But for maintenance charging or applications where you’re going to plug in a siliconchip.com.au Fig.1: the specified I/V curve for the supplied TUV NORD BL16P-12 panel compared to our measurements, made in full sun in early April. The mauve shaded area shows on which part of these curves the supplied charger will typically operate. Australia's electronics magazine July 2022  69 Six screw terminals are available on the side of the solar charger module for connecting solar panels, batteries and loads. flat battery and come back days later, it won’t make much difference. Solar charge controller This controller is quite small at 133.5 × 70 × 35mm and very light at 130g. Its model code is W88-C. The controller automatically detects and operates with either a 12V or 24V battery. As mentioned above, this is not an MPPT controller. Instead, it connects the solar panel to the battery using two paralleled Mosfets driven using pulse-width modulation (PWM). The Mosfets are switched with a variable duty cycle to maintain the required battery voltage. When a discharged battery is connected, the solar panel is connected continuously to the battery until the required end-point voltage is reached (typically around 14.4V or 28.8V). The duty cycle of the Mosfets is then reduced to a point where this voltage is maintained. Two USB Type-A ports are provided, rated at 5V <at> 2.5A total. But our tests showed that the maximum current that could be obtained before voltage dropped below 4.5V was 600mA. The short-circuit current is just 780mA. So the 2.5A seems ‘optimistic’. Still, they would be better than nothing if you had a flat phone battery and no mains power available. As the two USB ports are connected in parallel, if you are drawing 500mA from one, you can’t really use the other. Still, the second one might be useful to plug in a small LED light or similar. Connections to the solar panel, battery and load are via screw terminals along one side of the controller. The battery must be connected first before connecting the solar panel and load; disconnection is done in the reverse order. 70 Silicon Chip Reverse-polarity protection is included for the solar charge controller, and it uses Mosfets instead of diodes. These Mosfets are connected as ‘ideal diodes’ with a low drain-tosource resistance (Rds) of less than 11mW, so there is minimal voltage loss and heat produced. The same type of Mosfet is used for the charging connection from the panel to the battery, and the battery to the load. Heat dissipation The rear panel of the Solar Charger is the heatsink for the six Mosfets. These are pressed against the steel rear panel with adhesive thermal tape. This charger is likely to become very hot if used at its full ratings, but with the 16W or 32W panels supplied, the temperature rise is negligible, even with 10A drawn via the load connection. User interface A small LCD screen (34 × 22mm) shows the battery voltage at the top with solar panel, battery and load discharge icons below. An arrow between the solar panel icon and battery icon flashes during charging. Similarly, an arrow between the battery and load (shown as a light bulb) appears when the load is on. The battery voltage is shown to the nearest 100mV. The solar panel icon shows when a panel is connected and producing an output. The battery voltage icon is interesting in that it has five bars to show the state of battery charge, in addition to the voltage reading. There are three push-button controls on the front panel for Menu, Up and Down. The Down button also doubles as a load on/off switch. The display usually shows the battery voltage and returns to this screen automatically if the Menu button or Up/Down buttons are not pressed within five seconds. Australia's electronics magazine You can step through each menu item by pressing the Menu button. It cycles through the main display (showing battery voltage), the full charge voltage, the discharge reconnect voltage, the discharge disconnect voltage, load connect timer options (called the work mode) and finally, the battery type. To change any of these settings, press the Menu button to access that setting, then press it again and hold it for ~5s until the value flashes. The value can then be adjusted using the Up or Down buttons. Another long press of the Menu button is required to store the new value. Load output While you could connect a load directly to the battery, the load output on the charge controller provides valuable features. As mentioned, the load can be manually switched on and off with the right-hand push-­button except when making adjustments, when this button decreases the value. The maximum current that can be drawn from the load output is 10A. The main feature is that this load output will switch off the load when the battery charge falls below a preset voltage, thus saving the battery from damage due to over-discharge. The second feature is that the load can be switched off with an adjustable timer that starts counting down from dusk. Full voltage The full voltage setting is in the second menu and sets the voltage at which you want the battery to stop charging. Once the battery is charged up to this voltage, it is maintained at that same voltage. This is the only setting related to charging voltage. The battery is initially charged at a rate determined by the solar panel, until the battery reaches the full voltage. Typically the current needed to maintain the charge termination voltage is just that required to counteract the self-discharge current of the battery and any standby drain of the charge controller. That amounts to around 10mA. The specifications for the full voltage default values (double that provided when using a 24V battery) are somewhat confusing as they quote these as equalisation voltages. The default voltage is one of three values siliconchip.com.au depending on the type of battery selected. According to the user manual, they are for AGM batteries (B1), initially set at 14.4V; gel cell batteries (B2), initially set at 14.2V; and flooded batteries (B3), initially set at 14.6V. These voltages are too low for equalisation and are instead the full charge voltages. Typically, for equalisation, the charge voltage would be increased above 15V to ensure each cell in the battery becomes fully charged. This can produce a lot of gassing, so equalisation shouldn’t run very often. The so-called ‘equalisation’ voltage mentioned appears to be a misnomer in the user manual. The charger performs no equalisation. The full voltage for each battery type mentioned above is adjustable. However, we found a discrepancy in the B2 setting: we found that it was set by default at 12.6V and could only be adjusted downward from this to 11.5V, but not above 12.6V. By contrast, the B1 value could be adjusted between 14.4V and 13V and the B3 value from 14.6V to 13V. So if you are using an SLA (gel) battery, it would be better to use the B1 or B3 selection and set the full charge voltage to a more appropriate value like 14.2V. Note that the B1-B3 selections do not necessarily have to be used for AGM, gel and flooded batteries in that order. The selections are arbitrary and are determined by the voltage set for the connected battery type. You will probably need to compromise with the voltage settings. When charging a battery, typically, the voltage is raised until it reaches the bulk charge end-point voltage (around 14.0-14.6V) and then the charging current decreases to a low level. The charging voltage then drops to the float or trickle charge level, usually around 13.0-13.8V. However, that is not the case with this solar charge controller, as the full voltage is maintained. Many batteries have a maximum specified time at the bulk charge voltage (usually no more than eight hours), after which damage can occur due to outgassing etc. That is why a more advanced charger will drop the voltage once the battery is fully charged. Of course, with a solar charger, the maximum charge time is limited by the number of daylight hours available. But that could easily exceed siliconchip.com.au eight hours, and some batteries could have much shorter specifications for the amount of time they can spend above 14V. But with this charger, there is no other charging state. So you either set the charge voltage to the bulk charge level to fully charge the battery, or set it at the float level for the battery for long-term use. A higher voltage setting will have the battery charged closer to 100%, while a lower voltage will be more suited to lower float (maintenance charge) requirements. However, setting it to terminate at the float value will prevent the battery from reaching full charge if it is ever discharged. So if you are using the charger to maintain charge rather than for charging, set the voltage value to the recommended float voltage for the type of connected battery. Alternatively, if using the charger with a load such as solar lights, it may be better to set the full voltage to the recommended bulk charge voltage (or full charge voltage) for your battery type. So the setting really depends on your application. It would be wise to check the manufacturer’s specifications for your battery before making that setting. Load reconnect The next three menus are related to the load output. They set how the load is connected based on the battery voltage and light level, and for how long. The load reconnect menu selects the battery voltage that the load will be reconnected after being disconnected by a low battery level (see the next menu). It is initially set at 12.6V and can be adjusted between 10V and 13V. This setting should be high enough that the battery gains some extra charge from the solar panel if the load is disconnected, before reconnection. Load disconnect The next menu is similar to the above menu, except it sets the voltage below which the load disconnects. Initially, it is set at 10.7V, but you can adjust it between 11.5V and 8V. Ideally, this should be set to a higher value than 10.7V, as the battery would be almost fully discharged at this level and possibly already damaged. Around 11.5V would be a more practical value to prevent excessive battery discharge. Australia's electronics magazine For more information regarding battery voltage for charging and discharging, see: deepcyclebatterystore.com/ how-to-maintain-batteries/ Work mode This mode is for the load switching settings. This is initially set for 24H, meaning the load can be switched on at any time and will remain on continuously. Other options switch on the load from dusk for a set period in hours. When the time is set between one hour and 23 hours (1H to 23H), the load is powered for that period beginning at the onset of dusk. If you select the hours as zero (0H), the load is switched on over the entire dusk until dawn period. This is distinct from the 24H setting, when the load can also be on during the daytime. The controller detects dusk and dawn by monitoring the solar panel voltage, with 8V as the threshold voltage (or 16V for a 24V panel). Below 8V is dusk to dawn, whereas above 8V is daytime. Battery type Finally, the last mode before the main display reoccurs is the battery type. This is selected as B1-B3. You can set a different full voltage for each battery selection as detailed above. The maximum charging current for the controller is 30A. It will not come anywhere near this limit with the supplied panels. You would need over 300W of 12V panels or 600W of 24V panels to exceed it. Conclusion While the charger is a bit basic, these solar packages from Oatley Electronics are excellent value if you are looking for a solar charging system with around 12-24W of power, such as for some small outdoor lights or maintaining an infrequently used leadacid battery. The two packages are ● IT159PK1 with one 16W solar panel, the 30A regulator and 5m of Fig.8 cable, suiting 12V lead-acid battery charging: $39 plus P&P. ● IT159PK2 with two 16W solar panels, the 30A regulator and 5m of Fig.8 cable, suiting 24V leadacid battery charging: $54 plus P&P. For more information or to order these packages, visit Oatley’s website siliconchip.au/link/abes SC July 2022  71