Charge Controllers

A Morningstar TriStar 45 charge controller came with the boat. This charge controller is rated at up to 45A with 12V, 24V, or 48V battery banks. It was being underutilized with a 100W solar panel at 12V. This is a PWM controller and so there is no plan to continue to use it. Two Morningstar MPPT charge controllers have been purchased, one for each solar panel.

For the 24V and 12V battery banks additional Morningstar MPPT solar charger controllers will be used using a higher voltage battery as a charging source.

DC-DC Converters

There are a lot of DC-DC converters available in low power versions supporting an amp or two. Of those with higher power ratings, most are fixed at 12V or 24V output with the intent of driving a load directly rather than allowing a higher voltage to both charge a Li-ion battery and drive a load.

Brands such as Uxcel, Cllena, KNACRO, and others have no manufacturer web site, yet are sold on Amazon, Walmart, Newegg, and sometimes Alibaba and E-bay. They sometimes have inadequate specifications listed on the retailer listing. One brand Kohree has a web site with lighting products but no mention of electronic components. A look a Mouser Electronics, a well known electric and electronics component supplier, shows none of these manufacturers listed.

All of the 12V or 24V models step down to a fixed 24V or 12V so are intended to drive a load directly. These are not suitable to step down voltage to charge a battery which could then absorb a higher load. Though very inexpensive, these DC-DC converters will not be used except to directly drive 5V for USB or other 5V loads such as SBC (single board computers).

Victron has a number of DC-DC converters but since these don't support a charging profile. They do have a programmable target voltage so are more suitable for directly driving loads than for use to charge batteries.

Solar Charger Controller Types

There are two types of solar controllers, Maximum Power Point Tracking (MPPT), and Pulse Width Modulation (PWM). PWM are more simple and less expense. MPPT are more expensive but can extract more power from the solar panels. Both use PWM circuitry but the MPPT behaves differently most notably during the bulk charge stage. With a PWM controller the solar panel nominal voltage must be matched to the battery bank nominal voltage or the panel will not operate near its MPP. With a MPPT controller the solar panel nominal voltage can far exceed the battery bank nominal voltage. With the solar panel operating at MPP it can produce up to 20-30% more energy (kWh per day) in practice. The MPPT controllers can also use a higher voltage battery bank as a charging source for a lower voltage bank.

How Charger Controllers Work

All modern solar charge controllers use a three stage charge profile plus periodic, often configurable but usually monthly, equalization stages for lead acid batteries. The first stage is the bulk charge where the battery is discharged enough that the charging source (the panels) cannot bring the voltage to the optimal charging voltage. The bulk stage is current limited. The second stage is the absorbtion stage. When a configured absorbtion voltage is reached the charge controller limits voltage on the battery. During the absorbtion stage, current drops as the battery is no longer able to absorb charge. The next stage is the float stage. When current in the absorbtion stage reaches some low level, or after a configured time has elapsed in the absorbtion stage, the voltage is ramped down to a configured float voltage. Lead acid batteries can be held at the float voltage indefinitely without damage and very slowly absorb more charge if not fully charged.

Lithium ion (Li-ion) batteries work a little differently due to their internal battery management system (BMS). A Li-ion battery will begin charging when voltage sufficiently exceeds the internal battery voltage and can charge most efficiently within a certain voltage range. Typical voltage ranges for full charging and maximum efficiency fall within 14.4 to 14.8 volts for 12V batteries. The solar charge controller absorbtion voltage should be set to 0.1V lower than the upper limit on this range. The BMS will force the voltage down to the low range as long as the battery can absorb more current than the solar panels can provide. For controllers that use an absorbtion time limit, this time should be set for as long as possible up to 12 hours or the float voltage can be set to 0.1V above the lower voltage in the range. The BMS will protect the battery cells. If charging voltage remains high on a fully charged battery, or near fully charged, the BMS will leak some current so that other batteries in series can fully charge.

During the bulk charge stage the capacity of the solar panel is limiting the charge rate. During the bulk charge stage the solar panel and battery are affectively directly connected when a PWM controller is used. The voltage at the solar panel is usually well below the solar panel's maximum power point (MPP) during the bulk charge stage with a PWM controller. During the bulk charge stage a MPPT controller will use multiple PWM circuits, at least one to limit the current to the battery and one will keep the voltage at the solar panel higher and allow it to produce maximum power. With an MPPT controller the solar panel and battery will see different voltages in all charging stages. The benefit is greatest in the bulk stage and the battery may absorb up to 30% more charge during this stage. If the solar panel voltage is well above the battery float charged voltage, then with an MPPT controller charging can occur in low light conditions such as early morning, early evening, and hazy or partially cloudy days.

For MPPT chargers if the rated current capacity of the charge controller is exceeded current is limited. Most PWM controllers can't do this and therefore can be damaged if the charging source can produce more current than the controller is rated for (such as a battery bank used as a charging source). If the temperature of the charge controller or battery rises, most charge controllers limit current or shut down.

Charger Controllers Considered

After looking at capabilities, support, and cost the solar charge controllers that were considered were from the Morningstar Professional Series product line. The Morningstar TriStar MPPT and Morningstar ProStar MPPT and Morningstar ProStar (Gen3) were considered for various uses. All controllers in these product lines can be networked. Both MPPT and PWM versions of the TriStar and ProStar are available. The TriStar supports 12V, 24V, and 48V battery banks, higher maximum current (30A, 45A, 60A), up to 150V Voc input, better efficiency, and has larger heat sinks. The ProStar supports 12V and 24V battery banks, are less expensive, come in lower current models (25A and 40A for MPPT, 15A, 30A for PWM), have a lower Voc rating (120V for MPPT, 60Voc for PWM version), and small heat sinks.

All of the MPPT charge controllers can use a higher voltage battery bank as a power source. The maximum power delivered to the battery being charged can be limited by purchasing a solar controller with a lower current rating. The Morningstar controllers MPPT allow this but some, including Morningstar PWM would simply overheat and possible burn out. When driven by a higher voltage battery the MPPT capability is not needed but the current limiting to charge controller capacity is needed. If driven from the solar panels the added cost of the MPPT versions are well worth the cost due to improved charging efficiency.

Pricing

Two sources of charge controllers were considered: the AltE store and Wholesale Solar. Both are oriented toward DIY residential solar but carry some products that happen to be appropriate for marine use. Prices were higher on Amazon. Prices here are rounded to the nearest dollar.

Lowest costs (as of June 2020) were found on the altE store solar charge controllers web page. The TriStar TS-MPPT-30 is $382. The The ProStar MPPT 25 (PS-MPPT-25) is $318. The ProStar MPPT 25A with monitor (PS-MPPT-25M) is $390. The Panasonic 330 Watt HIT High Efficiency Solar Panel is $309 and the Panasonic 335 Watt HIT High Efficiency Solar Panel is $320.

Prices on the Wholesale Solar charge controllers web page were consistently higher. The TriStar TS-MPPT-30 is $385. The PS-MPPT-25 is $385. The PS-MPPT-25M is $395. The Panasonic 330 Watt HIT Panel is $390. The Panasonic 335 HIT Panel is not offerred.

An order was entered into the AltE online cart but held for months while they were checking whether they could get the Air Breeze 48V version. They never got back on the special order after multiple calls (though it was Covid-19 season) so the Air Breeze 48V order was placed through eMarine and the solar panels and controllers purchased from AltE. One spare ProStar MPPT was purchased.