Battery Monitoring and Protection
Two goals are supported by additional electronics related to battery charging and loads. These are battery protection and battery monitoring. Battery monitoring includes charge monitoring and load monitoring.
Commercial Solutions
Commercial solutions for battery monitoring and battery protection were considered first. Primary contenders were Morningstar, Victron, and Xantrex.
Battery Protection
At times it makes sense to prevent loads from running when battery capacity falls below some threshholds. An example would be large power draws such as AC/heat. Another example is charging the 24V or 12V bank from the 48V bank when the latter is low. This would preserve some energy for the auxilliary. Two products are meant to address this need.
Victron BatteryProtect
The Victron BatteryProtect supports a single load of up to 100A at 48V or 65A at 12V or 24V. The Victron Energy BP 48-100 (24-64V, 100A) costs about $150 and is less available than the BP-65. The BatteryProtectBP-65 (6-35V, 65A) costs about $60.
Morningstar Relay Driver
A more flexible product is the Morningstar RD-1 Relay Driver. The relay driver can support up to 4 outputs where each output is either open or provides a low power ground suitable to drive a solenoid. The RD-1 uses the Morningstar MeterBus (RJ-11 serial) to monitor voltages and other conditions reported by multiple charge controllers. Two or more conditions can be used to support a logical OR function. For example the 48V battery exceeding a voltage or the 24V battery dropping below a voltage can be used to enable charging of the 24V battery from the 48V battery. The RD-1 costs $145 from multiple sources. RD-1 outputs can also support three wire generator control. Up to four RD-1 can be chained on a meter bus interface.
Battery Monitor
A battery monitor provides battery state of charge (SoC) at a glance plus voltage, charge current, discharge current, and consumed Ah or Wh. One Xantrex Linklite came with the boat. This product supports batteries of 12V to 32V and can support up to 2 batteries. Victron Energy battery monitors can support battery banks up to 95V. The Victron BMV 700 supports one battery and sells for as low as $130. Victron BMV 702 supports two batteries and sells for as low as $178. The BMV-712 supports bluetooth, which I don't want for security reasons (and would go out of my way to not have). The BMV-700 series data sheet provides full specifications. The Victron ModBus register list is available from the Victron White Papers and Downloads page. To use ModBus TCP, a Victron Venus GX (about $300) or Victron Color Control GX (about $500) is needed (each can handle two BMV so one may need to be a BMV-702).
An alternate is the Xantrex LinkPro Battery Monitor (from $235 from Amazon and elsewhere). The Xantrex 854-2019-01 Data Link (from $180 on Amazon and elsewhere) may not be necessary. The Xantrex LinkPro Communications Protocol gives the pinout of the LinkPro RJ-12 connector with 3 pins for GND, TXD, and RXD and describes the interface as asynchronous "2400bps using 8 data bits, 1 stop bit and even parity". These are limited to 35V.
All of these battery monitors have one shunt and only measure current to the battery. If charge current and load current are equal the battery monitor reports no current flow. It is preferable to have an independent measure of total charging current, total load current, as well as independent measurement of each charging source and any very large loads.
Charge Monitoring
Multiple Morningstar controllers and relay drivers can be interconnected using a Morningstar MeterHub. The MeterHub has 6 RJ-11 ports. Two MeterHubs can be chained. Up to 15 MeterBus devices can be supported but need to be configured with different addresses. The configuration can be done with a PC using a Morningstar PC MeterBus Adapter and a generic USB serial port adapter and must be done before interconnecting with a MeterHub. The Modbus protocol is used. Another option is to use a Morningstar EMC-1 Ethernet MeterBus Converter instead of the serial port based PC MeterBus Adapter. This uses the IP based version of Modbus. Modbus over serial or Ethernet is supported by both FreeBSD and Linux, however it may be difficult to find documentation providing the Modbus target addressing for some Morningstar products.
Morningstar offers a few remote meters . It is not clear whether these can be used with the MeterHub. An email has been sent to Morningstar support to ask for clarification.
Morningstar Configuration
Morningstar MeterBus is capable of supporting up to 15 devices on a bus. The devices are interconnected using the Morningstar MeterHub. Up to four Morningstar MeterHubs can be chained.
A Morningstar EMC-1 Ethernet MeterBus Converter can be used to interface the devices on the MeterBus to Modbus over TCP/IP. Modbus is well supported. Morningstar support site has Modbus register assignments for TriStar MPPT, ProStar PWM (gen3), and Relay Driver. There are no Modbus documents for MeterHub, EMC-1, or remote meters. SNMP is now supported and might be a better choice for monitoring Morningstar products.
The ability to use a single EMC-1 to monitor multiple devices is limited and the limitations are not clear. The EMC-1 will be needed to at least set the MeterBus addresses on each device. To set the addresses, the EMC-1 needs to be temporarily connected to each device individually. The Modbus documents will be needed to set the addresses. It might also be necessary to program the Morningstar controllers while connected directly to the EMC-1 rather than through the MeterHub. Once set up, the devices can be interconnected on the MeterBus.
The EMC-1 and Modbus or SNMP can also be used to collect and archive data from the solar controllers. The computer used to do this will be single board computer (SBC) based on an ARM system on a chip (SoC) processor that is available with waterproof case. See the General Purpose Computers and Data Aquisition section for details.
The meters, RM-1 and TS-RM-2, cannot be used with a MeterHub. They must be attached directly to a Morningstar charge controller. This would require four expensive meters and the meters provide no archival of data.
Limitations of Commercial Solutions
None of the commercial products offers much in the way of data archival. These products are designed to give status at a glance, much like a fuel gauge on a fuel tank but little more. Some seem impossible to interface to provide external archival.
The charge controller products (meters) are designed to monitor charging performance but not battery health.
The battery monitors are designed to yield battery SoC and little more. To accomplish this a battery voltage is monitored and current to and from the battery. None of these battery monitoring products can accomodate separate charge and load shunts. Most do not support 48V battery banks. The motor controller has a battery shunt that is intended to serve this purpose (fuel gauge) but cannot be interfaced for data archival.
The Victron battery protectors can only monitor a single voltage, assumed to be the battery being protected. The Morningstar RD-1 is slightly more programable but designed to make use of voltages measured by the charge controllers and interface using the archaic Morningstar MeterBus, with remaining questions about whether the RD-1 could be programmed or driven through the EMC-1.
Commercial solutions exist but are piecemeal and generally not designed to be data aquisition front ends. They are expensive and their capability too limited.
General Purpose Computers and Data Aquisition
Battery protection, battery monitoring, charge monitoring, and load monitoring can all be provided using a very small general purpose computer, data acquisition modules, and solenoids to support battery protection. Alternately, battery monitoring and charge monitoring can be provided by a small computer and data acquisition modules with battery protection left to commercial products that won't be affected by a failure of the small computer.
This is a DIY solution. It requires the use of components that are not designed for marine use, requiring that they be kept in oversized air tight metal cases or plastic cases with metal lids with desicants and heat sinks. Fortunately these are very low power devices. It also requires some design and programming.
The DIY solution requires more work but should yield better results. The biggest risk is that the non-marine inexpensive components may fail. For battery monitoring this is not a major risk. Components are so cheap that a complete set of spares can be kept on hand. The greater risk may be in battery protection, either failure to prevent depleting a battery by not shutting off, or unintentionally shutting off loads including the chargers for the 24V and 12V batteries. A possible fallback could be manual override switches that bypass solenoids (or bypass the winding side). Solenoid failure could produce similar issues, but solenoids are also cheap and spares can be carried.
DIY monitoring solutions can also provide archival and display of data associated with navigation and ships instruments, such as speed over water, depth, pitch, roll, and yaw, GPS coordinates, compass readings, course and heading. The DIY monitoring section discusses the use of single board computers and microcontrollers to acheive this.