Solar Battery Management System
The German made solar charger, which uses fuzzy logic to track battery state of charge, fails to track properly when the batteries are fully charged; doesn't handle multiple batteries or multiple power sources; and doesn't provide prioritization of loads. This is more than enough provocation for a geek to build a battery management system.
The accurate tracking of the state of charge in lead-acid and other battery types is a significantly difficult problem, having many "solutions", none of which work well. The concept of battery state of charge is vague: the measures used can change as temperature and other environmental variables change. The tracking of charge state by counting Coulombs passing in and out of the battery is probably the simplest and most effective, but is subject to cumulative errors and needs to be regularly reset. In addition charge passed into a battery is not always stored and can simply pass though by means of side reactions such as electrolysis and grid corrosion. For this BMS project, the algorithms developed for tracking charge state still could be refined.
On this page the electronics associated with the BMS is described. As indicated above the requirements are:
The BMS has been designed to manage three batteries. In practice batteries can be connected together in parallel but this is safe only if they are of the same chemistry and age. It is better in fact to manage each battery separately. That way if a battery's state of health fails it can be isolated from the system and not adversely affect the other batteries.
The BMS has also been designed to manage two load priorities. Heavy current loads can be allocated low priority and will disconnected when the battery charge state becomes critically low. Low current loads can be allowed to remain connected for a longer time.
this design only one power source has been incorporated. In principle
it would be valuable to have other power sources available of the same or different types.
A major design decision to be made is how an electrical common is to be provided. In many solar charge regulators, switching and current measurements are performed at the negative side of the power feeds (as suitable low resistance N-type MOSFET switches are readily available). This however means that an electrical common cannot be established for the source, batteries and load and so creates design difficulties for the electronics used in measurement and control. For this design where the number of managed elements is greater than the usual three, current measurement and switching is performed at the positive side of the power feeds making the overall electronics design much simpler. to avoid the need for generating the high control voltages required by the N-type MOSFETs, switching is performed using P-type MOSFETs. These have higher on-resistance than the more commonly available N-type MOSFETs, but for the currents and voltages present in the application in mind (<20 amps and 10 to 15 volts) there are suitably low resistance P-type MOSFETs available at a reasonable price.
design currently envisages a central controller that measures voltages
and currents, and issues controls, directly to all parts of the
circuit, rather than having self contained modules with their own
processor. The latter system can be more flexible and modular but poses a
number of challenges with regard to reliability and communication with
a central processor. It would also have a higher current drain than a directly connected system
The overall design consists of:
The hardware layout is shown below:
This is a prototype board with space at the top left for a remote control receiver. The interface boards are hand wired.
This is the second version, needing the interface boards and switching board updated and the remote control receiver replaced.
Overall the system works satisfactorily but a number of issues remain: