Is Having Multiple Charge Controllers on a single 48V System/Battery Bank a "Bad Idea"?

Question

I have a 48-volt renewable energy power system in my house that is presently off-grid. It has: - a 6kVA inverter/charger that can connect to a generator or the grid (charge controller #1) - a 2kVA wind turbine with a built-in 48 volt controller - a 3kVA solar panel system with a 48 volt charge controller - a 1kVA water wheel that simply adds power to the battery bank through a 3-phase rectifier. These are all connected to a single 820Ah flooded lead-acid battery bank.

We have noticed that when the generator is running and charging the battery and the wind is blowing, we see the wind turbine switching from 'charging' to 'not charging due to high voltage' and the voltage of the system increasing.

It would seem that perhaps the charge controllers are competing with each other, each raising the voltage against the other until they hit a cut-off and then repeating the process.

As a result, should the charge controls be coordinated in some way so that only one charge controller is actually charging the battery at one time?

Obviously this is not ideal, because I'd like to be able to charge the batteries with all the available power if the sun is shining, and the wind is blowing etc.

Answer 1

I was headed this way until about 2009. In the most modern and expensive setups you might have all the devices communicating and coordinating. In most, each one has its charging parameters that it's trying to meet, and the only communication is the voltage on the bank.

With multiple, uncoordinated charge controllers you may exceed the optimum charging current, and you should expect to have disagreements among uncoordinated charge controllers (and a water turbine with no charge controller) as their charging set-points and algorithms are almost certainly different.

If you have the money to put into it, or you see it as saving money on the battery bank's lifetime, a set of networked, coordinated smart charge controllers would certainly work more optimally (and can all work at the same time) - but you should also realize that any system that won't leave you in the dark for large parts of the year is going to have some excess capacity when everything is going well, charging-wise.

You can't simply add more battery without making it really hard to get it fully charged and equalized, and you can't reduce power sources without finding yourself in the dark after a few dry, cloudy, windless days. So sometimes, you are going to be dumping excess power. If you can do something useful with that power, great; (laundry is a good one, often) otherwise it's just part of the cost of having power more of the time. In the case of a fuel burning generator and a wind turbine, common sense would be to shut down the fuel-burner on a windy day.

Answer 2

Sounds like contention between charge controllers. Your problem is whichever device raises the DC voltage for charging will cause the others to block or dump. So when your battery is low, whichever device kicks in first will remain the dominant DC supply until they run out of input (fuel/sun/wind/hydro) or they satisfy the battery charge.

Though there might be a way to fix this, albeit perhaps non optimal. I've never tried but it could work in theory. Many charge controllers have manual settings for connect and disconnect voltages to keep the batteries topped off. You could set each charge controller to different, yet very close voltages to establish a priority. Here's a general example of the controller settings:

- PV 48V (highest priority)
- Turbine: 47.5V (medium priority)
- Genny: 47V (low priority)

If your battery bank is low and the genny is running at night and the wind picks up, the wind controller will see a lower voltage and kick in to charge the DC. At the same time, the genny will see a rise in voltage causing it to think the battery is full and shut down letting the wind take over. Same goes for solar but solar will take precedence over the wind or you can reverse depending on which will deliver more current.

Some downsides would be non optimal genny charging and possible hunting as the PV/wind fluxuations can cause the genny to kick on and off. I'd call each of the manufactures and see what they say.

Thoughts?

Answer 3

I agree with ThreePhaseEel, this is a "terminal mode". Generator + wind + solar + water - your loads are more than the battery can absorb, since the battery is at or near capacity.

Your renewables are making the power, but it is more than either the house or the battery can use right now. How is your system designed to cope with that situation? Because - mind you - this should be a several times a week situation. Having excess power that needs dumping is the hallmark of a well-sized system. Your solar, wind and water all seem ample for an average home. You should be dumping, a lot.

It seems like right now, your generator is overpowering your renewables, causing the wind and solar to "back off", which is a total gong-show. At the absolute least, it should signal the generator to shut down. Unless you have a weather report in your hand that says otherwise... there is no earthly reason to run a generator if the battery is above 80% capacity. You should be saving that headroom to absorb the renewable power that comes available.

A hydro-generator should handle dump by shutting off: closing its input valve and capturing stream-flow in the upper impoundment, for future use. (this assumes you don't have year-long overabundance.)

Wind and solar cope with dump either by turning off, or diverting it out "Dump" terminals on the charge controller, which can go anywhere. For instance run dehumidifiers, electric heating registers or overcycle the air conditioning, effectively using the house as thermal storage.

If you have hydro and a paucity of water flow, a great use of "dump" is backpump water from the lower impoundment to the upper. This is called "pumped storage" and amounts to using gravity as a battery. And it can be a really big battery; it's just a question of how large you're willing to make your upper and lower impoundments. In fact, pumped storage can work for people who don't even have streaming water hydro, as long as they are able to build impoundments and get enough water to fill them and replace losses.

Answer 4

To answer the question, taking into account your answers and further research on the matter:

Multiple charge controllers are not necessarily a 'bad idea'. However, whichever controller has the highest threshold setting at that point in time will be adding power to the batteries. If they are not programmed in a coordinated way, that could result in batteries being overcharged.

In this case the manufacturer's recommendation is that the batteries are configured with the following charging parameters: - Bulk charge up to Absorption voltage of 58.8 V. - Float voltage of 54 V.

In this case, the solar charge controller is the primary charger as this is the biggest source of generation. It also has the capability to throttle back solar generation when targets are reached (which would include from other sources).

Where these targets cannot be met, (e.g. too much generation) the inverter can be configured to activate a 'dump load' if these targets are not met (e.g. the voltage goes above the maximum voltage for that stage of charging). However, in order for the inverter to know what the current state of charge is, that information needs to be communicated from the solar charge controller.

Both the solar charger and inverter have a communications protocol, which could be used to communicate this state-of-charge information with additional software/hardware.

The wind turbine has a factory set threshold of 57.2 V, which means that when the batteries are undergoing the absorption stage, the wind turbine will stop generating. During bulk (below 57.2) or float, the wind turbine will contribute power, but the solar charger will throttle back to maintain 54v. If that is not achievable (i.e. the voltage reaches more than 54v) then in theory the dump-load will activate.

The most elegant solution would be to have devices that communicated and all had configurable parameters, but for this you would need a master control system of some sort, and generation devices that were compatible with that same system.