Voltage cut out cut in

Hello guys, I have a project I need to undertake to connect a DC supply to a rather heavy load. That load would be to connect a 300 watt 12vDC water heating element during high level outputs from my solar array. I’ve looked at the LTC4365 in its PDF and it looks like this might be a good way to start this so far. Here are the parameters I need to work with. The array is a 600w system feeding into an MPPT controller to charge a 12v battery bank which is running an inverter. I have lithium batteries which like a peak input of 14.5~14.8. There are times my MPPT runs a tiny bit above the 14.7 range and this causes my inverter to shut down due to an over voltage sensed condition. So I want the LTC or appropriate IC and circuit to turn on power to the water heater if the DC battery voltage goes over 14.4 rather precisely and turn of if it goes below 14 +/- .2~v. ? There will be a thermostat on the water heater so if the water temp is good and the heater needs to shut down, I need another load that will take up power if the voltage is above the inverter cut off voltage. So it looks to me like another LTC circuit would be needed along side of this one that can continue to divert power from the bank, MPPT system?


Thank you for your post.

Based on what you have mentioned, it does sound like an MPPT system would be the way to go. This will likely take some research and development to come up with a proper layout and part numbers that match your ratings.

Hi, I have the MPPT system already installed and so far it’s working pretty fairly. The problem is with the inverter during mid day high solar levels. The controller lets the voltage going to the batteries go above the threshold the inverter can work with which causes it to shut down momentarily then come back on, it oscillates on and off. So I need to figure out a way to just divert some of the solar so that it powers up an in-tank heating element I have located. As it is right now, I have to monitor the system and disconnect the panels from the MPPT box during hours of high output if the batter Voltage goes above 14.5. I’m pretty sure when I install more storage batteries in the near future, this may not be an issue, I don’t know, either way, I still believe it would be nice to have this diversion circuit.

Greetings @92monterodo-bat ,

Perhaps you could put a diode in series with the inverter, in which case the voltage drop over the diode is enough to keep the inverter’s input voltage below the OVP. The bad thing is that the diode then consumes some power all the time, which you’d need to dissipate somewhere. Placing a power MOSFET in parallel with the diode and drive it with a comparator would solve that (partially), as the diode would be by-passed when the voltage is below a set value.

Cheers, heke

Hard to come up with a safe solution to this problem given the information provided. Seems like you’ve combined equipment that was not intended to work together.

What make/model is the MPPT and the other existing components?
That may help somebody come up with ideas for a solution.

I know rolling your own is appealing, you might be already aware of this but this is an extremely common application in Solar and Wind applications called “Dummy Load” or “Dump Load”, solutions for this exist.

There are some amount of companies that produce Dump Load Controllers aka Diversion Load Controllers and some MPPT controllers out there come with a Load Outputs built in.

Click here for an example of Diversion (Dump) Load Controllers

This would be akin to Dummy Loading, rolling off excess power, which is often done in wind power because the turbine needs a load so it doesn’t freewheel. Diversion Loading is appealing because it doesn’t waste energy and water heaters are a common application for this.

You are right Kristof.
Here, the dummy load would be the diode, wasting just a small fraction of the energy to keep the inverter running. I’d say series dummy load is better than a parallel dummy load.
Anyway, IMHO we should feed back the excess grid energy to wind turbines to adjust the rotation of the earth.
Cheers, heke

Yes and no to these comments; I’ve been tinkering with electronics for a good while. I went to school for it back in 79/80 but did not follow the field full time after that…I’m very aware of dump loads which is what I wish to do here and now and rolling your own is what I’m into. I just couldn’t do that with this MPPT controller, way out of my realm with all the software and circuitry it incorporates. The solar array consists of 6/ 100 w panels nominal 12v connected in 2x3 strings so nom. is 36v fed to an Epever commercially manufactured MPPT which is rated to accept up to 150 VOC of various watt levels depending on voltages. In this citation that would be 750 watts, next level up would 1480@24v nom… That would require me to install a 24v/12v converter, as this is an RV with mostly 12v systems along with the necessary AC circuits which I am powering with the inverter that isn’t happy all the time, just some of the time. I either have to be on site to disconnect the solar array or place an AC load on the inverter but this is heading for that state of broke down now as the fan fals to come on for cooling and it shuts down in excess heat mode. So now I know where some of that limited funding is going which will make it tougher on me for the coming month.I’ve considered a simple solution using a zener diode to trigger a transistor but not sure if that would be the most advantageous method. I thought about a comparator but a window circuit doesn’t truly fit my desire. I thought about the TL431 as an adjustable reference and maybe that would be a solution. Then I ran across the LTC4365 thinking that might be possible but not sure if this is what I’m looking for either. I’ve read about how a MOSFET might experience issues with shorting because of switching issues. I think the simplest method would be a zener with a relay but as with any relay, points burning is going to become an issue. That method is a crash bang which doesn’t sound really good. I have a pretty good supply of components to work with so I will just experiment for now I guess.

My MPPT has a switched load that functions during no solar periods and is rated at 10 A which is pretty much nothing more than an automatic light switch. There may be other features it is capable of but it would be done with code and more hardware which I don’t have at the moment and so I’m useless there. Now combining Pb/H2so4 batteries with LiFePo4 is not really the best idea but it does work, as the Li batteries have BMS in them that disconnects them during particular states, others out there are doing it in off grid systems, When the Li batteries drain to the point of disconnect, the Pb batteries continue to supply power until the Li units are back above cutoff V and if the Pb’s are down while the Li are up they will bring the Pb’s up, one fellow is running a large sailing yacht somewhere around the seas and posting his website. At this time, I;ve added a small 500w wind turbine I got ebay cheap and it works so so, wind resource here is not what it was in the last place I spent a lot of time at before moving back to the city and going back to work full time and back on the grid for some years. The down side of combining chemical comp is the Pb’s are much more limited to charge cycles whereas the Li are much better and are pretty stable compared to the others until cutoff occurs. Once that happens, the Pb’s are on their way out until a charge cycle comes back from wind and or solar

Kristof, I considered that Morning Star controller for a long time but instead chose to go with the Epever MPPT as the MorningStar was a PWM and not quite as efficient and function options in my mind, now as the same situation exists with my MPPT, this is the where and why I’m here now.

Just skimming the documentation of a few of the EPEVER MPPT charge controllers, it looks like they are designed to work with either lead-acid or lithium-based batteries, and that they have user-programmable max output voltages. That would seem to solve your problem. Which model do you have?

This link I found may be relevant for you. There appears to be some good info therem but be sure to read through the entire thread, as there is some conflicting info within.

It’s an AN6415 . I have the cables needed to connect it to my laptop along with the remote system option available which I do not have yet and the software should already be on my hard drive. The voltage settings are correct for proper charging so I don’t want to mess with those. I had the inverter opened the other day and there’s no pots or any components on the circuit board to adjust the parameters, setting over or under voltage. Also the fan system for it is not functioning anymore either so a replacement inverter is a must get item now. I’ve been wanting and needing this for some time now, just been delaying it. I now have a schematic for building a shunt control using a Zener, a couple of BC547’s and a big high power MOSFET (IRFP2907) this is a TO247 200 amp piece so that would be sufficient for the 300 watt heating element that needs to go in the water heater which I would need to order, the rest of the BOM I have on hand. As I mentioned, the settings for the controller are where they should be I just need to tap some of the solar output from the MPPT to the batteries during peak hours.