Auto switch for low voltage?

[LeoKa]

Maybe it does not exist and I am just dreaming, but I still ask the question here from the gurus. I am a live-aboard on the Columbia river.

I have two battery banks. Both lithium.
Bank 1 - is supplied by solar panels and Victron MPPT chargers. Bank is 680Ah total. Everything works, as long the sun is out. This banks handles 12V loads only.
Bank 2 - inverter/charger 3000W/150A/50A charges one large battery 400Ah (Renogy). Also provides the house with AC. This bank only supports AC loads, no DC to the house. It works smoothly now and I like the charging logic in it.

I have installed a DC/DC 12V charger (same brand as the inverter and battery) and set it to Lithium charging. The goal was with this to have charge from one bank to the other bank, in case there is no shore power for inverter/charger and the capacity goes down. Kind of buying time, before everything goes out, if I am away for longer time and no AC is coming in from shore. I tested it today and it works fine, as I hoped.
The only problem is that I cannot leave it on all the time, because it keeps charging the large battery, according to the settings. DC/DC charger has only two voltage settings for lithiums. Even the lowest setting is not low enough to let the large lithium to be used up, because it tries to keep it at 13.6V. This is fine, when I am on board, although the cooling fan keeps coming on, but it is suppose to be like that.
Is there a way to set this up, so the DC/DC could start charging only when the large lithium is close to the cut off voltage? Is there a voltage sensing switch, which could connect it and let it charge the 400A battery? (This could be useful when the shore power is out for longer time).
The solar panels are working, but in the PNW now not much is generated and I cannot add more to the existing 6 panels. So, I cannot get enough current coming down, which could charge charge both banks.
My goal is to be able to leave the boat in the marina for few months and not to worry about depleted batteries. Of course, if there is not shore power and no sunshine, all batteries will be depleted at one point, no matter what I set up.
At the last snow/storm week we had several power outages, plus the breaker in the marina's power station on our pier flipped and we had to live on batteries all night in the freezing temps. In case like this I can start my generator, but what if I am overseas?

 

[SteveK]

You have the D+ Terminal? It turns on when ignition run + is on.

I do not have ignition run + or a similar source found, (I have thought of) to turn it on when the ALTs are charging to pass current from start to LFP vis DC2DC. I have ordered some relays that close the positive circuit when grounded. Maybe this will work for me and you can use it as well.

 

[KnotYet]

I don't understand what load would be discharging the 400ah battery while you
are away. If I was going to be gone for a few weeks or months I would turn all
the AC loads off.

If you need AC power while you're away a simple solution would be to have the
DC-DC charger on a 12V timer set to an interval that wouldn't deplete those
batteries that solar charge. Some trial and error would determine the safe time.

 

[LeoKa]

You have the D+ Terminal? It turns on when ignition run + is on.

Yes, that is what turns on the the DC/DC charger.

 

[LeoKa]

I don't understand what load would be discharging the 400ah battery while you
are away. If I was going to be gone for a few weeks or months I would turn all
the AC loads off.

If you need AC power while you're away a simple solution would be to have the
DC-DC charger on a 12V timer set to an interval that wouldn't deplete those
batteries that solar charge. Some trial and error would determine the safe time.

I run a large fridge and a freezer all the time. I cook my own food, so I buy and store things in bulk. So, it is necessary to feed them for few weeks.
If my trip will go for few months, I agree with you, they should be turned off. In that case, only the dehumidifier needs to run. Steel hull, condensation, etc.
Your suggestion is a good one, I will look into it and test it out.

 

[mvweebles]

I assume from your post the DC-DC charger is a Renogy charger. I had one that failed and replaced with Victron. As SteveK states, the Renovy uses a D+ terminal to signal engine is on so it's binary. The Victron has adjustable voltage settings to determine when to consider engine is running and when to shut down charging. I am pretty sure (not positive) this would solve your problem.

Peter

 

[Bmarler]

This item certainly does exist, just google search for a dc volts set point meter.
Or dc volt meter with alarm output.
There will be something you could use to switch the d wire.

 

[LeoKa]

This item certainly does exist, just google search for a dc volts set point meter.
Or dc volt meter with alarm output.
There will be something you could use to switch the d wire.

Did you mean something like this?

https://www.ato.com/dc-voltage-monitoring-relay

with this

https://www.ato.com/100a-dc-contactor-single-pole-12v-24v-48v

I am not an electrician, so forgive my lack of knowledge.

 

[twistedtree]

Is it possible to combine the two battery banks into one? Then all these problems go away.

 

[LeoKa]

Is it possible to combine the two battery banks into one? Then all these problems go away.

It is possible, but not recommended.
Renogy battery likes Renogy inverter/charger only.
BigBattery's lithium battery is not compatible with Renogy inverter/charger. The two company has no interest to solve this discrepancy. Therefore, I decided to split the setup and BigBattery lithiums provide the 12V load and the Renogy lithium provides AC load only.

 

[Bmarler]

the first item, the voltage monitoring relay will work for sensing when you want to switch banks, but the other item, the contactor is too small, and it doesn't disconnect the first bank when you bring the other online.
ideally, you want the switchover to be failsafe, so you don't parallel the two banks at such a significant voltage difference.
you need something like a single pole double throw relay, if you can find one with a large enough amp rating. the amp rating would need to match the highest draw you might see, plus around 30% for safety.
a reversing contactor would do it, but i think there must be another device already made with this idea in mind. i've just never looked for one.

 

[LeoKa]

the first item, the voltage monitoring relay will work for sensing when you want to switch banks, but the other item, the contactor is too small, and it doesn't disconnect the first bank when you bring the other online.

How about this one? I have not seen the installation manual of it, plus there is no price. I just think this could be a solution.

https://www.intervolt.com/product/voltage-sensing-relays/

Rated for 150A. My dc/dc charger is 60A max and I have a fuse in front of it for 80A.

Installation manual:
https://www.intervolt.com/wp-content/uploads/2021/06/interVOLT-PSR-Manual.pdf

 

[Bmarler]

How about this one? I have not seen the installation manual of it, plus there is no price. I just think this could be a solution.

https://www.intervolt.com/product/voltage-sensing-relays/

Rated for 150A. My dc/dc charger is 60A max and I have a fuse in front of it for 80A.

Installation manual:
https://www.intervolt.com/wp-content/uploads/2021/06/interVOLT-PSR-Manual.pdf

That’s a nice VSR, but I have some concerns. You would need two of them to do the job, or another contactor that fires when this one alarms out.
In my mind, there’s to much room for error, and with a 100 amp rating, it could easily overload if the batteries were to hook up parallel at significantly different voltages.
There is a better choice out there.

 

[Bmarler]

Here’s an option, high current capacity, 12 volt coil. Use in conjunction with the little volt sensing relay you posted earlier.
No chance for the batteries to go parallel. Main bank to the normally closed contact, backup bank to the open contact. (Or vice versa depending on how the relay is set up.) it’s not really marine rated, you could put it in a box though.
https://www.electriccarpartscompany.com/spdt-single-pole-double-throw-400a-12000w-12kw-contactor

 

[LeoKa]

Here’s an option, high current capacity, 12 volt coil. Use in conjunction with the little volt sensing relay you posted earlier.
https://www.electriccarpartscompany.com/spdt-single-pole-double-throw-400a-12000w-12kw-contactor

Thanks. Before I'll ask too many questions how to set this up, I will read what I can on it.

 

[LeoKa]

That’s a nice VSR, but I have some concerns. You would need two of them to do the job, or another contactor that fires when this one alarms out.
There is a better choice out there.

I have found something interesting in my dc/dc manual. At the settings for lithium batteries, there are two options. One is to set the voltage to 14.2-14.6V range. The other is to set the charging voltage to 12.6-13.0V range.
My Renogy lithium is normally being charged to 14.6V, so right now, the dc/dc will try to push it up to that and keeps coming on using the BigBattery bank as source. The Renogy battery will shut down at 11.5V to prevent damage, if there is no charge coming in.

What if, I set the dc/dc charger to 12.6V and let it charge Renogy to this voltage? In case the Renogy battery is not being charged due to lack of shore-power, the dc/dc charger will come in and charge it to 12.6V until it can.
Of course this will not last forever, if there is no sun and the panels cannot keep up with charging two banks. Once the BigBattery bank is depleted, none of them will be charged and the loads will not be supplied.
However, if there is sun and the BigBattery bank is being charged daily, this total shut down scenario will be postponed, or completely avoided.

I understand that this is an extreme situation and hopefully never happens. Also, the AC load can be minimized by turning off the fridges and only a dehumidifier will be plugged in. I do have to test it out and see how long things can last this way. If I can manage the boat being without shore-power for a week, I think I have a workable solution.

 

[SteveK]

I think you have found a possible solution. This is what I am working on to avoid. Right now I have manual on/off for the DC2DC. I forgot to turn it off and it was sucking my start bat dry, so I know it will work for you.

 

[LeoKa]

I think you have found a possible solution. .

It worked!

I ran a test today. I let the Renogy battery only to supply AC. Once it reached 12.6V the dc/dc charger started to kick in, too. It slowed down the depletion of the Renogy, but did not stop it. There was no solar support, since it is dark outside. When we have long sunny days in the summer again, I will do another test to see, if the solars can provide enough to keep the Renogy supply AC loads day by day. We'll see.
Either way, this is my solution to the issue. This is all I needed.

 

[Bmarler]

It worked!

I ran a test today. I let the Renogy battery only to supply AC. Once it reached 12.6V the dc/dc charger started to kick in, too. It slowed down the depletion of the Renogy, but did not stop it. There was no solar support, since it is dark outside. When we have long sunny days in the summer again, I will do another test to see, if the solars can provide enough to keep the Renogy supply AC loads day by day. We'll see.
Either way, this is my solution to the issue. This is all I needed.

That’s terrific, and cheap too!

 

[Patrick4002]

we stay onboard and our service bank is loaded by solar during 8 months, we have then no shore power and want to start the gen when the battery bank is to low. i m made a study for the best solution (i am a industrial automatisation engineer) and the battery voltage is a good measurement if you get your battery complet disconected, a single load can change the voltage. i found the vectron battery monitor had a good solution, it determent the rest capacity of the bank and had a relais with gives contact on a pre defined low capcity and switch back off on a preset capacity. the problem is calibrating the monitor, it measure the current used by the battery with a shunt with is pretty accuret but not complete correct. to solve this you can set in the setting the voltage level for 100% battery loaded, this will calibrate the battery monitor every time you get a full loading cycle. got this installed last year and it works great. I had used 3 other battery monitor and the went out of sync all 3 after a month app. of operation. sorry for the bad english, Patrick