Question about dual battery MPPT controller and VSR

[tymonk]

Hello,

I recently installed a PV Logic dual battery MPPT controller, prioritising the domestic battery which it charges to 70% and then is meant to charge both batteries simultaneously. The manual states: "Please note this battery [Battery 2] must be on a separate electrical circuit and not connected back to 'Battery 1' in any way." It doesn't provide any warnings about what may follow if this happens.

Having left the boat on its swinging mooring last weekend, I now at home realise a small oversight. There is a dual sensing VSR (connect at 13.7, disconnect at 12.8). My fear is now that the if the solar generates a sufficiently high voltage the VSR will engage and I do not know what will happen then. Could it be that the controller simply won't charge properly, that it will damage the controller/batteries, or something worse...?

Fortunately the 2 panels are not big at all (a 5W panel and an unrated panel that came with the boat, that cannot be much more than 10-20W). My hope is that especially with it not being very sunny the controller will struggle to deliver over 13.7 anyway.

I'm due to go abroad for work tomorrow and wondering what the worst is that can happen and whether I need to see if I need to go back to disconnect Battery 2 (starter) from the controller. Ideally looking for some reassurance that it will probably be fine to leave for a couple of weeks

Thanks in advance.

 

[andsarkit]

At this time of year the small panels will deliver at most an average 2Ah a day. You haven't said what size and type your batteries are but it is unlikely to get the domestic battery up to 13.7V. Is it the 10A controller - spec below?
My 30W panel feeding 2 x 100Ah batteries without a controller sits at 13.1V in the winter ( the panel is not ideally oriented.)

Do you know how the controller measures 70% SOC? If it is less than 13.7 V then it will start to charge the second battery before the VSR cuts in and so it is even more unlikely to get to 13.7V on both batteries. This chart might help if you don't have a figure for 70% SOC.

If it gets to 13.7V when charging both batteries then it probably won't matter if the VSR cuts in as both batteries will be at the same voltage.
The best solution is to disconnect the VSR and allow the controller to look after the charging.

 

[jwfrary]

I doubt anything will harm.

You could switch the negative of the VSR with a relay via a feed from your ignition switch that way it will work when your engine is running but won't connect when the solar controller raises the voltage by itself.

 

[Alex_Blackwood]

Hello,

I recently installed a PV Logic dual battery MPPT controller, prioritising the domestic battery which it charges to 70% and then is meant to charge both batteries simultaneously. The manual states: "Please note this battery [Battery 2] must be on a separate electrical circuit and not connected back to 'Battery 1' in any way." It doesn't provide any warnings about what may follow if this happens.

Having left the boat on its swinging mooring last weekend, I now at home realise a small oversight. There is a dual sensing VSR (connect at 13.7, disconnect at 12.8). My fear is now that the if the solar generates a sufficiently high voltage the VSR will engage and I do not know what will happen then. Could it be that the controller simply won't charge properly, that it will damage the controller/batteries, or something worse...?

Fortunately the 2 panels are not big at all (a 5W panel and an unrated panel that came with the boat, that cannot be much more than 10-20W). My hope is that especially with it not being very sunny the controller will struggle to deliver over 13.7 anyway.

I'm due to go abroad for work tomorrow and wondering what the worst is that can happen and whether I need to see if I need to go back to disconnect Battery 2 (starter) from the controller. Ideally looking for some reassurance that it will probably be fine to leave for a couple of weeks

Thanks in advance.

Excuse my ignorance on these matters, but, why if you have a dual battery controller would you want to include the VSR in the system?

 

[VicS]

Excuse my ignorance on these matters, but, why if you have a dual battery controller would you want to include the VSR in the system?

For charging both batteries from the alternator or a single output battery charger perhaps ???

 

[Alex_Blackwood]

For charging both batteries from the alternator or a single output battery charger perhaps ???

Yes understand that, but in conjunction with the dual controller? should that not be connected directly to the batteries?

 

[PaulRainbow]

Yes understand that, but in conjunction with the dual controller? should that not be connected directly to the batteries?

Absolutely, fitting both is pointless.

I would fit the controller to the domestic battery only. Either that, or, connect the bigger panel to the domestic bank, using the controller and connect the 5w panel to the engine battery, without a controller.

In the meantime, i can't see how the current setup will cause any problems, it just means the solar controller might not be able to do what it's designed to do.

 

[VicS]

Yes understand that, but in conjunction with the dual controller? should that not be connected directly to the batteries?

Yes normally and that is what he has done but the instructions for the controller make it quite clear that there must be no connection between the two batteries :

"Please note this battery must be on a separate electrical circuit and not connected back to ‘Battery 1’ in anyway."​

​

"Please note: If charging two batteries that are interconnected on the same circuit (eg in parallel) only connect one battery to ‘Battery 1’ and do not connect the second battery to ‘Battery 2’. Battery 2 connection is only for charging a completely separate circuit with a battery on. For example the engine battery of a motorhome. "​

Presumably the OP already had the VSR then he recently installed the dual battery solar controller ......... without reading the instructions fully! You know what they say RTFM.. Read the FULL manual

But it does also say " The comprehensive self-diagnostics and electronic protection functions can prevent damage from installation mistakes or system faults. " Hopefully it means what it says and all will be well.

 

[Alex_Blackwood]

Yes normally and that is what he has done but the instructions for the controller make it quite clear that there must be no connection between the two batteries :

"Please note this battery must be on a separate electrical circuit and not connected back to ‘Battery 1’ in anyway."​

​

"Please note: If charging two batteries that are interconnected on the same circuit (eg in parallel) only connect one battery to ‘Battery 1’ and do not connect the second battery to ‘Battery 2’. Battery 2 connection is only for charging a completely separate circuit with a battery on. For example the engine battery of a motorhome. "​

Presumably the OP already had the VSR then he recently installed the dual battery solar controller ......... without reading the instructions fully! You know what they say RTFM.. Read the FULL manual

But it does also say " The comprehensive self-diagnostics and electronic protection functions can prevent damage from installation mistakes or system faults. " Hopefully it means what it says and all will be well.

Thanks VicS and Paul. I don't want this to escalate but I have a limited knowledge of VSR's. and would have thought that the VSR would need an input to operate and if it has no input present and the MPPT controller is connected, directly to both batteries they will be separate and the VSR will play no part. Perhaps like us the OP is unsure of exactly how he has got things connected?

 

[PaulRainbow]

Thanks VicS and Paul. I don't want this to escalate but I have a limited knowledge of VSR's. and would have thought that the VSR would need an input to operate and if it has no input present and the MPPT controller is connected, directly to both batteries they will be separate and the VSR will play no part. Perhaps like us the OP is unsure of exactly how he has got things connected?

Normal way to connect the controller would be directly to the batteries, same for the VSR.

If that's how the OP has them connected, when the controller output reaches the set point for the VSR, the VSR will close and parallel the two batteries. This will obviously conflict with the installation instructions for the solar controller, as Vic posted in #8

 

[Alex_Blackwood]

Normal way to connect the controller would be directly to the batteries, same for the VSR.

If that's how the OP has them connected, when the controller output reaches the set point for the VSR, the VSR will close and parallel the two batteries. This will obviously conflict with the installation instructions for the solar controller, as Vic posted in #8

Thanks Paul, I have it sussed now. The VSR operates on the battery voltage of the "Control battery " Not the alternator input voltage. That means that with a VSR any external charging device raising the voltage of that battery to the set point will connect the other battery(s) in parallel. It also means that the input from the alternator to the VSR will also be at "Control battery" potential, even with the engine stopped. Or so it would seem from the online info I have found today.
As I said previously, I have limited knowledge of these, I am aware of the principle but have never actually worked with, or even, seen one.
Never hurts to ask, "Every day is a school day"! Thanks again.

 

[tymonk]

Thanks all for your input. Before any of these responses came in I went to the boat and disconnected the controller from the engine battery.

Responding in order:
@andarkit: Not sure how the controller measures 70% SOC. Batteries are AGM and 75Ah (engine) and 85Ah (domestic)
@jwfrary: Thanks.
@Alex_Blackwood: As mentioned, it was an oversight. The VSR came with the boat and I want to get rid of it in due course and replace with a DC-to-DC charger. The fact that it was dual sensing slipped my mind when installing the controller (as for the time being I disable the controller when the engine is running). And response to your last post: Indeed, that is my understanding, except with a dual sensing VSR it's sensing from both batteries (e.g. if either goes above 13.7 or if either drop beneath 12.8)
@PaulRainbow: Thanks. See above.
@VicS: Indeed, the VSR was already there. I did read the full manual but didn't think through the full implications of the VSR...

Short term plan is to keep the VSR and the controller connected to the domestic battery, with long term plan to replace the VSR with a DC-to-DC charger that can make sure the alternator charges the domestic to the correct charging profile.

 

[andsarkit]

As Paul said, you can just connect the MPPT controller to the house battery and the alternator to the engine battery. As you have a dual sensing VSR then as either battery becomes charged then the VSR will divert excess charge to the other battery.
This is one of my winter jobs using a basic Victron MPPT 75/15 solar controller and a Victron Cyrix-ct 120A combiner. I just need to find somewhere to fit the extra solar panels.