Best way to combine Mains and Solar charging ?

[affinite]

I have a 520Ahr House battery bank (4 x Trojan SCS225 12v/130Ahr)
I also have a 200Ahr Thruster bank and a 100Ahr Starter battery
My mains charger (Christec CPS2/25) is only rated at 25A but has three independent outputs (one for each bank) and offers a 3 stage charging profile
I have approx 200W of Solar Panels connected to an MPPT Solar charge controller (cant remember the brand) which is attached to the house bank only.
I see about 9-10 Amps from this when the sun's out

I know that both chargers are somewhat under-rated (as is my 60A Alternator) compared to my bank size but I'm trying to get the best out of them without major upgrades.

When at anchor I leave the MPPT charger to give me what it can but if tied up at a quayside with shore power I want to run the mains charger so as to get the maximum charge.
My concern is that when the MPPT controller is in Bulk charge mode (at say 14.8v), the mains charger will be fooled into thinking that the battery is fully charged and so offer only a float charge.
I'll get 9-10A from the solar charger while the mains charger sits back with its arms folded in float mode. I would then only get a Bulk charge from the mains charger when the Solar charger has given up for the night.

So my question is;
What is the best way to operate these two chargers together?
Should I disconnect the MPPT controller when mains charging?

Interested in opinions and what others with a similar setup do ?
Thanks in advance

 

[Alan S]

If your batteries are at 14.8V they won't be able to take any more current from whatever source without pushing the voltage even higher. It just takes time to fully charge a lead acid battery.

 

[MarkCX]

I just leave all 3 charging sources connected and let them sort themselves out - I’ve had no issues this way.
If I switch on the mains charger when the MPPT is in absorbtion at 14.x volts, the mains charger just joins in and pumps in as many amps as it can too. Cloud goes over, mains pumps in more. When they’re both struggling to squeeze any more in, they both go to float at roughly the same time.
Don’t know if this is the ‘correct’ or most efficient way, but never had a problem.

 

[RAI]

When your solar is at 14.8 V, is that measured on the bank's terminals?

 

[affinite]

When your solar is at 14.8 V, is that measured on the bank's terminals?

Not measured. I assume that's the sort of voltage the MPPT charger would try to put out in Bulk phase.
I'm sailing a desk at present so this is all speculation/theory until I can get a flight to Greece

 

[RAI]

My guess is that your solar would be hard pressed to hold 14.8 V if the batteries are really taking much charge, 10 Amps isn't much for 520 Ah.
I follow MarkCX's routine. It seems to work with wet cells. Just keep the water levels up.
My last bank lasted 11 years before they really died. That collapse made me think the Victron mains charger had failed. 25 Amp doesn't warm a big dead bank much. New bank, charger happy. But I bought a bigger one anyway.

 

[Poey50]

It's the sort of thing that feels like it ought to be a problem but generally isn't.

What happens when I have multiple charge sources charging a single battery bank

 

[noelex]

The main problem is the battery charger will keep the batteries fully charged (providing your load does not exceed 25A), but most solar controllers will start a fresh cycle each solar day.

This means a fully charged battery will be kept at the absorption voltage (typically 14.8) for the absorption time (typically around 1-2 hours) by the solar controller. This overcharging is not optimal for battery health. For a short stay in a marina it will do little harm, but ideally especially for a longer stay this extra absorption stage should be turned off.

The easiest way is turn off the solar charging completely. The only danger is if shore power is disconnected without you realising this has been done the batteries could reasonably rapidly reach a low SOC. An alternative is to alter the battery set points so the absorption time is very short (say 1 min). The absorption voltage on the solar controller should also be reduced, the amount is not critical but about 0.4v is a good compromise. If this is done solar will still provide some back up charging if the battery SOC drops. The short voltage spike each day will do little harm and may even help some battery types by stirring up the electrolyte.

 

[Poey50]

Yes, you'd be unwise to have fixed absorption times on your solar controller. On the Victrons, for example, absorption is only for a fixed time if you override the default setting which would by default end absorption on the basis of the voltage of the battery or reducing tail current. In this way the battery state of charge is the controller.

 

[noelex]

Yes, you'd be unwise to have fixed absorption times on your solar controller. On the Victrons, for example, absorption is only for a fixed time if you override the default setting which would by default end absorption on the basis of the voltage of the battery or reducing tail current. In this way the battery state of charge is the controller.

The variable absorption time of the Victron solar controllers is a help but the absorption time is still too long for a fully charged battery. The default will produce a 1hr absortion time.

 

[Poey50]

The variable absorption time of the Victron solar controllers is a help but the absorption time is still too long for a fully charged battery. The default will produce a 1hr absortion time.

Tail current trumps absorption time - assuming Victron are telling the truth.

1 hour is the maximum absorption time for batteries reading >12.6 volts at start of day. Victron reports as follows "when the battery current gets below the tail current for one minute, the absorption phase will end. This setting can be disabled by setting it to zero." As said, you'd be unwise to switch to fixed absorption time under the OP's circumstances.

 

[noelex]

Tail current trumps absorption time - assuming Victron are telling the truth.

The tail current feature of the Victron solar controllers will only work if there is no load, or at least a very small load unless you have some option extras and have integrated these into the system (such as some of the Victron battery monitors).

If you are using the boat and don't have a Victron battery monitor system that is comunicating with the solar controllers the tail current feature will not activate to terminate the absorption phase and you will have some overcharging at the default settings in the OP’s situation.

"when the battery current gets below the tail current for one minute, the absorption phase will end. This setting can be disabled by setting it to zero." As said, you'd be unwise to switch to fixed absorption time under the OP's circumstances.

The controller also needs to maintaining the absorption voltage, before the tail current feature activates.

 

[affinite]

Thanks for the great advice
Its a long sorry story that I wont bore you with, but last winter I managed to trash my whole battery bank because of the failure of a single solar panel connection
When I get back to the boat in a few weeks I have to replace 6 failed batteries
To make matters worse its the second time its happened so I'm determined to turn over a new leaf and setup a more resilient charging system and take better care of the batteries
So, in addition to examining the charging setup, Ive also setup a remote management system so I can now remotely monitor the batteries and turn chargers on/off from home

 

[Poey50]

That's enough for me. I tend to believe the manual but others may be happy to follow you down this conversational rabbit-hole.

 

[TernVI]

I'd start by considering whether there is really a problem.
That will come down to how often you use how much of your capacity, how often you get shore power and whether it's imperative to charge in the shortest possible time.
I suspect you are spending a lot of time with a high % SOC , which will be making charging less efficient.
If you generally run the motor reasonably often (even if only for a short period) it might be worth upgrading the alternator or adding some sort of smart control or AtoB unit to 'assertively' bulk charge the batteries, if engine coincides with when the batteries are lowest.
You could improve charging efficiency by splitting the bank and discharging 1 battery to 80% instead of 4 to 95%. You have to analyse life predictions to see the cost implications in battery life.

I think I'd just fool the mains charger into bulk mode by switching off the solar when switching on the mains, if that works with the Cristec.

Just seen post 13. I would be asking if the big bank is needed. It's a lot of cash for something you can't be getting full advantage from because you don't really have the means to charge it?

 

[noelex]

That's enough for me. I tend to believe the manual but others may be happy to follow you down this conversational rabbit-hole.

There is nothing incorrect in the manual.

If you do not have the optional extras (such as Victron battery monitors) integrated into the system the tail current feature works by terminating the absorption phase if the current output of the solar controller is below the tail current value and the voltage is maintained at the absorption voltage.

The tail current value is different on the different sized controllers (it goes up with larger controllers) but is typically around 1A. If there is greater than a 1A load the controller will always be trying to deliver more than 1A so the tail current feature will not be activated.

If you are leaving the boat for long periods with no loads the tail current feature can be useful but not in other circumstances. The tail current feature can sometimes cause problems when other charge sources such as another solar controller or the engine alternator are being used. On most boat systems it is generally better to turn this feature off unless you are leaving the boat with no loads and shore power connected.

 

[affinite]

I'd start by considering whether there is really a problem.
That will come down to how often you use how much of your capacity, how often you get shore power and whether it's imperative to charge in the shortest possible time.
I suspect you are spending a lot of time with a high % SOC , which will be making charging less efficient.
If you generally run the motor reasonably often (even if only for a short period) it might be worth upgrading the alternator or adding some sort of smart control or AtoB unit to 'assertively' bulk charge the batteries, if engine coincides with when the batteries are lowest.

I do have a Sterling A-B thingy on the alternator - still rarely see over 25 Amps though

You could improve charging efficiency by splitting the bank and discharging 1 battery to 80% instead of 4 to 95%. You have to analyse life predictions to see the cost implications in battery life.

I think I'd just fool the mains charger into bulk mode by switching off the solar when switching on the mains, if that works with the Cristec.

I was wondering about that

Just seen post 13. I would be asking if the big bank is needed. It's a lot of cash for something you can't be getting full advantage from because you don't really have the means to charge it?

I was also wondering about that too

 

[RAI]

A big bank comes in handy for long overnight sailing trips. Depends on how much you use but you don't want to need to run an engine to power you through the night.
Unless you are driving a mobo of course.

 

[PaulRainbow]

Except in Winter, my solar controller is always on, my mains charger is always off. If the battery voltage falls below a set level the built in relay of the Victron battery monitor triggers a 12v/240v relay that turns the mains charger on. The relay can be set to turn close/open at particular voltages or stay closed for set times.

If the mains charger is on the solar controller "thinks" the batteries are charged and goes into float mode.

If the mains charger was to be left on all night (mine isn't usually, except in Winter) my solar yield is negligible, as there is nothing for it to do. If loads are turned on the mains charger will match them, still nothing for the solar to do.

In the Winter the mains charger is turned on and the solar yield is negligible. When the solar controller starts in the morning it may well stay at absorption for a short while, the effects of this are greatly exaggerated, IMO. So what is the volts are up, there is no current as the batteries can't take it. My domestic batteries are 4 years old, basic SLA leisure batteries, together with just 200w of solar panels they happily run the whole boat all Summer, unless there are 2 or 3 cloudy days, when the mains charge might kick in. So far, since March, the mains charger has come on twice, for an hour.

My engine battery is about 3 years old. This gets the same Winter absorption voltages as the domestic bank, but it also gets whatever absorption times that the domestic bank gets during the Summer, as there is a VSR in the system.

 

[jkraffty]

Would a Battery to Battery charger help the original poster? I understand they take the full current from the engine alternator (if needed), even when engine speed means that the Alt is not running at maximum output . I assume the BB charger overrides the Alt regulator to achieve this?