ACR vs. Dual Output MPPT

[DanielI]

I’m looking for a set-up to charge both battery banks from (initially) one panel. Currently looking at a 90W panel, and I have two 100Ah batteries, but in the future I may add both panels and batteries. I sail on weekends, so I have all week to top off.

It seems like the standard approach if you want both batteries to charge is to combine with an ACR. Since, with an ACR, the charge controller is going to be charging two different batteries with not only very different levels of charge, but also potentially quite different capacities, isn’t this going to be be very inefficient? Why aren’t dual output controllers more widespread?

As an aside, it also appears that you can’t use two separate controllers, one for each bank, without splitting the panels between them, which also seems suboptimal, because if one batter is mostly charged, the excess solar power can’t be shifted to the other panel.

Assuming I’ve got this correct, any recommendation on dual controllers. I like the fact that the Victron is NMEA 2000, but their only dual controller is PWM.

[CharlieJ]

Since, with an ACR, the charge controller is going to be charging two different batteries with not only very different levels of charge, but also potentially quite different capacities, isn’t this going to be very inefficient?

The short answer is no.

The conventional wisdom now is to have all charging sources supply the largest and generally most depleted battery on board - the house bank. An ACR or a battery to battery charger is then used to charge the starting battery or batteries.

Once the ACR senses that one of the batteries is > a threshold (approximately 13.0VDC for a short period of time), it closes and combines the batteries. Typically, the starting battery is only depleted a few Ahr so it responds to the charging evolution quickly and all of the charging current from the source flows to the house battery.

Literally 10's of thousands ACRs are installed and operating satisfactorily. They are truly set and forget.

[Stu Jackson]

Charlie's right.


But you don't need a VSR/combiner/etc. at all. You're a weekender. You can statt your engine at least 24 times on one of your batteries without going down past 50% SOC.


Or use the 1-2-B switch on B with all your charging sources going to your "house" bank. Which, based on the way you phrased it, appears to be only one of your two batteries.


For daysailing they should be adequate. For overnighting, not so much, specially if you have a fridge.



If your charging sources still go through your 1-2-B switch you may be interested in these links: (they come from Electrical Systems 101 http://c34.org/bbs/index.php/topic,5977.0.html)


OEM 1-2-B Switch Wiring History http://c34.org/bbs/index.php/topic,4....html#msg30101

1-2-B Considerations (New 2020 - Rodd finally got around to diagramming what I had done in the above link in 2009 )
https://marinehowto.com/1-2-both-bat...onsiderations/

Basic Battery Wiring Diagrams This is a very good basic primer for boat system wiring: http://c34.org/bbs/index.php/topic,6604.0.html

This is another very good basic primer for boat system wiring: The 1-2-B Switch by Maine Sail (brings together a lot of what this subject is all about)
http://forums.catalina.sailboatowner...d.php?t=137615

This is a newer primer for boat system wiring design with a thorough digram: Building a Good Foundation (October 2016)
http://forums.sailboatowners.com/ind.../#post-1332240

[DanielI]

Originally Posted by CharlieJ

Literally 10's of thousands ACRs are installed and operating satisfactorily.

Thanks. I figured the popularity of this probably meant I'm missing something, but the popular approach being wrong would not be unheard of!

So, let me see if I follow. The controller may not be optimized for the second (backup/starting) battery, but since that battery is at best only normally minimally discharged, if the ACR brings it online only once the house bank is also almost full, the mismatch is never a problem?

Originally Posted by Stu Jackson

But you don't need a VSR/combiner/etc. at all. You're a weekender. You can statt your engine at least 24 times on one of your batteries without going down past 50% SOC.

Or use the 1-2-B switch on B with all your charging sources going to your "house" bank. Which, based on the way you phrased it, appears to be only one of your two batteries.

For daysailing they should be adequate. For overnighting, not so much, specially if you have a fridge.

Thanks for the links. I scanned through, and noted several for followup reading.

As far as my current setup, yes, it is two banks with one battery each. And I do not have (or plan to have) a fridge. But I do intend to be onboard for extended weekends (4-5 days). I don't have very high power requirements, but I don't want to mount a slew of panels, either. My back of the envelope estimate is that I won't be able to keep up with my onboard usage. So my idea was to add a third battery, as a new backup bank, and combine my two current batteries in a house bank. That would give me 200Ah (plus whatever I generate from solar) to run off of, and then the solar could take its time recharging in the while I am not onboard.

So it sounds like the easiest first step is to just direct connect my solar controller to the house bank. If I set the switch to B when motoring, that will recharge the backup. When I get around to it, I can add an ACR without a problem.

It sounds like I could also wire my alternator to the backup bank (without an ACR)? Then I wouldn't have to worry about switch position.

Or wire both to the house bank, with an ACR.

For a weekender, I know I'm overthinking this, but I like playing with electricity.

[smac999]

how do both batteries charge from the engine? a dual output mppt does not solve that.

where as an ACR solves both.