MPPT 75/15 Load Outputs with DC to DC Charger

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solgato asked Mar 26, '19

MPPT 75/15 Load Outputs with DC to DC Charger

Did a search and didn’t find anything on this so thought I’d ask -is anyone using the Load Output to power a DC to DC battery charger, and if so what charger are you using?

I have found a few trickle type chargers that might work provided my panels are producing enough power and my onboard bank is charged.

Basically if possible, I’d like to have the ability to have an “auxillery out” if you will for charging batteries independent of my onboard bank.

As an example, I use my solar electric catamaran as a dinghy to get out to my moored Trimaran. My Tri has an onboard battery but only a small 1A solar trickle charger to maintain it. Sometimes it takes longer than I’d like for it to recover after heavy usage, but I don’t want to install more solar or a full system on the Tri since all it house battery does is power lighting, a stereo and USB ports. It would be nice if I could use my Catamaran, which has lots of solar and a full solar charging system, as a suplemental charger when it’s tied up alongside it. Plus my solar panels on the catamaran are mounted on a hard bimini top that can be angled to track the sun whereas the panel on the Tri is in a fixed position on the fore deck.

I did find a few posts asking about using the 75/15 load outputs as a charger which is not possible, but I’m thinking perhaps those 15A could be fed to a DC to DC Charger. One issue that comes to mind is that my system is 24V and I’d like to be able to charge 12V, I’d have to use a 24V to 12V converter.

MPPT Controllers

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Trina commented · Aug 05, 2020 at 12:49 AM

Can't you just use the Victron Orion DC-DC 24/12v 10a (or in the amps you want) wired to the MPPT direct-out? Set up a small input cord on the tri and plug it in when you pull alongside?

Everything else starts getting bulky and expensive (maybe like a 2nd MPPT and a select switch from your array to redirect some or all the hard-mount PV's to charge a different battery which might be the cheapest route to use direct excess solar).

I thought Guest made an auto-socket one that was cheap but it's just a maintainer which is probably all you'll find at a decent price and what I use for the smaller batteries. I use lithium type portables so their input auto-socket charging cords can handle 10-32v?

Multiple MPPT can charge same battery but the issue sounds like 24v vs. the 12v on the Tri. So either a switch on the PV direct cable and a secpnd MPPT for which MPPT they feed or a Victron Orion DC-DC 24-12v plumbed (if 's under 15a!) via the direct-out (to use solar first) and then sling a cord over to the tri when you pull alongside

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Trina Trina commented · Aug 05, 2020 at 12:53 AM

No... Would be ideal if they had less amp models - looks like 25a 40a and 70a only.

Otherwise would be perfect?

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Answer 1 · 2019-03-26T08:07:36Z

JohnC answered · Mar 26, 2019 at 08:07 AM

Hi Sol. I think I get where you're aiming. Trouble is, the LOAD terminals are connected to the batts too, and won't just dump 'auxiliary' or 'surplus' generation. It's just like connecting directly to the batts, except the load terminals are subject to low V cutoff protection if the batts get depleted. I doubt you let your 'dinghy' ever get that low anyway. :)

If no inverter in the picture, then yeh, dc > dc converter needed. Victron's Orion can do that, and is adjustable for Vout, but doesn't have the full charge algorithms so you'd have to adjust to a single V that suits. Nor do I know what happens to an Orion if you overload it - I suspect it just limits W, as I have one that I'm sure I've overloaded, but it never tripped.

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solgato commented · Mar 26, 2019 at 08:48 AM

Thanks JohnC.

I hear what you are saying about it being tied to my Catamaran bank. If my system continues to work the way it has when under load fully charged with ample sun, then I’m thinking this would be something I would do under the same conditions when on the water so as to not draw from my catamarans bank. And I like the idea of being able to set a threshold or alarm for the load outputs to protect the Cats bank from depletion.

I’ll have a look at the Orion, thanks.

I was eying the Optimate TM-500 which is good for a 2A charge from a 12-16V DC source. This along with the Trimarans 1A ondeck panel would would give the Tri’s onboard Group 27 house battery a 3A charge which would be perfect for my routine.

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Answer 2 · 2019-03-26T23:59:40Z

daven answered · Mar 26, 2019 at 11:59 PM

Your Tri is 24V and your solar electric catamaran is 12V. Why not wire the main output of the MPPT 75/15 to a switch where you can select it to output to your trimaran or catamaran and then change the settings of the MPPT appropriately depending on which battery bank is selected with the switch but be careful not to charge the catamaran at 24V. Alternatively, wire the 24V batteries to a second MPPT 75/15 and use it to charge your 12V catamaran batteries.

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solgato commented · Mar 27, 2019 at 12:46 AM

Thanks DaveN.

The Cat is 24V with two 75/15 in parallel each fed by 300W/60V of solar with a BMV-712 and shunt. I have a 24V to 12V converter wired to one of the 75/15’s Load Output to power USB charging ports and lighting. The other 75/15’s Load Output isn’t being used, so I’d like to use it for charging external 12V battery banks (and possibly 24V if I could find a 24V to 24V DC Charger) occasionally when conditions permit without it drawing from the Cat’s 24V bank, while also having the safety ability of setting an alarm or shutoff if conditions change and it starts pulling from the Cats bank.

Basically I want to use the second set of Load Outputs as an overflow valve directing the excess solar power not being used when the Cats bank is full, to a trickle charger for charging external batteries without it grossly skewing the behavior of my BMV and its monitoring and SOC predictions.

So, in order to do this I think I’ll need another 24V to 12V converter and a DC to DC Charger like the Optimate unit which needs 12-16V and 2.7A Max input to charge a 12V@2A.

The other reason for using the Load Outputs is to isolate the 2.7A max input that the charger needs. If the charger is the only thing on the Load Output of one of my 75/15’s, then I should think it will only ask for and receive the 2.7A it needs (provided the panels can supply it) without worrying about other loads upping that max. I think...

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Trina solgato commented · Aug 05, 2020 at 01:38 AM

I use a small Instapark 1205 PWM to charge my 8ah AGM's I use for a laptop (vs a mpre expensive setup).

1. Since you want to use excess off the current MPPT that has an unused load-out (@24v)And you want to

2) charge a 2nd battery.

3) your current is essentially a standard rate from the MPPT either solar or 28v give of take from the battery...

4) why not use a basic PWM that matches your tri's battery needs?

-the voltage is either within spec or will be excess within spec and doesn't need much MPPT conversion as you just want to keep the amps where you like -all easily done via the PWM's right?

hook up as many little pwm as you need so you could run or have a backup tri battery too.

your charging profiles and options will be based on the quality of the PWM charger you select but all decent ones ought to be able to be set up as you need.

?

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Answer 3 · 2019-03-28T21:58:48Z

ripper answered · Mar 28, 2019 at 09:58 PM

I had trouble using the orion 24 12 20 on the load out on the 75 15. The in rush current alone (so nothing else attached that needed powering at 12V) was so high the mppt switched off the load out because it detected short circuit or whatever. I changed to a 100 20 mppt, its load out can handle this orion, that solved this problem.

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solgato commented · Mar 28, 2019 at 11:29 PM

Thanks for sharing your experience ripper.

I’m using a 24V to 12V 10A 120W converter, not Victron but a common golf cart type, on the Load Output of one of my 75/15 to power USB charge ports without any issues.

I specifically chose one with with Amp output that was less than the 75/15 is capable of delivering. I wonder if the 20A rating was part of your problem since you aren’t having issues after moving up to a 100/20.

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ripper solgato commented · Mar 31, 2019 at 01:04 AM

Thing is though, the 20A are at 12V on the DcDc output. It is only 10ish on the 24V on the load out of the Mppt so well within spec. I think its more luck than anything else. I had a 1 ohm resistor in line for trial too. Worked also on the 75 15

, just got really hot under load.

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Trina solgato commented · Aug 05, 2020 at 01:59 AM

PWM -isn't a basic, decent PWM already a good DC-DC battery solution that can often be set to custom battery stage charging types?

I use a few various ones but most could handle the input from the MPPT load-out and all are cheap.

I'm excited because this solves an exact issue I was trying to solve on a smaller identical system I couldn't figure out until I tried to help you

Yay!

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Answer 4 · 2020-08-05T01:07:50Z

Trina answered · Aug 05, 2020 at 01:07 AM

(Solution above)

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Answer 5 · 2020-08-05T01:41:07Z

Trina answered · Aug 05, 2020 at 01:41 AM

Get a basic PWM and put it on the 75/15 load out. Set it to charge parameter of the tri battery and run the cord over when you tie up alongside.

How about that?

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Trina commented · Aug 05, 2020 at 02:53 AM

I have an Instapark 1205 waterproof PWM I use for my small AGM portables 24v max input limit though)and it works great so far (I use the auto socket on my direct load-out fpr 12v house).

This also looks good and netter for your needs of 24v+ feed; more amps for regular charging and can handle higher volt input:

HUINE 20A 12V 24V Auto IP68 Waterproof PWM Solar Charge Controller

= good specs for your setup (the minor parasitic draw won't matter as it's run off the direct load-out of the 75/15).

If those links didn't work just Google or Amazon.

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Answer 6 · 2020-08-05T18:39:45Z

Mark answered · Aug 05, 2020 at 06:39 PM

https://www.t6forum.com/threads/trickle-charging-starter-battery-psu-modification.19212/

this works a treat in my RV

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