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Old 10-10-2021, 09:36 AM   #1
Mini-Skoolie
 
Join Date: Jul 2021
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Possible to double up DC-DC B2B chargers?

I am helping build out a skoolie for full time living (my adult child) and I am choosing the best DC-DC B2B charger for for this purpose. We have an IC bus that has a 220A alternator and will be running a 280ah 24V LiFePO4 battery bank. We are also planning solar panels that total 750 watts. There are still a set of unknowns such as how much my adult child will be driving (ie seeing the country) vs. stopped (ie rock climbing for a week).

Because of the 24V, the two choices I am considering are a Sterling BB122470 or a Victron Orion-tr 12|24 - 15. The largest differences in my view are cost ($420 vs $220) and output amps (35 vs 15 @24v).

So my focused question is: Is it a feasible/reasonable to run 2 Victron units in parallel?

(My thought is to buy a single Victron unit now and then maybe after a few months of test-living, add a 2nd one in parallel to boost amps from 15 to 30 if that is needed.)

Other comparison comments or other options welcome.

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Old 10-10-2021, 09:44 AM   #2
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Location: So Cal
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Year: 1935
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Engine: 317 ci/tid / Isuzu
I would recommend that you pose that question to the mfg rather than your fellow skoolies. Even if you can double dip what will doing so do to your product warranty etc.

I chose the Sterling product and have had satisfactory service from it for several years FWIW.
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Old 10-10-2021, 09:51 AM   #3
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Quote:
Originally Posted by ol trunt View Post
I would recommend that you pose that question to the mfg rather than your fellow skoolies. Even if you can double dip what will doing so do to your product warranty etc.
I completely agree here. The manufacturer would be your best "first line" source of a good answer.

But, I'll try a second-grade answer. No, they won't work right. As each sends current into the battery bank, the resulting voltage rise will cause charge issues. If one's charging, the other will read something other than the real battery bank voltage (because one is raising that voltage due to charge current). You'll end up with dual, competing systems each coming on then turning off. I've encountered this with off-grid wind and solar hybrid systems...the same concept, though...two different charging systems feeding one battery bank.

On a related note: Are you locked into a 24V battery bank? There's really very little advantage to using something other than a 12V system in a 12V bus/motorhome.

EDIT to add some clarification: A higher voltage battery bank does mean lower current between the battery bank and the devices which turn that battery bank power into usable power. If you happen to have a bunch of 24V DC appliances, then you're all set and a 24V battery bank is just dandy. But normally, we find either 12V DC appliances or 120V AC appliances. A 24V battery bank would allow smaller diameter cables between the batteries and the inverter, but that run is (or should be) short and once the inverter's spitting out 120VAC the battery bank voltage is irrelevant. And if you have a 24V battery bank you need a converter to change 24V DC to 12V DC. Normally, I don't find any compelling reason to go with anything other than a 12V system in a vehicle.
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Old 10-10-2021, 01:05 PM   #4
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Yes you can parallel, and there is no IRL problem with any "confusion" along the way.

Charles Sterling is available by email to ask for confirmation.

Victron has its own community forums, and their techs do monitor threads
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Old 10-10-2021, 01:12 PM   #5
Bus Crazy
 
Join Date: Oct 2020
Location: Near Flagstaff AZ
Posts: 1,951
Year: 1974
Coachwork: Crown
Chassis: "Atomic"
Engine: DD 8V71
Quote:
Originally Posted by john61ct View Post
Yes you can parallel, and there is no IRL problem with any "confusion" along the way.

Charles Sterling is available by email to ask for confirmation.

Victron has its own community forums, and their techs do monitor threads
See...my answer was indeed second rate! Thanks John, for the real-world experience on that. My thoughts came from my experience with solar and wind dueling chargers. For those, we had to tweak the programming limits to make things work acceptably.

So, all, ignore my second rate answer above.
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Old 10-10-2021, 01:28 PM   #6
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Sure simpler to just have a single bigger current source, but sometimes that's too hard / expensive.

The real tests are, does the batt end up Full? Was it charged faster by having multiple disparate sources running concurrently?

How the sausage is made during the charge cycle really is not worth fussing about so long as you get there in the end.
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