Do I need a dc2dc unit?

[crystalclear]

Sorry, too lazy to search deeper than i have to find an answer....

my setup:
2 agm batts serviced by a victron mppt, monitored by a victron 712 - powered by 200w of solar

do i need a dc2dc charger as well?

i live in the sunny part of california.
i have a switched dual batt charger than i can turn on if there is zero sun and i need to charge the 2 house batteries by running the truck.

my understanding is that the dc2dc does a great job at properly charging my agms and keeping then optimal - which my victrons do.

 

[pugslyyy]

You need a DC to DC charger if you have lead-acid start batteries (you do), and LiFePO4 house batteries. If you don't have LiFePO4 now but might in the future, it might make sense to future-proof your build but isn't necessary today.

 

[crystalclear]

additional info: my starting batt is also agm, all 3 are odyssey pc1500s

and this is all in a 91 defender - so no ecu alternator foolery here

 

[john61ct]

You only need a DC-DC if a significant charge source delivers a voltage outside what your batt needs, or (usually with LFP only) the current drawn could damage your source.

Usually used for charging while driving off the alternator.

 

[john61ct]

Monitor your charge profile with an accurate DMM at the battery and an ammeter.

If all is within mfg spec then no DCDC needed, an ACR/VSR will do just fine.

 

[Rbertalotto]

There is nothing better than a DC to DC charger (Renogy) if you need to charge your batteries while traveling and the weather is not cooperating for your solar panels. I use my 40A Renogy DC to DC charger to charge my batteries while camping if I'm not getting enough solar. Simply start vehicle and let idle for 20 minutes or so and I'm back in business. No need to carry a generator any longer.
DC to DC Chargers have been around for years but until not they were crazy expensive. The new Renogy 20A and 40A versions are well under $200.

 

[Joe917]

Using your main engine at idle to charge your batteries is super inefficient and not good for the engine. Using a DC to DC charger in this situation makes it even worse. The typical DC to DC charger only puts out 20 to 40 amps . An alternator puts out 60 to 200. In a short idle charge you will only be in the bulk phase so you will charge slower with a DC to DC charger than with the alternator. On a longer drive where the batteries will get beyond the bulk phase the DC to DC is better.

 

[Rbertalotto]

I idle my engine in real world to charge a 100Ah AGM battery using a 40A DC to DC CXharger. Not hypothetical. It works great.

I put 40A into my AGM at bulk and can take it from 70% charge to 95% charge in about a half hour.

The truck has a 220A alternator. I've tested temperature of the alternator and there is no great difference from simply idling with out the DC to DC connected. Also tested it in my Toyota Venza with 150a alternator....Works great!

As far as efficiency, I never measured it, but I might wager that idling my truck isn't much different than the inefficiency of a small generator. All I know is the fuel gauge doesn't move on a 27g tank.

Many modern vehicles have "smart"alternators that no longer allow a simple current dump into the aux battery.

Sometimes Real World trumps theoretical......

 

[john61ct]

Of course it can be done.

More than say 100hrs per year standing idle would be pretty expensive

Not just fuel but wear and tear.

Plus if you care about climate change, IMO stupid and immoral.

A small inverter genset is a better solution, if you just don't drive enough to justify a bigger bank.

But expanding solar is always the best way.

 

[john61ct]

And has nothing to do with DC-DC already outlined the only criteria dictating when that is required.

Amps output can always be increased by buying bigger or parallel stacking.

But actually measure first, actual output from alt at normal rpm is usually much smaller than people think.

 

[vtsoundman]

[QUOTE="john61ct, post: 2767491, member: 159858"
Plus if you care about climate change, IMO stupid and immoral.
.[/QUOTE]

****** - climate change, immoral and stupid? Injecting statements like these on hobbyist forums kill forums.


To the OP: simple answer - do your batteries charge to 100% with your usage and solar controller? Then no converter required. Do you drive around a bit for a few hours every other day or so and need more juice? Then get a DC converter.

A DC/DC will charge your AGMs (or any battery for that matter) faster and better than the alternator alone. Most alternators will not properly care for deep cycle AGMs.

I have both solar and DC converters. Depending on my travel and usage, I'll use both. In no case will an alternator alone be sufficient.

sent via cone of silence

 

[john61ct]

It is not hard with the right vehicle to set up an alternator that is precisely tuned with an external VR to take perfect care of any bank chemistry and size without any DCDC, including limiting current when needed, dropping to Float when Full, etc.

Not cheap of course, nor am I saying it **should** be done in mainstream use cases.

Just correcting an over generalization.