DC-DC charger (B2B) options and general discussion

[Dzl]

Hmm, I can get a dual alternator kit, but 24v looks to require some major hacking and $$$ for a good one, and I'm seeing crap output at low speeds (idle). 12v is looking more appealing.

What is the advantage of 12V? This is probably stupidly obvious--but in case it isn't (sometimes stupidly obvious is easy to overlook)--at 24v half the current equals the same amount of power.

Still, rather than DC-DC why not a dedicated 2nd alternator with a regulator built for charging batteries? Seems way better.

I think an external regulator + second alternator is arguably "better" at least in some ways, but its also more involved/complicated and expensive (I think). On the upside, for higher output, or for much more advanced configuration, an external regulator is a good option. I believe they are somewhat popular in the marine world, not so much in the mobile world thus far.

 

[DerpsyDoodler]

...
Still, rather than DC-DC why not a dedicated 2nd alternator with a regulator built for charging batteries? Seems way better.

Seems more complicated. Probably better to upgrade the existing alternator so it can handle the load, and just add a dc dc charger. I’ve done nothing but grumble over the cost of one, but in the end I may just have to eat it. I haven’t given up yet, though.

 

[geoffire]

With 12v I can order a kit for $1000 that gives me the dual mount and a 200A (upgrades for more $) alternator with the charging specific external regulator. 24v there's nothing off the shelf so I'll need to fabricate a bracket with pulley and such and find a good 24v alternator and install the regulator.
The off the shelf kit for my vehicle (I think I can source all this for a bit less):

2005-2016 Ford truck & SUV Dual Alternator Bracket

2005-2016 Ford truck & SUV Dual Alternator Bracket

www.nationsstarteralternator.com

Output wise 24v looks to need higher speeds so less power at idle. Meanwhile about the highest I see in 24v without going to huge industrial units is 150A but I can easily get a 300+ amp 12v alternator. Given the choice of buying a reasonably priced kit or diving into the unknown and fabbing/hacking, I much prefer the kit.

Then there's the fact I'm not gaining much by going 24v anyways in this setup. It's a small vehicle and most the stuff will run on 12v - I can source 24v items as well (was planning to), but 12v is more common so I kind of prefer it besides the core power setup. Biggest reason I was leaning 24v was the more reasonable bms/inverter/solar charger expenses but we're planning minimal solar and <2000w inverter usage and no wall charging (unless I couldn't figure out the alternator situation).

As for why the dedicated alternator vs DC-DC? I think it depends on what kind of power you want. <600w? Just go DC-DC off your existing alternator, but I need a lot more than that. I don't think the large and expensive DC-DC units make sense.

 

[Mad_Texan]

I chose to focus on DC-DC chargers first because (1) they were designed explicitly for use with vehicle charging systems (2) they are the most popular solution currently (3) there are a manageable number of brands/products to cover (4) they are reasonably affordable and reasonably simple to setup.

A valiant effort on your part...

 

[geoffire]

Hmm, I'm realizing this really isn't so complicated. All that's really needed is current limiting (assuming same voltage alternator) to keep from destroying the alternator. Stepping up voltage (12v->24v) adds a significant stage of complication, but isn't rocket science. I'm a pure software engineer now, but I used to dabble in electronics, microcontrollers and the like. I can't say I need more projects however...

 

[Perrybuilds]

I think I found a better solution: Dual alternator with the second alternator setup for 24v with a fancy external regulator: https://shop.pkys.com/Balmar-MC-624-H-Regulator-24-Volts_p_1730.html

That's very old boat tech that was amazing but now it seems ancient compared to advance systems that use canbus controlled alternators to talk to your bms. Many companies that outfit overland rigs carry them for dual bracket applications. The nations kit sold in the van world is complete trash, it uses that regulator as well or a fancier one if you pay more. You need to be a scientist to program a balmar or just smarter then me because it looks tough. This setup has been a huge problem for vans cutting into coolant lines from the extra belt and no good hose routing choices. Most of the manufacturers will not warranty you anymore as well unless you get the option from them ( i.e. ambulance package etc). So for most people looking to keep things simple it's kinda a bad idea. Upgrading your existing alternator AND wiring to the battery is the cheapest and safest bet for most people. advanceRV and safiery are two brands out of a bunch that sell this stuff and have spoke about changes they have had to make to the alternator setups they implement.

Also I think it's good our resident librarian is keeping this list just to confirmed complete multi stage chargers. We would have to have a separate thread that would be giant to accommodate how to use these high amp buck boost converters safely. As most of them require specific communication protocols with BMS's to operate exactly as intended. Alternators aren't designed for such high continuous duty and smart chargers that are listed seem to all be safe on alternators if sized accordingly.

 

[trevor.the.van]

Anyone considered using a simple 13.8 regulated output from car battery to charge directly to LiFePo4?

I found this one that provides 20A output at 13.8v, with input from 8-40v.

WaterProof 8V/40V to 13.8V 20A 276W DC/DC Power Converter | eBay

DC/DC Converter Regulator 24V Step Down To 12V 20A 240W Waterproof. Waterproof DC-DC Buck Step Down Volt Converter 20V-72V 24V 36V 48V to 12V. Waterproof DC/DC Converter Regulator 12V Step Up to 13.8V 18A 248W.

ebay.us

 

[DerpsyDoodler]

Anyone considered using a simple 13.8 regulated output from car battery to charge directly to LiFePo4?

I found this one that provides 20A output at 13.8v, with input from 8-40v.

WaterProof 8V/40V to 13.8V 20A 276W DC/DC Power Converter | eBay

DC/DC Converter Regulator 24V Step Down To 12V 20A 240W Waterproof. Waterproof DC-DC Buck Step Down Volt Converter 20V-72V 24V 36V 48V to 12V. Waterproof DC/DC Converter Regulator 12V Step Up to 13.8V 18A 248W.

ebay.us

I do not know if what you ask is feasible, but 13.8v isn’t enough to charge a 12v lifepo4 battery full.

edit: derrrrrh that is what a stepup converter is for...lol. i’ll see myself out now.

 

[Dzl]

I've definetely considered/pondered it, but my depth of knowledge in electrical is not sufficient to feel confident going beyond pondering. I do not know if it advisable or doable or not. Not with those cheap converters at least. I am pretty sure (almost positive) Victron's converters (some of them at least) can be used in that manner, but at that point the price difference between a Victron DC-DC converter and Victron DC-DC charger (meant for alternator charging) is not that substantial.

If these cheap converters (from the link you posted) would work and don't introduce additional unacceptable risk, it would be a very cheap solution, and I actually think 13.8V (3.45Vpc) sounds like a good voltage for dumb fixed voltage (non 3 stage) charging.

 

[geoffire]

There are similar units with variable output (DerpsyDoodler linked one, I linked a similar but fully contained unit). I'm looking into how to use multiple of these in parallel, but have a bit more to grok...

 

[DerpsyDoodler]

I’ve come to terms with dropping the money for what I want here; however, by the time I am ready to purchase, maybe other viable alternatives will have made it to market.

 

[trevor.the.van]

I've definetely considered/pondered it, but my depth of knowledge in electrical is not sufficient to feel confident going beyond pondering. I do not know if it advisable or doable or not. Not with those cheap converters at least. I am pretty sure (almost positive) Victron's converters (some of them at least) can be used in that manner, but at that point the price difference between a Victron DC-DC converter and Victron DC-DC charger (meant for alternator charging) is not that substantial.

If these cheap converters (from the link you posted) would work and don't introduce additional unacceptable risk, it would be a very cheap solution, and I actually think 13.8V (3.45Vpc) sounds like a good voltage for dumb fixed voltage (non 3 stage) charging.

My plan would be to run a proper MPPT with solar charging up to 14.2 most of the time, but to get the boost when driving if the battery is low. So most of the time the battery will be maintained by a decent MPPT charger.

Also found another seller actually selling this as a charger, a 30A version.

DC-DC Charger Battery to Battery Charger DCDC 12V/24V 13.8V BC DC DC 30A ABR | eBay

When charging large battery banks they can be connected in parallel to boost the output (IE: 60A – 90A). The unit will start charging as soon as power is connected to the source battery, a disconnect is required by either Anderson plugs or a relay / solenoid.

ebay.us

Just trying to look at options on how to setup my next system.

I am currently using a 12-24v booster wired into the PV input of a small victron MPPT charging my LiFePo4 - and using that as a battery-battery connection. It's working quite well, and very cheap!

 

[trevor.the.van]

There are similar units with variable output (DerpsyDoodler linked one, I linked a similar but fully contained unit). I'm looking into how to use multiple of these in parallel, but have a bit more to grok...

DC-DC Charger Battery to Battery Charger DCDC 12V/24V 13.8V BC DC DC 30A ABR | eBay

When charging large battery banks they can be connected in parallel to boost the output (IE: 60A – 90A). The unit will start charging as soon as power is connected to the source battery, a disconnect is required by either Anderson plugs or a relay / solenoid.

ebay.us

These guys might have some info on parallel connection - I didn't go through their material, but they are a vendor I've used here in Australia many times - cheap, and mostly reliable goods

 

[trevor.the.van]

I do not know if what you ask is feasible, but 13.8v isn’t enough to charge a 12v lifepo4 battery full.

edit: derrrrrh that is what a stepup converter is for...lol. i’ll see myself out now.

I wouldn't use it as the primary battery charger - that would be solar or shore power - but rather than dumping $400 on a victron DC-DC, thought this might be a reasonable compromise.

 

[geoffire]

For a 12v system all you really just need current limiting to avoid killing your alternator and voltage limiting to avoid overcharging the battery - that device has both. 13.8V isn't high enough to fully charge, but assuming no further voltage drop to the batteries is enough to get you most of the way there (I'm assuming LiFePo4 here). As a secondary (or 3rd?) method of charging a 12V system that's totally worth considering.

Edit: It explicitly says parallel is fine, so that's a nice, but drawing a ton of power at idle is where you're likely to kill the alternator, much more than 30a to the batteries with the lights on and AC running, and the stereo... Going big means some sort of logic to be more intelligent than a set current limit I think.

 

[trevor.the.van]

For a 12v system all you really just need current limiting to avoid killing your alternator and voltage limiting to avoid overcharging the battery - that device has both. 13.8V isn't high enough to fully charge, but assuming no further voltage drop to the batteries is enough to get you most of the way there (I'm assuming LiFePo4 here). As a secondary (or 3rd?) method of charging a 12V system that's totally worth considering.

Edit: It explicitly says parallel is fine, so that's a nice, but drawing a ton of power at idle is where you're likely to kill the alternator, much more than 30a to the batteries with the lights on and AC running, and the stereo... Going big means some sort of logic to be more intelligent than a set current limit I think.

Agreed - was only looking at having one - and also a dash mounted toggle for it, because I'm a control freak

 

[trevor.the.van]

I ran a quick test on my 100Ah LiFePo4 battery using a bench supply this morning.

Ran 13.8 volts into it at max amps until amps dropped to zero - then disconnected and changed the volts to 14.6 and pushed more amps in. It literally took about 5 minutes before the amps dropped off again - so the extra amps past the 13.8 volt mark are pretty minimal.

 

[geoffire]

Yep:

Just be sure to top balance (and use a BMS) to ensure none of the individual cells are "running away" at that voltage.

 

[trevor.the.van]

Yep:

Just be sure to top balance (and use a BMS) to ensure none of the individual cells are "running away" at that voltage.

Certainly, I'm using a Daly BMS and top balanced the set when I built the pack.

 

[Annie C]

Jumping back in. DCDC/MPPT 50 has a common ground. I have seen plans that only connect positive starter battery to DCDC IN, no NEG. Starter has chassis ground.

Can one just ground the common ground out of the DCDC in with the NEG buss bar without running a sister NEG from the starter battery?