Why do I need a DC to DC charger?

[towpro]

I came to this group because of the tendency for truck campers to be more off grid oriented with knowledge about coach batteries.

Unless your using Lithium, why would you need a DC to DC charger between truck and coach? I know there will be some loss in the factory wiring (which gets smaller every year) but do I really need to limit current or voltage?

On my Ram it was not switched, and I knew to unplug the cord when camping to not discharge the truck batteries. Is the DC to DC charger only to prevent the truck battery discharge?

I was tempted to put a bluesea ACR on this line but they will conduct either direct either directions at a given voltage, IE my Camper solar would start to charge truck batteries which I don't need.

Really this question comes from I am wiring a new Toad. My Jeep Liberty I have the 12V feed wire from RV connected (fused) direct to the battery. Now I am wiring a new toad, a Ford C-max and I am reading about everyone using these special DC to DC chargers. But why do I need it? The truck alternator is only going to put out a max voltage, by the time it goes through the thin wire to the toad, its going to be even lower, so why do I need to regulated it?

[S_Davis]

time2roll wrote:
100 amps seems like a lot even on four batteries and especially a lot on the alternator.

The alternator can handle it, I have a high output alternator. I drive a couple hours a day and want to take advantage of that.

[time2roll]

100 amps seems like a lot even on four batteries and especially a lot on the alternator.

[S_Davis]

I am getting ready to install two of the 50amp Redarc dc to dc chargers on my truck, they are charging four Trojan T105s to start.

[toddb]

Kayteg1 wrote:
On practical note for B2B charger is inverter combined with converter.
So if you already have converter adjusted for lithium, or whatever batteries and on-board truck inverter >>> you already have the system. All it takes is to learn how to use it.
To clarify - I am talking about 400W inverter Ford installs on newer trucks and potential for adding 1 on truck who doesn't have it.
400W inverters run for like 35 bucks.

I don’t think you’ll find a converter that’ll run on 400w.

I use a small Orion 110w dc-dc that covers my fridge draw at about 85w. Runs on the factory wiring so no need for jumper cables.

[kohldad]

YOU ABSOLUTELY DO NEED A DC-DC CHARGER !

Prove it to yourself. Start your truck. Check the voltage at the battery. Immediately after starting it should >13.8V. Let the truck idle for about 10 minutes and check the voltage again. Probably <13.2V ! Have some one hold the engine at a high idle. I'll bet it is still <13.4 !

13.2V - 13.4V at your house battery bank will not let it discharge but it will NOT re-charge the house battery bank !

I don't need one but would like to have one for faster charging. My truck will recharge my battery faster than 10a/hr even when running the fridge on 12V. Now I did upgrade the wiring from the truck battery to within 10" of the camper battery to #2 and go through a 175A Anderson connector which helps greatly.

Most modern alternators have over 75% capacity at idle. The old days of having to rev the engine for a good charge are gone. At idle, my alternator turns 3,000rpm thanks to pulley size which is 90% of it's 220A capacity so has plenty of output at idle.

Now, I would like one so I can recharge a dual battery bank at 40A/hr when I upgrade my next camper to a DC compressor fridge as I hope to sit more and drive less.

[Kayteg1]

On practical note for B2B charger is inverter combined with converter.
So if you already have converter adjusted for lithium, or whatever batteries and on-board truck inverter >>> you already have the system. All it takes is to learn how to use it.
To clarify - I am talking about 400W inverter Ford installs on newer trucks and potential for adding 1 on truck who doesn't have it.
400W inverters run for like 35 bucks.

[NRALIFR]

In most cases, you should plan on installing heavier gauge wires than what you will find on the OEM 7-pin wiring harness. In fact, I wouldn’t recommend using the standard 7-pin RV plug at all for this circuit. The connectors are a poor design for this IMHO.

Also, don’t underestimate the total length of the wire required to connect the DC-DC charger, which should be as close to the battery it’s charging as possible, and the truck battery it will be drawing from (not the alternator!). You’ll be lucky if you can keep the circuit length under 25 feet on a crew cab pickup. Mine is closer to 30 feet.

This is from the Renogy installation guide. The recommendations for my Redarc were similar.



:):)

[time2roll]

towpro wrote:
I did not know that DC to Dc charges could RAISE the voltage?

Yes as long as the thin wire has 11 volts the DC-DC will charge the auxiliary battery properly at about 14.4 volts and once charged float at about 13.4 volts.

Here is the Renogy version to get more details. The 20 amp is generally plenty for a single battery.

https://www.renogy.com/12v-dc-to-dc-on-board-battery-charger/

[Kayteg1]

Beware adamis, that 7.3 powerstroke has very weird charging program, who is design for glow plugs protection.
On cold start alternator will not start charging for several minutes as 14-15 V wold shorten glow plugs life.
Took me a while to figure out why my truck, who was idled for 10-15 minutes every month would have dead batteries after 3 such charging cycles.

[adamis]

I'm surprised this wasn't mentioned already... A DC to DC charger on a camper that has solar is an excellent backup if the sun isn't shining. Think cloud / smoke coverage or camping in thick tree coverage. Your alternator is concerned about charging your starting battery. Hooking your camper battery straight to it leads to two different loads and the camper battery at best is maintained but charging properly can be difficult.

AGM and lead acid batteries on a DC to DC charger probably isn't as necessary but for LiFePo4 it is. They charge at a higher voltage than the standard alternators are going to provide. They are also expensive so a charger that is dedicated and has the proper charging profiles is a worthwhile insurance plan.

What the unit does is it regulates the voltage to the camper battery to be consistent with the charging profile appropriate for your battery type. It does that regardless of what the alternator is doing. By the way, the alternator voltage at least in my 99 7.3 Super Duty is anything but constant. It is constantly fluctuating and changing every few seconds. Not a great way to charge a battery.

The way these things regulate the voltage to be constant is they pull more current when necessary and through fancy electronic engineering that others could explain better raise keep the voltage constant on the output side. V=IR is the equation you are hunting for. If the voltage is dropping because of a load, you increase the current pull to maintain that voltage.

In short, you may not NEED one unless your use or battery type make it necessary.

[cewillis]

There's a lot of ridiculous advice in this thread. Maybe a little good too -- but I'm way too smart to keep reading when I see horse- I mean nonsense.

[NRALIFR]

Freep wrote:
Are they LiFePo4 batteries? That's the key here. You don't need a DC-DC charger for FLA or AGM but you definitely need it for LiFePo4 if you want to be able to charge the batteries from your truck.

I have to respectfully disagree with you here. I have a single group 31 AGM, and a DC-DC charger, and If I were asked whether to install solar or a DC-DC charger, I would recommend the DC-DC charger. With one caveat though: install one that has a solar input as well. DC-DC chargers are nothing more than an MPPT voltage controller. Install the right one, and it will do double-duty for you, and it will always prefer the solar panels over the truck alternator if the sun is shining. Up to the limit of what the panels can produce, anyway.

When I’m on the road, I’m running the fridge in either DC mode, or in AC mode from an inverter. Before I installed the DC-DC charger and was just relying on the truck alternator to charge the camper battery, I’d typically find the camper battery to be at 75-85% charged at the end of the day. And that was after installing 6 AWG charge wires from the truck to the camper battery. The fridge was working fine, but pulling in to a primitive camp without a fully charged camper battery doesn’t work for me.

With the DC-DC charger, I can run the fridge on the coldest setting while driving if needed, and there’s still enough charging capacity left to keep the camper battery as close to fully charged as possible.

Another unexpected benefit of the DC-DC charger is that the fridge DC mode works noticeably better than before. The voltage to the 12v heating element is being maintained at 2-3 volts higher than before, and it actually gets hotter, and can maintain a lower temperature inside the fridge, even in extremely hot ambient temps. It could never do that before. It worked so poorly I pulled the fuse on the fridge controller board for the 12v heater so I wouldn’t accidentally switch it to DC mode.

They’re getting more and more reasonably priced too. There just isn’t a down side as far as I’m concerned.

:):)

[Kayteg1]

Only 4-cylinder Sprinters of last decade has smart charging, so figuring it out might be useful information.
So is the Sprinter TV, or toad?
There are several ways to wire toad to TV.
Pretty popular is adding bulbs, or taillights who are powered directly from trailer pin.
This way toad electric system is not used and no worry about battery charging for weeks.
Than you might have electric toad brake, who needs to be recharged. But those don't take much current and standard 30 amp charging wire is plenty.

[towpro]

I did not know that DC to Dc charges could RAISE the voltage?

but anyway, as I did say in original post, I am hooking up a toad. it should be fully charged and I just don't want it to be dead after a days towing.
so were not charging batteries, were maintaining.

but by the way, it IS a Mercedes sprinter. Using the FACTORY 7 pin harness.
The RV side of the fuse line is already fused. I will install a 20A inline on the Toad side for safety and see how it goes.

Thanks everyone