Dual battery system - WITHOUT solar - DC only questions

[John61CT]

First actually measure voltage at the bank with heavy gauge wire. You'll get much more current that way, just as important as voltage.

Only spend the extra money on a DCDC charger if you really need it. That topic really deserves its own thread.

Also, a quality standalone solar controller will do a much better job than one included in any DC-DC charger.

 

[Ticklebellly]

If you take a 100 Ah battery down to 75%, you have about 25 Ah to replace.   Using the alternator, after charging for half an hour, charge current is likely to be as low as 1.5 Amps.    You will need to run the engine for a long time to fully recharge the battery.

A DC to DC charger will greatly reduce the necessary charge time.   A 20 Amp charger will still require over an hour to do the job.

Solar or a regimen of shore power charge every two days might be the go.

 

[king]

If you get a flooded battery you can test the state of charge using a $10 hydrometer.  Particularly with a first battery, it is good to be able to tell exactly what is going on.  Checking and refilling with distilled water takes 5 minutes per month.

I plan on getting the sealed kind with no water maintenance. So, to test those I will need a volt meter and an ammeter to measure voltage and current ? Will a multimeter do it all? I already have one of those.

 

[king]

There was no disagreement between us, just your limited understanding of how charging works.

The 5+ hours is about getting back to 100% Full.

20min may be enough to replace a certain amount of power used, but the batt may only be 75% full at that point.

The former is only required for bank longevity,  and 2-3 times a week may be sufficient for that, rarely do we achieve ideals in real life.

Thank you. I do understand now, and I was asking about how long it would take to get it to 100% full charge, because I know that is important unless I want to replace the battery every year.  I was just hoping there would be a quicker way to do it, but it seems solar is the better investment (rather than DCDC charger), so I'm shopping for a portable solar panel right now. Thank you for all your help.

 

[John61CT]

A DC to DC charger will greatly reduce the necessary charge time

completely depends, if the alt/vr are doing a good job and the wiring is heavy enough gauge then absolutely not, and possibly the opposite

> A 20 Amp charger will still require over an hour to do the job.

Once acceptance has dropped as you point out, even a 500A source won't speed anything up. If the endpoint is true 100% Full.

> Solar or a regimen of shore power charge every two days might be the go.

Yes, and even just once a week is better than perpetual PSOC abuse

 

[John61CT]

So, to test those I will need a volt meter and an ammeter to measure voltage and current ? Will a multimeter do it all? I already have one of those.

Ammeter, clamp style is convenient, google for UT203 for a super cheap one. Also shows volts.

 

[Ticklebellly]

completely depends, if the alt/vr are doing a good job and the wiring is heavy enough gauge then absolutely not, and possibly the opposite

You will have to explain exactly what a good job is.   In my tests, the DC to DC device is putting out 20 Amps an hour into a charge cycle while the Alternator is pushing less than 1.6 Amps in the same time frame.   In both tests I started with the battery at about 50 % SOC.   I have loaned my DC to DC to others so they can do a similar test while they decide on whether or not to fork out for a DC to DC charger.

 

[Weight]

I would like to know where the DC to DC charger gets the amperes it is using to charge the battery if the alternator is making less than those amps. Anyone?

 

[Trebor English]

To put out 10 amps at 14 volts, 140 watts, the dc to dc charger will take 150 or more watts from its input.  At 12 volts input that would be 12.5 amps.  If the engine is running the voltage regulator will notice the low voltage and increase the alternator field current so that it will produce 12.5 more amps.  If the alternator isn't producing 12.5 amps more the discharging of the starter battery will continue.

 

[MrAlvinDude]

As interesting (and as valuable) as the DC-DC charger insights would be, I think we might run the risk of getting sidetracked into  clarification of some details that the OP (original poster) might not necessarily benefit from right at this point in time.

So will it be okay with you guys to wait until the OP asks again, about this specific topic?

So what else besides:

12 volt AGM battery (100-125)
solar charge controller MPPT
solar panel (kit with fuses and cables)

would be wise to order, so a helper has all that is needed to do the install?

 

[Weight]

Some heavy enough wires, and then one size larger. Fuse, Fuse, Fuse, Fuse.

 

[Ticklebellly]

I would like to know where the DC to DC charger gets the amperes it is using to charge the battery if the alternator is making less than those amps. Anyone?

An alternator does not make amps.   The charge current into a battery connected to an alternator is determined by the voltage difference between Alternator output and the voltage at the battery terminals.   The charge current  into a battery connected to a DC to DC charger is determined by the voltage difference between the DC to DC charger output and the voltage at the battery terminals.   In this explanation, the energy (measured in Amps) comes from the alternator which responds to demand.   An alternator has a rated capacity to supply energy depending on the demand.

To extend and make relevant to the original question, as the state of charge of the battery being charged approaches full, the voltage difference between alternator output voltage and the battery terminal voltage reduces.   With a DC to DC charger in the mix, the DC to DC charger uses electronic magic stuff to increase the output voltage of the charger.   Refer now to the statement above about charge current value being a product of the voltage difference between the charging device output and the voltage at the battery terminals.

 

[HumbleBeginnings]

I am one of the few that DO believe solar is unnecessary in SOME situations... basically if your plans involve driving long distances most days anyway. However, that is not your case.

But consider this: an idling van is a very expensive generator! You would be basically running a very large inefficient generator at a high operating cost (in my case a 5.4L V8 generator).

So, why not an affordable generator instead?

I haven't run the numbers, but I'd be willing to wager a low cost generator (what was that Sportsman 1000W going for recently, $170?) would be cheaper to purchase and run than running your engine for the express purpose of charging your batteries. I don't have one but I think they use a tiny amount of gas, less than a gallon a day.

Just saying... if you'd like to avoid solar it's another option. Either would work.

 

[John61CT]

An alternator does not make amps.   

Of course it does!

It has a certain level of available current anyway.

The battery pulls the amps it needs, and the voltage regulator prevents overvoltage.

DCDC chargers work by **dropping** the voltage the VR sees to cause higher output than usual, and then if necessary they convert the voltage to what the battery needs.

There are times a DCDC charger is required, but very often they are not. There is no "electronic magic stuff".

Not all bank types require the same voltage profiles, not all alternator/regulators are the same, the distance vs size and quality of the wiring is a big factor.

In order to determine whether or not a DCDC charger is required, a lot more detail is required, and the owner should ideally have an ammeter.

 

[John61CT]

an idling van is a very expensive generator!

Yes, idling to charge is dangerous, crazy expensive, and often doesn't work at all.

A gennie is better, but only to get up to 85-90% Full.

Only solar or shore power are practical for the remaining 4+ hours required to get to 100%.

 

[Ticklebellly]

There is no "electronic magic stuff".

Of course there is.   Where do you think the Magic Smoke that powers all electrical equipment comes from?

 

[tx2sturgis]

So what else besides:

12 volt AGM battery (100-125)
solar charge controller MPPT
solar panel (kit with fuses and cables)

would be wise to order, so a helper has all that is needed to do the install?

Some type of battery tray or box, or hold-down arrangement.

A fuse panel, and cables for the battery to fuse panel, including an inline fuse.

Maybe a cigarette lighter port or two, for powering misc 12v items and charging a phone and laptop. 

Plenty of zip-ties, tape, cable clips, ring and spade terminals as needed. 

Tools! you gotta have tools!

 

[Blanch]

Have you considered a solar suitcase?
About $200 on Amazon and has the panels and controller built in to it.

I use a solar suitcase, it works great for me!


Sent from my iPhone using Tapatalk

 

[king]

You are talking about a minimal house battery. Rather than a DC/DC I would use this.  https://shop.marinehowto.com/products/blue-sea-7610-automatic-combining-relay

Spend more on a quality 120 volt charger.

This is another debate I have encountered and don't know which is better - VSR vs. isolator

I'm confused about the 120 volt charger - this is needed in addition to the list below?

As of now, the components I think I need:

Battery
Portable solar panel 
isolator (or VSR)?
charge controller

 

[John61CT]

isolator (or VSR)

Isolator, as with combiner, can imply old diode-based tech, not very precise terms.

VSR / ACR are more modern, two-way automatic joining based on voltage, any charge source.

A cheap ignition solenoid is enough if only using Alt charging.

DCDC charger when voltage needs modifying, more expensive often installed when not actually needed.