Recharging house batteries

countrykids wrote:
When leaving a campground with partially drawn down house batteries, the engines alternator will slowly recharge them.

Can I charge those batteries faster by starting the in house generator at the same time charging from two sources?

So. . . . . I've gotta ask the obvious (at least to me) question. Why are you leaving the campground without fully (re)charging the batteries? (assuming you have shore power and camped overnight)

Simple answer. NO. Two charging sources will compete with each other. The one with the highest charging voltage wins. It tricks the other charging source into thinking the battery voltage, or state of charge, is already high enough so it tapers the charge off.

Chum lee

pianotuna wrote:

pianotuna wrote:
I added a 2nd charging path using #8 wire--and I have manual control of the charging solenoids.

theoldwizard1 wrote:

Concur ! Modern vehicle charging systems reduce the alternator voltage to about 13.2V shortly after the engine starts. This will never fully recharge a house battery bank. You need a DC-DC charger.

Yes larger gauge wires helps, but you will never be able to get a house battery bank back to 100% SOC on 13.2V !

When I put in the 2nd charging path I had identical batteries for house and starter (marine cycle). I did see over 70 amps of charging.

I am not saying that you did not ! However, a lot depends on the YEAR, make and model of your tow vehicle.

pianotuna wrote:
Also, at that time, getting a dc to DC charging device was pretty much unheard of.

Yes, they have not been around that long and neither have vehicles with "smart" charginf system.

Two charging sources can charge faster under the right conditions. They do not necessarily "fight with each other" as some seem to think.
They are both looking to achieve a target voltage on the battery bank. If your batteries are low enough and your charging source limited enough that one charging source can not bring the voltage up to the set point then a second source will help.
Say your RV converter is set for 14.2 volts but running at full output the batteries are only at 13.2 volts then the vehicle's alternator can add useful current it it's voltage at the battery terminals is above 13.2. If the vehicle alternator is able to achieve 13.8 volts on the battery it will contribute up to that point after which only the RV's converter (set to 14.2) will contribute.
What happens for most people is that with the generator running the converter it already enough to get the battery voltage above what the tow vehicle can contribute thru the small wiring in the circuit so it's never really able to contribute any current.
Or, if your RV converter is only set to 13.8 volts if the tow vehicle is running and has the batteries up to 14 volts when you start your generator the batteries will appear charged to the converter and it won't contribute any current. Your generator will be doing nothing.

RJsfishin wrote:
No. depending on converter output and alternator output.
My 140 amp alternator will charge much faster than my PD 45 amp converter.
And 2 different charging sources will usually fight each other. The one w/ the highest voltage will do most all the charging

And my 80 PD-80 amp is faster than my alternator at charging house batteries.

So what is the moral of this...

A joke
The story goes researchers ask a bunch of men "Boxers or Briefs"
The young men said "Boxers"
The middle aged "Briefs"
And the old fogies "Depends"

It all depends on the size of the alternator. The size of the connecting wires, and their length, and the size of the converter and again the wires and length.

So the bottom line is this.

Using a CLAMP ON DC-ammeter Like a Crafstman 82369 or another like meter measure the current on .. likely the Negative lead with Generator running. Then with main engine running.

And finally with BOTH running.

https://www.amazon.com/Craftsman-Digital-400A-Clamp-Ammeter/dp/B003TXUZDM/ref=cm_cr_arp_d_product_top?ie=UTF8

dougrainer wrote:
It REALLY depends on how long you drive. If over 4 hours the Chassis alternator WILL fully charge the Coach batteries. Doug

I disagree !

Modern vehicle charging system cut the voltage back to about 13.2V after a few minutes of operation. This will NOT recharge a depleted house battery bank even after 4 hours of driving.

wa8yxm wrote:
Using a CLAMP ON DC-ammeter Like a Crafstman 82369 ...

No longer available.

I prefer the Uni-T B4Q094 UT210E Current Mini Clamp

(A decent clamp multimeter and an old fashioned, 12V incandescent test light should be in EVERY RVers tool kit !)

As you have probably figured out, they may both charge. Or the alternator may prevent the converter from charging. Or, the converter may prevent the alternator from charging.

The final outcome is with both TRYING to charge, the battery will charge at the highest rate that your system can charge it, and you don't really need to know which one(s) is/are doing the charging. And it can't hurt anything.

Good question "Can I charge those batteries faster by starting the in house generator at the same time charging from two sources?"

"GENERALLY SPEAKING" YES its like filling a swimming pool with two garden hoses versus only one HOWEVER this is all subject to the quality and capacity of your in house charger/converter as well as how your alternator is set up to charge the house batteries when driving.

If you use wire to basically place the house battery in parallel with the engine battery when driving, how many actual charging amps the house battery may get is subject to the wire size, length of the wire run, and the alternators regulation. In addition be sure to NOT overload the engine alternator. Often (subject to many variables) the quality of this charging scheme doesn't yield a high degree of quality or quantity house battery charging, but hey if it pumps any charging amps it can help. It will NOT be the same quality and completeness of charge a 3/4 State Smart Charger would provide. NOTE its best if the house and engine batteries are the same chemistry in this situation. While Ive used this method over the years and sure it can "help" it never seems to fully or completely charge the house batteries like my onboard 3/4 Stage Smart Charger can.

For a better system use a DC to DC Charger that shouldn't over tax your alternator plus provide a better quality charge to the house battery when driving.

John T BSEE,JD Retired Electrical Engineer and 49 year RV owner

dougrainer wrote:
It REALLY depends on how long you drive. If over 4 hours the Chassis alternator WILL fully charge the Coach batteries. Doug

Not always.

As I trundle down the road my solar system sends power TO the engine. I see zero charging from the alternator except just after starting. The ECM "sees" the chassis battery, and tapers accordingly.

I do see energy flowing from the alternator to the "house" bank if I start a large load such as the water heater.

eBay 280 amp brushless alt. Little brother to my 400 amp monster