RV Chassis & Coach Battery Charging Question - Sanity Check

Thread Starter

ischonfeld

Joined Jun 22, 2019
15
I’m looking to have the following solution “sanity checked” to be sure I’m not missing something.

Background: I have an RV with two totally independent 12V battery systems. System 1 is the chassis battery, alternator, engine, etc. – standard automotive system. System 2 is the coach system, with its own batteries, solar charging, shoreline charger (not plugged-in in this scenario), and second alternator/regulator.

The Problem: During the winter, the RV is rarely used. While I try to take it out periodically and run it, I don’t always do that enough and the chassis battery gets drained. In addition to not being able to start the RV, this isn’t healthy for the battery. The coach battery system is maintained due to the solar charging system (real system, not the tiny RV panel they sell that supposedly keeps your battery charged).

My solution: I want to connect the coach system to the chassis system through a bridge consisting of a 2.5 Ohm resistor, rectifier and fuse. My thought is that if parasitic load draws the chassis battery down, a small trickle charge will flow through the resistor/rectifier bridge. The maximum charge level for the coach system from solar is 14.0V. So if the chassis battery goes down below 13.4V (accounting for the rectifier voltage drop), a small charge current will start to flow. This current will increase if/when the chassis battery voltage drops. For example, if the chassis battery were to drop to 12.5V, then there would be approx. 360ma of trickle charge. When the vehicle is started, the chassis battery voltage might drop significantly. But even if it dropped down to say 6V temporarily, the flow through the resistor/rectifier bridge would be less than 3A. So I figure a resistor in the 25-30W range won’t get smoked.

In coming up with this, I’m also trying to avoid overcharging the chassis battery.

The chassis battery is buried under the driver’s side floorboards, so I have not been able to measure the parasitic load.

Does this solution make sense?

Since the nominal 12 car battery voltage is around 12.6V, should I use 2 rectifiers in series to change when the charge current starts to flow (approx 12.8V vs 13.4 with one rectifier)?

Thanks
 

LowQCab

Joined Nov 6, 2012
503
Both Battery Systems use the exact same Voltages.
You really should try to find the drain on the Engine Battery,
depending upon how long it takes to run it dead,
it should normally last at least 2-months running the usual electronic memories,
anything less indicates a problem that should be addressed.

A plug-in "Battery Maintainer" which remains connected for the entire "off-season" is the best solution.
They're dirt cheap.
A Diode will work to a degree,
but the Battery will last much longer if it is kept right around 13.8 Volts.
With freezing cold weather, the Voltage should ideally be closer to 14.1 Volts.
When a Lead-Acid Battery's Voltage is allowed to drop lower than ~13.5 Volts for extended periods
the Battery's life expectancy, and/or, Amp/Hour Capacity may be significantly shortened or reduced.

If the Battery has been allowed to go completely dead more than ~3 times,
or allowed to sit mostly dead for more than a month,
just go ahead and save yourself a headache, replace it with a new Battery,
because it has now been permanently damaged, and will have some degree of reduced performance.

Don't forget that there are certain circumstances where up to ~3 Amps of Charging Current could be demanded.
So you should go with a 5-Amp Diode, with the lowest Forward-Voltage Drop you can find.
I'd go with the Battery Maintainer.
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Thread Starter

ischonfeld

Joined Jun 22, 2019
15
You really should try to find the drain on the Engine Battery, depending upon how long it takes to run it dead, it should normally last at least 2-months running the usual electronic memories, anything less indicates a problem that should be addressed.
I try not to go that long, but a month between trips and the RV will start just fine. Too much more than that and between the cold and power required to start the diesel, it's a problem. Battery might not be dead, but not enough to start and attempting to start under those conditions probably isn't healthy for the battery. I also don't want to start it just to idle - instead I try to plan running it at a time when I can take it out and get the engine hot. Problem is, the week I plan on taking it out something comes up (snow, life, procrastination, etc. ) and it never gets run.

A plug-in "Battery Maintainer" which remains connected for the entire "off-season" is the best solution. They're dirt cheap.
Agreed. but not a great option in its current location.

If the Battery has been allowed to go completely dead more than ~3 times, or allowed to sit mostly dead for more than a month, just go ahead and save yourself a headache, replace it with a new Battery, because it has now been permanently damaged, and will have some degree of reduced performance.
Also completely agree, as I learned from my previous RV. Exactly the situation I'm trying to avoid.

With freezing cold weather, the Voltage should ideally be closer to 14.1 Volts.
This is great information. I was targeting something around 12.6V and thinking about using a pair of diodes to get the voltage drop from the solar (around 14.0V) down into that range. But if a higher charge voltage is better, that's good to know.

Thanks for your information. It helps.
 

LowQCab

Joined Nov 6, 2012
503
12.6 Volts might as well be dead, the Battery will not survive at that low of a Voltage.
Lead-Acid Batteries like to be fully charged at ALL TIMES.
Any "accumulated time" spent at less than ~13.5 Volts will shorten Battery life.
Low Voltage causes Sulfur Deposits on the internal Plates, which are extremely difficult to dissolve.
An "AGM" type Battery can withstand much more abuse,
( Optima Deep-Cycle Starting-Battery ), but they are very expensive, ( ~250.oo US ).
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I have a RV and had the same problem, I found that by turning off the isolators to both the habitation and main batteries the batteries stayed charged, I also have a relay that if the main battery is low you can link all the batteries together by pressing a switch on the dash which activates the Solenoid. you have to hold the switch so it doesn't stay closed.
 
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