EDIT:
I just realized there is this link. The subject C/10 design differs primarily from those described in the link in that it only supports C/10 charging currents for 200 Amp-Hr which is demonstrated to be fully adequate for haus battery charging while dramatically limiting the current-carrying ability of the components. It is quite similar to the second figure but essentially fully automatic.
https://www.sportsmobileforum.com/fo...ches-7991.html
Introduction
So I was prompted to write up this design analysis to both document the design I did on my own build as well as to solicit comments.
While this approach seems to go against "conventional wisdom", it does echo other experiences about this approach I found here.
https://www.sportsmobileforum.com/fo...tml#post152430
The benefits are fully automatic operation with manual override, lower cost, lower space claim, and more safety (less amperage). Feel free to comment.
BACKGROUND:
The old way (40 years ago) was to use an "isolator" which is a brute force device that uses power blocking diodes to direct current in only one direction toward the house battery. It also blocks current from coming from the car battery to the haus battery. The main issue with these devices is they can drop a volt or more between the car's charging system and the haus battery.
In more recent times, there has been a cottage industry of "separators" for RV applications. The Sure Power 1315 is one such device. Generally, these devices switch based on voltage and more sophisticated ones can switch based on current sense as well.
https://info.waytekwire.com/blog/bat...iary%20battery.
The earliest of these devices uses a Continuous duty solenoid to connect the Haus to the start battery with some type of low power logic designed to switch on the solenoid as sensed conditions would dictate. Ans we often find battery connect disconnect systems to include high amperage marine battery switches as well.
Max 300A Nickel-Plating Continuous Duty Solenoid Relay for Golf Carts, Winch, Marine on 12VDC Rush
https://www.amazon.com/Continuous-So...20756133&psc=1
Marine Battery Disconnect Switch, High Quality On Off Position Master Cutoff Switch, 6V, 12V, 24V, 48V, Battery for Boat, ATV, RV, UTV, Waterproof Isolator Switch, 275 / 1250 amps (1-2-1 and 2-Off)
https://www.amazon.com/-/es/Interrup...e&sr=1-20&th=1
I have had these devices some of which I installed myself on an old 1980 22ft RV. Going through the electrical design of the charging systems for my=y Van build I had to question the necessity of providing for switching and carrying large currents between the start and haus batteries. Why was it necessary? My conclusion has been that it is NOT necessary and only adds complexity, takes up valuable space, and is more to fail. or at least create unknown/unwanted behaviors.
DESIGN APPROACH
So the underlying approach in the following design is to design the start to haus battery connection based on a C/10 charging rate for the haus batteries. For 2X 100 amp hour batteries that is 200 amp-Hr/10 = 20 amps max charging current. This is what I will be assuming for this reference design.
Some RV's might have larger battery banks, and I should mention that this reference design is only for lead-acid compatible battery types where a relay connected the terminals of the batteries. If you are mixing Li-Ion haus with lead-Acid start battery then I would recommend something like this. The main issue with this device is that it limits you to small 12V RV solar panels.
DCC50S 12V 50A DC-DC On-Board Battery Charger with MPPT
https://www.renogy.com/dcc50s-12v-50...ger-with-mppt/
In my build, I have a 350W residential panel at 38V/9A (nominal) with two 80 Amp-hr lead-acid batteries in frame-mounted battery boxes. So the DCC50S doesn't really work in this configuration primarily because of the VOC of 38V on the panels. Instead, I have the Rich Solar 40A charge controller which is full-featured including load control but without the start battery connection features.
REFERENCE DESIGN
So I will do a brief description of the features. I refer to "separator" as the integrated design of the dash-mounted controls, #10 AWG cable with control relay, and two 30A resettable breakers. The calculations are shown in the third figure down.
1.) The separator is sized to safely charge C/10 for 200 Amp-Hr capacity batteries.
2.) Start and haus batteries are connected with a small 10 #AWG Marine Cable (for example $1.5 per foot) that is protected on both ends with 30 Amp auto-resettable circuit breakers. The 40 amp automotive relay will only be energized when the key is in "RUN". So unless you are in the habit of leaving your key on, the haus will be disconnected from the start battery.
2.A) The idea is to only try and charge the haus when the engine is running. I realized that even if the start battery is dead, if you have 7 volts to trigger the relay then you can connect the batteries and a charged solar bank will charge (30 amps max) the start battery as well.
2.B) Central control smarts reside with the driver. He/she can monitor the display and can see the obvious change in voltage when connecting or disconnecting the batteries. So typically when disconnected the haus battery bank might be at 12.1V. The engine is started and the start battery goes to 14V depending upon idle speed. When the connecting relay is engaged you will see the haus battery rise to from 12.a to say 13.5V as charge starts to flow to the haus.
3.) As sized, the auto resettable breakers will only allow 30A to flow to the haus. You can reduce this or raise it up to something less than 40A (the relay limit).
A subtle feature is that because I have selected a small 10 #AWG wire, there is a tendency for the more current being pushed,m to reduce the charging voltage at the haus batter which in turn limits the current flowing.
CONCLUSIONS
In this reference design I'm assuming 10 ft #10 AWG cable between start and haus batteries. In the second figure is a chart showing the self-limiting effect of the reduced wire size. With a #6 wire the maximum voltage to the haus bank is 14.35V at C/10 whereas with a #12 AWG cable the max voltage is 13.8V corresponding to SOC of 94% and 88 % respectively. So there re only a moderate reduction in the amount of SOC you will get from the alternator charging of your solar battery bank.
We can conclude then that with the alternator setpoint at 14.5V, you will never exceed 95% SOC in an event. Using the #10 AWG cable, will only lower the SOC to 91% . This is a modest price to pay for the much lower current ratings of the separator.
Operational Summary
Generally, the batteries can only be connected when the key is on and typically only when the engine is running. So the driver should note the solar battery voltage. With the engine running, activate the" separator (i.e. connect the start and haus batteries) The driver should be able to above a noticeable rise in the haus battery voltage to over 13v even at idle.
You can happily drive the vehicle knowing charging is limited to C/10 and there is no possibility of overcharging the battery systems.
After parking and shutting off the vehicle the 12V RUN will be off and the separator will disconnect automatically. This might be a preferred method of operation. "leave the switch on and allow the +12V RUN to control activation automatically.
If for some reason the start battery fails to crank the engine when attempting to start, leaving the key in run will allow the solar battery to charge the start battery (up to 30A) with is getting to 0.8C (for a 40 amp hr start battery).
If both battery banks are low, disconnect the solar to allow it to recharge and then turn on the key to charge when there is sufficient charge in the haus bank. To a large extent in normal circumstances, you would not need to ever turn the switch off to disengage the separator. This is essentially totally automatic with disabling overide
(i.e. disable the connecting relay).
Optional but Recommended Disconnects
As far as 12V cutoffs, I have added two that I use in my build. One is to cut off all DC loads in the haus and the other is to disconnect the solar panels from the charge controller. These are rated at 20A and 15A respectively.
Failure Modes:
A.) If the cable positive were t short somewhere in the 10 ft run then either or both circuit breakers should disconnect automatically. This is easily diagnosed by observing that the haus battery voltage does not rise when connecting the batteries.
B.) If the relay were to fail open, detect same as A.) replace the relay.
C.) If the relay were to fail open, detect an elevated haus battery when the engine is on but the switch is off. replace relay.
D.) If there was a double failure of the shorted cable and failed circuit breaker then you will have to rely on the 150A main fuse block terminal near the Econoline fuse box. There is no corresponding fuse in the haus.
It might be wise to put in 40A fusible links in series with the 30A circuit breaker.
Reference Bill of Material
I have added some representative parts I have used. I included links to the hydraulic terminal crimper which works well if you do a little practice and abide by the morning to stop pumping once the jaws come together.
T Tocas - Circuit Breakers, Panel Mount
https://www.amazon.com/-/es/gp/produ...?ie=UTF8&psc=1
RY116 Relay Standard Motor Products https://www.amazon.com/Standard-Moto...a-568855592562
Fastronix 30A 12V Type 1 Automatic Reset Circuit Breaker with Cover
https://www.amazon.com/-/es/gp/produ...?ie=UTF8&psc=1
10/2 AWG Gauge Marine Grade Wire Boat Cable Tinned Copper, Flat Black/Red
https://www.ebay.com/itm/14347062810...4AAOSwPHJdSrZ3
Yescom 10 ton hydraulic crimping pliers for battery cable terminals with 9 punches.
https://www.amazon.com/-/es/gp/produ...?ie=UTF8&psc=1
2 x 1/2-inch 3: 1 Heat Shrink Tubing with Large Heat Shrink Adhesive Liner by MILAPEAK (4ft, Black & Red)
https://www.amazon.com/-/es/gp/produ...?ie=UTF8&psc=1
One-hole standard barrel with window lug #10 AWG wire #10 stud hole.
https://www.alliedelec.com/product/p...E&gclsrc=aw.ds
Linear Mode
