Keeping two batteries happy (separated)

[truepath]

Please excuse this long post, but I thought a more comprehensive discussion of this topic might be of interest.

As is common with many RV's, I have two batteries in my RV, one for starting the vehicle and the other for house lighting, tv, etc. I started to have problems with my starting battery getting drawn down. After various diagnostic efforts I figure out that the diode based battery isolator had failed and no longer was keeping the batteries separated. This allowed the house battery to draw the starting battery down. I decided this might be a good time to look into the various battery separator options and have learned there are at least three with none clearly the best.

These are the options:

1. Solenoid. This is basically a switch that allows linking the batteries for charging both from either the alternator, generator or shore power (through a converter) and then disconnecting the batteries when parked. This seemed like a good choice since it can easily be wired to automatically link the batteries from the ignition or manually with a switch. Unfortunately, I have learned that many people have experienced early failure, but even more concerning was one comment I read that said if the two batteries are not roughly equal in charge, then when they are linked there is a massive movement of electrical power from the charged one to the other one (think hot wires, blown breakers, etc.)

2. Smart solenoid. This is a switch, but it has some intelligence that will block charge to the auxiliary battery until to main battery is at full charge and then I believe it meters the charge to the other battery. There are various options with these devices that will allow flow in both directions between the two batteries and allow use of both to assist with engine starting. One brand I have seen is Smart Power, but I'm sure there are others. These devices are more expensive than the basic solenoid and don't seem to have the same number of significant comments about failure, although there are some. They also seem to be more challenging to install.

3. Diode based isolator. These devices keep the two batteries separate through the use of diodes that allow a charge from the alternator to be delivered to each battery, but won't allow a charge to return from one battery to the other. These devices seem to usually function without much trouble, but mine did fail. Their downside seems to be the small voltage drop they cause in the charge from the alternator. They also don't allow the charging of the starting battery from the converter (either shore power or generator power).

I would appreciate comments, insights, clarification or recommendations.

[DryCamper11]

truepath wrote:
I've decided instead to use a SurePower 1315 bi-directional "smart" solenoid.

Does anyone know if this allows the RV batteries to assist during engine starting?

I note it's rated for 100A, which seems low for starting current. It also seems to have an optional override to engage by manual switch, if desired. Is it designed to carry starting current loads when activated with that switch? I assume it's not enabled during starting by default as the alternator won't be producing much voltage until the engine is running. I use my manually activated switch as a normal starting boost switch.

[DryCamper11]

truepath wrote:
I've decided instead to use a SurePower 1315 bi-directional "smart" solenoid.

I suspect that's a good choice. The parasitic draw should be low when not charging.

[truepath]

Well, here's the latest in my saga about battery separators. As mentioned the other day, I was going to just put in a manually (with a button switch) operated solenoid. The plan was to turn it on to connect the batteries when driving down the road and turn it off when parked. This would have allowed charging to the auxiliary battery (and refrigerator on DC) during drive times. Now I've decided instead to use a SurePower 1315 bi-directional "smart" solenoid. This will allow charging in both directions depending on power source--from the alternator to the auxiliary battery and from the converter (shore power or generator) to the starting battery while at the same time avoiding the issue of forgetting to disconnect the batteries. During long term off season storage, I'll disconnect both batteries from the separator to avoid whatever parasitic draw may be involved, if any. I do this anyway as a matter of practice when parking the RV for winter storage. The SurePower unit cost through ASE was a little over $50 so about the same as a high end solenoid.

Thanks everyone for the great discussion of this topic. I know that I have better insight on the subject now than before.

[ken_white]

MEXICOWANDERER wrote:
In the hope of learning something about the FET device are the following a reality?

Bi-directional sensing and operation?

Hosting a three bank (or more) system?

Mil Gracias

Here -> Website

And Here -> .pdf

[DryCamper11]

MEXICOWANDERER wrote:
Thank you for checking.

The Bi-directional feature is crucial for me.

You are welcome, although, as I said, I might have been wrong.


For you 1)automatic operation and 2) bidirectional and 3) low voltage drop are important, so you go with the smart solenoid based design. For those who want only 2) and 3), like me, a manually operated solenoid design is best (I also like having control, instead of having the circuit make decisions for me, but I realize it means I have to do more and will occasionally make a mistake). For those who will live with unidirectional, but want 1) and 3), the FET design will work well. There are lots of options out there, and in the end it seems we still have to decide what's important to us. That said, if I was asked by a new RV purchaser what I thought was the best, I'd probably suggest the solution you like - bidirectional and smart with no voltage drop.

[MEXICOWANDERER]

Thank you for checking.

The Bi-directional feature is crucial for me.

[DryCamper11]

MEXICOWANDERER wrote:
In the hope of learning something about the FET device are the following a reality?

Bi-directional sensing and operation?

Hosting a three bank (or more) system?

Mil Gracias

I read the posted pdf link. At least one model offered 3 bank system charging capability. It did not seem to offer bi-directional charging/ sensing. Thinking about the electronic design, that makes sense to me, but I could be wrong - there might be ways to hook it up to get bidirectional capability that I didn't see.

[DryCamper11]

MEXICOWANDERER wrote:
Ya might otter want to read a technical data sheet regarding the quiescent draw of a battery separator señor pianotuna

Pianotuna's point, as I took it, was that the OP was considering using a switch operated continuous duty solenoid (not a normal battery isolator). The OP seemed to want it set up so the solenoid was not powered from the ignition, and he wanted to leave it continuously on so that he wouldn't forget to turn it on.

In that configuration, I agree with pianotuna: 1) there would be a continuous parasitic draw to power the solenoid and 2) you'd run down the engine battery with the RV battery, potentially leaving you stranded.
If I misunderstood the OP's plan, or pianotuna's comments, I apologize.

[MEXICOWANDERER]

In the hope of learning something about the FET device are the following a reality?

Bi-directional sensing and operation?

Hosting a three bank (or more) system?

Mil Gracias

[ken_white]

I still prefer the FET option -> FET Isolator

FET's are voltage controlled devices that draw almost no current on the control port and drop very little voltage across the power ports...

[time2roll]

My Trik-L-Start works great. Limited to 5a but that is all I need.

[MEXICOWANDERER]

Ya might otter want to read a technical data sheet regarding the quiescent draw of a battery separator señor pianotuna

http://www.ase-supply.com/v/vspfiles/downloadables/13141315200.pdf



Sheemaniz I just couldn't resist. Forgive me!

[pianotuna]

Hi Onaquest,

The OP is considering switches that are NOT controlled by the ignition. Therefore he is creating an additional parasitic load whenever they are on.

[OnaQuest]

pianotuna wrote:
Hi,

My switched solenoids are off if the ignition is off. A continuous duty solenoid draws 0.88 of an amp. That is too large a parasitic load to live with.

I avoid forgetting to turn the solenoids on by leaving them in the on position.

I wouldn't call the continuous duty solenoids, driven by a BIRD, or those "so called" smart solenoids, parasitic loads. They are only energized when a charging source exists from either the alternator or converter. When they are OFF, so is the solenoid.