House batteries not charging

[Tom]

I received the following question via email. Please respond here and I'll ask the sender to look here for answers.

TIA

i have a 1998 tradewind disel pusher motorhome manufactured by national rv. my house batteries no longer charge when the engine is  running. i talked to a spokesperson from national and they beleive the interconnect solenoid for the batteries to charge is defective. this part,#RL1002,was manufactured by a company called Powertek who apparently is no longer in business. any suggestions on what i can do?(the person at national did not give me help with this problem)

thank you,

 

[Gary RV_Wizard]

Sorry, I don't have a source to give to you. I have an NRV motorhome myself and am starting to have problems with a charger controller relay sticking - has happened twice in the last 4 months. I may be facing the same problem soon myself.  National has these controllers custom built to their specs and I think the little companies that build them often disappear when the contract is finshed.

The relays themselves, however, are probably off-the-shelf parts and ought to be replaceable, either with the same part or an equivalent one.  There's nothing reallyunique about them, except to identify the particular one and its specs.  Getting somebody to work on the controller board (if you can't do it yourself) may be a serious problem, however. If you can find a local ham radio group or other club with some electronic "techie" types, maybe one of them could help out. Otherwise, maybe an old-fashioned tv repair place? Or ask at the local Radio Shack store if they know of someone who can fix circuitboards?

 

[King]

It would seem to me that a relay capable of isolating a charging source capable of over 100 amps would not be likely to be mounted on a printed circuit board.  What about replacing the present system with a dual diode isolator, available at most RV dealers?
Art

 

[Karl]

Art,

O.k., I'll play dumb - what is a dual diode isolator and how will it help him?

 

[Gary RV_Wizard]

There really is a large relay mounted (well, bolted) to the circuit board in this thing. It switches the chassis alternator charge current when the engine is running and lets the converter/charger provide charging in the default position. Mine is rated at 50 amps, even though the engine alternator is capable of much more (converter is 50A max). Don't know if it is regulated to 50A or they just figure that no more than 50is going to reach the house batteries anyway (chassis demands would eat up the rest of the potential 130A alternator output?).

There are also a couple of big circuit breakers on the thing. It's a combination electrical panel and circuit board. It also carries a dozen or so fuses for various circuits.

 

[King]

A dual diode isolator has three connections: one is connected to the alternator, one is connected to the normal chassis wiring to charge the chassis battery, and one goes to the house batteries, usually through a circuit breaker.  since the current can only flow one way through the diodes, the house batteries are isolated from the chassis battery, but the alternator can charge both.  No relays involved.

 

[Karl]

Art,

There may be a potential problem with that arrangement. Given that a diode has an inherent 0.6 volt drop, I'm not sure how the voltage regulator would handle that. Ideally you would want around 13.4 volts to charge the batteries and if the voltage regulator is set for that, the batteries would only see 12.8. Guess I need to do some research.

 

[John From Detroit]

There is at least one "Dual Diode" isolator that has 3 diodes, and 4 connections, the first 2 diodes and 3 connections are as described, the 4th connection (3rd diode) is for the voltage regulator sense leade, Thus solving the forward voltage drop issue.

That said, many cars today have alternators with internal voltage regulators so that don't work well

 

[King]

Since the present usual standard for alternator output is 14.5  (plus or minus about 0.3)  a .6 v drop will put you closer to your  desired voltage. 
Art

 

[Gary RV_Wizard]

The purpose of the relay in the National (and other) systems is to prevent the house charging system from attempting to charge the house batteries at the same time the chassis alternator is doing so.  Both systems are active if you run the genset while driving down the road or if you start the engine while plugged into shore power. The relay prevents the two 12V systems from interfering with each other at the house battery posts (something a isolator cannot do).

 

[King]

Why is that a problem?
Art

 

[Gary RV_Wizard]

Unpredictable and depends on the logic each charger/regulator uses - they can easily conflict with each other.  Maybe Charger #1 sees the voltage that Charger #2 is applying to the battery and maybe thinks the battery is already charged or overcharged - not too bad. Or maybe #1 is putting out a high voltage that actually back feeds into #2, reversing the polarity at that end - could be very bad. If one of them is a 3 stage charger, the staging modes will be all screwed up because of the output of the other charger - maybe causing over or under charging.

The chassis alternator tends not to be regulated very wisely - it mostly just pumps out a lot of amps at high voltage and hopes ha battery and whatever else is out there can deal with it.  Good enough for a quick recharge of the chassis battery after starting, but maybe not so great for a deep cycle's lifespan?

 

[King]

It seems to me that the situation you describe must certainly occur whenever an owner connects a charger to the house batteries.  Many of us do that prior to leaving on a trip to insure that the batteries are fully charged.  It would seem reasonable to design the converter/charger to be protected against such a fault, when on shore power or not, otherwise warranty replacement would be excessive.
Art

 

[Karl]

Art,

I think we're getting into the area of knowing something about your rig's electrical system and a bit of common sense. No charge controller can be expected to be intuitive with regards to other systems online at the same time. It's like having two thermostats in the same room; one calling for heat, while the other calls for cooling. Each charging system has its' pluses and minuses (no pun intended), but none can replace a logical reasoning process, i.e. three chargers will not charge the batteries three times as fast. In the final analysis, you need one sytem (relays, solid-state, microporcessors, etc.) to take control over the others to insure that the end goal is met - charging, but not over charging, the batteries. 

 

[Gary RV_Wizard]

It seems to me that the situation you describe must certainly occur whenever an owner connects a charger to the house batteries

Yes it can, but most car type chargers don't put out real high voltage or very much current, whereas the alternator pumps out around 15 volts and maybe 130-160 amps.  And yes, a well designed converter/charger would - or should - protect itself, but the manufacturer isn't going to honor any warranty claims if they find it fried from reversed voltage.

But I've worked a bit with automotive style electronics and have seen some really bizarre stuff. For example, when the starter motor cranks incredible spikes can surge through all the wiring, sometimes driving the +12V side to -60V! You really don't want to have that sort of thing pulisng into the Rv's mor emodest (and less rugged) 12V system. Ergo,a relay makes a lot of sense.

I'm not saying any system without a relay is fated to an early and smoky death, but it sure is taking some risks.