Alternator overcharging house batteries

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Harry B

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Jan 9, 2009
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Northern AZ
I am experiencing what seems like overcharging of my house batteries by the alternator.

Has anybody figured out or tried a way to regulate the charging of the house batteries and chassis batteries independantly being they have such different characteristics for charging. Usually when my house batteries are a ways down I will try to bulk charge via the inverter before we get underway.

Both the house batteries and chassis batteries were replaced at the same time last year. (2x 12v sealed starting batteries and 4x 6v wet cell house batteries). I am not having any problems with the chassis batteries.

Last year I discovered that my house batteries were not being charged by the alternator due to a bad contactor, which has been a common problem on the Winnebago diesels. I replaced the contactor last September after having driven 3600 miles with the alternator charging only the chassis batteries. I drove another 4500 miles with it fixed and my house batteries had excessive acid discharge again. I had absolutely no acid boilover when the contactor was defective  and the alternator was not able to charge the house batteries. Typical battery voltage supplied by the alternator when charging is about 13.8v, which seems normal.
 
Harry, if you think your alternator is over charging your house batteries, why would you exacerbate the problem by charging them before getting under way?
 
Most people don't drive enough hours to seriously overcharge the house batteries, no matter what the alternator does. That's why they don't try to regulate it.  13.8 is a tad high - 13.6 would be better - but typical of an alternator.

Can you disconnect the alternator-house charge circuit and see if the problem goes away? That would be conclusive.
 
Lou ....
My reasons for usually bulk charging the batteries via the inverter (when well depleted) is to ease the load on the engine alternator and to better balance the charge of the two very different types of battery sets. However I recently took a trip of 1200 miles in a couple of days and did not use the inverter for bulk charging and the result was the same, excessive gassing of the house batteries.

Gary ....
When the contactor was defective last year, it decoupled the house batteries from the engine alternator and I had no excessive gassing. The only way I was able to charge my house batteries last summer was via the inverter by means of shore power or the generator, until I replaced the contactor at my daughter's house.
When I thought back to last summer's trip and not having any problem going out west when the contactor was defective, is what finally clued me in as to what was causing the excessive gassing. So after our recent 1200 mile trip when it reoccurred again and the alternator now was charging both sets of batteries again is what substantiated my deduction that the alternator was causing the problem. Now how do I resolve it.

You are right about the duration of engine charge time, I usually drive about 200-250 miles a day on average.

 
Harry, do you feel that the 6V house batteries are as healthy as they should be?  I see they were replaced just last year.

Were you experiencing or aware of similar problems with your old batteries?
 
Lou ....
I replaced my  three 12v Trojan deep cycle batteries because one of the batteries had a bad cell. Those 12v deep cycle Trojans were only warranted for 1 year. To replace one of those Trojan 12v batteries was about $210 at the time. I decided to replace them all with four 6V deep cycle batteries as that would give me a matched set again and I would gain about 50 amp hrs in capacity at about half the price of replacing the three 12v deep cycle Trojans.
My chassis batteries gave up the ghost about the same time probably due to abuse while sitting on the dealer's lot. I was surprised they lasted as long as they did given I found them dead 2x while inspecting the coach prior to purchase. The dealer had that coach in inventory about 12 months.

Dan ....
Can't do that as you probably know it turns off everything, including the fridge and AC. That would mean no ice cream while it would be very warm inside  ;D
 
harry,
How about a 12v shutoff switch in the wire from the alternator to the battery? i saw one advertised by harbor freight recently for <$10. :)
 
Your choices are to disconnect the alternator wire to the house charge relay altogether or to put a manual disconnect switch on it. But how do you know when to disconnect? How long is too long? A 250 mile mile is not normally a problem. I guess I would put in a simple disconnect next to the relay but normally leave it open, so there was no charge. The switch would be there "just in case" I ever needed to charge from the alternator. If the relay is easy to get at, you could do the same just by disconnecting the alternator feed and tape it up. Reconnect if you ever really need it.

 
I know the alternator in my Workhorse Custom Chassis (GM alernator) is a bit on the "HOT" side.  However I've seen no evidence to suggest it's over charging seriously, I mean I add water to the house batteries (DISTILLED WATER) annually, (Or less, as needed

If it's not overcharging the sealed Maintenance free. for a fact it won't be overcharging the flooded wet cell batteries.
 
I know I could put a switch on the coil side of the contactor, which when open would prevent the contactor from energizing, thus isolating the house batteries from the chassis batteries/alternator circuit. I was just hoping of someone knowing about a device to automate this.
 
A lot depends on whether you want to treat the cause or the symptoms.

As Gary stated earlier, 13.8 is a tad high for long periods of charging, but it should not cause excessive gassing. IMHO

But, you could always put a diode isolator in-line with the house charge wire and drop the voltage.  The only "automatic" approach I can come up with.

I think the real problem may lie with the batteries.  Without knowing exactly how and when the problem first manifested it's self, it is difficult to say with conviction.  However, I would do the following:

I would not pre-charge the house batteries prior to a long drive.
I would immediately establish the correct water level in each battery.
I would carefully check the specific gravity of each cell of the fully charged and rested batteries.
I would perform equalization and de-sulfation cycles on all batteries.

If all the above procedures failed to show any problem, I would put in the diode.


 
I guess I am a bit ignorant on this one. I thought that an isolator in a MH was a given. I have experienced both types (diode and "smart"). Of the two I prefer the smart, but both would be better. Here is a link to the smart one Smart Isolator I am thinking along the line that your chassis battery is always (well almost) charging when the engine is running and the coach batteries are getting charged at the same time while not being used. So the coach batteries are overcharging. My thought would be to install an isolator (diode type) to keep the chassis battery from discharging to run the coach and a smart isolator for the charging of the coach batteries.

Just a REALLY weird thing I thought of. If by some crazy chance your 6V batteries were individually grounded (not just in pairs) they could be getting 12V to a 6v battery, even if the batteries were wired in series. .... I think. Never tried that experiment. Probably not though I guess it would short the second battery if that were the case... so enough of thinking out-loud and embarrassing myself. LOL
 
...a device to automate this.

The question is, automate based on what? Time? Miles driven? Voltage?  There simply isn't enough info to go on to be able to say when it overcharges, so it is hard to suggest what to monitor to stop it.

Lou's suggestion to bring the voltage down is probably the best approach. Somewhere below 13.6v it should not overcharge, if indeed the alternator is at fault at all.  I'm still not convinced that a continuous 13.8v for 6 hours a day is going to cause your problem.
 
RV Roamer said:
The question is, automate based on what? Time? Miles driven? Voltage?  There simply isn't enough info to go on to be able to say when it overcharges, so it is hard to suggest what to monitor to stop it.

Lou's suggestion to bring the voltage down is probably the best approach. Somewhere below 13.6v it should not overcharge, if indeed the alternator is at fault at all.  I'm still not convinced that a continuous 13.8v for 6 hours a day is going to cause your problem.
I agree. I don't think the voltage is the issue. I think the duration is. And charging when not necessary is the big culprit. A short drive may not show any symptom of charging while not necessary. But a long one would, especially with an efficient high amp alternator. I have seen similar problems with a bad chassis battery and the alternator continuously charging it, at the same time charging the coach batteries to the point that they boiled-over.
 
Low voltage (e.g. a bad battery) in the chassis system would indeed cause overcharging, but Harry says he just replaced the chassis battery.  It's a conundrum as to why the house batteries are gassing and I'll agree that overcharging is a likely explanation. It's the "why" that is escaping us, since the alternator does not seem to be excessive in voltage and the driving tine/distance is well within normal parameters.  High resistance in one or more of the house batteries could also cause it, but they are new batteries too. Maybe a poor connection at a battery post? Difficult to say without  more detailed measurements.
 
Couple of facts,
- The house batteries do not gas excessively (lets just say not at all) when they are charged via the inverter and the hydrometer shows they are being fully charged.
- I removed the house batteries and cleaned up all the cable and batteries, when I got home in November when I first noticed all the corrosion due to the gassing.
- Checked all house battery cells with an hydrometer and all show as being good
- Sprayed all cable connectors with connector sealer/protector so as to not have those impacted as badly as last year
- Took a trip of about 120 miles each way in January with the now clean again batteries - checked house batteries and they were still clean.
- Took a trip driving 575 in a day in and a half in April and the gassing was becoming evident again (top of batteries wet)
- At home the batteries are always being maintained by the inverter (coach is always attached to shore power while parked). No gassing problems at all.

I think I probably need to have a better look at my chassis batteries even though they are less than a year old (2x 1050 CCA). They are not giving me any problems. They are however sealed, so I can't use a hydrometer on them. I guess I'll pull them and check the voltage at rest and than get them load tested. Something is causing those house batteries to overcharge.
Maybe my chassis batteries are being overcharged as well but I can't see any evidence of it due to the fact that they are sealed.
 
as in my other post, I think that the problem stems from the charging of the chassis battery. Whether the situation is aggravated by a bad chassis battery is yet to be determined by the procedure(s) that you outlined, but even with a good chassis battery, while running the engine (using energy from the chassis battery) the coach batteries will also be getting a charge. If there is not an equal load (multiplied for more than one battery) on the coach batteries, then they will be overcharged. Yes there could be some resistance in a connection (corrosion preventative spray between the post and clamp, etc) but that would most likely cause an overcharge of the chassis battery instead. Of course having the chassis battery load tested etc is always a good idea. Also: just for giggles, make sure that none of the individual negative posts on the 6v batteries is grounded.

My main concern here is the fact that there is no true [charge] regulation between the classification of battery banks (one set is charged [ignorantly] as a result of the other banks need.

Therefore, I think that no matter the condition of the chassis battery, a good quality "smart" charge isolator is in order. And that is the main point of my post.

Of course, others have stated some very good cases for concern / areas to check etc, and the request for more information is valid to conclude with any certainty. There are many others with similar [physical] systems, and no gassing. So this is just one opinion based on logic.

BTW, 13.8 volts while charging is not high.
 
regularguy said:
as in my other post, I think that the problem stems from the charging of the chassis battery. Whether the situation is aggravated by a bad chassis battery is yet to be determined by the procedure(s) that you outlined, but even with a good chassis battery, while running the engine (using energy from the chassis battery) the coach batteries will also be getting a charge.
BTW, 13.8 volts while charging is not high.

Not sure I totally agree with your logic here.

Whether or not the chassis battery is accepting charge current because of load(s) won't make the idle coach batteries overcharge to the point of excess gassing at 13.8v.

The alternator doesn't force the coach batteries to accept current just because the chassis battery needs it.  There is a resistance mismatch somewhere in the system.  Either internal to one or more of the batteries or in the cabling.

OR - the voltages being observed from the alternator are not the true picture.
 
First: SOME rigs, use a volage "Sense" lead to read the voltage for the voltage regulator.. I've never seen one, but I'm told they exist.

However, first, Maintenance free (Sealed) bateries take the same or lower bulk and float voltages near as I can find as Flooded wet cells. so worst case is the starting battery needs less voltage

I've often posted this in another form..  but here goes

The argument is if the chassis battery is somehow defective it can cause over charging of the house battery:  This is.. basically... NOT TRUE

Here is why:

A battery cell has two failure modes.  Basically open (High resistance) and shorted.. THe case of very reduced capacity (The most common failure mode) is a High Resistance failure by the way, for the purposes of this post at least.


If the cell fails shorted, then you now have a 10 volt battery, this battery will suck all the free current from the 12 volt system, IT can take the house battery into deep discharge, but it will not let the system OVERCHARGE it, since it will be sucking all the charge (The isolator should isolate before deep discharge happens)

If the cell fails "High Resistance".. Well, then the voltage regulator will limit charge voltage, same as normal. so.. again no over charging will occure

It simply can not

If it's properly charging the chassis battery it can not, possibly, over charge flooded wet cell house batteries, can not happen.... UNLESS.... The house batteries have a shorted cell
 

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