RV Battery isolator?

[Hondafarian]

So this is more RV/ dual battery vehicle related but I know the brilliant mechanical minds on this forum will know the answer.
I have a 30 year old class A motorhome which I inherited from my father many years ago. It's been a real challenge to keep it on the road and campable but so far so good.
Problem is, when the engine is started I can tell from the voltmeter it initially starts charging the vehicle battery but will almost randomly switch over to charging the coach battery, and then switch back again. There's doesn't seem to be any pattern to it's switching and it can do it very quickly back and forth, several times within a minute. It seems to be a bad electrical part causing this.

Is it the battery isolator responsible? Where is this found and what does it look like? It's a 1994 Ford F53 motorhome chassis with the 460 V8.

 

[DannoXYZ]

Perfectly normal. It’s switching because charging battery “overtakes” other one as it charges. Then it switches to other battery to bring it up as well, which overtakes 1st battery, and it switches over again. Idea is to bring both batteries up to full charge.

Somewhere there’s a comparison taking place and when voltage differential gets over certain amount, it’ll switch. Might be useful to make this adjustable, so you can set it to 0,25v differential, or 0,50v, etc.

 

[Hondafarian]

Hmmm. I don't think this is normal. Last time I started it, the starting battery only charged for a few seconds before it switched to the coach battery. I know for sure the starting battery wasn't fully charged yet. It's an old battery and sits at barely 12 volts but always manages to start the engine. The coach battery was fully charged at 13.3 v. Isn't it supposed to split the charge between the two batteries? This has a very pronounced and rapid switching between the two with a loud click. And it can be only a few seconds before making another switch.

 

[DannoXYZ]

Maybe regulator is failing? It’s usually located betwen alternator and 2 batteries. Trace output from alternator and see where it goes. There should also be sensing lines from each battery to regulator.

There may also be combining relay between 2 batteries as well.

 

[JohnK]

Question - house batteries type, age and last tested ?
Engine battery age and last tested?

You may have a heads up here from that condition. Lead acid batteries can wreak havoc on a charging system when not optimum. These breathe and evaporate naturally. Some cells degrade too. If they dry too much the plates arc and reduce the cells capacity. Even if you refill it - it will fail.
If you kept it on shore power the coach inverter is constantly charging.

The relay solenoid is problaby the source of the click. It may be a good indication of battery problems.

What I found that I had to make sure my house batteries were optimum along with my engine battery. Both had to hold 13 volts and “each “cell tested good. The voltage sensor usually looks for the lower and minds it till it reaches 12.9 or better. A bad cell may cause an incorrect sensor reading and direct the charge to the lowest or resistant source. A failing cell would lose power affecting the reading pretty quick and flip back.

In short proper service and testing of the batteries is a must and the first thing I would check every spring. Regardless of use.

My class C Ford was acting oddly until I went through both batteries and made sure they were optimum. One of the house batteries had a bad cell and the system damn near boiled it dry and smoked. in part because the sensor read a lower voltage on that side and majority of the recharge was directed that way. Endless charging.
Ergo new house batteries and I don’t keep it on constant shore power when parked.

 

[Chris09]

There's likely a solenoid in the back of your battery compartment that switches between coach and chassis. The idea I think is that if you're not connected to shore power, you will only drain down the coach batteries and still be able to start the engine. Correction - if you're on shore power, It will switch between coach and chassis batteries, alternator will prioritize chassis then switch to coach.
Mine is an 04 and I could also hear the solenoid cycling in no specific pattern. Turned out it was working correctly.

 

[dduelin]

I used to work for West Marine and both the battery combiners and battery isolators we carried were used in 12v systems but it was a long time ago and I'm hazy in the details.

The rule of thumb is with one charge current source (alternator) is to charge multiple battery banks in combination and discharge banks in isolation. A combiner senses individual battery voltage and if there is a combiner in the RV it allows current from the alternator to flow to both batteries until they reach a full charge then diodes or a relay shunts current to the lower of the two batteries until it too is fully charged. If one or the other battery is in poor shape and has lost much or most of it's reserve capacity it reaches 12.9v quickly and the combiner shuts off current to it but the battery quickly falls below the threshold value and the current is switched back on to it. The clicking noise you mentioned would be that relay working overtime.

When the engine is off the combiner opens a break between battery banks so you can't pull the start battery down if you drain the house battery(ies). The batteries should be isolated from each other when no start current is present.

The old fashioned way to do this was with a large battery selector switch. It's large, generally round, orange or red face, and marked Both, 1, 2, Off. Combiners and isolators remove the need for the operator to manually select the charge current path (If the switch was turned OFF with the engine running it could ruin the alternator).

I would follow the wiring from the alternator and look for that manual switch or a black box or device between the alternator and starting battery. It should have large terminals with large wires. One red/positive lead will go to the alternator and one will go to the RVs house battery (ies).

 

[Hondafarian]

This is great information, thanks everyone. John may be onto something by pointing the finger at the batteries. The coach battery is a one year old LiFePo4 in good shape. I'd be concerned about the charging differences in the FLA starting battery and the coach battery but it did this same thing when I had FLA batteries for the coach side.

The starting battery, otoh, is a 10 yr. old costco miracle battery. I keep waiting for it to give up the ghost but it always starts strong even after not starting for weeks on end. At rest it sits at just barely 12 volts which is why it's a miracle it can start that big engine at all. How do you test a battery for a bad cell? Does it require a load tester? I wouldn't want to replace the miracle battery unless that really is the culprit. Or is just the fact it sits at 12v an indication of it's demise?

 

[dduelin]

This is great information, thanks everyone. John may be onto something by pointing the finger at the batteries. The coach battery is a one year old LiFePo4 in good shape. I'd be concerned about the charging differences in the FLA starting battery and the coach battery but it did this same thing when I had FLA batteries for the coach side.

The starting battery, otoh, is a 10 yr. old costco miracle battery. I keep waiting for it to give up the ghost but it always starts strong even after not starting for weeks on end. At rest it sits at just barely 12 volts which is why it's a miracle it can start that big engine at all. How do you test a battery for a bad cell? Does it require a load tester? I wouldn't want to replace the miracle battery unless that really is the culprit. Or is just the fact it sits at 12v an indication of it's demise?

Are you sure that the two banks are separated at start, maybe the brand new coach battery is feeding the system, making that start battery last forever. A 12 V battery showing 12.0 is for all intention purposes dead. If you don’t have a load tester and you wanted to definitive test just drive the coach to an auto parts store that offers alternator and battery checks. They’ll do it in the parking lot while you watch.

 

[DannoXYZ]

Disconnect coach battery and see if 12.0v Costco miracle battery still starts engine.

 

[JohnK]

I’d agree try starting without house battery
Some coaches have a dead man switch that over rides the solenoid to get power to the engine or to the generator. Others some how run the two sources in parallel - so you might be getting assist from the house batteries.


Couple of ways to test the lead acid battery is to meter it disconnected from the chassis. Pull the negative side. If the meter reads steady with no fall off it should be good but should read at least 12.8. If it falls off it’s bad.
Hydrometer tests the cell for correct ph they should all be the same.

Life po batteries have built in circuitry that protects the battery. They shut down if to high or low voltage. If your inverter panel shows the house battery at 13.3. The battery is charged optimum.

I’ve heard some RV owners having issues with them, as the house charger might not be programmed for this type of battery and the

MyCclass is 10 yes old and it’s house charger is a little more modern circuitry - the LifePo’s I have two 12volt these handle everything ok and the only thing is I have to keep them from weather below 20’ f. Exposure for long periods and do not charge them when this cold.
That’s why I test both every year - I figure my engine battery will fail first.

 

[DannoXYZ]

LiFePO4 battery’s full-charge state of 14,4v is higher than lead-acid’s 13,8v. So they’ll always be slightly under-charged. That’s Ok as that will extend their lifespan.

 

[Chris09]

Or you could measure DCV across the coach batteries and see if the voltage drops down while cranking the engine.

 

[Hondafarian]

All great ideas, thank you. I'm all but positive (pun alert) the starter is not drawing on the house battery. The volt meter for the house battery doesn't drop when I crank the engine. Also, I found out the hard way the new LiFePo4 battery is very sensitive to big draws and the BMS shuts it down immediately. I know this because when I try to start the generator, which draws off the house batt., the battery shuts down. The work around has been to have the engine running while starting the generator which allows enough electrons running around to not shut down the house battery.

Unfortunately the moho is stored 20 miles away so I don't get to it as often as practical but plan to run up there tomorrow after purchasing a hydrometer and run all these tests as well as tracing down all the components involved in the DC charging process.

 

[Hondafarian]

Nabbed a hydrometer then rode the ST up the mountain pass on Saturday to diagnose the moho. I cleaned up the starter battery, added a bit of distilled water to several cells, then checked all the cells with the hydrometer. All but one cell was in poor condition. (1225?) One was in fair condition (1175? I may have reversed those numbers) The battery sat at 11.8 volts. Inexplicably, as always, it started the engine right up and without drawing on the house battery. I let the engine idle for appox. 15 minutes. It started charging the battery at 14.6v then slowly dropped down to 14.3v. It never once switched over to charging the house battery, which was completely charged as there's a solar panel on the roof. After shutting off the engine, the battery showed 12.6 v then slowly began to drop to 12.2v when I last checked it before leaving. The sky was threatening rain I didn't want to get stuck in it on the bike.

In other words, I couldn't replicate the problem it's had in the past. I'm stumped. Could just cleaning up the battery and adding a smidge of water make that much of a difference?

 

[JohnK]

Filling the cells up will improve the charging process. However you may have a damaged cell that was to dry it would resist charge and arc. Damage would be indicative to falling voltage after you charged it. All 6 cells should test close. May have to replace that soon or keep a battery tender on it, solar or powered

Definitely an annual maintenance to check batteries. If not bi annually.