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Old 09-30-2017, 02:55 PM   #1
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**Starter Battery** Blues?

Hey.
Hopefully a somewhat straightforward one here.

Question:
Has anyone had the situation where it appears their *starter battery* was overcharging?

I have an older SMB (1995 vintage) that still has the original SurePower isolator under the hood. I've learned that these are known for having a 0.7volt drop in terms of what's supplied to them and what they actually deliver to the house battery on the other end.



I've had the van for about 3 years now, and have already replaced the starter battery one time (the one that came with the van was pretty old and didn't last that long after I got the van) --- but after only a bit over 2 years, the new starter battery I have in there now was wet on its top the last time I looked under the hood. Tested its resting voltage and it came in at only 11.49 volts.

Starting the van up, I then immediately measured 14.8 volts at the starter battery terminals, and the house battery showed it was receiving 15.0 volts (still not sure how that makes sense...). This settled down in a minute or two to show 14.6 at the starter battery and 14.8 at the house battery. (Side note: how the heck does the house battery get MORE volts than the starter battery, if the SurePower isolator is supposedly creating a 0.7volt drop.....)

SO --- I've started to wonder....
.....if the fact that one alternator does all the charging for both the starter AND house battery can lead to situations where the starter battery receives *too much voltage* while the depleted-from-dry-camping house battery is being recharged.

Does this make sense?
If, after dry camping for a night....the house battery is partially run down/depleted, and thus "thirsty for amps" when you start the van and run the alternator charge circuit.....then won't the voltage regulator on the alternator sense this and raise the output voltage accordingly, to begin to aggressively charge the fairly-deeply-discharged house battery? And...won't this unfortunately subject the nearly-full starter battery to more voltage than it probably wants or needs? And possibly boil it/overcharge it?

As I understand (or misunderstand...) things at the moment, it seems like this fairly-standard RV arrangement of "one alternator, two batteries with very different states of discharge" is a setup that is almost guaranteed to subject the starter battery to overcharging/excessive voltage quite often.

Hope to be set straight here by more enlightened minds than mine.

Thanks / Happy Saturday everybody
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Old 09-30-2017, 05:01 PM   #2
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First, the diode drop is from the alternator to the batteries, so they both should be seeing a drop, Your voltages seem high for you're alternator considering the drop. You might want to check out the terminals on isolator and make sure that they are not shorted, or if your alternator is putting out 15.05 -07 volts

While not perfect. two dissimilar batteries in parallel this is pretty common. In general usage this should not cause you to overcharge your starting battery. The current should go proportionally to the battery that is discharged.

-greg
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Old 09-30-2017, 05:18 PM   #3
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Quote:
Originally Posted by Scalf77 View Post
First, the diode drop is from the alternator to the batteries, so they both should be seeing a drop, Your voltages seem high for you're alternator considering the drop. You might want to check out the terminals on isolator and make sure that they are not shorted, or if your alternator is putting out 15.05 -07 volts

While not perfect. two dissimilar batteries in parallel this is pretty common. In general usage this should not cause you to overcharge your starting battery. The current should go proportionally to the battery that is discharged.

-greg
Hey thanks for the reply Greg.

>> I'll check the alternator output separately to see what that is.
(I assume this is done by testing it directly at the positive output terminal and grounding the test meter to common ground (frame/body of van.) By the way --- these alternators have integrated regulators, do they not?)

If the alternator is putting out 15.05 -07 volts (I am guessing you meant -0.7 volts) as you mentioned --- what would that indicate? Healthy voltage range? Or too much?

If the terminals in the isolator are shorted:
Should I understand that correctly to mean....that if they are indeed shorted, that the house battery and starter battery are both "permanently" linked to each other? They don't de-couple from each other when the engine turns off and alternator current stops flowing? Such that dry camping is effectively running down *both the house AND starter battery*?

Still really curious how the house battery sees more voltage than the starter battery sees. Definitely don't want to "boil" another starter battery either, I'm very keen to understand what's truly going on with this.

Thanks again Greg!
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Old 09-30-2017, 09:27 PM   #4
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I have no idea how you could have higher voltage after the diodes, but make sure you have a solid ground on your meter leads when measuring voltage on each side. Perhaps your ground is not as good when measuring the start battery voltage. Idealy, use the same ground for each measurement. Meanwhile, it sounds like your alternators voltage regulator may be bad, causing overcharging.
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Old 09-30-2017, 09:43 PM   #5
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Mike, a couple of notes:

1. As Greg mentioned, the .7 volt drop applies to BOTH the house and chassis batteries; not just the house battery. Think of it this way: The alternator supplies X volts to the isolator. The isolator eats .7 volts, and then ships X-.7 volts to the chassis battery, and X-.7 volts to the house battery.

2. In your shoes, my next test steps would be as follows:
(Actually, in your shoes--assuming that you are an average size guy--I'd probably be tripping and falling on my face, but that's another issue entirely...)

a. Disconnect everything from the isolator. Check for continuity between the two output terminals on the isolator.

a.1. If you have continuity, then the terminals are shorted, and that isolator is done. End of test. Time to get a new isolator, or move to a separator.

a.2. If you do not have continuity, proceed to Step 2.

2. Reconnect to the isolator. Measure the following voltages. Ensure that you have a quality ground for each test. Variations in your ground quality will produce unreliable results.
- Alternator output terminal to ground.
- Isolator chassis battery post to ground.
- Isolator house battery post to ground.
- Chassis battery positive post to ground.
- House battery positive post to ground.

That should produce some interesting data. If the voltage is the same at both isolator posts, that's good news. If it is .7 volts less than at the alternator, that's also good news. If there is a difference between the voltage at the isolator post and at the corresponding battery, then you have line loss.

With this data set in hand, you should be in a position to start isolating (hehe) the issue.

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Old 09-30-2017, 10:04 PM   #6
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Quote:
Originally Posted by Scalf77 View Post
First, the diode drop is from the alternator to the batteries, so they both should be seeing a drop, Your voltages seem high for you're alternator considering the drop.

So both/all batteries are downstream of the isolator?

So it's alternator-isolator-starting battery/house battery.....??

Can you confirm that it's not alternator-starting batt-isolator-house battery?
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Old 09-30-2017, 10:12 PM   #7
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You need to explain how you are measuring the two batteries, but the difference between batteries is not unusual. The diode drop is between the alternator and batteries not necessarily between batteries. So your expectations would that they would be the same . The difference could be attributed to wire size, dirty terminals, etc. You could also be using different tools. The isolator has a an A (alternator) , 1 (starting battery), 2 (auxiliary battery) terminal. The voltage drop should be between A & 1 and A & 2. Depending on your year it is possible that you have an E terminal.

hope that makes sense. Below is a procedure to test the isolator.


INSTRUCTIONS FOR TESTING A SURE POWER ISOLATOR WITH OHMMETER*:
1. Remove all wires from the isolator.
2. Using a needle movement ohmmeter RX-1 scale or a digital ohmmeter diode scale, hold the Red* probe on the terminal "A" and with
the Black* probe touch terminal #1 and #2, and the "E" terminal for 3A isolators (group 2), and the "R" terminal for (group 3) isolators.
A good isolator will show a current flow from "A" to #1, #2 and "R", and no current flow to "E".
3. Next, hold the Black* probe on the "A" and with the Red* probe touch terminal #1 and #2 (terminal "E" and "R", if used). A good
isolator will allow no current flow from "A" to #1, #2 or "R" and will show current flow from "E" to "A".
4. Hold one probe on the aluminum heat sink, being sure there is contact by scratching through the protective coating. Then touch
with the other probe, terminals "A", #1, #2 (the "E" terminal for 3A isolators [group 2] , the "R" terminal for group 3 isolators). A
good isolator will show no current flow.
5. Colored terminal indicates "E" post on group 2 isolators and "R" terminal on most group 3 isolators.
*On some import ohmmeters, the red and black probes are reversed for these tests.
**If using a digital ohmmeter, a diode scale MUST be used.




greg
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Old 09-30-2017, 10:35 PM   #8
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Thanks everyone....
....this is good stuff to digest and then go test with.

Boywonder -- I thought the same thing you did, that the charging circuit loops were laid out this way:

Loop 1: alternator --> isolator --> house battery
and
Loop 2: alternator --> starter battery.

But.....they're not? Perplexing. (To me at this point in time anyway.)

Still trying to picture why the starter battery would need to be looped into the isolator circuit whatsoever....? Hoping to learn why that is necessary (and is apparently reality.) I am learning that tidbit for the first time this evening. Must admit it sounds odd to me though!
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Old 09-30-2017, 11:17 PM   #9
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Here's a good read on isolators and separators from Greg. Should help make sense of the wiring.

http://www.sportsmobileforum.com/for...ches-7991.html
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Old 09-30-2017, 11:18 PM   #10
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Because it is the diode going to the starter that blocks the current from going to the aux. Each diode allows voltage to flow from alternator to the battery. It is the diode going to the other battery that blocks the current from that battery to the other.

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