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Battery Isolator Lock-Out relay on firewall


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Hi All,

On my 2000 Diplomat firewall, I have a stencil that says "Battery Isolator Lock-Out", and a bunch of wires.

Today I found a photo online of a Diplomat firewall WITH the relay connected! (photo below)

So now I know which wire goes where.

BUT, I don't actually know:

  1. what this particular relay does (functionally)
  2. the relay part number

Anyone have any insight into either question?


- John

John Taylor
2000 Diplomat


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Hi Richard,

That is my guess too. I’m not sure who removed it or why... (I’m the 2nd owner, and the first owner “fixed” a lot of things by unplugging them)

...like “the jacks are down BONG” (in reality, low hydraulic fluid), “...so let’s disconnect the warning BONG...” kinda stuff.

I’ll keep hunting, I think you are on the right scent...

- John


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John, that's always the issue with RVs n things.  As you can see looking at any one post, there are lots of opinions, guesses, and way off the charts ways to repair somthing and many, many.. levels of workmanship by owners and even certified repair presons/shops.

As with anything be educated in it: Buyer Beware !

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The problem on the system can be over taxing the battery charging alternator on the engine while long distance driving.  I my case, the Lithium battery bank for solar panels is so large, it can pull way more energy/amps than the alternator can supply on a single steady charge so, something has to be installed to keep from cooking the alternator and that is an isolator of some sort.  I installed a Li-bim (Lithium battery isolator manager) in place of the old isolator system components.  Basically what it does is allow charging of house Lithiums, chassis lead acid batteries and then gives the alternator a rest (Cooling) these generally operate on a timing system rotating among all 3 stages while you drive. To be clear,  I removed 3 major original pieces, charging 3 wire brick (trickle charge chassis batt. maintainer). (Isolator delay relay delay Diesel) And battery isolator relay.  LiBim replaced battery isolator relay.  My point is: All of these orignal components work well for lead acid or agm batts. Also including shore power charging.  (All components are needed obviously). One more thing, I have found that many of the small black relays on the coach system are the same and available at places like Amazon.  You have to look for small print.  I would be interested to see an expansion of my thoughts on this incase more detail is needed.

Hope this helps




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Hi Greg,

Many thanks for the explanation. It all makes sense.

I too replaced those components for those same reasons (slightly different components, same idea )  so it makes sense why the missing relay seems to have no effect.

Multiple charging sources into multiple battery banks with differing chemistries is challenging. The low internal resistance of lithium banks means monitoring the charging sources so they aren’t stressed and won’t melt.

Greg, Question: do you still run a mix of lead acid/agm with your lithium banks?

I’ve thought about eliminating the entire lead acid portion for a lot of reasons (huge weight reduction, much simpler system), but have held off for two reasons:

(1) my current LFP chemistry does not like cold, and I live in Colorado Springs at 7200’ above sea level. I keep the house banks heated using battery heating pads. Effective, but a hack, and also subject to failure.

(2) starting the main engine of of a LFP bank can be a problem, even at proper temperatures. Normal current discharge for even the most egregious loads (in my case, the A/C and microwave) is still fractional C (under 240A at 12V). Starting circuit would have to be much higher, if only for the cranking time (1000A at 12V for up to 15 seconds I’m guessing). That makes fusing the starting and house circuits into two paths, and complicates life.

BUT, I could see adding an additional lithium bank (dual use - house and starting bank), but using is in stead of LiFePO4(LFP), adding a  LiFeYPO4 (LFYP) bank. The yttrium allows lower temperature charging and discharging, exactly what I need. And, I could simplify my charging setup, as they have the same profiles.

I would keep the existing LFP banks (sunk cost, work fine), but add another dual use bank of LYFP that could be fused for time limited high current discharge connected to the starter circuit (main engine and genset). 

The LYFP bank, on its own BMS, etc, would also be useful in extreme cold situations (albeit at a much reduced capacity) until the main LFP banks were within charging/use temperature range.

Greg, (and others reading this) Does this make sense, or am I over thinking it, and there is a better way?

Thanks again.

- John

Edited by StellaTariche
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  • 4 weeks later...

I am using LFP for the house and still have the huge lead acid batt. for the chassis.  I know battleborn has recently introduced a heated version of Lithium batt. but, I haven't studied them or any results/opinions yet. Seems B-born just added a heater to each batt.   I have no knowledge of the LFYP batts. as of yet.     

After about 5 years of trouble free use of LA chassis batt. I am very happy and it will be some time before I would consider making a change.  (zero corrosion and tiny amount of maintenance & water).



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ooohhh, I just started looking into replacing my two big starting batteries with a super capacitor. 

Very interesting.  Many truckers are doing it.

Maxwell makes a single super capacitor which will put out 1800 CCA for 10 seconds, with multiple repeats allowed before recharging the super capacitor is needed.

And, it is in parallel with your main house bank. They claim that a house bank with 7v (really dead) is enough to charge the super capacitor in about 15 min.

$800 form multiple sources.

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