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Topic: Mysterious voltage on Engine Batteries (Read 536 times) previous topic - next topic

Mysterious voltage on Engine Batteries

In preparation for replacing my two Optima yellow tops with three Optima red tops, I removed the yellow tops from the RV. After more than a day, they still both showed more than 13 volts DC. I expected any surface charge to have dissipated by then. Why are they reading so high? After charging my three red tops, they quickly went to about 12.7 volts and slowly decreased from there.
 
One of the reasons for replacing the engine batteries was because starting the engine quickly pulled the voltage very low (6 - 8 volts), from which they soon recovered. I thought the trouble was with the batteries. Now I am wondering if something else is drawing the voltage during engine cranking. Anybody got any ideas about what else I should investigate before using my new red tops to avoid ruing them? (Although the initial testing of my old yellow tops does not look like they really needed replacing.)
 
And then I have the final mystery: After removing the yellow tops, I noticed the voltage reading on my two plugin DC meters on my dash. The coach batteries read about 13.4 volts (The  Magnum display showed 13.7 volt float charge). The meter for the chassis batteries read 13.2 volts, without any batteries in the RV! The only thing that comes close to making sense is that I still have my Echo Charger hooked up; is this what is causing my "ghost" voltage? Is this going to cause a problem hooking up my new battery cables and bus bars?
 
Any advice or explanations gladly accepted.
 
Thanks,
 
Trent

Trent and Jean Eyler
2000  U295  4003  WTFE  ISC  350
Build#5603 MC#17385

Re: Mysterious voltage on Engine Batteries

Reply #1 –
Quote from: TulsaTrent –
I expected any surface charge to have dissipated by then. Why are they reading so high?

Test the old batteries with a load tester - see how they do.
100 Amp 6/12V Battery Load Tester
500 Amp Carbon Pile Load Tester




The only thing that comes close to making sense is that I still have my Echo Charger hooked up; is this what is causing my "ghost" voltage?

Disconnect/turn off the Echo Charger, then see what your meter reads?
 
1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Surely, no age has been more fraught with insecurity than our own present time."

Re: Mysterious voltage on Engine Batteries

Reply #2 –
Yes the Echo Charger would be trying to put voltage to the batteries even if no batteries were hooked up in the coach
previous 1984 35 ft ORED 250 HP 3208 Cat       
previous 1998 40 ft U295 CAI 325 hp Cummins
previous 2003 40 Ft u320 build #6140 450 Cummins M11.                                                         
1999 Mazda Miata
Ron, Nancy, Tipper the cat, Max The dog
1997 U 270 36 ft build number 5174 8.3 Cummins

Re: Mysterious voltage on Engine Batteries

Reply #3 –
Can you take the two yellow tops to a battery shop and have them load tested to find out what condition they are in?
John Duld
1995 U320C SE 40'

Re: Mysterious voltage on Engine Batteries

Reply #4 –
I bought a "cheap" battery tester that has done a good job of testing batteries for me. With the tester, I have identified a couple of Red Tops that have failed. Good batteries have registered "cranking amps" on the meter that are comparable to published numbers for the batteries. Bad batteries have show good resting voltages, but failures on the "load test." The "weenie little" test can't put much "real" load on the battery, but it does seem to provide a sufficiently valid measurement to classify a battery as good or bad.
You may want to buy a cheap battery tester (or borrow a good one) and test the batteries. Your local auto supply will probably test batteries for free. Also, clean all the connections you can find. Clean the surfaces on the cable lugs, as well as terminals and clamps.

The last time I had a serious lack of voltage for starting, the culprit was a dirty connection between the lug on the ground cable and the battery post clamp. Everything looked good. I had pulled clamps, and cleaned posts and clamps. I finally isolated the problem by measuring the voltage across the connection. With any significant load (key on and running lights and headlights), there was a significant voltage drop between the lead battery clamp and the copper lug on the cable. I used sandpaper to clean the lug and the area of the clamp that mates with the lug. We started up and set out on our adventure.

I also added additional 4/0 cables from batteries (+) to starter (+), from starter (-) to chassis, and from batteries (-) to chassis. The improvement in cranking speed is significant.

I found a "phantom voltage" on my coach when I was installing an automatic generator starter. With the coach unplugged and ALL batteries disconnected, I found 8.5 volts on some wires in the dashboard. The voltage dropped to zero when I put a blanket over the single solar panel on the roof.  :facepalm:
J D Stevens
1997 U295 CAI 36' Build #5085
2002 Subaru Outback
Motorcade 16869
Bellville, TX

Re: Mysterious voltage on Engine Batteries

Reply #5 –
Quote from: TulsaTrent –
In preparation for replacing my two Optima yellow tops with three Optima red tops, I removed the yellow tops from the RV. After more than a day, they still both showed more than 13 volts DC. I expected any surface charge to have dissipated by then. Why are they reading so high?...

Trent - Just replaced my 10 year old original Yellowtop D31T starting batteries.  I removed the batteries and took them to Pep Boys and had them tested.  While the surface charge was 13+ all 3 load tested at much less than half their cranking capacity.
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Mark & Mary Benko
Former coach: 2005 U295 3823
Jeep Cherokee, Honda Fit