Skip to main content ↑ ↓
Topic: Battery detective help needed. (Read 485 times) previous topic - next topic

Battery detective help needed.

When I turn the ignition I'm showing 11.3V. With the boost I'm getting 11.9V. The chassis  batteries themselves are showing 12.59V. The house batteries are 12.79.
With the engine running I'm getting 12.64V in one red top and 14 in the other. Both house batteries are getting 14.28. Both red tops are two years old but have been completely discharged in the past. I had them load tested and were told they were good. I had the alternator and battery isolator replaced a year ago. When plugged into house power it doesn't look like the chassis batteries are charging. If I use a separate charger they'll take a charge.
Is it normal to lose voltage between the battery and the ignition. Is it possible I have a bad battery even though it tested as good when load tested?as always any ideas are appreciated.
Build 5477
99 3602 WTFE U270C

Re: Battery detective help needed.

Reply #1 –
There is SOME voltage loss between batteries and dash gauge-- long runs of "not overly large" wire.

BUT, be careful here.  With ignition turned on (not to start), it is quite likely that you have activated the very large draw intake manifold heater. If that is the case, 11.3/11.9 is not unreasonably.  Yes, if intake manifold heater is not engaged, that is low.

You need to find out why the two red top (assume chassis batteries) read so differently. Something is AIN'T right.

OE, your coach had no means of charging the chassis batteries from shore power or generator.  Most have installed "work arounds".
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Battery detective help needed.

Reply #2 –
Quote from: goldbehen –
With the engine running I'm getting 12.64V in one red top and 14 in the other.
You have a bad connection somewhere. Even if one red top is bad, they are connected in parallel and with the engine running you should be reading exactly the same voltage on both batteries which is actually the charge voltage from the alternator.
1994 U280, Build 4490
Deming, NM.

Re: Battery detective help needed.

Reply #3 –
Maybe I should swap the red top positions to see you if I get the same reading. If something feeding both batteries wasn't right wouldn't I see a abnormal reading in both batteries?currently they're wired positive to positive and negative to negative. Not sure if that's in series or parallel.
Build 5477
99 3602 WTFE U270C

Re: Battery detective help needed.

Reply #4 –
That is (correctly) IN PARALLEL.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Battery detective help needed.

Reply #5 –
Make sure the main neg cable from the start batteries comes off one battery and the main pos cable comes off the other battery.  It wouldn't hurt to carefully disconnect the start battery cables and clean every connection and then reassemble. 

With just two batteries rotating them won't make any difference.  With three it does.
Roger Engdahl and Susan Green
2001 U320 3610 #5879 (Home2) - 2014 Jeep Cherokee or 2018 F150
Hastings, MN

Re: Battery detective help needed.

Reply #6 –
I cleaned the terminals and made sure everything is snug. I'll try another cable to see if it makes a difference. Also I thought when the inverter was set to share power I was charging my chassis batteries. Is that not the case?If not my work around is to separately charge them.
Build 5477
99 3602 WTFE U270C

Re: Battery detective help needed.

Reply #7 –
If the red tops went totally flat they are dead.  Killed two sets the same way.  Dead is dead
"Riding and rejoicing"
Bob
1997 U320 40' Mid entry, build 5132,  wtbi ce27, 4th owner
2007 Solara convertible
2 prodeco tech outlaw ss electric bikes

1095 watts solar
08 Ls 460 and a sc430
2000 Ford F-250 superduty 4x4

Re: Battery detective help needed.

Reply #8 –
Quote from: goldbehen –
I cleaned the terminals and made sure everything is snug. I'll try another cable to see if it makes a difference. Also I thought when the inverter was set to share power I was charging my chassis batteries. Is that not the case?If not my work around is to separately charge them.

It's hard to tell from a distance but if both of your batteries were at a low state of charge, the first one in the pair may have been hogging all of the current.  Only time will tell.

A good technique for quickly checking the patency of connections is this:  With current flowing, test  across the connection with a volt meter.  Any voltage indicates loss in that connection.  Remember, with 12 VDC systems every one-tenth of a volt is important. 

P.S. I use the boost switch to keep my chassis battery charged.  While they all look like Ford starter relays, some are designed to be on all the time.
1992 Foretravel Grand Villa
U225 SBID Build No. 4134
1986 Rockwood Driftwood
1968 S.I.A.T.A. Spring
1962 Studebaker Lark
1986 Honda VF700C
1983 Honda VF750C
Charlie, the Dog was broken out of jail 24 Oct 2023
N1RPN
AA1OH (H)e who must be obeyed.

Re: Battery detective help needed.

Reply #9 –
Quote from: goldbehen –
Also I thought when the inverter was set to share power I was charging my chassis batteries. Is that not the case?
On most inverter/chargers "Power Share" does not mean "share the battery charging power between all the batteries"

"Power Share" usually means limiting the AC power consumed by the battery charger so as to avoid exceeding the breaker rating on the shore power pole.  See example below:

"So in our scenario, if you are on a 30 amp incoming power source, and you set the combi-inverter/converters power share or incoming breaker setting down to say 15 amps or lower instead of the default setting of 30 or 50 amps, the charger will limit the amount 110VAC amperage it will pull from the incoming power supply and slow the charger rate down and thereby allow it to "share" some of the available 110VAC with your other power needs inside the RV. It will just take longer to charge the batteries at the lower rate."
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
"Nature abhors a vacuum"