Different voltages when batteries connected

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bongojoe
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Different voltages when batteries connected

Post by bongojoe » Mon Jun 03, 2019 10:27 pm

I have a weird issue I can't make any sense of.

When my starter battery is connected (in parallel) to my leisure battery, I get quite different voltage readings from the terminals on each. With the engine running, the SB is 13.8v while the LB is around 12.9v. With the engine off, the LB reads about 0.5v less.

As I understand the world of electrics, both should read the same voltage regardless of the charge state of either. Both are under the bonnet (so only a few feet apart) and the connecting cable is fairly beefy. I have a 30A fuse at each end, neither are blown.

I have a voltage sensing relay and wondered if something was up with that but I tested it and it appeared okay, then I took it out of the picture and connected the two batteries directly (with a 30A fuse at each end). Readings were the same.

The SB is a relatively new Bosch, the LB is a 120Ah AGM battery which is less than a year old. I've had my fridge running for almost 24 hours off the LB so I'm sure it's holding a charge.

Am I completely misunderstanding the fundamentals of electricity or is there something weird going on?
1999 2WD 2.5L Diesel AFT
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g8dhe
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Re: Different voltages when batteries connected

Post by g8dhe » Tue Jun 04, 2019 10:07 am

Several things here, yes the voltage across the two batteries when the SC relay is operated i.e. charging should be very close. Given the difference I would suggest that the two batteries are not yet connected! Check the the fuses with a meter, make sure the contacts are clean, after that suspect the relay itself, you should be able to hear the relay click at least when the voltage on the SB exceeds the switching voltage, sometimes this might be higher than expected i.e. closer to 14 volts than the float voltage, some relays have an indicator LED to show when operated, try revving the engine a bit until the alternator is running above 14 volts, if its hasn't switched by then the relay is faulty.
Geoff
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bongojoe
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Re: Different voltages when batteries connected

Post by bongojoe » Tue Jun 04, 2019 10:13 am

Thanks Geoff. I disconnected the VSR on the LB side and tested voltage before and after starting the engine—0v before and 13.8v after, so it does seem to be switching on. I also checked the fuses visually and with the meter and both are fine.

Also wired the SB directly to the LB bypassing the VSR (with fuses still in place) and the voltage drop was the same as via the VSR.

I'm sure in the past they have read the same voltage whenever the engine is running so I'll keep investigating and see if I can figure out what's going on. I'll disconnect everything else connected to the LB and see if that makes any difference.
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g8dhe
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Re: Different voltages when batteries connected

Post by g8dhe » Tue Jun 04, 2019 10:48 am

In that case check all the cable terminations and mating surfaces as unless you were very heavily charging the LB the voltage should be within 0.1 volts or so merely voltage drop along the cable basically, so with little current flowing they would be identical, and don't forget the earth/chassis connections as well!
Geoff
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