Different voltages when batteries connected
Posted: 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?
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?