bp71k5
3/4 ton status
Would a bad battery cause it to charge higher than average? I'm guessing if I measure the post on the alternator directly, it'll be 15V. Should I try measuring voltage at a higher rpm to make sure it's not going over 15.5?
CK5
Register an account today to become a member! Once signed in, you'll be able to participate on this site by adding your own topics and posts, as well as connect with other members.
Why is my battery bubbling?
- Thread starter bp71k5
- Start date
Would a bad battery cause it to charge higher than average? I'm guessing if I measure the post on the alternator directly, it'll be 15V. Should I try measuring voltage at a higher rpm to make sure it's not going over 15.5?
First let me state that the alternators job IS NOT to charge a battery but rather to maintain the charge that is in the battery. If you took for instance a dead battery and then jump start the vehicle you are now forcing the alternator to do something it was never intended to do (to charge). There is a difference between 1-2 volts of "charging" versus 0-18 volts of charging. If you need a better explanation then call me and i'll be glad to explain it over the phone.
bp71k5
3/4 ton status
I understand the issue with trying to charge a dead battery with the alternator. I'm more interested in knowing for sure whether the alternator is still ok before I hook up this new battery. Luckily I found a bunch of battery acid damage that needs fixing before I replace the battery. That gives me a few more hours to check things out.
As long as the voltage is somewhere between 13.5-15.5 volts with the engine running the alternator is good. If the voltage is too low then it can't maintain the charge in the battery and if it's too high it will boil the battery.
only one problem if he is using a AGM battery, they don't really like to see 15v for more than a few minutes.
If it's only a few minutes to top off the battery after a deep discharge from starting the engine, it's OK.
If it stays above the 14.5v for extended periods of time it will kill a AGM battery.
Ask the people where you bought it to confrim.
Regular lead acid battery can take the 15.5v and will only suffer from a little dehydration which you monitor and replenish when needed.
Not so with AGM.
Exactly, AGMs are a little less forgiving, but they are worth it. The key, is the voltage measurement at the battery.
For long battery life, you are concerned with what the battery sees, not what the truck does.
If an accurate meter at the battery never goes over 14.5 for very long, it does not matter to the battery what the gauge in the dash says.
If the gauge in the dash is different from the rest of the readings in the truck with a good meter, I would suspect the gauge in the dash.
However, you start seeing different voltages in different parts of the truck, you need to check your grounds and power connections.
A tenth of a volt is not a problem, but if its a 1/2 volt or better, then you need to check.
For long battery life, you are concerned with what the battery sees, not what the truck does.
If an accurate meter at the battery never goes over 14.5 for very long, it does not matter to the battery what the gauge in the dash says.
If the gauge in the dash is different from the rest of the readings in the truck with a good meter, I would suspect the gauge in the dash.
However, you start seeing different voltages in different parts of the truck, you need to check your grounds and power connections.
A tenth of a volt is not a problem, but if its a 1/2 volt or better, then you need to check.
bp71k5
3/4 ton status
Now you guys have me worried about the 0.2V extra I have. I'm going to hook things back up with the new battery and see where the voltage ends up. If its too high then, I'll have the alternator tested or replaced.
When batteries get old, the internal resistance gets higher, so it has a lower voltage under load and a higher voltage under charge. A new battery will run your headlights for 30 minutes and still be over 12V. An old battery also does a much worse job of filtering, so you get a lot more sinusoidal and "other" noise in the system, which can confuse a simple meter and some of the electronics. I bet the voltages will be a lot more stable after you swap. Just figure out where your dash gauge sits when you get close to 15V and keep an eye one it.
