Correct use of batteries

rwidman said:

BTW: That would be "AGM", not "AMG".

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JD *DOES* drive a Mercedes, so we should excuse the Freudian slip.

Major difference of opinion here so who is right Rick or Ron.

Does the low battery discharge the charged batterie.

Or does the two combined act as a more charged battery.???

SD

skipperdude said:

Or does the two combined act as a more charged battery.???

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That was not the statement. Look at how current flows through parallel resistance in a circuit.

But, there is more power available in pair of batteries if one is half charged and the other fully charged than if one is "dead" and the other fully charged.

As I understood Rwidman. When two batteries are combined the less charged battery will draw down the charged battery.

I thought the implication was this happens instantaneously. Therefore combining batteries is pointless. Am I right or wrong.

Rick. What if both batteries are partially charged?

SD

Combining batteries is not pointless. If it were we would not have a group of cells combined to form a battery.

If both batteries are partially charged then you have less power available than if they were both fully charged, that's all.

If the batteries were in series it would be different.

Say you need 12v to the starter and had two 6v batteries in series. If one were low (or dead) the low battery would adversely affect the other when trying to use because the voltage of the combine would be lower than optimum and the starter would turn too slowly to start.

This isn't the case in a parallel circuit. While the voltage is the same (assuming they are all generally the same voltage to start), the amperage supply is deeper, and thus will provide increase current to the load. Yes, the voltage would be lower, but only to the average of the two voltages and not the sum. (Did I say that right? Been a long time since college.)

If I understand it correctly, the big deal with mixing dead batteries and good ones (or having a dead-ish one among your cluster) is during the state of charging.

Now I am totally confused.

If you have two 12 volt batteries. One is say 11.5 volts the other is at 10 volts.

If you combine the two what do you get?

I know you will not get more than 12 volts. But will it give you any extra juice to start your engine?

Sd

Boy, you're testing the old brain cells today... It depends on the internal resistance of the lower voltage battery, but generally it will be closer to 11v. What is happening is the current from the higher source is driving into the lower one. Joining two batteries of different voltages would cause a current to flow between the batteries as follows.. I = (Vbatt2 - Vbatt1) / (Rbatt1+Rbatt2)

skipperdude said:

Now I am totally confused.

If you have two 12 volt batteries. One is say 11.5 volts the other is at 10 volts.

If you combine the two what do you get?

I know you will not get more than 12 volts. But will it give you any extra juice to start your engine?

Sd

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I think of it this way: Voltage differential is synonymous with the size of a water pipe. Current is the water flowing through the pipe. Undersized cabling is like having ridges inside the pipe causing increased resistance to the flow of the water. If the voltage is high (big pipe) and the cables are adequately sized (smooth inner walls of the pipe), then the current flows with less resistance. That's why our alternators put out 14V (+/-) for a 12V system, so current will flow from the higher voltage alternator to the lower voltage battery.

If you have batts with 11.5V and 10V, you've really got 2 low batts. A fully charged battery in a steady state should read 12.6V. The 10V batt may have a bad cell or 2 cells shorted out causing the low voltage. Optimally, each cell should provide 2.1V. If one is dead, you'll get 10.5V at best. But let's look at the situation with these numbers.

If you put a voltmeter on each batts when separated, you'd see 11.5 and 10V. The instant you combine them, you'd start to see the 11.5 gradually decrease and the 10V may or may not slowly increase, depending on it's condition. The 11.5V decreases because the 10V batt places a load on it and the current flows from high voltage to low voltage.

If the 10V batt is your start batt, it's too low to start the engine. By placing the switch to "ALL", you allow the better 11.5V batt to assist in the process. But it must still flow "through" the high resistance of the 10V battery to get to the starter. It would do a better job of starting if the 11.5 could provide the start power without the 10V batt in the circuit, but few if any of us have redundant cabling to allow is to completely bypass the bad batt without recabling or use of jumper cables.

rwidman said:

There are many "maintenance free" flooded batteries. Many cars and trucks come with them. "Maintenance Prevented" might be a better term but they are out there.

Car Battery Maintenance: To Water or Not to Water?

BTW: That would be "AGM", not "AMG".

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Wow what a bunch of legal speak. Who wrote that Congress? " "But in a properly designed, voltage-regulated system, water loss should not cause maintenance-free batteries to fail," said Mr. Hart." Unfortunately this is not the case in most boats. You can't stick them in there and forget them. Had a friend at the last marina who was going crazy because when he took the boat away from the dock nothing worked. I asked if he had checked the water in the batteries and he said he didn't have too because they were "maintenance free". I checked they were dry and he replaced them and all was well.

One more thing. When the battery dies in a car or truck you pick up your phone and call. Someone will come get you and within an hour or so you are on your way. Boat, not so much.

Thanks about the AGM and AMG. I know but I'm a little slow some days. The front spoiler on my MB is low enough I don't need the air dams the AMG's have.

I didn't mean to imply that "maintenance free" sealed flooded cell batteries are good, I just felt it should be pointed out that they are pretty common.