Charge 2 x 6V on 12V Charger

(...)

Most of the time, we would 'get away' with doing that. Ideally, one would measure the charge on each battery first, to assure they were within a few tens of millivolts of each other before strapping them together.

If not, a C/1 resistor in series would limit the current between them until they were safely within 'paralleling' range.

--Winston

The arithmetic works the same way for the first few milliseconds of charging or discharging at 10 A. R=E/I, in theory.

At a 10 A charge however, the user can be fooled by the higher 'surface charge' voltage measured on the cell, thus overestimating it's 'equivalent resistance'.

There is also the danger of underestimating the 'equivalent resistance' due to the current flow, especially with smaller capacity batteries. Hey, it is just a thought experiment meant to reveal the danger of connecting mismatched batteries.

--Winston

Yep. But Phyllis was too dense to grasp the leveling scheme ;-) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

An amusing thought, make a smart charger that bypasses some charging current around the "better" unit until leveling is accomplished.

Solution left as an exercise for the student :-) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

"John Fields"

** Cannot work.

The terminal voltage will obviously be the same immediately, but very little charging will go on.

... Phil

"Jim Thompson"

** Cos it is bullshit.

Dummy, one will charge the other until they equalize. The down side is that large currents are possible.

--- Let's say we have two lead-acid batteries, one of which has been discharged to 12V, and the other to 10V.

Let's also say that they both have internal resistances of 0.1 ohm:

12V 11V 10V / / / . +-[0.1R]-+-[0.1R]-+ . |+ | |+ .[BA1] Vt [BA2] . | | | . +--------+--------+

Since the batteries are in parallel, Vt will of course be the same for both batteries, but because of the internal resistances of the batteries and the difference in their internal voltages, there will be an EMF of 2 volts impressed across the sum of the internal resistances, 0.2 ohms, which will cause charge to be transferred from BA1 to BA2 at the rate of 10 coulombs per second, which is 10 amperes.

That's quite a bit more than "very little charging", yes?

-- JF

Is Phyllis really that ignorant? Or is she just trying to challenge Larkin for the title ?:-) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

"John Fields"

** ROTFL !!

A " thought experiment " involving SLAs !!

Einstein would be pissing himself.

... Phil

A " thought experiment " involving Phyllis !!

Einstein would be pissing himself.

= psychopath

** Capacitors do something like that.

But not SLAs.

.... Phil

Battery voltage, measured at the battery posts with a DMM, drops significantly when a heavy load is applied, demonstrating the effect of internal resistance.

That voltage drop is significantly lower with a fully charged battery than it is with a discharged battery. It is why you can't start your car when the battery is discharged too far, yet you can read ~12 volts on the DMM when the starter load is not present.

Whatever "much" means in your statement "I don't think ESR changes much with state of charge.", the change in ESR is the difference in being able to start your car with a charged battery, and not being able to start it with a too far discharged battery.

If that were true, there would be no voltage drop when a load is applied to the battery, and the voltage drop would be less with a discharged battery than it would be with a charged battery when. Exactly the opposite is true. Electrically, a battery is an ideal voltage source with a series internal resistance. A discharged battery has a higher internal resistance than a charged battery.

The current starts high and decreases as the battery charges. That's no mystery when you understand a battery in terms of an ideal voltage source in series with an internal resistance.

The battery attempts to push current into the charger, and the charger attempts to push current into the battery. The charger "wins" because it is at a higher voltage. Current flowing into the battery from the charger lowers the internal resistance in the battery. The current that the battery is able to exert against the charging current increases, due to the lowered internal resistance. As the battery current in opposition to the charger current increases, the current from the charger to the battery decreases.

Agreed.

My guess is that they are already in series in whatever device they are part of. Setting my guess aside, there are devices that use 2 6V SLAs in series and charge them in series, so it has been done as a practical matter. Specifically I know of emergency lighting that is configured that way.

Agreed. And in the emergency lighting devices I mentioned above with two identical SLAs installed at the same time, one battery usually fails before the other. I maintain a bunch of those lights at church. The batteries last about

5 years, and I replace the pair when they fail. I find in testing that I can replace one and the other is good. Putting the bad one back in with the new one, the light still fails. (However, I get rid of both old batteries.)

Ed

Conservation of energy and Ohm's law don't work in your universe, huh, Phyllis.

--
Hey, Jim, why not bury the hatchet?

We all make mistakes and the more gently we're corrected the less
likely we are to go Columbine.

I _would_ like a statement from Larkin that he's withdrawing from cluck-cluck-cluck, then I'll bury the hatchet... like the butcher in "Boardwalk" ;-) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

(...)

You are joshing, but I wonder if the electrical characteristics of a relatively discharged battery vs those of a charged battery could be used to that end. Ferinstance, is there a pulse waveform that would be ignored by the charged battery but would tend to charge the 'flatter' battery connected to it in series?

Hrm.

--Winston

Since it would get i x t at every charge, it would not be stored in a consistently discharged state.

NT

(...)

In a chart referenced below, ESR does increase by 71% from full charge to well flattened. At it's highest, though, it is only 15 milliohms! More about this later.

(...)

We see a chart that shows a wheelchair SLA internal resistance as a function of state - of - charge. (Chart titled "Changing Resistance with SoC".)

I see that the internal resistance *does* increase as the charge depletes. I also see that it changes from 8.75 milliohms at full charge up to 15 milliohms with a well flattened battery.

Ohms law says that 11.8 V and 15 milliohms would still allow the battery to source 120 amps into a load requiring 10 V. Clearly this battery is not going to do anything of the sort, so we have to look for an additional series resistance if we are going to create a good arithmetic model.

That is what I hoped to do by adding my "Equivalent Resistance" to ESR.

(...)

That is a way of looking at it, I guess.

I could substitute an electrolytic capacitor for the battery and see that charge current starts high and then diminishes, too. It is because of the diminishing relative voltage, not the increasing current of the capacitor opposing that of the charger, though. I conjecture the same is true for the battery.

You appear to agree because you say: "The charger "wins" because it is at a higher voltage."

(...)

I didn't get that from what the OP stated.

If capacity and state of charge are matched, one can use SLA batteries in series or parallel.

This tends to support my earlier statement about no two batteries aging in exactly the same way and the attendant 'death spiral' when one of the series batteries can no longer charge at the same rate as it's partner.

Could one double the service life of these batteries by re-hydrating them say, annually?

Thought-provoking, wot? :)

--Winston