# Charge 2 x 6V on 12V Charger

Started by December 18, 2011
```On Wed, 21 Dec 2011 13:42:21 -0500, ehsjr <ehsjr@nospamverizon.net>
wrote:

>Winston wrote:
>> ehsjr wrote:
>>
>>> Winston wrote:
>>
>>
>> (...)
>>
>>> Whatever "much" means in your statement "I don't think
>>> ESR changes much with state of charge."
>>
>>
>> In a chart referenced below, ESR does increase by 71%
>> from full charge to well flattened.  At it's highest,
>>
>> (...)
>>
>>> 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.
>>>
>>>> I think that my 'equivalent resistance'
>>>> of a relatively discharged battery looks
>>>> lower than the 'equivalent resistance'
>>>> of a fully charged battery. 'Equivalent
>>>> resistance' is just ESR plus (the battery
>>>> voltage divided by the charge current).
>>>
>>>
>>> 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.
>>
>>
>> http://batteryuniversity.com/learn/article/how_does_internal_resistance_affect_performance
>>
>>
>> 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.
>
>Right. The series internal R is not linear, which you can
>see in the graph.  What the graph does not show is that it
>varies with both state of charge and the discharge current
>used to make the internal resistance measurement. The higher
>the discharge current, the greater the internal R.  With
>small enough batteries, you'll never get 120 amps.  However,
>if you examine a fully charged car battery, you'll find it
>can produce *well* in excess of 120 amps, briefly.
>
>>
>> That is what I hoped to do by adding my
>> "Equivalent Resistance" to ESR.
>>
>
>That's probably as good an approach as any to accommodate
>the non-linearity of the beasts.  The numbers change
>with state of charge, temperature, the amount of current
>going out (or in), electrolyte specific gravity (which
>itself changes with charge/discharge).
>
>
>> (...)
>>
>>> As the battery current in opposition
>>> to the charger current increases, the current from the charger
>>> to the battery decreases.
>>
>>
>> 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.
>
>Just another way of looking at the same thing.  You could
>even look at it as electrons repelling each other.  As
>more and more electrons are added to a plate, they exert
>a greater force against the addition of even more electrons.
>
>
>>
>> You appear to agree because you say:
>> "The charger "wins" because it is at a higher voltage."
>
>Yes.
>
>>
>> (...)
>>
>>> My guess is that they are already in series in whatever
>>> device they are part of.
>>
>>
>> I didn't get that from what the OP stated.
>>
>>  > 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.
>>
>>
>> If capacity and state of charge are matched,
>> one can use SLA batteries in series or parallel.
>>
>>>> I have two 5 Ah SLA batteries of the same lot number
>>>> from the same manufacturer. They were both used
>>>> in burglar alarms. One of the batteries measures
>>>> 6.36 V and the other 6.05 V because only the second
>>>> alarm was turned on. Can I expect these batteries
>>>> to perform to spec if I use them in series? No.
>>>
>>>
>>> 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.
>>
>>
>> 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.
>
>Right. I guess that in a purely theoretical world two SLA's
>could be 100% identical and age at the exact same rate.
>However, my experience supports your statement - in the
>real world it certainly appears they age at different rates.
>
>>
>>> 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.)
>>
>>
>> Could one double the service life of these batteries
>> by re-hydrating them say, annually?
>
>I've done extensive experimenting attempting to desulfate
>SLAs taken from the emergency lights and from UPSs. I
>tried adding water to only one of them. The desulfating
>produced some small improvement in some batteries, but
>not enough to justify the time invested, nor to produce
>a battery useful for the application.  But the learning
>was worth the time. :-)  The water experiment was only
>for an experiment - I would not use an SLA that had been
>opened.  I should also note that batteries used in emergency
>lighting are a really poor candidate for desulfating.  The
>very nature of the application should prevent sulfation in
>the first place, as these batteries are under constant
>charge.
>
>I'll have to take a look at what they say in the link you
>provided - but I still won't use an SLA that's been opened!
>
>Ed
>
>
>>
>>
>> Thought-provoking, wot?  :)
>>
>> --Winston

It occurs to me that you don't need to know much about battery
chemistry/behavior... just devise a shunt scheme that diverts all
current (but say C/10) from the battery with the higher voltage until
they come up equal.

...Jim Thompson
--
| James E.Thompson, CTO                            |    mens     |
| 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.
```
```Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:
> On Tue, 20 Dec 2011 20:41:45 -0800, Winston <Winston@BigBrother.net>
> wrote:
>
>> Jim Thompson wrote:
>>
>> (...)
>>
>>> 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 :-)
>>
>> 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
>
> Would I josh ?:-)
>
> How does the voltage regulator in your car work?  It tailors the
> current to match a voltage profile versus temperature.
>
> Just make a shunt that works oppositely.
>
>                                         ...Jim Thompson

I suspect a high accuracy, sharp kneepoint, current limited zener (or
equivalent, of course) would be all that you need.
```
```On Wed, 21 Dec 2011 20:21:49 GMT, Ralph Barone

>Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:
>> On Tue, 20 Dec 2011 20:41:45 -0800, Winston <Winston@BigBrother.net>
>> wrote:
>>
>>> Jim Thompson wrote:
>>>
>>> (...)
>>>
>>>> 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 :-)
>>>
>>> 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
>>
>> Would I josh ?:-)
>>
>> How does the voltage regulator in your car work?  It tailors the
>> current to match a voltage profile versus temperature.
>>
>> Just make a shunt that works oppositely.
>>
>>                                         ...Jim Thompson
>
>I suspect a high accuracy, sharp kneepoint, current limited zener (or
>equivalent, of course) would be all that you need.

Something like that, but accomplished with comparators, and other
"smarts".  You have to track temperature.

...Jim Thompson
--
| James E.Thompson, CTO                            |    mens     |
| 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"
>>>
>>>>> Let's say we have two lead-acid batteries,
>>>>
>>>>
>>>>** ROTFL !!
>>>>
>>>> A " thought experiment " involving SLAs !!
>>>>
>>>> Einstein would be pissing himself.
>>>
>>> ---
>>> Why is that?
>>
>>
>>**  FFS  -  get real.
>

I am laughing at your pig ignorance.

...  Phil

```
```On Dec 21, 2:21=A0pm, Winston <Wins...@BigBrother.net> wrote:
> NT wrote:
> > On Dec 21, 6:07 am, Winston<Wins...@BigBrother.net> =A0wrote:
>
> (...)
>
> > I was thinking of flooded batteries, but of course 6v units are often
> > SLA. Its not hard to add a drop or 2 of distilled/deionised per cell
> > though.
>
> A user that refuses to balance the charge on two
> batteries (for whatever reason) is likely to
>
> Pull the other one. =A0:)

I certainly cba to equalise the load on 2 batteries running separate
items. Can you? But I've added water to SLAs before, and much more.

> >> We have three choices:
>
> >> 1) Over - charge the top SLA and dry it out in an
> >> =A0 =A0 =A0attempt to get the necessary i x t to the bottom
> >> =A0 =A0 =A0SLA.
>
> > overcharge happens on the first charge cycle only. That much doesnt
> > dry a battery out.
>
> The series batteries are unlikely to equalize
> on the first charge. =A0I don't think they would
> equalize at all. As Ed mentioned yesterday afternoon,
> one of the batteries is likely to fail.
> That would be the one with the lower charge.
>
> >> 2) Leave both batteries disconnected until the
> >> =A0 =A0 =A0charge of the top SLA diminishes to that of the
> >> =A0 =A0 =A0bottom SLA, risking sulfation of both batteries.
>
> > usually not very practically useful
>
> It is risky and uneconomical, though. =A0:)
>
> >> 3) Match the charge on both batteries before
> >> =A0 =A0 =A0connecting them together.
>
> > few end users are willing to.
>
> Are these the same users that pry open an SLA to

few of all end users
```
```Jim Thompson wrote:
> On Wed, 21 Dec 2011 20:21:49 GMT, Ralph Barone
>
>> Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:
>>> On Tue, 20 Dec 2011 20:41:45 -0800, Winston <Winston@BigBrother.net>
>>> wrote:
>>>
>>>> Jim Thompson wrote:
>>>>
>>>> (...)
>>>>
>>>>> 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 :-)
>>>> 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
>>> Would I josh ?:-)
>>>
>>> How does the voltage regulator in your car work?  It tailors the
>>> current to match a voltage profile versus temperature.
>>>
>>> Just make a shunt that works oppositely.
>>>
>>>                                         ...Jim Thompson
>> I suspect a high accuracy, sharp kneepoint, current limited zener (or
>> equivalent, of course) would be all that you need.
>
> Something like that, but accomplished with comparators, and other
> "smarts".  You have to track temperature.
>
>                                         ...Jim Thompson

My 1957 Jaguar XK150 had a 12V, (positive ground), electrical system.
This was made up of two 6V lead-acid batteries, one under the fender of
each front wheel.  These were charged, (and discharged), in series with
no problems.  The only difficulties I experienced were in showing a AAA
tow-truck driver how to hook up for a jump-start. :-)

--
Virg Wall
```
```On Thu, 22 Dec 2011 08:16:12 +1100, "Phil Allison" <phil_a@tpg.com.au>
wrote:

>"John Fields"
>>>>
>>>>>> Let's say we have two lead-acid batteries,
>>>>>
>>>>>
>>>>>** ROTFL !!
>>>>>
>>>>> A " thought experiment " involving SLAs !!
>>>>>
>>>>> Einstein would be pissing himself.
>>>>
>>>> ---
>>>> Why is that?
>>>
>>>
>>>**  FFS  -  get real.
>>
>
>

---
Here's my post, again:

"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."

There seems to be a bone of contention here, so what do you think AE
---

>     I am laughing at your pig ignorance.

---
that have to do with conventional current flow?

--
JF
```
```Ralph Barone wrote:
> Jim Thompson<To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com>  wrote:

(...)

>> Would I josh ?:-)

Look! A Joshasaurus!  :)

>> How does the voltage regulator in your car work?  It tailors the
>> current to match a voltage profile versus temperature.
>>
>> Just make a shunt that works oppositely.
>>
>>                                          ...Jim Thompson
>
> I suspect a high accuracy, sharp kneepoint, current limited zener (or
> equivalent, of course) would be all that you need.

I had the same idea from Jim's comment, except for
the power level.  I suspect the zeners would have to
be rated to easily pass full alternator current
with plenty of headroom in the middle of a Mojave
summer.  :)

The voltage regulator in my late lamented POS
BSA 500 cc single was just a power zener
bolted to the frame in parallel with the battery.

--Winston <-- I'm just saying
```
```VWWall wrote:

(...)

> My 1957 Jaguar XK150 had a 12V, (positive ground), electrical system.
> This was made up of two 6V lead-acid batteries, one under the fender of
> each front wheel. These were charged, (and discharged), in series with
> no problems. The only difficulties I experienced were in showing a AAA
> tow-truck driver how to hook up for a jump-start. :-)

You can bet the batteries had equal state of charge
when they were installed.  Otherwise you would have
an unreliable electrical system on your hands.

No. Wait.

--Winston<-- You in the back. Stop giggling.
```
```NT wrote:
> On Dec 21, 2:21 pm, Winston<Wins...@BigBrother.net>  wrote:

(...)

> I certainly cba to equalise the load on 2 batteries running separate
> items. Can you? But I've added water to SLAs before, and much more.

Yes but you are about as normal as I am.
:)

(...)

>> Are these the same users that pry open an SLA to