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Charge 2 x 6V on 12V Charger

Started by Olin Frank December 18, 2011
NT wrote:
> On Dec 20, 9:35 pm, Winston<Wins...@BigBrother.net> wrote: >> NT wrote: >>> On Dec 20, 2:53 pm, Winston<Wins...@BigBrother.net> wrote:
(...)
>>>> Which is unfortunate because, in our thought >>>> experiment, the top battery requires no more >>>> I or T and the bottom battery requires nearly >>>> a full dose of both I and T if it is to perform >>>> to spec and indeed, survive over the long haul. >> >>> /If/ the batteries are discharged asymmetrically, one will eventually >>> need water adding. That's all. Normal maintenance.
SLAs are sealed. Folks who *do* add water to them are not engaging in 'normal maintenance'. (I confess that I add water to my 'sealed' car batteries and get excellent service from them. No one calls me 'normal', though.) :)
>> I don't believe the 'bottom' battery would survive. >> Stored in a consistently discharged state, it would >> sulfate and fail. > > Since it would get i x t at every charge, it would not be stored in a > consistently discharged state.
It is in series with the 'top' battery which is very nearly 100% charged. The ratio of charge on each battery isn't going to change much. We have three choices: 1) Over - charge the top SLA and dry it out in an attempt to get the necessary i x t to the bottom SLA. 2) Leave both batteries disconnected until the charge of the top SLA diminishes to that of the bottom SLA, risking sulfation of both batteries. 3) Match the charge on both batteries before connecting them together. I know what I would do. :) --Winston
On Dec 21, 6:07=A0am, Winston <Wins...@BigBrother.net> wrote:
> NT wrote: > > On Dec 20, 9:35 pm, Winston<Wins...@BigBrother.net> =A0wrote: > >> NT wrote: > >>> On Dec 20, 2:53 pm, Winston<Wins...@BigBrother.net> =A0 =A0wrote: > > (...) > > >>>> Which is unfortunate because, in our thought > >>>> experiment, the top battery requires no more > >>>> I or T and the bottom battery requires nearly > >>>> a full dose of both I and T if it is to perform > >>>> to spec and indeed, survive over the long haul. > > >>> /If/ the batteries are discharged asymmetrically, one will eventually > >>> need water adding. That's all. Normal maintenance. > > SLAs are sealed. Folks who *do* add water to > them are not engaging in 'normal maintenance'. > > (I confess that I add water to my 'sealed' car > =A0 batteries and get excellent service from them. > > =A0 No one calls me 'normal', though.) =A0 :)
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.
> >> I don't believe the 'bottom' battery would survive. > >> Stored in a consistently discharged state, it would > >> sulfate and fail. > > > Since it would get i x t at every charge, it would not be stored in a > > consistently discharged state. > > It is in series with the 'top' battery which is > very nearly 100% charged. The ratio of charge on > each battery isn't going to change much.
Yes it is, see below
> We have three choices: > > 1) Over - charge the top SLA and dry it out in an > =A0 =A0 attempt to get the necessary i x t to the bottom > =A0 =A0 SLA.
overcharge happens on the first charge cycle only. That much doesnt dry a battery out.
> 2) Leave both batteries disconnected until the > =A0 =A0 charge of the top SLA diminishes to that of the > =A0 =A0 bottom SLA, risking sulfation of both batteries.
usually not very practically useful
> 3) Match the charge on both batteries before > =A0 =A0 connecting them together.
few end users are willing to.
> I know what I would do. =A0:)
NT
On Wed, 21 Dec 2011 12:13:34 +1100, "Phil Allison" <phil_a@tpg.com.au>
wrote:

> >"John Fields" >>> >>>** Cannot work. >>> >>>The terminal voltage will obviously be the same immediately, but very >>>little >>>charging will go on. >> >> --- >> Let's say we have two lead-acid batteries, > > >** ROTFL !! > > A " thought experiment " involving SLAs !! > > Einstein would be pissing himself.
--- Why is that? It's a rather simple experiment which can easily be performed in real life without people in free-falling elevators, so I can't understand why you think Einstein would be pissing himself over its description. -- JF
"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. ... Phil
On Wed, 21 Dec 2011 12:18:41 +1100, "Phil Allison" <phil_a@tpg.com.au>
wrote:

> ><krw@att.bizzzzzzzzzzzz> = psychopath >>> >>>> I think he meant to connect them in parallel, sans charger, until they >>>> leveled off to the same voltage, and then to connect them in series >>>> and charge them. >>>> >>> >>>** Cannot work. >>> >>>The terminal voltage will obviously be the same immediately, but very >>>little >>>charging will go on. >> >> Dummy, one will charge the other until they equalize. > > >** Capacitors do something like that. > > But not SLAs.
--- Why not??? Since they both start off with different voltages and, when they're connected, charge is transferred until the potential difference between them is no longer sufficient to push electrons around, I fail to see - in that sense - why a lead-acid battery is different from a capacitor. -- JF
On Wed, 21 Dec 2011 23:25:34 +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.
--- Instead of epithet, why not support your argument with fact? -- JF
NT wrote:
> On Dec 21, 6:07 am, Winston<Wins...@BigBrother.net> wrote:
(...)
> 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.
http://www.instructables.com/id/Refilling-SLAs-Sealed-Lead-Acid-battery/ A user that refuses to balance the charge on two batteries (for whatever reason) is likely to follow this process? Pull the other one. :) (...)
>> We have three choices: >> >> 1) Over - charge the top SLA and dry it out in an >> attempt to get the necessary i x t to the bottom >> SLA. > > 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. I 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 >> charge of the top SLA diminishes to that of the >> bottom SLA, risking sulfation of both batteries. > > usually not very practically useful
It is risky and uneconomical, though. :)
>> 3) Match the charge on both batteries before >> connecting them together. > > few end users are willing to.
Are these the same users that pry open an SLA to add distilled water? Just asking. :) --Winston
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 -- | 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.
On Wed, 21 Dec 2011 09:47:33 -0700, 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
If there's an SLA Spice model around that actually models charge and discharge characteristics I'll show you how easy it's be. ...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.
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, > though, it is only 15 milliohms! More about this later. > > (...) > >> 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
> > http://www.instructables.com/id/Refilling-SLAs-Sealed-Lead-Acid-battery/ > > Thought-provoking, wot? :) > > --Winston