# Charge 2 x 6V on 12V Charger

Started by December 18, 2011
```NT wrote:
> On Dec 19, 4:39 pm, Winston<Wins...@BigBrother.net>  wrote:
>> NT wrote:

(...)

> We've no idea what the OP's discharge conditions are

Would it be good practice to assume that
the OP's discharge conditions are optimal?

I don't think so.

>> The top battery will have 51% of the charge voltage
>> across it and the bottom battery will have only 49%.
>>
>> If we begin charging them at say 10 A, the battery
>> charged to nearly 100% will get about 3 W more
>> charge power than the relatively discharged battery.
>
> Pdiss is usually  a non-issue

A large fraction of that power is not dissipated
but continues to charge the battery beyond 2.12 V
per cell.  This is a largely useless 'surface charge'
but my point is that the battery does not know when
to stop charging (until the electrolyte boils off
that is).  :)

>> Successive charge and discharge cycles will
>> cause the difference in state - of - charge
>> to widen until eventually, the bottom battery
>> sulfates and fails.
>
> no, when the fuller cells are fully charged, further current only
> charges the less charged ones. Charging equalises.

For sustained, large amounts of charging current, yes.

We 'get away' with a long duration, high
current 'equalizing charge' with wet cell
batteries (and the accelerated conversion of
electrolyte to vapor 'gassing') because we
can carefully top up the cells with distilled
water to keep them from drying out.

We have no way to replenish water in sealed
lead acid batteries, so we don't use an
aggressive equalizing charge on them.

Beyond a threshold of difference of charge,
it's a race between the weak tendency of the
charges to equalize and the strong tendency
of the higher voltage battery to 'hog' the
charging power (in series).

(...)

>> One can equalize the charge between them and connect
>> them in parallel safely.  Many diesel - powered cars had
>> two 12 V batteries in parallel, for example.
>>
>> Here, we see three batteries in parallel for trailer use:
>>
>> http://www.etrailer.com/Merchant2/graphics/00000001/pics/q/u/qu22749_...
>
> Sure, but its not appropriate advice for someone that is discharging
> them separately, and doesnt even know whether they can be connected in
> series or not.

'Blanket' statements can be misleading, with costly consequences.

SLA batteries in good condition, of the same description and age,
that have been operated under identical circumstances
can *generally* be placed in series to form a battery of higher
voltage or in parallel to form a battery of higher current.

Anyone doing so will want to perform 'due diligence' to assure
themselves that they are being safe and will get the
desired electrical characteristics because no two batteries
age in exactly the same way.

--Winston
```
```ehsjr wrote:
> Winston wrote:
>> NT wrote:

(...)

>> Two 6 V SLAs.
>>
>> One is charged to nearly 100% or say 6.3 V.
>> We place it in series with one that is charged
>> to about 35% or say 6.05 V
>>
>> At 10 A discharge we can think of the top battery as
>> a resistor measuring 0.63 ohm and the bottom
>> battery equivalent to a 0.60 ohm resistor.
>>
>> The top battery will have 51% of the charge voltage
>> across it and the bottom battery will have only 49%.
>>
>> If we begin charging them at say 10 A, the battery
>> charged to nearly 100% will get about 3 W more
>> charge power than the relatively discharged battery.
>
> No it won't. The battery that is discharged the most gets the
> most charge power.

In a parallel circuit, yes.
These batteries are in series, though.

> The lower the charge on a battery, the higher
> its internal resistance.

I don't believe that is true. I don't think
ESR changes much with state of charge.

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).

Watch the ammeter next time you charge a
battery and see if charge current starts
out low and then increases as the battery
charges, or starts high and then decreases
with increasing battery charge.

(Then you say:)

> The .63 ohm equivalent top battery will
> receive higher power than the .60 ohm equivalent bottom battery.
> P = I^2 * R, so the top battery gets more.

I agree with your third statement but not your
first or second.

:)

> Seems moot anyway, as the op specified batteries of the same
> brand, capacity and usage.

Note that the OP did not mention state of charge.
One would not place them in series (or parallel)
without being sure they were at the same charge level.

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.

> If one wants to discuss batteries of
> different usage/discharge level/specs/whatever, then it is
> best to independently charge, where the charge can be tailored to
> the individual battery.

I agree. That is the most prudent approach.

It is also moral and correct to match the battery's
state of charge, then charge them in series or
parallel using the proper sequence for the
chemistry in question.

--Winston
```
```"Winston"
>
> Two 6 V SLAs.
>
> One is charged to nearly 100% or say 6.3 V.
> We place it in series with one that is charged
> to about 35% or say 6.05 V
>
> At 10 A discharge

** Should be " charge " -  right ?

...   Phil

```
```"Olin Frank"  wrote in message news:4eede3f9.10529890@news.tpg.com.au...
>
>If I have two 6V SLA's of the same brand, capacity, and past usage,
>can I safely series wire them to charge with my existing 12V charger?
>
>Olin Frank

Generally it is not a big deal. After all, those 6 SLA's are actually 3 2V

In any case the issues is that one or more cells maybe "dead" which can
cause problems. If the cells are functional and you have some way to monitor
the temperature or have a safe location for charging and can deal with
possible destruction of the whole battery then it will be ok.

For example, suppose one cell in a two cell battery has a short. What
happens? The other cell is being charged at twice the voltage(and hence
current) it should be. This will cause the battery to heat up and a possible
explosion, meltdown, fire, etc.  With a proper charge this shouldn't happen
as the full discharge voltage is generally higher than if a dead cell
occurs, e.g.,

Full charge: 4.2V
One cell shorted: < 2.1V

So a decent charger should be able to easily detect a dead cell this case
and refuse to charge.

Understand that most batteries are composed of multiple cells so what you
are doing is not any different than what the manufacturer does except they
has a single container.  In some cases such as LI it maybe a different
story.

If you take any due precaution you should be just fine.

--

```
```On Dec 19, 10:09=A0pm, Winston <Wins...@BigBrother.net> wrote:
> NT wrote:
> > On Dec 19, 4:39 pm, Winston<Wins...@BigBrother.net> =A0wrote:
> >> NT wrote:
>
> (...)
>
> > We've no idea what the OP's discharge conditions are
>
> Would it be good practice to assume that
> the OP's discharge conditions are optimal?
>
> I don't think so.

agreed

> >> The top battery will have 51% of the charge voltage
> >> across it and the bottom battery will have only 49%.
>
> >> If we begin charging them at say 10 A, the battery
> >> charged to nearly 100% will get about 3 W more
> >> charge power than the relatively discharged battery.
>
> > Pdiss is usually =A0a non-issue
>
> A large fraction of that power is not dissipated
> but continues to charge the battery beyond 2.12 V
> per cell. =A0This is a largely useless 'surface charge'
> but my point is that the battery does not know when
> to stop charging (until the electrolyte boils off
> that is). =A0:)

Pdiss is still a nonissue

> >> Successive charge and discharge cycles will
> >> cause the difference in state - of - charge
> >> to widen until eventually, the bottom battery
> >> sulfates and fails.
>
> > no, when the fuller cells are fully charged, further current only
> > charges the less charged ones. Charging equalises.
>
> For sustained, large amounts of charging current, yes.
>
> We 'get away' with a long duration, high
> current 'equalizing charge' with wet cell
> batteries (and the accelerated conversion of
> electrolyte to vapor 'gassing') because we
> can carefully top up the cells with distilled
> water to keep them from drying out.
>
> We have no way to replenish water in sealed
> lead acid batteries, so we don't use an
> aggressive equalizing charge on them.

agreed

> Beyond a threshold of difference of charge,
> it's a race between the weak tendency of the
> charges to equalize and the strong tendency
> of the higher voltage battery to 'hog' the
> charging power (in series).

charging power is a non-issue, charge is what counts, i x t. Both get
the same, inevitably

> (...)
>
> >> One can equalize the charge between them and connect
> >> them in parallel safely. =A0Many diesel - powered cars had
> >> two 12 V batteries in parallel, for example.
>
> >> Here, we see three batteries in parallel for trailer use:
>
> >>http://www.etrailer.com/Merchant2/graphics/00000001/pics/q/u/qu22749_..=
.
>
> > Sure, but its not appropriate advice for someone that is discharging
> > them separately, and doesnt even know whether they can be connected in
> > series or not.
>
> 'Blanket' statements can be misleading, with costly consequences.

Recommendations to connect lead acids in parallel, without the
necessary knoweldge or checks, is liable to lead to a bad result.

> SLA batteries in good condition, of the same description and age,
> that have been operated under identical circumstances
> can *generally* be placed in series to form a battery of higher
> voltage or in parallel to form a battery of higher current.
>
> Anyone doing so will want to perform 'due diligence' to assure
> themselves that they are being safe and will get the
> desired electrical characteristics because no two batteries
> age in exactly the same way.

I doubt the OP will. People don't usually.

NT
```
```NT wrote:
> On Dec 19, 10:09 pm, Winston<Wins...@BigBrother.net>  wrote:

(...)

>> Beyond a threshold of difference of charge,
>> it's a race between the weak tendency of the
>> charges to equalize and the strong tendency
>> of the higher voltage battery to 'hog' the
>> charging power (in series).
>
> charging power is a non-issue,

Not 'absolute charge power', I agree.
'Relative charge power' is the core issue, though.

> charge is what counts, i x t. Both get
> the same, inevitably

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.

It would be much safer to just remove the batteries
from the series string long enough to charge the
'lower' battery independently until it's state
of charge matched that of the upper battery,
as Ed alluded to earlier.

(...)

>> 'Blanket' statements can be misleading, with costly consequences.
>
> Recommendations to connect lead acids in parallel, without the
> necessary knoweldge or checks, is liable to lead to a bad result.

As is true for series - connected batteries.  :)

(...)

>> Anyone doing so will want to perform 'due diligence' to assure
>> themselves that they are being safe and will get the
>> desired electrical characteristics because no two batteries
>> age in exactly the same way.
>
> I doubt the OP will. People don't usually.

Thus the value of 'Plain Applied Technology'.
Our discussion will show up in subsequent searches
and will highlight this important consideration
so folks who want to do it right can continue to
inform themselves.

This is good.

--Winston
```
```On Dec 20, 2:53=A0pm, Winston <Wins...@BigBrother.net> wrote:
> NT wrote:
> > On Dec 19, 10:09 pm, Winston<Wins...@BigBrother.net> =A0wrote:
>
> (...)
>
> >> Beyond a threshold of difference of charge,
> >> it's a race between the weak tendency of the
> >> charges to equalize and the strong tendency
> >> of the higher voltage battery to 'hog' the
> >> charging power (in series).
>
> > charging power is a non-issue,
>
> Not 'absolute charge power', I agree.
> 'Relative charge power' is the core issue, though.

its irrelevant. charge i x t is what counts, and thats invariably the
same for both bateries.

> > charge is what counts, i x t. Both get
> > the same, inevitably
>
> 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.

> It would be much safer to just remove the batteries
> from the series string long enough to charge the
> 'lower' battery independently until it's state
> of charge matched that of the upper battery,
> as Ed alluded to earlier.

If the op had a 6v charger, I dont think the question would have been
posed.

> (...)
>
> >> 'Blanket' statements can be misleading, with costly consequences.
>
> > Recommendations to connect lead acids in parallel, without the
> > necessary knoweldge or checks, is liable to lead to a bad result.
>
> As is true for series - connected batteries. =A0:)

worst case outcome: normal maintenance required after a while.

> (...)
>
> >> Anyone doing so will want to perform 'due diligence' to assure
> >> themselves that they are being safe and will get the
> >> desired electrical characteristics because no two batteries
> >> age in exactly the same way.
>
> > I doubt the OP will. People don't usually.
>
> Thus the value of 'Plain Applied Technology'.
> Our discussion will show up in subsequent searches
> and will highlight this important consideration
> so folks who want to do it right can continue to
> inform themselves.
>
> This is good.

NT
```
```NT wrote:
> On Dec 20, 2:53 pm, Winston<Wins...@BigBrother.net>  wrote:
>> NT 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.

I don't believe the 'bottom' battery would survive.
Stored in a consistently discharged state, it would
sulfate and fail.
If it did survive, then the 'top' battery would have
to be over charged pretty consistently in order to
fully charge the bottom battery.
I don't think it would be wise to abuse either
battery in these ways.

I've never added water to an SLA.
I see that it is possible though.

That will be what I learned today.

>> It would be much safer to just remove the batteries
>> from the series string long enough to charge the
>> 'lower' battery independently until it's state
>> of charge matched that of the upper battery,
>> as Ed alluded to earlier.
>
> If the op had a 6v charger, I dont think the question would have been
> posed.

Lots of us have lab supplies that can be used to
top up the charge on a battery until it can be
paralleled with it's mate for equalization.

Lots of us are virtually enmeshed in abandoned
wall-wart supplies that can be pressed into service,
for that purpose, with a series resistor and careful
monitoring.

These aren't called 'battery chargers' though.  :)

>>> Recommendations to connect lead acids in parallel, without the
>>> necessary knoweldge or checks, is liable to lead to a bad result.
>>
>> As is true for series - connected batteries.  :)
>
> worst case outcome: normal maintenance required after a while.

Worst case outcome, failure of the lower battery
due to sulfation and / or failure of the upper battery
due to the effects of over charging.

--Winston
```
```On Tue, 20 Dec 2011 13:35:18 -0800, Winston <Winston@BigBrother.net>
wrote:

>NT wrote:
>> On Dec 20, 2:53 pm, Winston<Wins...@BigBrother.net>  wrote:
>>> NT 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.
>
>I don't believe the 'bottom' battery would survive.
>Stored in a consistently discharged state, it would
>sulfate and fail.
>If it did survive, then the 'top' battery would have
>to be over charged pretty consistently in order to
>fully charge the bottom battery.
>I don't think it would be wise to abuse either
>battery in these ways.
>
>I've never added water to an SLA.
>I see that it is possible though.
>
>That will be what I learned today.
>
>>> It would be much safer to just remove the batteries
>>> from the series string long enough to charge the
>>> 'lower' battery independently until it's state
>>> of charge matched that of the upper battery,
>>> as Ed alluded to earlier.
>>
>> If the op had a 6v charger, I dont think the question would have been
>> posed.
>
>Lots of us have lab supplies that can be used to
>top up the charge on a battery until it can be
>paralleled with it's mate for equalization.
>
>Lots of us are virtually enmeshed in abandoned
>wall-wart supplies that can be pressed into service,
>for that purpose, with a series resistor and careful
>monitoring.
>
>These aren't called 'battery chargers' though.  :)
>
>>>> Recommendations to connect lead acids in parallel, without the
>>>> necessary knoweldge or checks, is liable to lead to a bad result.
>>>
>>> As is true for series - connected batteries.  :)
>>
>> worst case outcome: normal maintenance required after a while.
>
>Worst case outcome, failure of the lower battery
>due to sulfation and / or failure of the upper battery
>due to the effects of over charging.
>
>--Winston

Connect the two 6V batteries in parallel until equalized, then charge
in series.

...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"
>
> Connect the two 6V batteries in parallel until equalized, then charge
> in series.

** The OP does not have a 6 volt SLA charger -  he has a 12 volt one.

Most SLA chargers work as a current limited PSU with a fixed voltage limit
of 13.7 volts for a nominal 12 volt battery.  13.7 volts is carefully chosen
so 6 fully charged cells will draw only a small current -  the terminal
voltage must be raised to 15 or 16 volts to overcharge the battery.

When a discharged battery is connected, charging commences at the current
limit and as voltage rises it gradually tapers off until at full at charge
( ie 13.7 volts ) it is only a trickle.  The limit current value depends on
the amp hour rating of the cells so charging is completed in about 2 to 4
hours.

So, if you connect the wrong voltage battery ( 6V instead of 12V ) high
current will flow until the battery explodes  - the better designs recognise
when the voltage across the battery is dangerously low and refuse to apply
charge.

The interesting question is what happens if one or more cells in a battery
are in a much lower state of charge when connected to the charger.

If it is only one, charging will drop to a trickle with that cell not fully
charged. This is because 5 fully charged cells require 13 volts to be
applied to pass significant charging current and the remaining cell requires
about 2 volts. The total needed of 15 volts is simply not available.

A similar scenario applies for 2 out of 6.

However, if it is 3 out of six charging will still continue at a slow rate
until all 6 cells are topped up.

...   Phil

```