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'[OT]: Charging batteries of different voltages'
2001\05\06@221706 by

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PICers,

I have this battery power supply that has a 12V gel battery in series
with a 6V gel battery. The idea was to have a 12V & and 18V rail.
(Could've been done better but i'm stuck with it for now.) OK, I'm
sick of clipping the charger terminals onto each battery separately so
I figured a clever(?) way to switch the batteries in series when
on-load and in-parallel for charging whilst disconnecting the load
device. Used a 4PDT switch. Seems to work too, at least the switching
side of it.

But how to charge a 6 volter & a 12 volter from the same source? What
I did was put in a 5W Zener, so that, on-charge, the cct looks like so
:-

SEE GIF IMAGE ATTACHED.

(Pardon for the uncompressed image but the BW is only 5K and zipping
didn't make much difference.)

Trouble is the 6V battery seems to charge OK but the parallel combo
seems to stick on 12.2 volts. Probably is the max terminal voltage of
the 6V battery clamping the applied volts to that level? Which means
the 12V one won't charge fully - it needs to get to 13.2, am I right?
In any case the 12V one is gonna top up the 6V little one when in
parallel till the ZD+6V=12V terminal volts, thus further limiting the
amount of charge you can pump into both. (OK, I know - make sure the
6V terminal doesn't short to GND & that the 6V doesn't go real low or
kaboom.  Don't worry, both batteries are separately fused)

What to do? I'd *really* like to charge both the suckers together, to
save a lot of messing about with more wires, separate charging lines
etc.

Thanx for insights - Ian

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part 3 105 bytes
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How to charge lead acid batteries

You need two numbers: charge voltage (assuming you want 100% charge) and
float voltage (assumes you will use a trickle charge rate to keep the
batteries "floating" near full charge to prevent self discharge.

For 12V lead acid (either wet or gel cell) batteries, the charge voltage is
14.8V and the float voltage is 13.8V. (not counting temperature extremes,
other unknown factors, etc.)

If you bring up a discharged battery to 13.8V, it will only be about 80%
charged.  It needs to be brought all the way to 14.8V before allowing to go
back down to 13.8Volts.

In your circuit, I guess you are charging with a low current source,
probably to avoid roasting the zener diode. The "6" volt battery at float
is 6.9V and with the zener voltage is 12.5Volts. As you transition from
float to charge, the current begins to rise. (Continuous charging at higher
voltages begins to dry out the electrolyte and seriously shorten the life
of the battery.)

So it looks like your 6 volt battery and zener are clamping the system
charge voltage way too low for the 12 volt battery to be happy.  A
simplistic answer is to size the zener up to around 6.9 volts. And test.
Maybe you'll get lucky... but due to the unequal capacities of the two
batteries, you will have to charge the 6 volt battery longer than it
'wants' to get the 12 volt battery charged.

Best regards,
Tom Messenger

At 12:16 PM 5/7/01 +1000, you wrote:
>PICers,
>
>I have this battery power supply that has a 12V gel battery in series
>with a 6V gel battery.
---snip---
{Quote hidden}

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>For 12V lead acid (either wet or gel cell) batteries, the charge voltage is
>14.8V and the float voltage is 13.8V. (not counting temperature extremes,
>other unknown factors, etc.)

So you say that to charge a gel cell battery, I need a supply of constant **voltage** at 15 volts, and it should be shut off when it rises to 14.8v? Do I need to control the current too, or only the voltage?

Stupid question, I know. But every one says different things about lead acid accumulators, and I NEED to know the truth :o)

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Er, sorry. Of course, what I *meant* to say was size the zener up to around
7.4 volts, *not* 6.9volts. This will allow the 12 volt battery to come up
high enough to get a full charge.  Your charging system will need to
provide enough  current to satisfy the total of both batterie's currents
added up.

TM

At 08:03 PM 5/6/01 -0700, Tom Messenger wrote:
>How to charge lead acid batteries
>
>A
>simplistic answer is to size the zener up to around 6.9 volts. And test.
>Maybe you'll get lucky... but due to the unequal capacities of the two
>batteries, you will have to charge the 6 volt battery longer than it
>'wants' to get the 12 volt battery charged.

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Lead acid batteries are charged in voltage mode - as opposed to current
mode charging for NiCads. The way this works in practice is that a very low
battery *does* pull lots of current - for a short while. As soon as it
starts to charge, it's terminal voltage will rise up to nearly the charger
voltage.  For example, if your charger is set to 15volts, usually you will
see the battery voltage quickly rise up to 14volts or so.

A typical cheap circuit is a LM317 set to about 15 volts and a series
resistor going to the battery.  When the terminal voltage gets to
14.8volts, the '317 is switched to 13.8 and the battery then is in float
mode. If you don't need float mode, just take the battery off the charger.

For more info, check the net for manufacturers suggested charge methods. I
learned my stuff from Gates and General Electric apnotes years ago.

Best regards,
Tom Messenger

At 12:26 AM 5/7/01 -0300, you wrote:
>>For 12V lead acid (either wet or gel cell) batteries, the charge voltage is
>>14.8V and the float voltage is 13.8V. (not counting temperature extremes,
>>other unknown factors, etc.)
>
>        So you say that to charge a gel cell battery, I need a supply of
constant **voltage** at 15 volts, and it should be shut off when it rises
to 14.8v? Do I need to control the current too, or only the voltage?
>
>        Stupid question, I know. But every one says different things about
lead acid accumulators, and I NEED to know the truth :o)

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>A typical cheap circuit is a LM317 set to about 15 volts and a series
>resistor going to the battery.  When the terminal voltage gets to
>14.8volts, the '317 is switched to 13.8 and the battery then is in float
>mode. If you don't need float mode, just take the battery off the charger.

Only this? But how the 317 is switched? No zener or comparator to change the output value?

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At 12:43 AM 5/7/01 -0300, you wrote:
>>A typical cheap circuit is a LM317 set to about 15 volts and a series
>>resistor going to the battery.  When the terminal voltage gets to
>>14.8volts, the '317 is switched to 13.8 and the battery then is in float
>>mode. If you don't need float mode, just take the battery off the charger.
>
>        Only this? But how the 317 is switched? No zener or comparator to
change the output value?

Yes, with a comparator or an opamp. Or a TL431... use your imagination! I
only meant to describe the general "algorithm" of how it's done. Of course,
as the saying goes, the devil is in the details. But then that's the fun
part... ha ha. ;)

For you, just dig through your 'junk box' of old parts and make a charger
out of whatever you come up with! But... make sure to control it with a
*PIC* and tell us how it works.

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At 12:16 PM 5/7/01 +1000, Ian Hynes wrote:
>PICers,
>
>I have this battery power supply that has a 12V gel battery in series
>with a 6V gel battery. The idea was to have a 12V & and 18V rail.
>(Could've been done better but i'm stuck with it for now.) OK, I'm
>sick of clipping the charger terminals onto each battery separately so
>I figured a clever(?) way to switch the batteries in series when
>on-load and in-parallel for charging whilst disconnecting the load
>device.

Why not charge at 18V? (more precisely, 2.25V x 9)
Adjustable regulator, 18VAC transformer, bridge, cap, couple resistors, and
a protection diode across the reg.

>Used a 4PDT switch. Seems to work too, at least the switching
>side of it.

You're lucky it's a "break before make" switch.
The other type exists, and it would have been briefly, spectacularly,
interesting.
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Ian Hynes wrote:
>
> PICers,
>
> I have this battery power supply that has a 12V gel battery in series
> with a 6V gel battery. The idea was to have a 12V & and 18V rail.
> (Could've been done better but i'm stuck with it for now.) OK, I'm
> sick of clipping the charger terminals onto each battery separately so
> I figured a clever(?) way to switch the batteries in series when
> on-load and in-parallel for charging whilst disconnecting the load
> device. Used a 4PDT switch. Seems to work too, at least the switching
> side of it.
>
> But how to charge a 6 volter & a 12 volter from the same source? What

An easier way is to use a 18v source, (+ allow for
overhead) and leave the batteries in series.
You can put a parallel resistor across each battery
and this will not affect the initial charging much
but works well for the topping off stage and trickle
charging, to make sure both are equally and fully
charged. Like a 60 ohm resistor across the 6v
battery and a 120 ohm resistor across the 12v battery.

Obviously these stay with the charger, and you need
three terminals from the charger.
I've used this system before with 2x 12v batteries,
(24v total) and it works great. I've also seen similar
systems on old industrial chargers.
-Roman

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Alexandre Domingos F. Souza wrote:
>
> >A typical cheap circuit is a LM317 set to about 15 volts and a series
> >resistor going to the battery.  When the terminal voltage gets to
> >14.8volts, the '317 is switched to 13.8 and the battery then is in float
> >mode. If you don't need float mode, just take the battery off the charger.
>
>         Only this? But how the 317 is switched? No zener or comparator to change the output value?

Hi Alexandre, I have a few home made chargers for the
12v gell cells we use on tools in the workshop and for
charging customer's batteries.

Allow for the 12v gel battery to be fully charged at 14.2v.
If it's a 4Ah battery, you want to trickle it at about
10%, ie 400mA.

So imagine this simple circuit, (you can use a 317 or
even a 7815 regulator):

15.2v (regulated voltage)
|
2.5 ohm resistor (400mA @ 1v)
|
14.2v (charged battery) + terminal

So when the battery is charged it will safely trickle
charge and it can be left overnight. (10% current is
safe to leave charging).
Also, when the battery is discharged the circuit looks
like this:

15.2v (regulated voltage)
|
2.5 ohm resistor (3.28A @ 8.2v
|
7v (very empty battery)

So it will charge the battery quickly. In reality
my supplies are regulated at 1A max (using 7815)
so the empty battery is charged at 1A max.
I use 10W resistor for the series resistor, with:

(always 15v regulated 1A supply)
2.7 ohm (for 4Ah and 7Ah batteries)
5 ohm (for 2Ah batteries)

These chargers have been use for years and are
simple, cheap and great for hands free and overnight
use. And good for my dumb apprentices to use. ;o)
-Roman

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>These chargers have been use for years and are
>simple, cheap and great for hands free and overnight
>use. And good for my dumb apprentices to use. ;o)

Hmmm, these seem to be nice, I'll build one and see what happens, thanks Roman!

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Tom,

Good points, those. I just need to be able to charge the thing more or
less conveniently while I figure out a "better way".

OK, in the short term, suppose I substitute a 7.4V zener and put a
blocking diode in the 12V battery arm? That way the 12V 7AH can't
discharge thru the ZD+6V 4AH battery. The 2 batteries would then take
current separately off the external charger. Trouble is the little 6V
fellow would fill up before the bigger 12V one was fully charged.
Maybe I could put a 2PDT switch in the 6V arm and disconnnect it
manually when it had enough?

This is all pretty stop gap - the battery pack has to power a camera
in a remote location and for some weird reason I figured an 18V rail
was needed. Bad idea. What I'll do "sometime" is build a switchmode
supply with an 18V o/p, run the camera & etc off the 12V battery, fire
up the 18V rail from that & I can just have a single 12V charger.
That's in the long term, though ...

Ian.

Tom Messenger wrote:
{Quote hidden}

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On Sun, 6 May 2001 20:34:32 -0700 Tom Messenger <kristTHEGRID.NET>
writes:
> Lead acid batteries are charged in voltage mode - as opposed to
> current
> mode charging for NiCads. The way this works in practice is that a
> very low
> battery *does* pull lots of current - for a short while.

And, of course, this is another use for a series light bulb! The lamp
will increase resistance to limit the current. As the current drops, the
lamp resistance drops so the voltage drop across the lamp drops even
further than it would if a series current limit resistor were used.
The light bulb also serves as an indicator. If it's glowing, the battery
isn't gully charged yet...

Harold

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