# Battery question: Can this be done?

Posted by Jim on August 17, 2010, 1:14 am

Let's say you had two 12 Volt batteries in series, giving you 24 volts
to power lights, inverters or whatever.

Now, let's say you have the following to charge the batteries, which
are now at this time separated into two each 12v batteries:

1 each  65 watt panel with Morningstar Sun Guard Controller

1 each  55 watt panel with Morningstar Sun Guard Controller

Is there any way to hook things up so that if the two batteries are in
series giving 24vdc, that the controllers can be hooked up so that
they think that each controller is charging just one 12v battery?
That is, say one panel would charge one 12v battery and the other
panel would charge the other 12v battery, so you could power say small
12 volt loads off each separate battery but have the heavier 24v load
across both batteries?

If that didn't work, could the two panels be put in series, and use
just one 24v controller to charge both batteries as one 24 v battery?
I seem to have read somewhere that putting unequal size panels in
series is not a very good idea.

Any ideas?

Jim

Posted by Josepi on August 17, 2010, 1:45 am

It could be done with each panel charging each battery if you only use small
amounts from the 24v series connection. If you do any deep discharge or
repeated cycling the larger capacity can reverse charge or damage the
lighter battery.

If full charging is accomplished each charge cycle the cells should stay
equalized but this is not usally a good idea, generally for normal usage of
batteries.

If you series the PV panels the rated output will only be the current of the
lighter panel. PV panels act as constant current generators and the larger
one will never reach it's full rating. If the batteries are the same
capacity, rating, brand and age, I would series your panels (24v)and use it
that way, derating the larger panel to the smaller one's rating. This
to individually drain one bank.

Let's say you had two 12 Volt batteries in series, giving you 24 volts
to power lights, inverters or whatever.

Now, let's say you have the following to charge the batteries, which
are now at this time separated into two each 12v batteries:

1 each  65 watt panel with Morningstar Sun Guard Controller

1 each  55 watt panel with Morningstar Sun Guard Controller

Is there any way to hook things up so that if the two batteries are in
series giving 24vdc, that the controllers can be hooked up so that
they think that each controller is charging just one 12v battery?
That is, say one panel would charge one 12v battery and the other
panel would charge the other 12v battery, so you could power say small
12 volt loads off each separate battery but have the heavier 24v load
across both batteries?

If that didn't work, could the two panels be put in series, and use
just one 24v controller to charge both batteries as one 24 v battery?
I seem to have read somewhere that putting unequal size panels in
series is not a very good idea.

Any ideas?

Jim

Posted by Martin Riddle on August 17, 2010, 2:06 am

Two possibilities,
The controllers can be put in Parallel  with one another, so you can
charge 12V bank then use a 12 - 24v dc-dc converter to get 24Vdc.

The other is as your last bit said, get a 24v charge controller, and put
the panels in series. The 55w panel will limit the current output of the
65w. ( as if they  we two 55w panels) Then use a 24 - 12vdc converter

You need to use a 24vdc load.

There is another way, but the batteries will be unbalanced and as a
result may get damaged.

Cheers

Posted by Martin Riddle on August 17, 2010, 2:09 am

Oh, forgot.  Here is a Velleman converter
http://www.bgmicro.com/psdc20.aspx   But it is only 80w continuous.

Cheers

Posted by Jim on August 17, 2010, 11:17 am

On Mon, 16 Aug 2010 22:09:23 -0400, "Martin Riddle"

Martin;

That would work, and there are higher power versions out there used
in marine radio field which one can sometimes find at Hamfests.  I
never thought of that route.

The question of the two batteries was mostly
out of curiosity and the answers explained why it was not a good
idea.

I'm just getting into solar and am asking questions before deciding
which way to go next.  I'm saving for a 200 watt panel and MPPT
controller next.  Our utility bill has gone ballistic since
Katrina, Rita and Ike.  The local utility has the politicians in it's
pocket, and they have added all kinds of surcharges onto the
ratepayer, while the CEO and executive bonuses have set all time
records, and they jet
around the world at our expense.  Shareholders have not missed a
dividend payment either.

Guerilla solar is my goal as finances permit.  I now have two 12v deep
cycle batteries with one panel charging each battery through two Sun
Guard controllers.  It's running some lights, a fan, and my ham radio
equipment if we lose power.  The heat almost killed me after Katrina,
we were some time without ANY electricity and I don't want to go
through that again.  I now have a Honda generator running on Natural
Gas (which has never gone out yet), but solar would be better as there
is always lots of sunshine after a hurricane comes through.

Guerilla solar would be useless at night duing a prolonged power
outage like Katrina, so panels could be switched to battery bank
service, so the
generator would not have to run all the time or the NG fuel supply is
shut off.

During Katrina, one of my neighbors had generator, but soon ran out of
gas and the nearest gasoline was two states away.  Several failed due
to oil problems.  Most people don't realize mfrs call for oil changes
on smaller generators every 25 hours or so.