Noob battery question..(kinda long.)

[ptaylor]

Hey gang,

I've got a small 12V setup,with a 10W Solec panel,and a bunch of small
assorted batteries,of different ages,sizes,etc. (all 12V) a couple
2.5AH,a couple 4.5AH,17AH,etc. and I just added two car batts. (one is a
normal starter battery,and one is a "marine" battery.) both rated about
50-55AH. (about 140AH total.)
This leads me to my question..
I don't want to just parallel all of these different size,age,etc
batteries.(Do I?)

I figured out a way to isolate them from eachother,using 2 diodes per
battery (or 1 bridge rectifier for each pair of batts.)

So,I've got some bridge rectifiers,ranging from like 4A to 35A rating.
(one of my main hobbies is electronics,and I have plenty of parts around!)

So,using a few bridge rectifiers,I've tied one (~)AC terminal to the (+)
positive of one battery,and the other (~)AC terminal to the other
batteries positive.Now using the (+) terminal of the bridge rectifier we
have the output point of both batteries (minus one diode drop ~0.7V),It
is electrically very similar to them just being wired straight in
paralell,except they are isolated from eachother (the stronger batteries
can't try to "charge" a weaker battery.)
We can use the (-) terminal of the bridge rectifier for our "Charge
input" (solar PV panels,microhydro,etc.),of course the batteries will
see 0.7V LESS than the voltage on the (-) input terminal. (likewise,the
output will be 0.7V lower than the batt's voltage(s),1.4V loss from the
charge input,to the loads,but I've actually measured more like 1.0V in
practice here.)
Using schottky/fast/ultrafast diodes can lower the losses in the diodes
to about 0.2-0.3V each,So obviously these would be a better
choice.(lower voltage drop/less loss) now a 0.4-0.6V overall voltage
drop from "Charge input" and "output" (the bridge's - to + terminal.)
isn't that bad at all.(Still about the same as one normal diode,or a bit
less!)
I've been using this system for a while now,and it seems to work well..
If one battery craps out,and decides to hog current,it won't drain the
other batteries,since they are all isolated from one another with
diodes.(unless you've got a shorted diode or two.)
What it *WILL* hog is the charge input current.
I had one small SLA batt. die on me,and I noticed that the battery bank
didn't seem to be charging up much,despite giving them what should have
been a decent charge.(I also have a 13.8V 2.5A regulated supply to
"supplement"/charge on cloudy days,or times of heavy use.)
So I switched off the charge input (to prevent false readings) and got
my DMM and started checking the voltage of all the batteries,one-by-one.
Sure enough,found one that finally gave it up,around 8V..so I unplugged
it,and turned the charging source back on...I'm Back in business!

Anyways,I'm wondering if there are any major flaws with my
diode-isolation idea?
(I'll draw up a schematic if someone likes,but be warned,I can hardly
draw stick-people!)
It seems to be working well for me so far.
I don't really see any major flaws,maybe a couple minor
disadvantages..But considering the *whole system* cost me under $50,I'm
not complaining!
The small batteries were all aquired from local surplus places (UPS
batteries,etc.)I bought one car batt for $20,the rest
(inverter,panel,and the second car battery) were given to me by friends!
(They all know I'm interested in thier random electronic "junk".) ;-)
I've also recently bought a 24V 400W UPS for a whopping $5. The
batteries were dead (12V 6.5ah),but thats no matter,I've got the 2 car
batteries that I've used for it. (tested it once during an outage,works
great! the runtime meter says "99:00" remaining!)
I've not yet re-charged the car batteries with the UPS,as I fear the
internal charger may overheat,and release it's "magic smoke". No big
deal,I've got the solar,etc. for re-charging them!

This stuff rocks!

Now that I have sufficient battery storage(~140AH),I need to find some
more cheap/free solar panels!

The system is running a couple 12V PC case fans,a cellphone charger,and
modem, 24/7 without any real problems.(I do need more solar/etc. though)
I also have a 400W max. car amplifier connected up for my stereo's
subwoofer,I don't use it a whole lot,so it's not much of an issue.
Also,rarely used..Misc. radio equipment CB,Scanner,etc. (nice to have
during an outage/storm aswell!)

Whew..anyways..I suppose I'll let your eyes rest..I've been tinkering
with this setup for,wow,almost 2 years now! and have found it quite
fun,and I really enjoy my own spare power during power outages,etc! I
can't wait to go off-grid! (unfortunatly,thats'probably still a few
years away.)
Did I mention that this stuff rocks?

 

[Richard P.]

(I had to reply to the spirit of your message)...

It sure does! I have been "playing" around with solar for past several years, gradually building up
a small collection. I started with those little 5 watt "battery saver" panels gradually collecting
up to "professional" Siemens 50 watt jobs. Now that i'm finished school and working full time, i'm
ready to move up to the big time and start getting some larger batteries, sine wave inverter, and
full sized solar panels.

I can't answer your battery question because trying to understand all those numbers while in my
current state of mind (neo-citran induced trippy cold medication) makes for one $#@! of a head ache!
<grin>. Sounds like it's working for you, though, and that's the main thing. Until recently I have
been using those 7 Ahr SLA's found in computer power supplies and building emergency lighting
systems. I use them to power up small radios and stuff. Had to recycle all but one of them out
recently as they shorted internally and had one burst on me. They were all freebies so it was no
financial loss to me. But the experience (and fun) I gained from them all has finally led me to the
point where I am going to invest in some Surrette S460's 6volt deep cycle batteries. Several recent
and extensive weather related power outages in my area proved how nice it is to be able to still
have light, communications, and entertainment while my neighbours sat in the dark with dangerous
open flame lighting (candles). Seeing the power bill dropping is kinda nice too. Wish there were
subsidies in my country so that I could cover my roof with PV and perhaps some wind too..

But I must confess, experimenting with them has been a thrill... is there a Solar-holics
Anonymous....??

I've got a small 12V setup,with a 10W Solec panel,and a bunch of small
assorted batteries,of different ages,sizes,etc. (all 12V)> Did I mention that this stuff rocks?

 

[JB]

If the batteries are all the same type (for example SLA) you could parallel
them without major disadvantages. The disadvantage is during the float
period where you'll have a bit more self discharge.

If the batteries are of different types (SLA, gell, car lead batteries, or
even worse if you mixed them with Ni-Cd, Ni-MH or Li-ion) you shouldn't use
them and your system won't be any better.

The major flaw of your system is that you don't control the charge of the
batteries. How do you know if you're overcharging them? You'll reduce those
batteries life.

I think you'd be better served paralleling the betteries (being all of the
same type) and use a charge regulator.

But as you said, the batteries were quite cheap. One regulator costs you
almost as much as the total cost of your system. Now you have to balance
between a good rest of life for your batteries spending a bit more money, or
keep replacing your almost cost free batteries. But if you are an electronic
hobbyst and have access to an electronic junk yard you may build a cheap
battery charger yourself.

JB