Mixing battery sizes in one battery bank?

39 Posts
Sep 25, 2010 09:07 am
Mixing battery sizes in one battery bank?

I have a 12V battery bank, with two 100Ah AGM batteries, and two 200Ah AGM batteries.  Is this advisable, or should they all be the same size? (I've been told the latter) Should the bigger batteries be on the outside ends of my parallel string?
 
46 Posts
Sep 27, 2010 05:46 pm
Re: Mixing battery sizes in one battery bank?

hi matthew, yes they should all be the same size. whats more, they really should all be about the same age (or at least the same usage profile). all you will be doing is reducing the capacity of the 200ah batteries, and eventually potentially causing undue wear too to undercharging. two separate battery banks by adding an additional pwm (or other) charge controller? any way, best of luck. -m
 
6 Posts
Sep 29, 2010 07:51 pm
Re: Mixing battery sizes in one battery bank?

Hello there:

If you are running in series , ie a 24 / 48 volt system then no you will have big problems.  the lower capacity ones will discharge more than the higher cap ones and you will end up damaging them.

If you are running a 12 volt system and are running them in parallel, ... yes it can be done but as said before is not the best move.  Even if they were the same Ah rating, having 4 batteries in parallel tends to cause problems with charging properly.  You would end up doing more maintenance in the long run, isolating the to charge them up properly this way.

It can be done but won't be the best way to do things.
Take care
Aaron
 
220 Posts
Sep 30, 2010 05:15 am
Re: Mixing battery sizes in one battery bank?


hey guys,

agree that it's not the best or a "by the book" situation.

sometimes in the real world we do what we have to with the resources we have at hand.

we can make things less bad and more good by (electrically) moving the battery terminals off the batteries and onto a common busbar.

in a setup like this we want to pick a cable size, length and matching lugs and use the same for all our runs to the buss. (all cables #2awg and five foot each would be reasonable) if we are doing an inverter i would suggest a maximum size of 2.5kw connected with 4/0 cables from the buss.

some scribbles for a visual...



a pic of a battery combiner that supplies 500+ amps on a regular basis...


it's all good fun!
cheers, dave


*just one guys thoughts here and always open for a rethink.
 
39 Posts
Sep 30, 2010 06:58 pm
Re: Mixing battery sizes in one battery bank?

Thanks Dave for the ideas.  Curious- why do you have 3 red cables coming from your positive bus bar in your battery combiner pic?
 
39 Posts
Sep 30, 2010 06:59 pm
Re: Mixing battery sizes in one battery bank?

Thanks Aaron for the advice- really helpful. -Matt
 
39 Posts
Sep 30, 2010 07:00 pm
Re: Mixing battery sizes in one battery bank?

Ok, thanks Michael for the advice! -Matt
 
39 Posts
Sep 30, 2010 07:10 pm
Re: Mixing battery sizes in one battery bank?

by the way, how do you post pics or diagrams in your response, Dave? I'd like to do the same. -Matt
 
220 Posts
Sep 30, 2010 11:55 pm
Re: Mixing battery sizes in one battery bank?

Curious- why do you have 3 red cables coming from your positive bus bar in your battery combiner pic?

cheers matthew,

 the three large red and black cables are separate 4/0 inverter feeds. that system is running two 2.5kw and one 1.5kw @12v inverters.

one 2.5kw MSW to a 1800w pizza oven and other small loads..
a second 2.5kw MSW inverter to a large commercial coffee maker and a bunch of smaller loads and the 1.5kw pure sine wave for the band electronics/amps ect. the system runs a coffee house and we go thru about 5kwhs over a four hour period.

by the way, how do you post pics or diagrams in your response, Dave? I'd like to do the same. -Matt

matt, i'll put up a separate post on that..was never sure if they allow that on this forum but have been sneaking some pics in here and there and the administrator has not said anything or banned me yet so it may be ok? Wink 

dave

edit: added "Pictures and Sketches" post.
http://www.altestore.com/forums/Renewable-Energy/RE-General-Discussion/Pictures-and-Sketches/index.php/topic,2272.0.html
« Last Edit: Oct 4, 2010 06:27 pm by david ames »
 
39 Posts
Oct 4, 2010 07:02 am
Re: Mixing battery sizes in one battery bank?

What do you suggest using for a bushing behind those custom bus bars when mounting them?
 
220 Posts
Oct 4, 2010 04:18 pm
Re: Mixing battery sizes in one battery bank?

 hi matt,

 they make insulated standoffs for this kind of thing..i'm cheap though and used some blocks of HDPE (a cut up cutting board) this cutting board material is nice as we can cut any shape we need..i use this stuff in some of my antenna work as it is rugged, weatherproof, virtually indestructible!

cheers, dave kb1mzf
 
39 Posts
Jan 27, 2011 05:45 am
Re: Mixing battery sizes in one battery bank?

Thanks for the tip, Dave!
 
2 Posts
Feb 15, 2011 11:16 am
Re: Mixing battery sizes in one battery bank?

Best read on the subject I have found:

Abstract – The operation of a 24V VRLA battery
formed by two strings of dissimilar cells and
monoblocs in a 100% d.o.d C3 discharge/charge cycle
service has revealed not unexpectedly that an
asymmetric current flow from the strings occurs but
that its implication on ultimate cycle service life is less
significant than the strict rule of “do-not parallel
dissimilar strings” would indicate

Document found at:
http://neuralfibre.com/paul/wp-content/uploads/2007/05/can-we-now-sin.pdf

Rich
 
26 Posts
Feb 16, 2011 10:12 pm
Re: Mixing battery sizes in one battery bank?

Richard, excellent scientific study!  The results are intuitive to me... the lower impedance/capacity string should be no worse off than if it was the only string.  The addition of a higher capacity string should protect the lower capacity one from discharging too far.  You could almost think of the higher capacity string as being a charging source once the lower capacity one is discharged far enough.  Meanwhile, the higher capacity string is protected from deep cycling, increasing its lifespan as well.  The addition of a desulphator should protect the higher capacity batteries from issues stemming from too shallow of discharges.

Theoretically, you could even out the charge/discharge pattern by increasing the impedance of the lower impedance string by perhaps using a longer cable or smaller gauge cable.
 
2 Posts
Feb 16, 2011 11:59 pm
Re: Mixing battery sizes in one battery bank?

I thought I would post reference to the article before going forth and saying other posts are an outdated opinion.  I suspect it will be years before it is commonly being done to mix battery sizes and ages.

My solar application is for remote monitoring.  In that application I designed the system to reach 100% DOD at 2 full days of overcast using a 33ah agm 12 volt with a 65 watt poly panel.  But I sure prefer to be at 4 days gloom before shut down so last year I carried in a very old 65 ah agm and connected it in parallel. It worked just fine.

This year I updated the charge controller to one with a low voltage disconnect that is current compensated.  I may miss the hardware fuse but we will see.
This is what the battery looks like before jumping other batteries onto it in parallel:
http://home.comcast.net/~soundfiles/sd722bat.JPG

Rich
 

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