Two strings of slightly different GC2s?

2twisty2twisty Solar Expert Posts: 199 ✭✭✭
Some background:

I've been off-grid since August 1, 2014. Initially my system was 12v with 8 US Battery 1800XC2s. I then moved to 48V with the same batteries. About a month ago, I added a second string of Duracell GC2s.

I was out doing some battery maintenance the other day and took some SG readings. These batteries were NOT fully charged at the time. I was moving my batteries around and checking their cabling, etc and decided to spot-check a few cells. In the string of Duracell GC2s, I was seeing 1.225, and in the US Battery GC2s I was seeing 1.210. I spot checked about 4 cells in each string and the SGs were very consistent in each string.

The Duracell batteries are listed as 220Ah, and the US Battery ones are listed as 208Ah. If they both gave up similar amounts of energy during the last discharge, it makes sense that the US Battery GC2s would have lower SGs than that of the Duracells.

Is this a problem?

Would there be any reason or advantage to mixing the battery types in the strings, so that there are 4 of each battery type in each string? This way the strings would be "balanced."

Please advise.


  • Blackcherry04Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: Two strings of slightly different GC2s?

    Well, they are not that far apart. Have you done a EQ on them all ?? It will take 30 + cycles on the new ones to get to full capacity. I'd EQ them and see where they are after you'v cycled them a few times. Being lower in amp hrs is small, but it could show up in the SG level. I can't see where mixing them in the strings would hurt. As you use them they should balance out more to each others capacity, the SG's may be a different issue for a while. Get them up to 1.265-1.270 and they will be fine.
  • jcheiljcheil Solar Expert Posts: 722 ✭✭✭
    Re: Two strings of slightly different GC2s?

    I had a friend that had a 12v system with 6 trojan T105's and then added 2 trojan T145's.
    One day when charging he got a temp warning and the T145's were smoking hot.
    Nothing was wrong with them physically, the only thing we assumed is that somehow those 2 had a different internal resistance and were not getting along with the T105's. Pulled them out and everything went back to normal.
    I know you see warnings on every device now-a-days stating not to mix battery types, perhaps they are right.
    Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software
  • BB.BB. Super Moderators, Administrators Posts: 30,721 admin
    Re: Two strings of slightly different GC2s?

    More or less--Keep an eye on them. As long as you have the same batteries in your series string... Adding the second (or more) parallel strings of same chemistry batteries--You will probably be OK. Just get a DC Current Clamp type meter and make sure that the batteries are sharing reasonably during charging/discharging (which is sounds like you are doing now).

    in 3-5 years or so--You may start having one or two battery failures--And you will have to decide if you want to replace the few failures (with more to follow), or replace the entire bank at once (picking new batteries and possibly different batteries--Depending on your power needs).

    For me, one of the big issues with "old vs new" and "mix" of batteries--As things fail (normal aging/cycling life)--You are left with the decision of replacing a battery at a time (and debugging another failed battery a few months later), or replacing the entire bank (plus those several new batteries you have recently purchased). I am a fix and forget type of guy... But if you don't mind working around the bank and/or are cash limited, you may be better off replacing piece wise and extending your initial investment.

    No absolute right or wrong--Just more shades of grey.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
Sign In or Register to comment.