Old bats along with new bats?

Chris11Chris11 Registered Users Posts: 126 ✭✭
I put new Costco 6 volt bats, 4 of them, in my system.  All are working great but being a cheap old fart I hate taking back the old bats to Costco for a $60 core refund since they still have some ability to hold juju.  I know it's not a good idea to mix old and new bats but ....lol ... the new were about 25.4 volts and the old were about 24.8 volts.  Seems to me my olduns are still usable. 

Whatcha think?  Can I add my old ones back in a parallel configuration, retaining the 24 volt configuration?  Or would it be a super bad idea?

Thanks, Chris


  • BB.BB. Super Moderators, Administrators Posts: 32,639 admin
    edited June 2020 #2
    Keep "new batteries" in their series strings. And "old batteries" in their common series strings. (don't mix old/new batteries on same series string).

    They will work, and as long as you monitor the batteries (water usage, specific gravity, quick per 6 volt battery DMM voltage sanity checks).

    Wire your parallel strings per this website suggestions:


    At some point, your old batteries will carry less of the loads, and your new batteries carry more--And you probably will get a slightly shorter life with your new batteries vs if they where part of a matched set. And your battery storage will tend to degrade "sooner" as the old batteries eventually age/cycle out.

    You may get into the issue of (say) you have a 6 year life for your batteries, and the new batteries will (hopefully) last near 6 years, and the 3 year old batteries will last ~3 more years (made up numbers). And you end up having to debug/replace 1/2 the batteries 2x as often (i.e., 1/2 battery replacement every 3 years, vs full battery replacement every 6 years).

    So, not a "bad" setup (old+new)--More maintenance, less than optimum battery bank capacity, possibly slightly (somewhat) shorter life of the new batteries as they cycle more than the older batteries.


    PS: I should add--Assuming batteries are the same chemistry/construction and hopefully mfg/model... You don't want to mix FLA with AGM or GEL (as an example).
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  • Chris11Chris11 Registered Users Posts: 126 ✭✭
    Whoa....I love it.  I was sure everyone was going to yell at me, "don't mix 'em"...lol. Thanks so much Bill...I'm going to put them, and yes, they're all Interstate 6 volt GC batteries, in parallel.  You made my day...err, night.  Thanks again. Chris

  • clockmanfranclockmanfran Registered Users Posts: 92 ✭✭✭
    I agree with Bill.

    Its very nice to have deep pockets and buy the latest sexy batteries but in reality Its cost effectiveness that counts.

    I have a 1300ah 48v battery, that's 12 strings of 4off 12v 110ah sealed ultra deep/marine use golf cart type batteries, yep that's 48off 12v batteries!. I have the ones with  threaded studs. 

    Yes, as Bill says keep old batteries in their own string, keep new batteries in their strings.

    I used to be a believer in used/recycled Fork lift batteries that came in their own container, the problem was there was always one or two cells/batteries that were hanging in their, and a few years later they became a problem as they pulled the complete fork lift battery pack down.  Then the pack had to be split down and etc etc.

    I have also had GEL/AGM batteries, ie real non liquid gel matts inside, these were ex out of service date mobile mast power back up supplies. They were very cheap at the time, but again it was getting strings that matched each other and one would not pull the others down. 
     However my conclusion, I would not recommend GEL/AGM batteries as they are not good at being drawn down and charging up quick, in my experience GEL/AGM like a long gentle soak charge, to get the best out of them. 

    So after 15 years of experimentation i am happy with my present situation of the Golf Cart batteries, there readily obtainable, their cost effective to replace a string of 4, there easy to move about on your own, and importantly they are constantly available. 

    Yes Bill, still researching that book about 'How to make your own 2v 200ah Plante Battery', preliminary it looks good but a fair bit of development/experimentation is required regards containers against weight and Ah output.  

    Everything is possible, just give me Time.

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    3off Hugh P's 3.7m dia wind turbines, (12 years running).  ... 5kW PV on 3 Trackers, (8 years) .... 14kW PV AC coupled using Used/second hand GTI's, on my OzInverter created Grid, and back charging with the AC Coupling and OzInverter to my 48v 1300ah batteries. 

  • mcgivormcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    Although it is possible to parrallel used with new batteries if all are the same chemistry, which is confirmed to be so, even down to the same type, however there is one thing to consider, this is the actual condition of the older string. By this I mean are they still healthy enough to transition into float under normal charging conditions, if so they could be a good candidate. However if not they are not, through degredation, they may have developed  minor short circuits between the plates, this would cause a parasitic load on the newer ones through the cascade effect, the newer ones attempting to keep the older ones charged after solar charging has terminated, which may manifest over time.

    Flooded cells are prone to shedding material which acclimate in the wells below them often this could leadi to buildup causing localized heating, due to short circuit between the plates  of individual cells. As there is no mention of actual age, cycle depth, operational temperature or if flooded or otherwise, it would be wise to do an individual battery and cell test to determine if they are in fact healthy enough. Furthermore it would require constant checks in the future, in the form of routine maintenance to determine if the condition of the older ones are still up to the task, after, if that is the route chosen.

    An Infrared thermometer would be an invaluable tool to help keep tabs on whether there are hot spots developing, in one or more cells, if this is the case it would be prudent to eliminate the damaged string to preserve the newer one.

    In conclusion, it's my opinion, that mixing old and new may be possible but not something I would do, or recommend, especially to a novice, there are way too many variables 

    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
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  • InCogKneeToeInCogKneeToe Registered Users Posts: 73 ✭✭
    I have to agree with Clockman. Both with Used and AGM batteries.

    Used, we picked up 3V Batteries from a Telephone Company, (yes 2 cell) and 1 weak one would draw down the whole string Quickly.

    AGM, has made it's way into my Work. Auto Sales. Vehicle sitting on the lot, in minus 40 temps, go dead. Jump starting, brings them back to life after running them awhile. (as long as they were not flat dead). with Flooded. AGM although they don't care if they freeze or not, take a lot to bring them back to life.

    I have 6 AGM Truck Batteries in my Shed, just for a light/saw every now and then. They were all replaced because Testing Failed, in New Trucks. Slow Charge over many days, then an AGM compatible Charger saved them. But they don't like a deep discharge.
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