Replacing a discharged battery to the bank

Hello again everyone! I find myself here asking another question I cannot seem to answer on my own anywhere on the web.

I have a 440ah battery bank (4 12v 110ah AGM batteries wired in parrallel.... I know, I know, I shouldn't have more than 3 in parrallel)

I'd like to take one battery away at a time and use it in my canoe with my electric outboard trolling engine for a day of fishing and exploring from time to time, but wanted to make sure there would be no issues with replacing the drained battery to the bank... My plan is to hook up my multimeter to ensure I don't drain the battery all the way so it stays in decent condition, return that battery that is drained to the other 3 fully charged batteries on my off grid system, then take out a different battery the next time so I'm not putting more use on any one single battery.

My questions are

1) Is this ok as long as I rotate which batteries I'm using and have my usual equalization of the battery bank every month?

2) Is there any danger to any of the batteries by replacing a drained battery at say 11.7ish volts to others that are in float or absorb at 13.6 or 14.4 volts?...
a)Will my charge controller be confused by the voltage difference and use too high a voltage for the "full" batteries? (Tristar MPPT-45 charge controller)
b) Will the other batteries move too many amps too fast into the drained battery shortening its life due to too fast of a charging process?

Thanks in advance for your helping me think this through or providing any insight.


Rory

Comments

  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: Replacing a discharged battery to the bank

    Using it is not a problem and shuffling the bank is a good thing. I'd pull your Charge Controller off the others and charge it first before introducing it back with the others. If you have a charger then use it. I'v done it off my truck when camping back in the day, but even then I'd disconnect the main truck battery after I started it. 110 AMP batteries are not that forgiving to take a bunch of over-charge trying to bring it back up.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Replacing a discharged battery to the bank

    Depending on the brand/model of AGM batteries--They can accept (or source) upwards of C*4 worth of current (not great to hit a battery with so high of current, can cut battery life--but it can be safe as long as you respect the maximum current limits of the battery specs from the Mfg.).

    Your bigger problem will probably be making sure your one battery and its wiring can absorb the full charging current of your Charging Source without overheating the wiring. Until the batteries are more or less back "in sync" (i.e., if you have a 50 amp battery charger, the wiring to each battery need to accept 50+ amps from the charging source).

    In some ways, I would tend to look at using a DC to DC battery charger instead of removing and replacing a battery in my main battery bank. The chances of shorting the wiring or spot welding a wrench to the top of the battery bank (or causing a battery explosion) is always there when dealing with high current circuits (1,000's to 10,000's of amp into a dead short with your AGM battery bank).

    But, it sounds like a reasonable use of your battery bank for another function.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Replacing a discharged battery to the bank

    i don't think i'd make doing that a habit as it puts a higher cycle strain on one battery. you could alternate what batteries you use to help the over using of any one battery. of course, i concur that it must be charged fully and properly before you put it with the other charged batteries. watch out for any inequalities in the batteries and if you see any inequalities that normal charging doesn't fix then stop the swiping of individual batteries from the bank. remember you are playing with the equality of the batteries to each other and agm batteries aren't quite like fla types as i don't recommend eq charging them unless as a last resort on aged batteries.
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: Replacing a discharged battery to the bank
    niel wrote: »
    watch out for any inequalities in the batteries and if you see any inequalities that normal charging doesn't fix then...
    Niel, how do you watch for inequalities in an AGM battery? Obviously specific gravity is not an option. Perhaps end amps (while charging battery in isolation) or perhaps using a clamp ammeter (when charging or discharging it as a member of its bank)?
    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • SolaRevolution
    SolaRevolution Solar Expert Posts: 410 ✭✭
    Re: Replacing a discharged battery to the bank

    Balancing and monitoring the charging and discharging of 4 parallel sealed batteries is tough. Even with new batteries there is often an uneven amount of current in the parallel strings. I have occasionally rotated batteries to try to help them even out. Checking for inequalities with a clamp on amp meter may help but there are other factors involved.

    IMO, doing load tests on each string individually gives a pretty good indication. Try a 30 minute C10 load on each string and compare starting and ending voltages of each battery.

    Alex
  • OffGridRory
    OffGridRory Solar Expert Posts: 26
    Re: Replacing a discharged battery to the bank

    So is the consensus it is fine to do so long as I alternate which batteries I'm using and don't do it too often? Or would the safe way to play it be to just pony up and buy another AGM battery to be used for the trolling motor?
  • SCharles
    SCharles Solar Expert Posts: 123 ✭✭
    Re: Replacing a discharged battery to the bank

    Just charge up the canoe battery prior to wiring it back into the string. Shouldn't be too difficult. Although, thinking about disconnecting and re-connecting a battery from the string regularly, I would vote for the "buy another, dedicated battery" idea.
  • OffGridRory
    OffGridRory Solar Expert Posts: 26
    Re: Replacing a discharged battery to the bank

    Ok, so I need to charge back up my 12v 110ah AGM battery after its discharged before adding it back to my battery bank of 3 other AGM batteries.... Since I have little experience charging batteries from other than solar charge controllers I want to make sure I have an acceptable charge controller for this method... I've always tried to take really good care of my batteries thus far (Always charged with no more than 700w solar on an MPPT charge controller, never had more than 1500w drawn from them at one time, RARELY dropped below 12.5 or 12.4v) so I want to make sure at least the recharging of this electric outboard idea is going to be easy on the one I'm taking out then recharging with a standard houshold battery charger... Is this a respectable one?

    http://www.amazon.com/Schumacher-SSC-1000A-SpeedCharge-Battery-Maintainer/dp/B0009IBJE4/ref=sr_1_7?ie=UTF8&qid=1334681924&sr=8-7

    I notice it has pretty much the same specs as a PWM charge controller, but these things are so much cheaper than solar charge controllers it worries me a little...

    My plan is to use my existing bank and inverter (when the sun is shining) to recharge this battery to capacity before returning it to the bank that evening...

    Any objections before I shop around for one of these things and start pulling batteries?
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: Replacing a discharged battery to the bank

    The Schumacher charger would work - IF you select the proper (AGM) battery type and charge rate each time.

    I would also add to the discussion: Consider using Anderson Power Pole connectors to connect and disconnect batteries from the bank, boat, and charger. Get rid of the arc welding problems with the wrench. More work to set the connectors up at first, but much easier on the long term.
    http://www.solar-electric.com/sb-175-1-0.html
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
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