Reduced effeciency when connecting batteries in parallel?


I am working on a small 12V system for a friend, it will be powering an off-the-grid surveillance system for 24-hour wildlife research. The problem is that when testing the system I am seeing a large difference in expected performance between running on just one battery at 35Ah and two batteries at 70Ah.

The system is pure DC with no conversions and has a 0.436 Amp draw (24-hours everyday). For this amount of draw I calculate that one battery will last for 4.3 days before needing to be charged; and indeed, when testing the system on one battery it runs for 97hours - just at 4 days. This outcome gives just 1% error from expected results, this is great!

Doing the same calculations for a 70Ah system, or two batteries, I expect the system to run for 8.5 days - but it reaches 11 volts at 158hours, about 6.5 days - a 30% error! This lower performance of the parallel connection occurs over and over during testing.

Any ideas what might be happening here, and how I might resolve the problem?


  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Reduced effeciency when connecting batteries in parallel?

    Welcome to the forum.

    Let's do some backward math, starting with 0.436 Amps and multiplying by 24 hours: 10.464 Amp hours, or 1/3 the capacity of your 35 Amp hour battery.

    Now, what are you recharging it with? Because if you let that run 4.3 days or 97 hours you get 45/43 Amp hours. So something is not right because you can't draw a battery down below zero.

    In fact you shouldn't be drawing it down below 50% SOC, so two such batteries @ 75 Amp hours total should last about 3.5 maybe 4 days.

    How many times did you try this experiment? If you've pulled that one battery down to dead several times it probably no longer has its full capacity no matter what you do to it in charging.

    I'd suggest you get two new 35 Amp hour batteries and a 130 Watt panel and 10 Amp charge controller to keep it going day after day.

    Also make sure the batteries are not 'laddered' in their wiring, but rather connected like this diagram shows (although it depicts two battery strings of two in series):
  • BB.BB. Super Moderators, Administrators Posts: 32,319 admin
    Re: Reduced effeciency when connecting batteries in parallel?

    Also, check the circuit current draw... Some circuits will draw more current when the battery voltage falls, other may draw less current as the battery voltage falls. All depends on the details.

    Also, what type of battery are you using exactly (brand/model number/specifications). Also, it is not unusual for batteries (or even cells in lead acid batteries) to have a spread in capacity of +/- 10%, or to have an increase of 10% or more in capacity from "new" until they been cycled several dozen times.

    Temperature also makes a big difference in battery performance too (hot batteries run lower voltages, but have more apparent AH capacity. Cold batteries run higher voltages and have less AH capacity).

    Do you have a Doc Watson or equivalent Battery Monitor (DC AH/WH meter)? Very handy for these types of tests (not recommending websites or meters specifically--Just some suggestions for research--The Doc Wattson and Turnigy meters are pretty popular).

    Also, for longer battery life, you should avoid taking Lead Acid batteries much below 50% SOC and never below ~20% SOC (cell voltage reversal on the "weak cell" will kill the battery).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PhotowhitPhotowhit Solar Expert Posts: 5,729 ✭✭✭✭✭
    Re: Reduced effeciency when connecting batteries in parallel?

    I suspect, as 'Coot did, that you put your previous battery in parallel with another battery.

    I do think you might have seen the runtime demonstrated for assorted reason, drawing less than 1/20th of the batteries rated capacity, Batteries are typically rated at capacity drawn over 20 hour and will exhibit more capacity when drawn down over longer time. As the camera becomes warm it might draw less current.

    Recharging of a deep cycle battery is different than a car type battery, so I would be curious as to what type of battery charger you have used on the battery?

    You typically do not want to run even a deep cycle battery down below 20% and if you intend charge the batteries, you will want 2 sets of batteries, even the most forgiving of lead acid batteries will require some time to recharge properly.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former, 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • TeerlincTeerlinc Registered Users Posts: 3
    Re: Reduced effeciency when connecting batteries in parallel?

    Ah, yes. Well, the scientist running the show does run the batteries down as far as the camera will allow. The batteries are paid for by the state and he is less concerned about the real life of the battery than he is about how much footage he can get before he has to replace the batteries. He does not want solar panels in the field, especially one as large as a 130w, because it may attract predators. Anyway, I am sure that I have connected the batteries correctly - when the batteries are connected in parallel, what EXACTLY happens if one battery has a slightly faster discharge rate than the other battery?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Reduced effeciency when connecting batteries in parallel?

    The lower battery will become an instant drain on the higher battery, taking away some of its stored capacity in an attempt to charge up. It is less than 100% accurate science determining how bad this will be.

    If you can't talk him into solar recharging, then go with a much larger battery capacity so that it is not drawn down to dead in the allotted time frame. It would be a good idea to put a small charge controller on it with an LVD and LOAD terminals to power the camera, causing it to shut down before the battery is ruined. Something like this: Note no solar panel is required; the controller will simply shut off the LOAD terminals when the battery gets low.
  • TeerlincTeerlinc Registered Users Posts: 3
    Re: Reduced effeciency when connecting batteries in parallel?

    Very nice! I was not aware that we could use a charge controller in this way, thanks for the link!
  • mike95490mike95490 Solar Expert Posts: 9,511 ✭✭✭✭✭
    Re: Reduced effeciency when connecting batteries in parallel?
    Teerlinc wrote: »
    ... does not want solar panels in the field, especially one as large as a 130w, because it may attract predators....
    Yep, those 2 legged predators. If I catch one at my panels......
    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 ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    gen: ,

  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Reduced effeciency when connecting batteries in parallel?

    if you're using the batteries like throwaway primary batteries then there isn't any sense in using a controller as you have no intention of recharging anyway. just get a battery with enough capacity that will last until you visit to replace it.

    i should point out that if you only drain the battery to its 50% point and replace it that you can take the battery back with you to get a proper charge. that means you need 2 batteries with each having a capacity 2x that which the load requires in the allotted time before replacement and the batteries will not be ruined.
Sign In or Register to comment.