Parallel or Series

adas Solar Expert Posts: 136 ✭✭✭✭✭
Aloha, as a offshoot topic of my earlier thread.......

What is better to do, run batteries in parallel or series. given the parameters below:

Two used 24v electric forklift batteries salvaged from a junkyard. These are junked from a forklift dealer because they will not hold a charge for a constant 8 hours of use in a store like Home Depot or Costco. I have used such batteries in my forklifts for four to 8 MORE years as long as I can live with batteries that will not last all day.
Anyways these batteries will not be the same voltage at full charge, due to their age. They may have a weak cell, etc. so the batteries may be not be the same voltage and amperage rating due to their age. I have two batteries fully charged and each cell is a bit different, from 2.08V to 2.10 up to 2.18v one up to 2.2v.

So what is best to do? Run them parallel 2 x 24v or series 1 x 48v. I bought my MX60 and can run 2 x 30v panels to charge the 24v or 3 x 30v panels to charge the 48v.
I have seen comments both ways discussed here. What is best for my system?



  • nigtomdaw
    nigtomdaw Solar Expert Posts: 705 ✭✭
    Re: Parallel or Series

    The series or parallel question is a little bit redundant because the voltage question should be focussed on your final nominal voltage that you require for your eventual system. As you are aware panels can be recon figured and added to quite simply and your MX60 is a flexible chap ! Going 48 v has the advantage of wire sizing and savings for a larger system. But if you intend to use an inverter/charger then this item calls the base voltage shots and cant be reconfigured at a later date.

    However I think you need to look long and hard at trying to use batteries which you are aware that are already failing. Given the cost of your investment in a MPPT charge controller and solar panels it seems a litle fuel hardy to then store it in a failing battery bank.

    Would you put expensive petrol (gas) in to a leaking or contaminated fuel tank ?

    I am a where that money is a consideration. I'm running on a 2nd user battery bank but without any known failures ie capacity SG and voltages all good, I did have a couple of duff units but as i bought spares and they are 2v cells they were easy to weed out. One duff cell in your 24 v units will drag the banks complete efficiency downwards. When you spend good $ on the generating of power seems foolish to not match it with quality storage. IMHO
  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: Parallel or Series


    There are pros and cons to series vs. parallel. For your situation, I’d be concerned about wiring the batteries in parallel and having the weaker battery discharge the stronger one. Therefore, eveything else being equal, a series configuration might be better.

    Note, however, that the MX60 does not operate in auto-sweep MPPT mode when its bulk (MPPT) mode output current is <5 A. This condition is not uncommon when using small PV arrays (<~400 W STC) and 48 V battery banks. Instead, the controller sets the PV array at the percentage value entered into the controller’s “U-Pick” setting. The default value is 77%, but I found that settings of 80% or 85% usually work better (more bulk-stage output current from the controller to the batteries).

    Jim / crewzer