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Due to availability and not practicality, I am attempting to build an as large as possible battery bank from 16 AH AGM batteries.

The end goal is a large system that is charged with a solar system that is scalable for future upgrades, while also able to be grid charged. The inititial solar kit I'm planning to use is a 540 watt system including a 40 amp charge controller. The manufacturer informed me I would not be able to link more than 3 batteries in parallel on this 12v system, but general research hasn't been able to provide me with any reasoning for this other than uneven charging.

On a side note, I am considering using a bus bar system with jumper wires for each battery. If I opt to use this system it is my understanding that I create the option of easily monitoring individual battery voltages, as well as fuse each battery.

Highly motivated to move forward with this project but I would like to do so with a solid plan in place.

Thank you for reading, looking forward to input!

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  • \$\begingroup\$ No obvious reason to limit bank number. Choose appropriate terminal block , AWG size and length. Use mV drop on ground wires to sense current sharing during charge. \$\endgroup\$
    – D.A.S.
    Commented Oct 11, 2019 at 16:51
  • \$\begingroup\$ Each new 16AH battery ESR is about 16 mohms. The busbar connections ought to be << 1 mohm. The cable AWG mOhm/m * length determines I*R voltage drop and wiki shows these values. Welding or jumper cables might be suitable. but 0.1V drop @ 40A means 2.5 mOhm overall loop resistance. \$\endgroup\$
    – D.A.S.
    Commented Oct 11, 2019 at 17:13
  • \$\begingroup\$ Sunnyskyguy: the internal resistance of the batteries is "ultra low" to the point where at constant voltage the charge current capacity is unlimited. When I referenced jumper cable I simply meant the link from terminal to bus bar. my intended bus bar is salvaged bars from converter boxes which I believe to be coated copper (I am going to verify before use). The bars are approximately 1 in by .25 in in size with .25 in holes drilled for connections. \$\endgroup\$
    – Joseph Thurlow
    Commented Oct 12, 2019 at 18:14
  • \$\begingroup\$ You can measure ESR of batteries and cables with a known current such xx mV drop / xx A =ESR , unlimited current is not realistic. Each cell in series will add ESR. \$\endgroup\$
    – D.A.S.
    Commented Oct 12, 2019 at 20:07
  • \$\begingroup\$ So being wired entirely in parallel I am confused why the ESR is being emphasized here. Obviously I want as little resistance as possible to allow for larger charge currents but these batteries will offer no issues there. They could easily withstand 100 amp current and my plans really only include up to about 80 amps charge or load current. Maybe some clarification on the importance of your input? My electrical theory is not up to par \$\endgroup\$
    – Joseph Thurlow
    Commented Oct 12, 2019 at 21:12

1 Answer 1

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Uneven charging is the reason. The risk you haven't addressed is the possibility of improperly charging one of the batteries when they are connected in parallel. In particular, you have no way to guarantee that the charging current to each battery is below its specified maximum current. If one battery is older, or has been discharged further, then it may draw more than an equal share of the charging current. Worse, if one cell in one battery fails shorted then that battery will draw the lion's share of the current which may cause overcharging and the release of hazardous amounts of hydrogen gas.

One way to prevent overcurrent is to limit the total charging current to no more than the maximum charging current for just one battery. Unfortunately, this will result in slow recharge times, particularly as the number of batteries increases.

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  • \$\begingroup\$ Hi Elliot, Thanks for the feedback. I have the data sheet from the manufacturer and the current from a constant voltage charger is unlimited due to the ultra low internal resistance of these batteries. could I not prevent one battery with a bad cell from drawing too much current by fusing one side of each jumper to the bus bar? In the design of my box for these batteries I have incorporated a spark free fan that draws fresh climate controlled air and vents to the outside. I believe that should eliminate the fear of H gas buildup. With all this in mind do you think it would be a safe operation \$\endgroup\$
    – Joseph Thurlow
    Commented Oct 12, 2019 at 17:49
  • \$\begingroup\$ I'm not willing to say that anything would be safe because I don't have all of the information and I can't see the batteries with my own eyes. Fusing is certainly a good idea, but can you use fuses with a low enough amperage to protect from overcharging and still draw enough current during discharge? I really think you need to find out why the manufacturer suggests no more than 3 batteries. \$\endgroup\$
    – Elliot Alderson
    Commented Oct 12, 2019 at 18:16
  • \$\begingroup\$ It is the manufacturer of this solar panels and the charge controller not the manufacturer of the batteries that poses the issue of not linking more than 3 in parallel. \$\endgroup\$
    – Joseph Thurlow
    Commented Oct 12, 2019 at 19:52

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