If you haven’t read LEAD-ACID BATTERIES 101, please click on the link and read it.  It contains a lot of great information and it lays the ground-work for understanding other lead-acid battery topics.

Battery Banks

There are many reasons you would want to connect two or more batteries together.  You can increase Voltage or increase Storage by connecting batteries together.  First some rules:

  • Only connect the same Type batteries together.
  • Only connect the same Capacity (amp hour) batteries together.
  • Only connect the same Age batteries together.
  • Use the same gauge and length of cable to connect batteries together.
  • Only connect fully charged batteries together.
  • If you have limited knowledge and expertise, Don’t connect more than Four batteries together for a battery bank.


Mixing different type, size and age of battery will cause many problems, such as unequal charging and discharging which will significantly shorten battery life.

Cable gauge is also very important.  I use #0 AWG (single ought) for my battery bank.  Check a wire size guide for your application.  Using small gauge cable can cause overheating.  Using different length cables can cause charge and discharge differences in the batteries.  You can have the cables made to custom lengths or you can buy ready made cables.  Here are pre-made cables that will work for most applications.   Red #0 AWG 12 inch Cable and Black #0 AWG 12 inch Cable.  #00 cables are actually the same price if you want to go one gauge larger.


Battery Bank Connections

You can connect batteries in Series or in Parallel or in a combination of both.  Depending on how you connect them will give you very different results.


Series –  To connect two batteries in series, you connect the Positive (+) terminal of battery #1 to the Negative (-) terminal of battery #2.  When you connect batteries in series the voltage increases but the capacity stays the same.  Connecting two 6 volt, 100 amp hour batteries together in series would give you a 12 volt, 100 amp hour battery bank.



Parallel – To connect two batteries in parallel, you connect the Positive (+) terminal of battery #1 to the Positive (+) terminal of battery #2 and you connect the Negative (-) terminal of battery #1 to the Negative (-) terminal of battery #2.  When you connect batteries in parallel the voltage stays the same but the capacity increases.  Connecting two 12 volt, 100 amp hour batteries together in parallel would give you a 12 volt, 200 amp hour battery bank.




Combination – It is possible to have batteries connected in series and in parallel in the same battery bank.

If you have four 6 volt 100 Ah golf cart batteries that you wanted to use for a 12 volt battery, you’d connect two of the four batteries in series, connect the other two batteries in series, and then connect the two sets of batteries in parallel.




Connecting Three & Four Batteries – If you connect more than two batteries together, it is important how you connect them.  You need to connect them in a way that balances the discharging and charging of each battery in the bank. If not connected correctly you will put uneven stress on the batteries and you will decrease the storage capacity of your battery bank and the life of the batteries in the battery bank.


Worst Way – The worst way to connect batteries in a battery bank is the way it is most shown on the web.  The problem with this way of connecting batteries in your bank is that the first battery in the bank is worked twice as hard as the battery at the far end of the bank.  This is true for both charging and discharging.






Better Way – By making a small revision in the connections, having the Positive (+) and Negative (-) leads at opposite ends of the battery bank, the battery bank is almost in balance.  The draw is almost even from all four batteries.  A very simple change with a very big improvement.  Check out Smart Gauge for the exact draws and differences between these two methods.

A float charger or charge controller from solar panels should be connected to the unused terminals on batteries #1 and #4.  This will help balance the bank during charging.




Best Way – If you want a truly balanced battery bank, connect each Positive (+) terminal to a common Positive (+) lead and each Negative (-) terminal to a common Negative (-) lead.  All 8 cables should be the same length.  Some experts wonder if the additional cost and work is worth a small increase in efficiency.  Unless you have large batteries that are heavily worked, it probably isn’t worth the time and cost.






Battery Bank Monitoring

To maximize battery life deep-cycle batteries should not be discharged past 50% (60% – 75% is better).  You can use a hydrometer to measure the specific gravity of the electrolyte solution and determine the SOC (state of charge).

State of Charge

Specific Gravity


























There is an easier way to check the state-of-charge.  You can attach a battery monitor to your battery or battery bank.  I have one attached to my battery bank and it makes checking the battery bank’s SOE very easy.  The Equus Battery Monitor has a digital readout for voltage.  Based on the chart a 12.7 voltage reading means a 12 volt battery bank is fully charged.  A 12.4 voltage reading means a 12 volt battery bank is 75% charged.  A 12.2 voltage reading means a 12 volt battery bank is 50% charged.


Equus 3721 Battery Monitor   Displays the condition of the battery and charging system.

For 12-volt negative ground systems.  Easily plugs into a cigarette lighter socket.  Head pivots for easy viewing.  Currently $15 shipped.


You will need to a cigarette lighter socket adapter.  Roadpro 12v Battery Clip-On and Cigarette Lighter Adapter  Currently $5 shipped.



Connecting an Inverter

In most cases an inverter is connected to a battery bank to convert 12 volt battery power to 115 volt power.  Most inverters connected to a 3 or 4 battery bank will be rated at 1500 watts or more.  Be sure to read the wiring guide that comes with your inverter to select the proper wire gauge to use between the battery bank and inverter.  Try to keep the inverter as close to your battery bank as possible to avoid power loss.  I keep my inverter next to my battery bank and I oversize my connection cables (#0 AWG).


Finally place a Single ANL Fuse Holder with a 300 Amp ANL Fuse in line on the Positive(+) cable between the battery bank and inverter.  The fuse will protect both the battery bank and inverter from damage due to shorts, surges, or overloads.  Be sure to keep some extra fuses around in case one burns out.


Battery Bank Do’s

  • Inspect and service your batteries every three months, more often if the batteries are exposed to heat and hot temperatures.
  • Make sure plates are covered by electrolyte solution.
  • Only use Distilled Water when adding liquid to electrolyte solution.
  • Keep your battery as fully charged as possible.
  • Use a “Smart” Three-Stage charger to float charge your battery bank.
  • Reduce your average depth of charge.
  • Make sure batteries and the battery bank are fully charged before adding a battery to a battery bank.
  • Use #1 or #0 AWG cable or thicker to connect batteries in a battery bank.
  • Connect a 300 amp fuse to the Positive (+) lead between the battery bank and the inverter.


Battery Bank Don’ts

  • Don’t add more acid to the electrolyte solution.
  • Don’t use tap-water to add fluid to the electrolyte solution.
  • Don’t use an unregulated, high output battery charger to charge your battery.
  • Don’t overcharge your battery.
  • Don’t discharge deep-cycle batteries more than 50% (only 40% is better).
  • Don’t mix different size, type, and age of batteries in a battery bank.
  • Don’t rely solely on solar panels and a charge controller to keep your battery bank charged.  There are many days that the sun doesn’t shine.

I highly recommend Bill Darden’s Battery FAQ.  CAR AND DEEP CYCLE BATTERY FAQ


Ed Rogers
Copyright “Keep It Simple” 2013. All Rights Reserved.

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