Connecting batteries in parallel

There are two ways to wire batteries together, parallel and series. The illustration below show how these wiring variations can produce different voltage and amp hour outputs.

In the graphics we’ve used sealed lead acid batteries but the concepts of how units are connected is true of all battery types.

Wiring batteries in parallel and series
Different wiring configurations give us different voltages or amp hour capacities.

This article deals with issues surrounding wiring in parallel (i.e. increasing amp hour capacity). For more information on wiring in series see Connecting batteries in series, or our article on building battery banks.

Connecting in parallel increases amp hour capacity only

The basic concept is that when connecting in parallel, you add the amp hour ratings of the batteries together, but the voltage remains the same. For example:

  • two 6 volt 4.5 Ah batteries wired in parallel are capable of providing 6 volt 9 amp hours (4.5 Ah + 4.5 Ah).
  • four 1.2 volt 2,000 mAh wired in parallel can provide 1.2 volt 8,000 mAh (2,000 mAh x 4).

But what happens if you wire batteries of different voltages and amp hour capacities together in parallel?

Connecting batteries of different voltages in parallel

This is the big “no go area”. The battery with the higher voltage will attempt to charge the battery with the lower voltage to create a balance in the circuit.

  • primary (disposable) batteries – they are not designed to take a charge and so the lower voltage battery is likely to overheat, it may leak or bulge and in extreme circumstances where the voltages are very different, it may explode.
  • secondary (rechargeable) batteries – these only fair a little better. The lower voltage battery is not designed to charge above a certain point, but the higher voltage battery will try anyway. The result can be over heating, leaking or bulging in the lower voltage battery and/or overheating in the higher voltage battery as it drains rapidly. Again, the larger the difference in voltage the greater the chance of fire or explosion.

It’s worth pointing out that many people accidentally connect batteries of different voltages in parallel every day. For example:

  • If you mix brands even of the same labelled voltage – you can experience problems. Due to different manufacturing processes, the exact voltages of batteries from different producers can vary slightly. This means a 1.5 volt battery from brand X could actually be 1.6 volts, while a 1.5 volt battery from brand Y could be 1.55 volts. If these were connected in parallel, you are unlikely to see fireworks, but would experience other issues.
    • for primary (disposable) batteries – the stronger battery would still try to charge the weaker one reducing the lifespan of both.
    • for secondary (rechargeable) batteries – the stronger battery would charge the weaker one, draining itself and wasting energy.
  • If you connect rechargeable batteries in parallel and one is discharged while the others are charged – the charged batteries will attempt to charge the discharged battery. With no resistance to slow this charging process, the charged units can overheat as they rapidly drain and the discharged battery can overheat as it attempts to charge at well above its design capabilities.
  • If you mix batteries of different ages – the older batteries will always have a lower voltage as all batteries self-discharge over time. Even rechargeable batteries will not recharge to the same level as new ones.

As such, the following guidelines are important:

  • With primary (disposable) batteries – only use batteries of the same brand and age (ideally from the same packet). If this isn’t possible, double check the voltages of each unit with a voltmeter.
  • With secondary (rechargeable) batteries – only use batteries of the same brand and age and make sure all the units are fully charged before connecting them together in parallel. If you are uncertain about the state of charge, either connect them individually to a charger until the charger confirms they are fully charged, or check the voltage with a voltmeter.

Connecting batteries of different amp hour capacities in parallel

This is possible and won’t cause any major issues, but it is important to note some potential issues:

  • Check your battery chemistries – Sealed Lead Acid batteries for example have different charge points than flooded lead acid units. This means that if recharging the two together, some batteries will never fully charge. The result here would be sulfation of those that never reach a full state of charge, reducing their lifespan.
  • Double check voltages – if you are using batteries with different amp hour capacities, it is highly likely that the voltages will be different (even if the stated voltage on the labels match). Check this with a voltmeter or you will experience problems (covered in connecting batteries of different voltages in parallel above).

It is for these reasons that you are advised to use batteries of the same brand, voltage and capacity. Failing to do so (if you don’t have the knowledge and tools to check what you are doing) could create a potentially dangerous circuit.

Was this article helpful?

Related Articles

11 Comments

  1. Peter

    I was looking on amazon for a converter to allow me to use a AA battery in place of a D cell. Many of the ones sold seem to use 3 AA hooked in parralel. This seems very dangerous to me, so I didn’t buy them. What would happen if someone put an alkaline and a NiMH in together?

  2. BatteryGuy Editor

    Peter,
    You never want to mix battery chemistries together. NIMH is meant to be recharged and Alakaline used and thrown out. Very bad idea. If an Alkaline battery were to be charged with a NIMH in a device with a charging circuit, it would probably explode in the device and ruin the product it was in. If it were a standard Lithium battery charged within a device, it could create a fire. In a device not meant to charge the batteries where you mixed Alkaline and NIMH chemistries, one would negate the other battery and damage the device or batteries. NIMH on their own can replace Alkaline, but have to be charged once they run down, the benefit being a longer lifetime. However, never mix the chemistries.

    To replace an Alkaline D Cell with AA Alkaline batteries you would need approx. 6 or 7 AA Alkaline batteries. You would run them in Parralel and it would be based on the AH rating of the AA -vs- the AH rating of the D cell. Most AA cells are around 3AH and most D cells are around 20AH.

  3. mark

    batteries connected in series, Why does it not increase a/h please explain.

    1. BatteryGuy Editor

      EXAMPLE: Two 6 Volt 4.5AH SLA batteries wired in Series would be a total output of 12 Volt 4.5ah. A battery has two terminals, one that gains electrons and one which gives electrons. Within the battery an electrochemical reaction occurs to produce electrons. Since the resistance of a battery is low, when connected in series, an increased concentration of electrons goes to the negative terminal. Once you connect wire from the positive (+) terminal of battery #2 to the negative (-) terminal of battery #1 the concentration of electrons shift toward the negative terminal and join forces in battery #1 as they flow forward.

      Why do the electrons not move to the positive (+) terminal from inside the battery? – This is because of a formation of a magnetic field / barrier which only allows the flow of current in one direction. Since the electrons cannot move against the flow, they move with the current and exit the only viable way via battery #1 through the positive red wire to the object being powered, compensating for the loss of electrons from the positive (+) terminal. The electrons have now combined forces for a total output of 12 volts 4.5ah.

  4. Robert P DiPuppo

    I have a secondary sump pump that is supposed to pump water out of the sump pit in the event I lose power while I’m not at home. This is very important as I live in an area that when the wind blows a little we can lose power. I test this unit monthly and change batteries every 2-3 years. Recently a friend who is a plumber removed a similar system from a house and the former owner gave him permission to take the charger, battery case and battery. He gave it to me and now I want to hook these units together. The batteries are the same voltage, deep cycle marine batteries, however they are from different manufacturers, and I have no idea how old the gifted battery is although it looks newer than my battery which is 18 months in service. My question is this: How do I hook the charging system (which delivers a trickle charge and disconnects when the battery is at maximum) to the batteries when they are in parallel hook up? (I assume parallel is best for this application because the pump is designed for 12v DC so combining 2 would give me longer service at the same voltage. Correct?) Do I need both chargers, one for each battery? [NOTE: Since my battery is 18mos old and I don’t know the age of the other battery, I am content to wait a couple months and buy 2 new identical batteries. I have no long trips planned and I have a generator to power the pump in case of an outage. Unfortunately it’s not a whole house generator but one step at a time.] I would appreciate it if you gave me a sketch or schematic (of the circuit) of how to hook the batteries in parallel with the chargers and the pump. And Thank You Very Much, your time is much appreciated. I’ve had several floods in my basement including 1 with the back up pump installed. The motor seized and my now finished basement needed new drywall and flooring.
    Thank you once again!

    1. BatteryGuy Editor

      ROBERT,

      YOU DON’T WANT TO SET UP THE BATTERIES YOU CURRENTLY HAVE IN PARALLEL. NEWER AND OLDER BATTERIES AS WELL AS DIFFERENT BRANDED BATTERIES HOOKED TOGETHER IN PARALLEL WILL CAUSE THEM TO FAIL QUICKLY. I WOULD WAIT UNTIL YOU HAVE NEW BATTERIES OF THE SAME AGE, BRAND, VOLTS AND AH RATING. ONCE THESE WERE TO BE HOOKED TOGETHER IN PARALLEL, YES THIS WOULD INCREASE THE PUMPS RUN TIME WHEN THE POWER WAS DOWN. FOR A CHARGING SYSTEM, YOU JUST WANT ONE. YOU WOULD HOOK THE CHARGING SYSTEM TO THE LEAD BATTERY FOR IT TO CHARGE BOTH. AS TO A CHARGING SYSTEM, IT NEEDS TO BE ABLE TO HANDLE 12 VOLTS AND WHAT EVER THE COMBINED AH RATING OF THE BATTERIES IS. TAKE THE NUMBER OF BATTERIES AND MULTIPLY IT BY THE AH RATING OF A SINGLE BATTERY TO GET THE TOTAL AH RATING THE CHARGER WILL HAVE TO BE ABLE TO HANDLE. THERE IS A SMALL PROBABILITY THAT ONE OF THE CHARGING SYSTEMS YOU HAVE IS OVER RATED NOW IS RATED AND MIGHT WORK, BUT YOU WOULD NEED TO CHECK.

  5. Lou

    It’s time to replace my 3 12 v batteries wired in series (36V system) with new ones. When removing the old batteries do I remove all 3 negative leads first then do all the positives? Or do I remove one batteries neg. then the same batteries pos. then move to the next and the final battery? I’ll reverse what ever method you recommend when installing of course. Any other tips?Thanks

  6. Kevin Wallace

    I’m upgrading my power in my truck camper because replaced my absorption fridge for a compressor fridge. I bought a new AGM group 31 battery 12 volt. I would like to buy one more new AGM battery but half the size. Both new from same manufacturer same model and wire them parallel. I don’t have space for 2 group 31. What do you think? Thank you

  7. Mark

    Please is it possible to connect two 6volt, 4.5 ah battery together to give 12volts and 9ah?

  8. Chris

    If i have two batteries connected in parallel, i want to charge them together by connecting the charger to 1 battery’s positive and the other battery’s negative correct? Also when I’m using them to power my trolling motor do i connect it the same way with the positive of one battery and the negative of the other one?

  9. Nathan Garcia

    Let’s say I’m using lithium ion tool batteries to power a mini bike I am putting 2 20v 4Ahr in series to make 40 v then I want to put two 20v 2ah in series but parallel with the first two. Do I need to put diodes between the two 40v banks?
    Thank you in advance.

Leave a Reply to Nathan Garcia Cancel Reply