Connecting batteries in series

There are two ways to wire batteries together, parallel and series. The illustrations below show how these set 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.

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

Connecting in series increases voltage only

The basic concept when connecting in series is that you add the voltages of the batteries together, but the amp hour capacity remains the same. As in the diagram above, two 6 volt 4.5 ah batteries wired in series are capable of providing 12 volts (6 volts + 6 volts) and 4.5 amp hours.

This is where most tutorials end, but what happens if you wire batteries of different voltages and amp hour capacities together?  Most people simply answer by telling you “Don’t do it!” … but why not?

Connecting batteries of different voltages in series

In theory, a 6 volt 5 Ah battery and a 12 volt 5 Ah battery connected in series will give a supply of 18 volts (6 volts + 12 volts) and 5 Ah. A 6 volt battery is often three 2 volt cells and a 12 volt battery is usually six 2 volt cells. Therefore, all you have done is connected nine 2 volt cells together to get 18 volts … so what’s the problem?

The reality is that no 6 volt battery is exactly 6 volts and no 12 volt battery is exactly 12 volts. Individual cell voltages differ, even with batteries of the same brand and manufacturer. A 6 volt battery might have a cell voltage of 2.2 volts and a 12 volt battery might have a cell voltage of 2.1 volts. This can however be fairly easy to read with a volt meter if one was to check.

Matching amp hour ratings is much more difficult. The 6 volt battery might really be a 5.2 Ah, while the 12 volt battery might be 5.5 Ah. Amp hour ratings are also much harder to test without accurately discharging both units at the same rate under the same conditions and accurately measuring the results.

You also need to check with the manufacturer on how they arrived at their amp hour rating, because different manufacturers use different methods – not all 5 Ah batteries are 5 Ah in the way you might think. Some manufacturers will claim their battery is 5 Ah using the “20 hour rating”, while others will say their battery is 5 Ah using the “100 hour rating”. For more on this subject see Which deep cycle ah battery.

Furthermore, these ratings and behaviors can be different depending on the structure of the battery. A flooded lead acid battery may have different discharge and recharge patterns compared to a sealed lead acid battery.

What do these issues mean in practice?

The first practical outcome is that the amp hour capacity will be the lowest of the batteries connected together.  In the example above, this would be the 5.2 Ah battery. Not a disaster if you were only expecting 5 Ah, at least not a problem right away.  If you were to connect a device to the battery bank it is capable of powering (say a 0.5 amp bulb) then it would work.

The real problems arise during discharging and recharging cycles (if the batteries are rechargeable) .

Discharging

During discharge the weaker battery will run flat first. As batteries discharge, their voltage drops. When this voltage drops in a device below a certain point, the auto cut-off may engage, switching off the item or causing it to refuse to operate. Its one reason why the ignition lights in a car might turn on, but the starter motor wants nothing to do with you.

These built in cut-off points are there because batteries have a shorter life if they are run completely flat each time. In fact, if you look closely at some manufacturers who claim their battery will last for thousands of cycles, they clearly state something along the lines of “when discharged to 80% State of Charge”.

In our example, we are powering an 18 volt device, which may have a cut off at 16 volts. Our smaller 6 volt battery as it drains might drop to 5 volts, but the 12 volt battery (which is actually in this example 12.6 volts) still has enough charge. Meaning the total voltage being supplied is 17.6 volts (5 volts + 12.6 volts).

The 6 volt battery should be disconnected by now, but the circuit is being kept alive by the larger 12 volt unit as the smaller battery continues to drain, moving far below its design capabilities.

This is not an immediate disaster for disposable batteries, but for rechargeable batteries you will dramatically shorten the life of the battery as well as its ability to recharge.

Disposable battery issues

When the weaker battery is almost completely drained, the stronger battery will attempt to recharge it in order to keep the circuit alive.

Attempting to recharge disposable batteries can lead to a build up of hot gases internally, which can cause the case to crack and leak. In extreme cases, it could catch fire or explode.

Reverse Polarity

When some rechargeable battery types (the emphasis on some) have been completely drained, there is no chemical difference between the negative and positive plates. In our example, the 6 volt battery would hit this point first, but the 12 volt battery is keeping the circuit alive and would start attempting to recharge the smaller battery.

By forcing current through the dead battery in this way, it can reverse the terminals of the weaker battery – positive becomes negative and negative becomes positive. Now, in effect, we have the 6 volt battery positive terminal connected to the 12 volt battery’s positive terminal. Not good.

In most circumstances, both batteries would be almost completely dead by this point. Their ability to explode dramatically would be low, but you might see leaks caused by hot gases venting as this person found inside a child’s toy or as witnessed by batteries connected in series in this clock.

However the bigger the difference between the two batteries, the more potential for a dramatic event!

Recharging

Assuming nothing has exploded, but the 12 volt battery eventually dropped in voltage to a point where the device cut off the supply, you are left with a fairly flat 12 volt battery and a very flat 6 volt battery. Time to recharge.

As the batteries charge, their voltages rise again and this time the smaller battery charges faster. Most chargers, like various equipment, have a cut off point. In our example, if both batteries were fully charged, they would actually give off 19.2 volts (12.6 volts + 6.6 volts) but our charger wants to cut off at 18 volts (or a little over).

The smaller battery will get to 6.6 volts faster, but because the overall circuit has not hit 18 volts, the 6 volt battery will then start overcharging and possibly result in internal damage. To get to the chargers cut off point, the larger battery only needs to achieve 11.4 volts.

The result is an overcharged 6 volt battery and an undercharged 12 volt battery. Undercharging on a regularly basis also causes internal issues such as sulfation.

Summary

In short, connecting batteries of different voltages in series will work, but damage will be done to both batteries during the discharge and recharge cycles. The more one is damaged, the more the other one will be damaged and both will need replacing long before needed.

The greater the difference between the batteries capabilities, the faster this damage will occur.

Even if you could get both a 6 volt and a 12 volt battery with exactly the same cell voltage, a problem would arise due to the small difference in the amp hour capacity, a rating very difficult to measure. This would shorten the life of the smaller battery through the over-discharging and over-recharging described and shorten the life of the larger battery through under-charging.

Connecting batteries of different amp hour ratings in series

In theory a 6 volt 3 Ah battery and a 6 volt 5 Ah battery connected in series would give a supply of 12 volts 3 Ah (the capacity of the weaker battery always restricts the circuit) and if you did so it would work and nothing would explode (to start with).

But, as covered above, 6 volt 3 Ah batteries are not exactly 6 volts and 6 volt 5 Ah batteries are not exactly 6 volts. Using different batteries increases the chance of this voltage mismatch. The result is exactly the same, therefore as connecting batteries of different voltage in series (see above). However, if it were possible to find two batteries or cells that both had identical voltages, what would happen then?

Discharging

The voltage of batteries drops as they are discharged. Most battery operated devices are designed to recognize this drop in voltage and stop operating. So, a 6 volt device may stop working when the battery supply drops to 5 volts. This fail safe is designed to stop excessive discharge of the battery which would shorten its life.

In our example, the smaller 3 Ah battery will drain faster (it’s just simply a smaller batter) and its voltage will then drop. However, the larger 5 Ah battery will still be maintaining its voltage, allowing the overall circuit voltage to be enough for the device to continue drawing current.

The result is that the 3Ah battery will discharge far below the point it is designed to withstand. If it runs completely flat, reverse polarity (see above) is possible.

Recharging

The smaller 3 Ah battery would recharge faster and recover its 6 volts. However, the 5 Ah battery would not be fully charged by this point and the charger, seeing 12 volts has not yet been achieved, would continue to charge the circuit. The result is overcharging of the 3 Ah unit causing it further damage.

Connecting batteries of different voltages and amp hour ratings in series

As covered in the section Connecting batteries of different voltages in series above, the greater the differences in either voltage or amp hour rating, the more the discharging and recharging is unbalanced and the more damage you do to the batteries through over-discharging and over-charging the weaker ones and under-charging the stronger ones.

Small differences can lead to reverse polarity that causes leaks or bulges. Very large differences can result in explosions. This is why the short answer to connecting differently rated batteries in series is “Don’t”.

The age factor of batteries

When connecting batteries in series, the general advice is to use batteries of the same ratings and the same make and model in order to minimize differences in exact voltage and amperage. Note, we say ‘minimize’, because even batteries coming off the same production line can vary slightly in these measurements.

Another factor is battery age.

The older batteries get, both in terms of time since they were manufactured and in how many times they have been discharged and charged, the more this affects their real voltage and amp hour capacity. This means that if you have two batteries in series of the same voltage and amp hour capacity that you have been using for a while, but replace one with a new unit, what you have in reality is one battery with a higher voltage and amperage (the new battery) than the other older battery.

The result is that the older unit will incur greater damage through over-discharge and over-charge, while the newer one will be damaged by under-charging.

In the case of disposable batteries, the older battery might split and leak when it runs completely flat and the newer unit tries to recharge it.

Best practice when connecting batteries in series

As discussed in this article, the closer the voltages and amp hour capacities of the various batteries wired together match, the less damage they will do to each other. Age also plays a part in these ratings and this is why it is usually recommended that you:

• Only use batteries of the same voltage and amp hour capacity from the same manufacturer and brand
• Replace all the batteries at the same time
• Replace all the batteries with ‘new’ ones (the same batch number or use by date)

Not following these rules does not mean that your batteries in parallel won’t work, just that it will cost more in the long run as the batteries will need to be replaced more frequently. There is also an outside risk of explosion if you have to many batteries of varying volts and amps or to big a variance from one battery to the other.

When you can mix different rated batteries in series

While the answer to connecting batteries with different ratings is usually “Don’t”, it should really be “Don’t without balancing circuitry”. Balancing circuitry monitors individual batteries or cells to ensure that the entire circuit shuts down when the voltage of the weakest cell or battery falls to a certain point. Balancing circuitry also assures that each battery or cell is fully recharged.

Related Articles

1. Christopher Brown

We have 3 12volt hooked in a series to make 36bolt, but the middle battery has reversed polarity without wrong charging wondering why and which buttery causing this?

2. Fred Barnes

I have an ezgo 48v battery series. When checking the batteries with a voltmeter, I get good readings on all, both individually and in series. Except one. When I check the negative of battery one against the positive of battery 2, I get no reading. Any thoughts?

1. Kris

You are only reading across the connecting cable, the difference between each end of the cable is 0v.

3. Jamie

Great article. You seem to be more in depth than others I’ve read. Would you say no to running a kids power wheels 6v in series with the existing 12v to make it 18v. Looking for a little more speed and fun. Im not worried about the battery life so much. Just safety.

1. BatteryGuy Editor

This could be done in Series, but the AH amperage ratings for each battery would need to be the same to prevent irregularities. 18 volts should increase speed without putting too much strain on the motor or gears, but can’t guarantee it wearing out quicker. Make sure there is an inline fuse to protects against power surges. Also, more speed means more potential for a child to in up in an accident.

4. Denis

Hi ! Is it possible to connect 2 batteries in series 12v 100amp/hours with one of the same battery 12v 100amp/hours in parrallel .because my inverter doesnot take 36 v so 24v is ok but want to make sure if it,s ok . please let me know .

1. Rafael velasco

I was connecting two batteries in series for a 24v solar system. I got a spark which I kind of suspected, But then I can hear the battery bubbling and steam coming out of the battery. Yes battery was hot. The 4 gauge battery are toast. Help me understand what I did wrong. If you can

5. DJ

My friend and I bought 12v SLA batteries at about the same time. He got a pair with 200 AH each and I got a pair of 100AH each. I set my pair up in series to get a 24V 100AH battery backup. He has since upgraded and I’d like to get his old batteries. If everything works, I guess my best bet is to swap my batteries out for his and keep mine on standby. Is that right? We live in a third world situation and I don’t think I can source ‘balancing circuitry’ here. Also, what size fuse and where in the circuit should I be placing it? Any help would be greatly appreciated.

I am setting up an off-grid solar/wind turbine system for my garden office. I have a bank of 3 x 12v batteries and a 12/24v inverter so how would that work if they are in series or should I install them in parallel, advice would be good.

7. Evert Spies

Your wiring diagrams show NO for Series-Parrarell circuits. Would it not be fine if all cells were of equal voltage and amp hour rating?

8. james Winks

Dear Battery guy
I have a solar battery bank of 8 six volt batteries. I have one battery with a bad cell. I’m not financially ready to buy a new bank. If I continue using the system will the bad battery ruin the seven others? And is there anything I can try to fix the bad cell? They are Trojan L16 RE-B 370 AH

9. John

I have a 2 volt 400AH battery that I want to charge every six months or so. I have a 12 volt trickle charger, a 6/12 volt charger and a spare 12 volt car battery. I only want to stop the 2 volt cell from dying from not being charged occasionally. Can I do this with what I have?

10. Kit

OK – 2 – 12V in series, 200Ah, 2520W = 1 – 24V, 200Ah, W? Does the wattage double like the volts or remain the same like the Ah? LFE batteries

11. virangi silva

I have 3.8V battery and 3.85V battery. (different mAh) So I want to know, can I connect these two batteries in series? will it damage batteries?

12. Hollister

Great article, thank you. Like the person with powerwheels, I have been toying w/ a similaridea for a “boost button” for an ebike or scooter. The main difference would be that it would only be connected for short periods w a momentary switch. Also I would use separate chargers to charge them correctly in disconnected state. Maybe use a diode to insure the bigger battery doesnt end up wasting power trying to charge smaller battery? A typical use case for me might be 48v 13ah main w a 12v 8ah booster. Might be better if i could make the booster a bit beefier at 13ah to match… but if Im only using for short temp bursts do you think it matters that much? Thank you, Hollister

1. BatteryGuy

We do not supply ebikes so have no expertise in this area. I’d recommend asking the question on an ebike forum

13. Ron Dockery

Is it possible to wire 3 batteries in series for a 24v system to run lights and heat tape

14. William hodgkins

I had a scooter batter pack at 36 volts short and now puts out over120.volts. Hooked my drill and it ran strong ‘.any ideas. I could use it for a emergency light back up?

15. Wali Rashid

Using six SLA 12v 35ah batteries in series to acheive 72v need to power my ehub motor.
Problem I’m having it’s not knowing how to parallel them as well. Every attempt so far ends with sparks flying and cable ends fusing to battery terminals!. I’ve tried neg to pos and got 72v
I’ve series 2 together to create 3x 24v packs then series connect all 3 sets together and still acheived 72v. I just don’t know how to do the parallel method without making sparks
fly.

16. Jim White

I’m connecting 6 8volt batteries in series to get 48 volts for my ez-go txt. I know when I connect the batteries, I will have an open plus cable and an open neg cabel. What I ddont know is what connects to the open posts? Is it a charger? What wire connects? What about the accessories? There is a three wire step down from 48 to 12 volt gadget. Where do I connect that? Thanks.

1. BatteryGuy

Your probably going to find the answer you need in an electrical forum is we do not deal in inverters or other items for such a complex set up.

17. Gerry

With 2 – 6V identical batteries in series and being charged with 12V charger at say 10 amps , is EACH 6 v battery charging at 10 amps or??

18. Emil Iannace

I have a (60 volt) golf cart, can I connect sixed battery in parallel with the last battery to keep my 60 volts but get more range

19. Ron

I understand how to connect batteries (rechargeable NIMH) together to get the voltage I want but I need to know how to connect them without welding. Do you know a good way to do this?

1. BatteryGuy

An electrical forum would probably be a good place to ask for options.

20. Mike

Very useful article – Can you take a 12 volt supply by using one of the batteries of the two that are connected in series to give a 24 volt supply?

1. BatteryGuy

If the setup uses 2 x 12 volt batteries then, once disconnected, each battery will output 12 volts.

21. Michael

Thank you for a great article, could you please advise me why it would be necessary to wire rechargeable batteries in series and parallel? There are a bank of 6 batteries totalling 14.4 volts.
Also what would be the voltage of each battery.
Kind Regards
Michael

22. Robert

I’m building an emergency well pump power system. I wouldn’t use this unless the grid was down for over a day, which had never happened. I would connect two different type (LiFePo4 and flooded) 12V batteries with similar amp hour ratings in series. The 24V power station would go through an inverter to drive a 230V 1/2 hp submersible well pump for about 15 minutes once a day. That would give me 100 gallons of water in a surface tank.
I can recharge the batteries as separate 12 V batteres with solar panels.

1. BatteryGuy

Don’t use different chemistries when connecting in series. Although they may have the same voltage and amperage ratings on their labels these will not be their true outputs. Even identical batteries from the same manufacturer differ from batch to batch. Essentially this means you will have ‘different’ batteries connected in series and one will damage the other.

23. Jensen

I have been running multiple batteries of different voltages in series with no issues what so ever. It’s all in the technique and extra steps required to successfully run different voltages in series. I currently run 84v on my custom built ebike and run 2 to 3 batteries in series from packs I made from failing old ebike battery packs from a factory. I put balance cables on the custom packs and charge them separately with a balance charger. I also put battery low voltage alarms on the the custom batteries and when they get to 3.3 v they sound off an alarm and I know it’s almost time to go home disconnect the batteries and charge them separately. All 3 batteries are above low voltage when I’m finished and I charge all separately till full and reconnect them in series when i’m ready for a ride. Been doing this for years and have had no issues with any of the batteries what so ever. So it is not completely 100% true about batteries not being able to run in series with different voltages and have a healthy batttery. That is tru if you permanently connected all the batteries and tried tp charge them all together at once with one charger. My method is not easy but is 100% doable and your always left with healthy batteries as long as u use a low voltage alarm on the smallest battery.

24. Carl Parkinson

Hi battery Guy, I have been using a battery bank in a forklift truck, 24/2v cells for energy storage
With my solar Pv hybrid 48v inverter system. Amusingly I also disconnect the truck and use it as a forklift as and when required, the system gives me this facility.
Having been forced to up-spec the truck, the replacement is 80v ! 40/ 2v cells, I took a punt on this, with my favourite problem solving phrase in mind, Necessity is the mother of all invention !
Would you have any suggestions how I could maintain the dual function of this battery bank ?
Great article by the way !
Kind regards Carl.

1. BatteryGuy

Not sure what you are asking here. Do you want a battery bank that can switch between 40 volt and 80 volt output?

25. Daniel

I am in the process of powering my Garden railroad with batteries . due to size and output 3 6volt batteries would be perfect . can 3 batteries be wired in series ?

1. BatteryGuy

Yes – theoretically any number of batteries can be wired in series as long as they have the same voltage and amperage ratings.

26. Rich

I’ve got a Yeti 1250 solar charger with an internal 12v 110ah battery, and chained to that is another 110ah battery, both batteries are Mighty Max brand. The Yeti is connected to two 100wat slar panels.
So far so good.
I also have a third 12v batterry, but it’s only100ah (Ritar brand)…. Would adding this battery to the chain be too much of a missmatch? Or do you think it would be alright?

1. BatteryGuy

Yes – it’s a mismatch. In any setup (parallel or series) all the batteries should be the same voltage and amperage. Ideally they should all be from the same manufacturer and the same batch (as each factory batch produces small variations).

27. Tim

Good morning

we bought a sailing boat with a Torqeedo inboard moter running from two Vetus 220Ah marine batteries in series. The system was installed in 2018, rarely used and the batteries charged with a sterling 24v-20a charger and two 60W solar panels with an MPPT controler. The solar panels were found not to work and I’m not sure how regularly the batteries were charged with the streling charger.
I took the batteries to a garage to test the CCA – one tested at just over 1000 the other 900.
will I cause further damage by continuing to use them in series? can I rejuvenate them by charging them separately with the sterling charger? should I scrap them and buy one Lithiam 24v 200Ah or two lithium 12v 100AH batteries? any help much appreciated!

1. BatteryGuy

CCA stands for Cold Cranking Amps. It shows how well a battery would work when, say, starting a petrol engine. It is not relevant if your motor is electric. You need to fully charge both batteries separately and then see if there is a difference in the voltage or ah output. If there is and you continue to use them one battery will damage the other because they are now essentially ‘different’ batteries that should be wired in series or parallel. Reference: https://batteryguy.com/kb/wp-content/uploads/2016/01/ah-battery-banks-cheat-sheet.jpg

28. john

So i have an electric Scooter and all the 12v batteries are wired together in series. I want to install a 12 volt device. If i hook up to the center battery and just measure between the + and – on just one battery would i still get 12 volts or would the voltage be added together even though i an just only connecting to the terminals on one battery.

29. Jon P Laird

In my boat, there are two 12 v batteries in series to provide 24 v to the trolling motor. Can I drop a 12 v line from one of these to power things that I don’t want to run off the seoarate starter motor battery?

1. BatteryGuy

If two 12 volt batteries are connected in series they will output 24 volts. If you attach anything else to these batteries the output will also be 24 volts, not 12 volts.

30. Tom

A friend asked me to wire in 6 batteries into his golf cart. The old battery were removed and traded in on the new ones. I never saw the original layout. I wired them in series, however the cable to the reverse switch had a tag stating to wire it to the negative of battery # 4. My question,
6 batteries: which is #1 ?
The one the negative lead went to or the one the positive leads goes to?

1. BatteryGuy

It would probably be best to contact the manufacturer of the golf cart to get a correct wiring set up. Otherwise if you get it wrong you could damage the batteries or the golf cart motor.

31. Toqeer khan

Thanks,
I have a set 4 batteries with 12v, 250 AH to get 48v
For APC UPS, but unforunately one battery damaged after about one year and 3 months now I have to add one 12 v battery but as per your recomendation I should purchase all 4 batteries new to connect in series.
What should I do with 3 healthy batteries ? It is big financial loss for me. Is there any solution to save this loss

1. BatteryGuy

Not really. If you continue with your current set up the 3 healthy batteries will not remain healthy for long and you will have to replace them anyway. When you do you will then start damaging the single battery that you replaced 3 months ago and so the cycle will continue. Over the long term it will cost you more than simply replacing all 4 batteries at the same time.