Charging two 12 volt 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.

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