(Updated December 2015)
Interconnecting batteries in series or parallel has been debated since the 1920s, but campsite and forum mythology continues over this mostly non-issue.There are two main ways of doing this: end to end (series) and side to side (parallel). The former is additive in respect to voltage; the latter is additive in respect to current. Multiple batteries may be used to increase capacity, or to increase voltage but, however interconnected, their stored energy remains absolutely the same.
Broadly speaking, the amount of energy that a lead acid battery can store is related to the quantity of lead it contains.( Lithium-ion batteries have a totally different chemistry but the effects of connecting in series or parallel are as in this article.)
Whichever is used is often not so much an issue of, ‘is one way inherently better than the other?’ It is more a case of the most suitably convenient way of handling and locating them.
Each way (interconnecting batteries in series or parallel) has its set of good and bad aspects – but not the same set of good and bad. Asking which is ‘better’ is like asking if buying a poodle is better than buying a fish. The main differences are explained below.
A 12 volt 110 amp hour deep-cycle battery weighs about 30 kg. Its 220 amp hour equivalent weighs about 60 kg. To ease handling, or obtain greater capacity or higher voltages, it is common to use several smaller interconnected batteries in series or parallel – or a combination of both. No matter how interconnected, two 6 volt 200 amp hour batteries in series, or two 12 volt 100 amp hour batteries in parallel will (when fully charged) will result in 200 amp hours at 12 volts.
For 24 volt systems, 100 amp hours can obtained from four 6 volt 100 amp hour batteries in series, two 12 volt 100 amp hour batteries in series, or two paralleled pairs of two 12 volt 50 amp hour batteries in series. In all cases the amount of energy stored is identical.
The main problem with series connected batteries is that charging and discharging is inherently limited to the condition of the ‘weakest’ cell. Even with single batteries, for optimum charging, every cell must be of identical size and have identical charge acceptance. The effect on charging is even more so when batteries are series-connected. They must be of identical type and amp hour capacity.( This is particularly an issue with lithium-ion (LiFePO4) batteries – these must also have individual cell monitoring that ensures all are at equal voltage).
If you were to series connect a 12 volt 100 amp hour battery with an otherwise similar 12 volt 200 amp hour battery you would have 24 volts but at only 100 amp hour. This is because once the 100 amp hour is fully charged, it inhibits the 200 amp hour battery from charging any further.
Likewise, simply tapping 12 volts from one of two 12 volt series connected batteries is a total no-no! This is because the battery that is less drawn on becomes fully charged first and inhibits the other from fully charging thereafter. The only way to remedy that is disconnect them and charge each separately. Obtaining that 12 volt can be done but requires special equalising units (such as from Redarc and GSL Electrics). These systems are commonly used on boats where 24 volts is used for winches, but 12 volts for most else. To do this please see http://caravanandmotorhomebooks.com/12-volts-dc-from-24-volts-dc/
It cannot logically be argued (but often is) that batteries should not be series-connected because a 12 volt lead acid battery is in effect effect six nominally two volt batteries (cells) connected in series. (A 12 volt lithium-ion battery has four nominally 3.2 volt cells in series). The still heard argument that batteries should not be parallel connected is disproved telephone exchange batteries having been connected successfully like that for over 110 years!
Few if any battery makers are opposed to parallel charging as such, and most recommend how to do it. General Electric says ‘there are no major problems with parallel charging.’ Exide is a little more cautious. It advise ‘up ten batteries may be interconnected without problem as long as certain precautions are followed’.
Paralleled lead acid batteries have socialist tendencies. Each takes according to its needs, and gives according to its means. If two unequally charged batteries are paralleled, the more highly charged will slowly discharge into the less highly charged until their voltages are equal. There is no problem parallel charging similar type batteries of the same voltage but of widely different capacities. They look after themselves. ‘Each draws a proportionate share of the available charge, and all reach about the same level of charge at roughly the same time,’ (says Ample Power Company). They discharge in much the same way. As also does Exide, Ample Power emphasises that to have even-charging voltage available, paralleled batteries that are spatially apart are best connected via equal length and size cables.
If 24 or 48 volts is required it is fine to parallel-connect sets of series-connected 12 volt units.
What happens when a battery fails
This is where arguments about interconnecting batteries in series or parallel may get heated.
Traditional starter batteries tend to fail instantly when active material progressively shed from the plates piles up in the bottom of the cell. It rises until reaches and shorts out the plates – killing the battery as it does so. Such plate shedding in deep-cycle batteries occurs naturally. As a battery’s capacity is related to the amount of lead that’s left, shedding causes a gradual loss of that capacity (or not so gradual if the battery is regularly over-discharged).
‘Cell shorts can occur particularly if a deep-cycle battery is left for a long time without being charged. Then dendrite [a crystal with a tree-like structure] is formed during recharge and this causes a low resistance path through the cell,’ (General Electric 1979). The result is the same as with a starter battery, but takes longer to happen. This ‘shorted cell’ failure is the most commonly heard argument against paralleling batteries. ‘Just imagine,’ some say, ‘what happens if a fully charged cell in a big battery shorts itself out.
What actually happens (in say a 100 amp hour battery) is that current will flow in that cell at about 100 amps.This is not a huge amount of energy but it’s enough to bring the electrolyte to the boil within about 10 minutes. As it boils away, current flow slows down and eventually stops. In the meantime the cells either side heat up and as their electrolyte boils away too, they stop conducting.
The argument is often then extended to what happens with other fully charged batteries paralleled across the one with the ‘shorted cell’. Here, this is like applying about 12.5 volts across a (now five cell) 10 volt battery. It is much like charging a 12 volt battery at 15 volts. The already faulty battery will simply be a warm dead battery.
The main risk is that hydrogen is created. In practice, as long as a battery compartment is well ventilated, the likelihood of danger is remote. ‘Since the early 1960s, when we designed our first battery charger, we have witnessed no dangerous situation that resulted from a cell short,’ says the Ample Power Company.
Summary (interconnecting batteries in series or parallel)
The best way to view interconnecting batteries in series or parallel is as follows:
Parallel connection is usually convenient for 12 volt use, and parallel connected pairs of (two) series-connected 12-volt batteries for 24-volt systems.
Big property stand-alone solar systems usually run at 48 volts. This is usually done by parallel connecting strings of four series-connected 12 volt batteries.
The above applies to all batteries: conventional lead acid, gel cell, AGM and lithium-ion. See also http://caravanandmotorhomebooks.com/lithium-ion-batteries-in-caravans
No matter how connected, any combination of the same batteries will always result in the same amount of stored energy.
If you liked this article you will like my books. All are written in the same technically accurate but down to earth plain English.
A great deal of (constantly updated) information regarding batteries and their charging for camper trailers, caravans and motor homes is in my Caravan & Motorhome Electrics. That for solar use in the above is in Solar That Really Works! and for solar in home and property systems is in Solar Success.
• Ample Power Company 1990. Parallel Batteries, Seattle, Washington.
• General Electric 1979. The Sealed Lead Battery Handbook, Publication BBD-OEM-237, GEC, Gainesville, Florida.
• Linden. D 1984. Handbook of Batteries and Fuel Cells, 2nd Ed McGraw-Hill, New York.
• Also used for general reassurance: Barak M 1980. Electrochemical Power Sources: Primary and Secondary Batteries, 1st ed. IEE UK and New York.