Updated March 2016
Interconnecting batteries in series or parallel is totally feasible but its best to know how it works – and the limitations of each. Collyn Rivers explains.
Interconnecting batteries in series increases voltage, the current remains as before. Interconnecting batteries in parallel increases current. the voltage remains as before. No matter how connected however their stored energy remains absolutely the same.
The most common need for series connection is that most rechargable batteries are two, six or twelve volt. Some vehicles have 24 volt systems and usually have two 12 volt batteries in series. Further, many largish stand-alone solar systems use 48 volt battery storage and have four 12 volt batteries in series.
The most common need for parallel connection is in systems above about 120 amp hour. A 12 volt deep-cycle 120 amp hour battery weighs about 32 kg. For ease of handling it is common to connect multiple such batteries in parallel.
Interconnecting batteries in series or parallel – the pros and cons
Each way of (interconnecting batteries in series or parallel) has its pros and cons – but not the same pros and cons. But if one needs higher than twelve volts, and/or a lot of stored amp hours there is no choice but to do one or other (or both).
A commonly heard case against series connected batteries is that charging and discharging is inherently limited to the condition of the ‘weakest’ cell. Whilst true, it overlooks that (say) 12 volt batteries consist of four series-connected cells anyway. It does however necessitate series-connected batteries to be be of identical type, amp hour capacity and condition. This is particularly so with lithium-ion (LiFePO4) batteries. These must also have individual cell monitoring that ensures all are at equal voltage.
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 sooner. It then inhibits the other from fully charging thereafter. The only way to remedy that is disconnect them and charge each separately.
Obtaining 12 volts from a series pair can be done but requires either specialised equalising units (such as from Redarc and GSL Electrics). Or by a 24 volt to 12 volts dc-dc converter. These systems are commonly used on boats where 24 volts is used for winches, but 12 volts for most else. See http://caravanandmotorhomebooks.com/12-volts-dc-from-24-volts-dc/
Parallel connected batteries (or paralled strings of series connected batteries) must be all of the same voltage, but may of widely varying capacity.
Battery makers are rarely opposed to parallel connection but 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 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 much the same way. As also does Exide, the Ample Power company emphasises that to have even-charging voltage available, paralleled batteries that are spatially apart are best connected via equal length and size cables (i.e. as shown above).
If 24 or 48 volts is required it is fine to parallel-connected sets of series-connected 12 volt batteries. That show above is a bank of 16 batteries (each of 12 volt). They are connected in series/parallel to provide 48 volts at approximately 960 amp hours. It is in the author’s previous all solar house north of Broome.
Interconnecting batteries in series or parallel – what happens when a battery fails?
Traditional starter batteries tend to fail instantly. Over time, active material shed from the plates piles up in the bottom of the cell. As a battery’s capacity is related to the amount of lead that’s left, shedding causes an ongoing loss of that capacity. Or not so gradual if the battery is regularly over-discharged. The battery is usually then replaced. If however the shed material rises high enough to short circuit the plates the battery fails instantly.
If a deep cycle battery is left for a long time uncharged, dendrite (a crystal with a tree–like structure) is formed during recharge. This results in a low resistance path through the cell. The result is much as with a starter battery, but takes longer to happen. 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 probable 100 amps.This is not a huge amount of energy but it’s enough to bring the electrolyte to the boil. As that happens current flow slows and eventually stops. In the meantime the cells either side heat up. As their electrolyte boils away too, they stop conducting. The argument may then be extended to what happens with possibly fully charged batteries paralleled across the one with the ‘shorted cell’. But here, this is like applying 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 become a warmish 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
Parallel connection is convenient for large capacity 12 volt use and, as parallel connected pairs, of series-connected 12-volt batteries for higher voltage large capacity 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, when Interconnecting batteries in series or parallel, 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 cabins and RVs is in Solar That Really Works, and for solar in home and property systems is in Solar Success. My other books are the Camper Trailer Book, and the all-new Caravan & Motorhome Book. For information about the author Click on Bio.
• 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.