Updated 31 May 2017
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, yet current remains as before. Interconnecting batteries in parallel increases current. Voltage, however, remains as before. No matter how connected, their stored energy thus remains the same.
The most common need for series connection is that most batteries are two, six or twelve volt. Some vehicles have 24 volt systems. These typically have two 12 volt batteries in series. Many stand-alone solar systems, however, use 48 volt storage. These typically have four 12 volt batteries in series.
The most common need for parallel connection is in systems above 120 amp hour. A 12 volt deep-cycle 120 amp hour battery weighs about 32 kg. To ease handling it’s common to parallel multiple such batteries.
Interconnecting batteries in series or parallel – the pros and cons
Each way of interconnecting has its pros and cons. But not, however, the same pros and cons. Nevertheless, if one needs over twelve volts, and/or substantial capacity, there’s little choice. One must increase voltage or current. Or both).
One argument against series connection is that charging and discharging is limited by that of the ‘weakest’ cell. It overlooks, however, that most batteries over 2 volts consist of series-connected cells. Such series-connected batteries must be of identical type, capacity and condition. This is particularly so with LiFePO4 batteries. These also need monitoring to ensure all cells are at equal voltage.
Tapping 12 volts from one of two 12 volt series connected batteries is a no-no! This is because the battery that is less drawn on becomes fully charged sooner. This inhibits the other fully charging. The only remedy is disconnect, then charge each separately.
Obtaining 12 volts from a series pair can be done. It, does however, requires either equalising units (e.g. Redarc and GSL Electrics). Or a 24 volt to 12 volts dc-dc converter. These systems are commonly used on boats. There, 24 volts is used for winches, but 12 volts for most else. See 12-volts-dc-from-24-volts-dc/
Batteries parallel connected
Battery makers are rarely opposed to parallel connection. Most show specifically 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. Each gives according to its means. If two unequally charged batteries are paralleled, that more highly charged slowly discharges into the less highly charged. That continues until voltage is equal.
There is no problem parallel charging batteries of the same type and voltage but different capacities. They look after themselves. ‘Each draws a proportionate share of the available charge. All reach about the same level of charge at roughly the same time,’ (says Ample Power Company). They discharge much the same way.
Ample Power company emphasises to connect paralleled batteries via equal length and size cables.
If 24 or 48 volts is required it is fine to parallel-connected sets of series-connected 12 volt batteries. That shown above is a bank of 16 batteries (each of 12 volt). All are connected in series/parallel to provide 48 volts at approximately 960 amp hours. Pic: 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 battery capacity relates to the lead that’s left, shedding causes ongoing loss. That is rapid if the battery is regularly over-discharged. The battery is usually then replaced. If, however, shed material rises high enough to short circuit the plates, the battery fails instantly.
If a deep cycle battery is long uncharged, dendrite (a tree–like crystalline structure) forms during recharge. This causes a virtual ‘short cicuit’ across the cell. It kills the battery instantly.
Such failure is the most common argument against paralleling batteries. ‘Just imagine,‘ some say, ‘what happens if a fully charged cell in a big battery shorts itself out.’
Shorted battery cells – what really does happen
What actually happens in (e.g. a 100 amp hour battery) is that current will flow in that cell at a probable 100 amps. This is not a huge amount of energy. It may nevertheless cause the electrolyte to boil. As that happens current flow slows. It eventually stops. Meanwhile, adjacent cells heat up. As their electrolyte boils away, they stop conducting.
The argument may then be extended. ‘What happens with possibly fully charged batteries paralleled across one with a ‘shorted cell’. This, however, is like applying 12.5 volts across a (now five cell) 10 volt battery. It’s like charging a 12 volt battery at 15 volts. The warm dead battery become a slightly warmer dead battery.
The main risk is that hydrogen is created. But as long as a battery compartment is ventilated, danger is remote. ‘Since the early 1960s . . . 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 use. Parallel connected pairs, of series-connected batteries, are convenient for higher voltage large capacity systems.
Many big property stand-alone solar systems 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 LiFePO4.
See also Lithium-ion batteries in caravans
Any combination of the same batteries will always result in the same amount of stored energy.
Interconnecting batteries in series or parallel – further information
If you liked this article you will like my books. Batteries and their charging is fully covered Caravan & Motorhome Electrics. That for solar in cabins and RVs is in Solar That Really Works. That home and property systems is in Solar Success. My other books are the Camper Trailer Book, and 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.