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. Current remains as before. Interconnecting batteries in parallel increases current. Voltage remains as before. No matter how connected, the stored energy remains the same.
A common need for series connection is that most batteries are two, six or twelve volt. Some vehicles, however, have 24 volt systems. These typically have two 12 volt batteries in series. Many stand-alone solar systems use 48 volt storage. These typically have four 12 volt batteries in series.
At common need for parallel connection is in systems above 100 amp-hour. A typical 12 volt deep-cycle 100 amp hour battery weighs about 32 kg. To ease handling it’s common to parallel multiple such batteries. Lithium batteries of similar capcaity are one third the bulk and weight.
Interconnecting batteries in series or parallel – the pros and cons
Each way of interconnecting has its pros and cons. But not 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.
A minus of series connection is that usage is limited by that of the ‘weakest’ cell. Series-connected batteries must thus 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 rarely oppose parallel connection. Most show how to do it. General Electric says ‘there are no major problems with parallel charging.’ Exide, however, is a little more cautious. It advises ‘up to 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.
For 24 or 48 volts it is fine to parallel connect series-connected 12 volt batteries. That shown above is a bank of 16 batteries (each of 12 volt). They connected in series/parallel to provide 48 volts at 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 may fail instantly. The cause is active material shed from the lead plates piling up in the bottom of a cell. Battery capacity relates to the lead that’s left. Shedding thus causes ongoing loss. That loss is rapid if the battery is regularly over-discharged. The battery is then replaced. If shed material rises high enough to short circuit the plates, the battery fails instantly.
If a deep cycle battery is long uncharged, dendrite (a 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 forum 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 happens
What really happens in say) 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. Furthermore, as their electrolyte boils away, they too stop conducting.
The argument may extend to: ‘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 systems. Parallel connected pairs of series-connected batteries are convenient for higher voltage large-capacity systems.
Many property stand-alone solar systems run at 48 volts. This is usually by paralle- 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 batteries in travel trailers
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 in Caravan & Motorhome Electrics. That for solar in cabins and RVs is in Solar That Really Works. That for 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.