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INTERCONNECTING BATTERIES Batteries are curious animals that do not necessarily behave as one might suspect. Ample Power company says: 'some of the worst people to ask about batteries are those who work in general electronic disciplines . . . general electronic knowledge just isn't sufficient ... even those working in battery distribution channels cannot be relied upon to dispense correct and meaningful battery information'. This leads to the reasonable question, 'if that's so, why believe what this article says either'? You don't necessarily have to, but most of what's in here is basic stuff that's been known for the better part of a century. I'm also quoting references. I usually check references anyway - but I don't often quote them because most are in technical gobbledegook or read as if translated from German by someone with an inadequate grasp of English. Series or Parallel This article is about interconnecting batteries. There are two main ways of doing this: series and parallel, and there have been articles and letters about which is 'better' since the late 1920s. Over the decades campsite mythology has all but buried the realities involved. This article may help reveal the facts. The Basic Issue The energy that a battery can store is closely related to the quantity of lead it contains. A 12-volt 100-amp/hour deep-cycle battery weighs about 35 kg; its 200-amp/hour equivalent weighs about 70 kg. To ease handling, many people prefer several smaller interconnected batteries rather than a single big one. It may also not be possible to obtain what you want any other way. Multiple batteries may also be used to increase capacity, or to increase voltage but, however interconnected, their stored energy remains the same. Thus, 200 amp/hours at 12 volts can be obtained from two 6-volt 200 amp/hour batteries in series, or two 12-volt 100 amp/hours in parallel. Both configurations store 200 amp/hours x 12 volts = 2400 watt/hours. For 24-volt systems, 2400 watt/hours can be 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. Is one way inherently better than the other? Or is it a case of lead-acid horses for motorhome courses? What Happens when a Battery Fails Starter batteries ultimately fail quickly, and often instantly. Material progressively shed from the plates falls to the bottom where it piles up until it reaches and shorts out those plates. Then Bingo! It's dead. 'Deep-cycle batteries [on the other hand] mostly fail through corrosion of grid material, and material shedding from the plates. But [unlike starter batteries] shed material is not a significant cause of cell failure, because deep-cycle batteries have bigger and better plate separators. Grid corrosion is caused by the normal charging/discharging cycle. Eventually there's no grid left to corrode at that point the battery goes open circuit' (Linden 1984). The effect is like disconnecting one of the leads. Plate shedding in deep-cycle batteries occurs naturally and, as a battery's capacity is a function of 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 manifest. This 'shorted cell' failure is the most commonly heard argument against paralleling batteries. 'Just imagine;' people say, 'what happens if a fully charged cell in a big battery shorts itself out.' But forget imagining - let's look at what actually happens. First, consider a cell shorting internally in a single 100-amp/hour deep-cycle battery. Conductive material (eg. dendrite) has bridged the cell's plates. This has much the same effect as bridging that cell's terminals externally. The bridging material has some resistance and current flow will be limited to about 100 amps. As Peukert pointed out way back in 1897, a 2-volt 100-amp/hour cell discharging at 100 amps is good for about 50 amp/hours. Thus the equivalent of 200 watts for 30 minutes (ie. 360,000 watt/seconds, or 360,000 joules) will be dissipated in that cell. That's not as much energy as it sounds, but it's enough to bring it to the boil within about 10 minutes. As electrolyte 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. Meanwhile a lot of hydrogen and oxygen is created. This is not a problem if the battery compartment is adequately ventilated, but it may be a big one if it's not, and there's a source of ignition. If ignited, hydrogen sort of fizzles at 4% concentration, but goes off with considerable violence at 10%. This rarely happens. But it can. Despite this, people build RVs without ventilated battery compartments (one blew the front off an almost-new caravan earlier this year). One builder allegedly maintains it's safe to totally box them in - and is claimed to have said that he's 'built quite a few' that way! Myth Buster One A cell is a cell is a cell. The above scenario is totally independent on whether the batteries are charged in series or in parallel. The next scenario is the one that keeps people awake at night. 'But what if paralleled batteries were to discharge through a shorted cell in a connected battery? Would this not result in megamps flowing through the crook battery, a serious bang, and the scenery covered in bits of what was once a Winniswagden?' It could be if that battery compartment were not ventilated - but the bang would once again be due to lack of battery enclosure ventilation - not whether the batteries are in series - or in parallel. And Winniswagden battery enclosures are ventilated. Myth Buster Two A 12-volt battery has six cells and is normally charged at up 14.4 volts. A six-cell battery with a shorted cell is, in effect, a five-cell battery. If it has charged 12- volt batteries connected across it, those further batteries will attempt to 'charge' it at about 12.6 volts. Thus 12.6 volts divided by 5 (2.52 volts) will be applied across each of its remaining five cells. The result will be just like charging a six-cell (12-volt) battery at 2.52 x 6 = 15.12 volts. This is no problem and here's why. As many readers will know, deep-cycle batteries like being equalised occasionally (equalising is the lead acid equivalent of blowing out the carbon by giving the rig a thrash down the expressway). Equalising a 12-volt battery involves connecting over 16.0 volts across it for an hour or two - so 15.12 volts is clearly safe. Applying 12.6 volts across our now 'five-cell 10-volt battery' is thus just the same as 15.12 volts across a six-cell battery. And, as happens with equalising, the battery only accepts 3- 5 amps charge current anyway so the extra current flowing through the shorted cell will make next to no difference to its internal heating. After a time the faulty battery will behave like any other battery that's being equalised: it will boil like the witches' cauldron in an amateur production of Macbeth. And presents much the same level of toil and trouble (unless you are sitting on the battery and smoking at the time). The battery may end up more equalised than it had in mind, but who cares. Instead of a cold dead battery you have a hot dead battery. And a hopefully reinforced belief in the need for battery ventilation. It is theoretically possible for a cell to short, but the discharge through that shorted cell will only be marginally greater (typically 3% more current flow) with any number of other batteries paralleled across it. 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,' (Ample Power Company 1990). Charging Problems Few if any battery makers are opposed to parallel charging. General Electric states unequivocally: 'there are no major problems with parallel charging,' (General Electric 1979). And that quote refers to sealed batteries, where the consequences of a cell short are greater. With sealed batteries, world authorities, Ronald Hamel, Alvin Salkind and David Linden are a little more cautious. 'Sealed lead-acid batteries can be charged or discharged in parallel. When more than four strings of cells are paralleled it is advisable to use diodes in both the charge and discharge path. Discharge diodes prevent a battery discharging into another battery should a cell short-circuit in that battery. Charge diodes will prevent a battery with a short-circuited cell accepting all the charge current,' (Linden 1984) and thus preventing other batteries from being fully charged. A few minor areas need consideration. Lead acid batteries have socialist tendencies. If two unequally charged batteries are paralleled, the more highly charged will discharge into the less highly charged until their voltages are equal. Apart from the above 'there is no problem parallel charging similar batteries of the same voltage but of 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,' (Ample Power Company 1990). They discharge in much the same way. To ensure they have even charging voltage available, paralleled batteries that are spatially apart are best connected via equal length and size cables. Series Connection Charging series connected batteries is far more of a problem because charge acceptance is inherently limited to that of the 'weakest' cell in the chain. Even with single batteries, for optimum charging, every cell must be of identical size and have identical charge acceptance characteristics. But even here there are inevitable differences, eg. the specific gravity of the electrolyte is bound to vary over time from cell to cell. The effect on charge acceptance is emphasised even further when whole batteries are series-connected. This occurs even between new batteries of a similar size and type. If they are different the effect can be profound. You can readily parallel a 100-amp/hour battery with an otherwise similar 200-amp/hour battery to obtain 300 amp/hours. But if you series-connect those same batteries you'll have close to a 100-amp/hour battery - because once the 100 amp/hour is fully charged, it will inhibit charging the 200-amp/hour battery thereafter. This is why tapping 12-volts from a 24-volt battery is such a no-no! Unless you 'switch sides' every few seconds, those batteries can never charge fully thereafter. The one that is less drawn on becomes fully charges first and inhibits the other thereafter from further charging (unless separated and equalised). Some people appear to get away with it but this is usually because they are not otherwise placing much demand on their battery and don't notice the inevitable loss and accelerating battery degradation. So do please stop recommending it to innocents on web sites on the basis 'that it works fine for me'! Such 'advice' is odds-on to do them a costly disfavour! (Tapping a battery bank can actually be done, but requires a special equalising system which costs about the same, and has similar efficiency as a 24-12 volt dc/dc converter. These systems are being increasingly used to retrofit boats where 24- volts is increasingly favoured to reduce voltage drop with anchor winches etc. I'll cover this in a future Tech News). Personal Summary My own preference is for parallel connection for 12-volt mobile uses, and parallel connected pairs of (two) series-connected 12-volt batteries for 24-volt systems. For big battery systems such as I have at home (twenty-four by 360 amp/hour two-volt cells providing 8640 amp/hours) I have no choice but to use series connection, but I am always concerned that differences between the cells inhibits even charging. I equalise them every two months and the immediate increase in effective capacity is obvious. References • Ample Power Company 1990. Parallel
Batteries, Seattle, Washington. |
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2002-2009 -
Collyn Rivers - Caravan and Motorhome Books - www.caravanandmotorhomebooks.com |
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