Battery Myths Busted

Battery Myths Busted


by Collyn Rivers

Myth 1: A close-to-flat starter battery can be partly revived by feeding it an aspirin and letting it sleep. 

This is a classic example of arriving at a partially true answer for totally wrong reasons. That battery can also be partially revived by playing it a Mozart concerto, reciting a short story, or arranging a Reiki (‘hands-on healing’) session! The true reason why this vaguely works is that after a heavy discharge, a starter battery’s remaining  energy is depleted around the terminal and plate areas. After resting, the remaining charge (that can be seen as within the electrolyte) becomes more evenly distributed and the battery may partially revive. 

Aspirin in itself does nothing – it’s the sleep that does the job. About 30 minutes’ rest usually helps, which is also about the length of a typical concerto, a decent short story and a Reiki session (the latter is also aided by the warmth of the practitioner’s hands).

Myth 2: It takes several hours to recharge the starter battery after starting a big rig.

This is an understandable myth probably reinforced by the heavy starter cables. In reality starting an engine requires several hundred amps, but only for two or three seconds. Heavy cables are needed to carry that current, but the energy used is about the same as that drawn by an interior light for 15-20 minutes. This energy is typically replaced within a minute or so of starting the engine (the author’s 51 kg former girl friend could hand start his 1929 4.5 litre, four-cylinder Bentley with ease. The energy used was adequately replaced by a gin and tonic). 

Myth 3: My standard alternator will fully charge my auxiliary 120 Ah conventional deep-cycle battery (from half-charge) within three or so hours of driving. 

The alternator will charge it to 60 per cent within 15 minutes, and to around 70 per cent about an hour later. It takes several more hours to reach 75 per cent and even longer to reach 80 per cent. In practice, few such batteries ever exceed 70 per cent. This does not affect the starting system. AGM or gel cell batteries reach a higher state of charge (and faster) from the alternator voltage available. The now available dc-dc alternator chargers, however, enable that 50 per cent onward charge to be done well with that time claimed.

Myth 4: Installing a bigger alternator will speed up charging. 

True, but only if the battery is close to flat. Then, it will charge faster to about 40 per cent. Over that, charging is a function of alternator voltage, so increasing its size will make next to no difference. The most practical way to increase charge rate nowadays is via the dc-dc alternator charging mentioned above.

Myth 5: I get more power from two 12 V batteries by connecting end to end (in series). That way I get 24 V, not just 12 V.

The world does not work like that. This myth assumes voltage equals power: it does not. No matter how one interconnects those batteries, the energy stored is just the same. As Malcolm Fraser once said, “There’s no such thing as a free lunch.” 

Myth 6: I can charge my auxiliary battery from the vehicle’s alternator via 4 mm auto cable. 

Auto cable is rated by the size of the hole you can poke it through! So-called 4 mm auto cable has a mere 1.8-2 sq mm of conductor. The ‘4 mm’ relates to its overall diameter, including the insulation. In effect it is the size hole it can just be pushed through. That cable size will limit charging to an amp or so! Depending on the cable length, to do this job properly you need anywhere from 8.0 sq mm to 25 sq mm cable. 

Myth 7: I sometimes need to charge my auxiliary battery by running the engine for a bit. To speed this, I set the throttle so the engine runs at half speed. People say this damages the engine though.

A vehicle alternator produces about two-thirds maximum output at idle. Little is gained by exceeding the idle speed by more than 10-15 per cent. Regarding damaging the engine, this is something that city commuters do twice each working day, so it isn’t that bad, particularly with the very high quality of today’s oil. 

Myth 8: I give my auxiliary battery a good workout every month. It stands to reason that, like people, they need exercise. 

Battery-wise, this is mostly a myth. Lead acid batteries are like ageing Labradors: they like being kept up with food (i.e. charging) but otherwise left alone. Their ideal life is not to be used at all (provided they’re fully charged), but who needs a 40 kg 12 V battery as a pet! NiCad batteries however do prefer being fully discharged before recharging. 

Myth 9: A standard lead acid battery must not be charged in parallel with an AGM battery.

Partially true, but needs qualifying. Like greedy people, AGM batteries tend to grab more of what is available (in this case, the available charge). If charged in parallel with a conventional battery, this is likely to reduce the charge available to that battery. Battery makers therefore warn against this practice. Technical writers (including myself) used to do likewise.

However, if the conventional battery (particularly the starter battery) can be given first go at the cake – i.e. charging priority up to a safe starting voltage of about 13.6 V, it is then safe to switch an AGM battery in parallel. This can readily and reliably be done via a heavy duty solenoid that senses starter battery voltage and closes when above that voltage. By 2005 or so this practice became virtually the norm. Auto electricians are totally familiar with these relays and hundreds of thousands of vehicles are now set up this way, worldwide.

The above situation is changing, however, as by 2014 EU regulations will require variable voltage-controlled alternators, possibly as low as 12.7 volts. This is already happening and companies such as Redarc are now warning of this.

Myth 10: It is a waste of money buying ‘top quality’ deep-cycle batteries – they all last about the same time. 

This depends almost entirely on how batteries are used and maintained. If they are abused by persistent and severe over-discharging or left only partially charged over time, the statement is substantially true. 

However, if used and maintained correctly, high quality deep-cycle batteries (such as Rolls, Surrette, etc.) will more than repay their extra cost. In practice, few conventional batteries are correctly used and maintained, so while in no way validating this myth, many users are better off buying cheaper deep-cycle batteries. Gel or AGM cell batteries will accept some abuse with less ill effect, but both are affected by heat. If the battery is located under the bonnet a Gel or AGM cell will have a much shorter life. 

Collyn Rivers is an ex automobile research engineer, who now writes books in this and similar areas. Many of his published articles can be found on

For the full article download Issue 19 of iMotorhome eMagazine by clicking on the link below.

iMotorhome eMagazine Issue 19 - 16 February 2013 iMotorhome eMagazine Issue 19 - 16 February 2013 (8116 KB)

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