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Two 12 volt batteries in parallel
As sequel to the 12 volt replacement for the twin 6 volt batteries on chrome bumpered V8s, Any Miller pondered in a posting on the V8BB whether two 12 volt batteries could be installed in parallel to increase the cold start cranking power. (Jun 05)

Andy Miller noted "someone suggested today that two 063 batteries could be used in place of the 6 volt ones but wired in parallel and not in series as seen in the original twin 6 volt installation. Two 063s would provide all the cranking power and reserve capacity that any V8 would need! It would also be a very simple and quick job, merely needing adaption of the link cable with the addition of a second earth strap so there would be two earth cables. Would there be any issues arising from any differential between the internal resistance of two batteries wired in parallel, or of too low an overall impedance to the alternator output causing a problem?"

Brian Moyse (BRG RV8 1714) from Surrey responded that it is "not something I have ever thought of doing as my experience with the 6 volt batteries on my MGBGT 1800 was good, mainly because my batteries were isolated by the safety switch I fitted and used when the car was garaged. In principle I cannot see any major problems arising from the twin 12 volt in parallel installation. Industrial storage battery systems are often designed with parallel connected cells to increase the AH (amp hour) capacity. The reduced impedance seen by the alternator will merely result in a higher float charge output required from the alternator but this would be well within its overall rated power output, which in any case will be current limited by the regulator (AVR). Charge rates to each battery will be determined by any imbalance in their individual impedances and this may result in them being maintained at different percentage states of charge".

Bob Owen (Blaze V8 1625) from Berkshire commented "Yes, I think this is a good idea. In fact it is better to have two batteries in parallel than two in series since matched charging is not a problem. When in series the capacity of the two batteries needs to be

closely matched otherwise the first to be charged will gas while the second one finishes charging. This precludes the use of sealed batteries in series. When in parallel the first to be charged merely accepts less current so sealed batteries can be connected in parallel. Of course, for load sharing and life considerations it is better if the two batteries are nominally the same".

Bob Owen added a couple of further points on the twin 12 volt batteries in parallel idea:

  • Ideally, for load sharing, the two batteries should be strapped with two identical leads and then diagonally opposite posts used for the earth and starter feeds - this symmetry ensures identical series resistances outside each battery.

  • The best place for an isolating switch is in the earth lead. This then protects the "hot" terminal of the battery - for example should a spanner be dropped and bridge the hot terminal to chassis it would cause no problem, not the case if the isolator is in the "hot" lead. However, the disadvantage of the isolator in the earth lead is that it makes it more difficult to run non-isolated equipment since you must provide them with individual earth returns back to the battery earth post. Equipment that you may wish to keep non-isolated includes the clock and alarm system, and in my case the interior lights since these provide part of the alarm system by monitoring battery voltage deviation - opening a door switches on courtesy lights which loads battery and causes a voltage shift which triggers the alarm. Bob notes "So, for the slight disadvantage of reduced hazard prevention, I personally have found it more convenient to put the isolator in the "hot" lead. This also is more convenient for wiring as the hot lead is the driver's side (for right hand drive cars) and so the isolator can be conveniently positioned on the vertical panel behind the driver's seat and so be easily accessible while sitting in the driver's seat. Non-isolated equipment is then operated via a fused take-off from the hot terminal but earthed to chassis locally as normal.