At 12:31 PM 1/6/2009 -0700, David Waterworth in South Africa wrote:
>>"I am in the process of installing 2 x 12-volts in parallel. Each battery is 40 amp-hr so x2 = 80 amp-hr. Two batteries to keep it looking original.

First off, two 12-volt batteries will not look original. Aside from obviously different physical features on the battery case, there will also be (minimally) one additional cable between the batteries to connect them in parallel to have 12-volts total (2x12 in series = 24 volts).

>>"There is a remark in one of your articles, ET 203B, that worry's me. 'CONSIDER GENERAL ADVICE WARNING AGAINST CONNECTING BATTERIES IN PARALLEL'. Can you tell me what this warning is?"

Batteries may occasionally develop an internal short as a failure mode. When this happens the battery is not only dead, but electrically it looks like a solid metal buss bar. If you have two batteries connected in parallel with heavy cables, and one of them shorts out internally, it is equal to throwing an iron bar across the power terminals of the other battery. Shorting out a good battery has a huge power potential, enough power to melt metal, draw a welding arc, or potentially self destruct in catastrophic explosion. You would like to avoid this if at all possible.

That said, there are a substantial number of electric cars with multiple batteries connected in parallel. These setups have the potential to generate thousands of amperes current with a dead short. In that case Internal shorting of one battery can potentially destroy several other batteries in a matter of seconds with even greater fireworks. The solution to this problem is installation of current limiters or large fuses between each battery to prevent discharge rate that could be faster drain than the power draw of the starter motor (or traction motor). If one battery shorts internally if can blow a big expensive fuse, but it saves the rest of the equipment and protects human life.

If you intend to do this I suggest you check with an RV dealer who has experience with multiple batteries in parallel used to power accessory equipment in the RVs. These setups will commonly charge the batteries through diodes so one battery cannot discharge into the other. The vehicle engine or starter typically draws current from one battery while accessory equipment runs from the other battery. Either battery can be run down flat without discharging the other battery. This precludes drawing cranking current from both batteries at the same time, but the vehicle can be set up with a crossover switch so if one battery goes dead the other battery can be switched in to start the car. This does not allow the batteries to be connected together bypassing the diodes.

This is when I should ask why you think it might be a good idea to want dual 12-volt batteries?

These days we have access to modern alternators that can have give pretty much as large power production as you would ever need in a car (or even a large bus or airplane). The modern trend is toward using smaller batteries for less weight. Modern batteries can produce large cold cranking current from a fairly small package. Check out the small battery used in a Mazda Miata these days, and forget about big batteries or dual batteries.

There are a few things that determine how small a battery you can get away with, the first being cold cranking capacity. An MG only requires about 200 amps for cranking in cold weather. You might do that with a fully charged garden tractor battery in good condition, but you probably wouldn't get good cold cranking from a small motorcycle battery.

The next item of concern in modern cars is parasitic power draw when the car is not running. This is mostly a small draw for all sorts of on-board electronic equipment used to maintain computer memory for radio, engine control computers, theft alarms, remote controls, etc. The traditional rule is to size the battery to survive for 30 days under parasitic power draw and still be able to start the engine. If you park a new car for 6 to 8 weeks, don't count on it starting without first recharging the battery. For our old MGs that have near zero power draw when parked, we can maybe park it for 6 months over winter storage and still have it start in the spring, so it doesn't need a big battery for that either.

The remaining concern arises from leaving the lights on when you park the car. Parking lights, dash lights, and headlights on low beam will draw about 12 amps from the battery. A 40-ampere-hour battery in good condition might hang in there for two hours or more and still allow you to start the car, but at four hours you're screwed for sure. If you had a 100 ampere-hour battery it still would not hold up to 8 hours discharge with the head lights on. In years past this used to be a big problem requiring you to get a jump start if you left the lights on (not to mention being hard on the battery going to full discharge). Many modern cars now have automatic headlight shut-off happening a few minutes after you park the car. This may not be an easy conversion for an older car. But it is easy to install a headlight warning buzzer for about $10 that will buzz if you leave lights on with ignition off.

There is also a neat battery tender gadget available that can monitor the charge level of the battery, and it will disconnect the battery from the main cable if voltage goes below some preset point. This will allow you to start the car later even if you left the lights on and parked it for several weeks. This gadget will also commonly have a remote control that fits on your key ring for manual battery disconnect and anti-theft features.

There might be some advantage to using dry cell batteries for low maintenance and long life, but the dry cells may not be cost effective when you figure out how many wet cell batteries you can buy for the same price. As far as I know, the only good reason for using a large battery or dual batteries is for some intended continuing power draw when the engine is not running, like running a refrigerator and lights for night time camping or tail-gating at a football game.

The MGA originally had dual 6-volt batteries because larger single 12-volt batteries were in short supply when the cars web built. The other reason for using a large battery (or even dual small batteries) disappeared with introduction of the high output small alternator in the mid 60's (made possible by high power diodes). The MGA originally had a 19 or 22 amp generator. If you switch on every accessory in the car and drive with the lights on the generator can hardly keep up with steady power consumption, let alone recharging the battery after hard cranking for starting. If you use a couple of auxiliary driving lights it could have a small net negative current flow with full generator output. With 4 or 5 amps negative current at the batteries you might run for 4 or 5 hours before you have to switch something off to allow recharging of the batteries. Doubling the size of the batteries could allow you to drive through the night with everything switched on. This is why larger batteries and oversize generators were popular on rally cars in the 50's and 60's.

Today a small battery will start the car, and a small alternator will allow you to drive indefinitely with everything switched on. So the only thing left to worry about is leaving the lights on when parked. You wouldn't want to carry batteries large enough to leave the lights on for 12 hours unattended, let alone over a weekend. The better solution is the self-sensing automatic battery shut-off switch.