Two Batteries?

[DPellegrin]

The primary reason for two batteries in parallel is to maintain an acceptable voltage drop during cranking. A starter motor is a shunt wound DC motor. At the instant the starter is engaged there is quite literally a "dead short" until the motor starts to turn. As the motor starts to turn a counter electromotive force (CEMF) is created. Yes, the starter motor actually becomes a generator. The faster the motor turns the higher the CEMF and the lower the amount electrical current (heat) used by the starter motor and the lower the voltage difference between what the battery is producing and what the starter motor is generating (Ohm's Law). The bottom line is using one battery is much harder on the starter motor than using two batteries because the starter motor sees more current and therefore creates more internal heat than if you used two batteries to crank the engine. Using one battery significantly reduces the life of the starter motor.

 

[RHestand]

The primary reason for two batteries in parallel is to maintain an acceptable voltage drop during cranking. A starter motor is a shunt wound DC motor. At the instant the starter is engaged there is quite literally a "dead short" until the motor starts to turn. As the motor starts to turn a counter electromotive force (CEMF) is created. Yes, the starter motor actually becomes a generator. The faster the motor turns the higher the CEMF and the lower the amount electrical current (heat) used by the starter motor and the lower the voltage difference between what the battery is producing and what the starter motor is generating (Ohm's Law). The bottom line is using one battery is much harder on the starter motor than using two batteries because the starter motor sees more current and therefore creates more internal heat than if you used two batteries to crank the engine. Using one battery significantly reduces the life of the starter motor.

Thanks for the great post. This kind of info. is hard to come by, and is what makes TDR such a great site.

Ray

 

[MHalloran]

pepboys and auto zone in my area,are about the same price on group 27 batterys,79. 00 each,,walmart is 59. 00 on the MAXX brand 750 cca,, i run the MAXX brand in both my hot rods 1000 cca and one car sits in a trailer most of the year,,,great batteries,,i just hate walmart,,,

I just picked up two of the Maxx batteries from Walmart also. Can't beat the price and they're guaranteed for 3 years. Yeah, Walmart. Go figure.

 

[BHaden]

. Using one battery significantly reduces the life of the starter motor. [/QUOTE]





I would think that it would also significantly reduce the life of that single battery if it needs to put out close to full amperage rating every time you hit the key.

 

[DPellegrin]

BHaden-You got it right! Fact is batteries don't like to be cycled. The deeper the discharge the shorter the battery life.



regards,

 

[JLEONARD]

Single battery as stock in my first gen. 185K miles. . original starter.

Just put in a new battery... 4 years sevice from an Optima red top. I am very pleased with that performance.

 

[tgordon]

Question for you electrical experts on here, or those who know a way lot more than me Optima has three differant batteries, one is a standard, one is a deep cycle, and one is in between. The deep cycle can be puchased in 1000 CCA size (I don't know if it will fit in the truck), while the others I can only find in 750. Which would be a better fit for the use and demands our trucks put on them, the deep cycle with higher CCA, or the more traditional at 750?

 

[Skydiver]

The primary reason for two batteries in parallel is to maintain an acceptable voltage drop during cranking. A starter motor is a shunt wound DC motor. At the instant the starter is engaged there is quite literally a "dead short" until the motor starts to turn. As the motor starts to turn a counter electromotive force (CEMF) is created. Yes, the starter motor actually becomes a generator. The faster the motor turns the higher the CEMF and the lower the amount electrical current (heat) used by the starter motor and the lower the voltage difference between what the battery is producing and what the starter motor is generating (Ohm's Law). The bottom line is using one battery is much harder on the starter motor than using two batteries because the starter motor sees more current and therefore creates more internal heat than if you used two batteries to crank the engine. Using one battery significantly reduces the life of the starter motor.

Automotive starters are series wound. Extemely high torque at low rpm's. The CEMF statement is correct.



Caution: Never connect a starter to a battery for more than half a second. Series wound motors do not have a top speed. They will spin in excesss of 20,000 rpm and grenade.



A starter must have a load to operate safely.

 

[DPellegrin]

Skydiver- Good catch about series vs. stunt. I mistakenly thought that automotive DC starter motors were like the Delco-Remy MT 40 starters that were used on the Cummins 855/N14 engines I used to rebuild "back in the day. " I know that in the late 70's heavy trucks were going away from 24 volt starting and the series-parallel switch. The 31 size battery had been popularized by Delco Freedom batteries and heavy trucks were going to 12 volt starting. I guess I'm not sure of the specific design of the starter motor used on a ISB Cummins. In any case the battery does not care about the design of the motor. The purpose of the post was to comment on the need for 2 batteries.



regards,