12 Parallel-Same Batteries?

[Strikenmike]

Greetings: New to forum, be patient. I have searched and found some good diagrams on hooking up two 12v batteries in parallel. However, I vaguely recall someone telling me that if you do this the batteries need to be identical in every way.

True or false? Same class? AGM to AGM, Deep Cycle to Deep Cycle...etc.

Please advise.

By the way what is the practical advantage of doing this. Will my 40hp E-TEC charge both batteries, will I be able to charge both batteries at the same time from one connection? What are the advantages.

Thanks

 

[wingless]

Re: 12 Parallel-Same Batteries?

Yes, use identical batteries w/ identical date codes when connecting in parallel.

That will minimize the inter-battery current during discharge and during recharge.

The recharge requirements are defined for each battery chemistry. It is likely that the 40hp engine alternator will have insufficient output to charge the batteries to the battery manufacturer's preferred / recommended profile. Here is a very good AGM battery manual from Lifeline.

The disadvantage of improper charging will be decreased battery life and potentially less Ah capacity available when required.

A properly-selected plug-in charger can restore the batteries to fully charged once AC power is available.

 

[Silvertip]

Re: 12 Parallel-Same Batteries?

In a perfect world the above is true. Fact is if the batteries are at least the same chemestry, approximately the same capacity, approximately the same condition, they will live together just fine. The down side is that when the batteries are not charged over an extended period, the weaker battery in the pair will suck enough life out of the stronger battery until both are equal. There is a simple way to eliminate this equalization process. Install a dual battery switch or an automatic charge relay (ACR). The dual battery switch lets you select either or both batteries for charging, starting, or running accessories. The ACR does essentially the same thing but unless it has a combine, feature, you cannot use BAT 2 to start the engine. A 40 HP E-tec will keep two batteries topped off provided the boat gets regular use and runs are not just 10 minute jaunts to the bar and back. Two batteries in parallel still provide 12 volts to the system but the capacity is doubled (provided both batteries have the same capacity). To the engine, the two batteries look like one great big one.

 

[Strikenmike]

Re: 12 Parallel-Same Batteries?

Yes, use identical batteries w/ identical date codes when connecting in parallel.

That will minimize the inter-battery current during discharge and during recharge.

The recharge requirements are defined for each battery chemistry. It is likely that the 40hp engine alternator will have insufficient output to charge the batteries to the battery manufacturer's preferred / recommended profile. Here is a very good AGM battery manual from Lifeline.

The disadvantage of improper charging will be decreased battery life and potentially less Ah capacity available when required.

A properly-selected plug-in charger can restore the batteries to fully charged once AC power is available.

Thanks, does identical include the class of batteries eg 25 or 27?

 

[wingless]

Re: 12 Parallel-Same Batteries?

Thanks, does identical include the class of batteries eg 25 or 27?

Yes, identical in every way possible. Buy the same batteries at the same time having the same date code.

When I replace my six group 31 batteries, I do all six at once.

My latest batteries are the Lifeline GPL-31XT AGM batteries because they have 25% more Ampere-hour capacity than all the other brands.