Optima battery combination question.

[John]

I sent an E-mail to Optima asking them a question. I thought I would post it here too to see if anyone has any input.

“I have a question about combining different models of Yellowtops.
I’ll start by explaining my current setup.
I have my normal lead acid in the factory spot under my rear seat. (Not ideal, I know) I also have an auxiliary Yellowtop D31T in the trunk. They are separated by a 200 amp relay. The trigger for the relay is hooked to the “run” ignition circuit, but I also have a delay module that delays the “run” signal by about 7 seconds so the batteries aren’t connected until after the car is already running to charge.
My current starting battery needs to be replaced and my D31T is also worn out.
I planned on getting two D31T batteries, but I noticed you make the DH6, which would be a direct fit for my starting battery.

Here is my question.
Is it OK to combine a DH6 and a D31T?

I want the maximum amount of runtime I can get.
I considered doing 2 DH6 batteries, but that loses me 30 minutes of rated runtime. That is a bit more than I would be comfortable with losing.
If I can combine a DH6 and a D31T I would only lose about 15 minutes. Not ideal, but would save the hassle of battery compartment modification.

If combining the two models isn’t an option, I will probably get 2 D31T batteries and modify the factory battery location.
I am considering adding an additional auxiliary battery for a total of 3, but undecided yet.

If it is of any consequence, the vehicle is a 2008 Cadillac DTS with the limo option alternator and upgraded power wiring.

If you can answer this or forward it so an engineer or other person that can answer this it would be helpful.

Thank You”

Topic

[Richard Kirshy]

This setup has some questions.
To begin with, it is always best to have a matching set of batteries.
It is also wise to have them charge at the same rate.
By your description you have neither.
I have a pair of group 34 yellow tops in a dual configuration that are coupled when in the ignition is in the run / start .
Both batteries work to start the engine and charge together thus having as close to the same initial current draw / drain and charge.
I’ve had this setup with the original batteries for 10 years so far with no issues.

In your setup, you always draw down the starter battery but your secondary is always topped off.
I do not understand why you have a 7 second delay.

The issue with that is the charging system is working to recharge the starter battery while also overcharging the secondary battery.
Another possible issue is if your vehicle has a “smart” charging setup from the factory.
These smart charging systems do not fully charge up AGM batteries.

In general a dual battery setup is to provide power via the secondary battery for acc. so as to not deplete the starting battery.
What are you doing with a Cadillac DTS that requires such a setup other than perhaps a stereo?

[Author]

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