Sundeck Dual Batteries - can one be cranking and one deep cycle?

[jebjam]

I have a 2008 240 Sundeck currently with two marine cranking batteries, I use a NOCO genius 2 bank charger and the boat is on shore power when in the slip.
One of the batteries needs to be replaced. I recently changed my camper to lithium and have a deep cycle battery that's less than a year old sitting on the shelf.

Is there any reason I can't replace the current bad battery in my Sundeck with the deep cycle battery, leaving me with one older cranking battery which seems just fine and a newer deep cycle?

I typically turn the batteries off when in the slip, attached to the smart charger which has worked very well. I use both for starting and running and one or the other when moored off shore running sound, water pump etc. It seems a capable cranking battery will be just fine for starting and the deep cycle battery will be perfect when moored, are there any issues in the Sundeck with having different batteries on board but using both to start and run? I don't believe they are connected in parallel on the Sundeck but the boat's 40 miles away so I can't just walk out and check.

 

[paulswagelock]

It will be fine.

 

[jebjam]

Thank you, that's what I wanted to hear! They are connected in parallel after all btw.

 

[techmitch]

Thank you, that's what I wanted to hear! They are connected in parallel after all btw.

If they are connected in parallel then they should both be the same type and both replaced at the same time.

 

[paulswagelock]

Are they connected in parallel or do you have a 1/2/both/off switch? If you have the switch, don’t run in both, use them as starting and partying. If they are truly in parallel, should be same size, type and age.

 

[jebjam]

I have the 1/2/both/off switch. I thought parallel but that is incorrect - the negative terminals are connected but the positive goes through the switch. I realize one is for starting the other for partying, and I can connect to shore power for charging when in the slip, but don't I need to run on both to charge when I'm away and not able to connect to shore power? My local boat mechanic advised there would be no problem running on both, but I'm questioning that exactly because they are not the same. Can you fill me in on the trouble/harm with running on both?

 

[ColoSR]

Running on both with two different ages/types is bad for the batteries. The alternator thinks it is charging one battery, and keeps charging the newer one even after it gets to full charge. This boils the water in the battery and shortens it’s life. If you have 2 batteries of similar size and vintage you don’t have this problem on both because they both reach full charge about the same time.

there is a widget called an automatic charging relay (ACR) that will allow the boat to charge both batteries even when running on 1 or 2. it monitors the charge on the first battery, and closes the connection to the 2nd battery once the first one is fully charged. You can also get a different switch that has starting and gauges on battery 1 and house for battery 2 (stereo). In this configuration you are better off with a starting battery and deep cycle battery for the house.

https://www.bluesea.com/products/7610/SI-ACR_Automatic_Charging_Relay_-_12_24V_DC_120A

 

[paulswagelock]

I have the 1/2/both/off switch. I thought parallel but that is incorrect - the negative terminals are connected but the positive goes through the switch. I realize one is for starting the other for partying, and I can connect to shore power for charging when in the slip, but don't I need to run on both to charge when I'm away and not able to connect to shore power? My local boat mechanic advised there would be no problem running on both, but I'm questioning that exactly because they are not the same. Can you fill me in on the trouble/harm with running on both?

ColoSR explained it well, different batteries will charge at different times and rates, potentially overcharging one and undercharging another if run on “both”. Running on “both” also has the potential to let a failed battery draw down a good one leaving you stranded. Just not a great idea all around, except it does recharge both batteries when not on shore power. Yandina makes a low cost and effective solution to charge both while running but not switching to both called the combiner 100. It does the same thing as the ACR mentioned.
Is it the end of the world doing as you suggested with the spare battery of different specs and running on both, no, but it introduces failure risk. Might be fine and last for years that way, hard to say. But you now have the knowledge to understand the risk, what are the alternative solutions, and can make a more educated decision.

 

[jebjam]

Thank you for the easy to understand explanations. Happy to learn that the risk is to the batteries not the boat's electronics. I'm familiar with the need to not overcharge a battery and invested in an on board smart charger for exactly that reason. Never really thought about that happening while running, the lakes we boat on are relatively small so I'm not typically running for more than an hour at a time and until now the batteries have been the same.
Run on one, float on the other, and charge in the slip it is. Thanks!

 

[Arminius]

Charge one battery at a time or get 2 chargers. Chargers can be confused in the same manner as the alternator discussed above.

 

[Leardriver]

I have always had one of each battery. Running them on the both position defeats the purpose of having two. When you unintentionally run one down, you have a brand new charged one waiting to save the day.