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Old 07-29-2020, 04:15 AM   #1
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Connecting 2 batteries of different capacity


I understand that connecting 2 battery of same voltage but different age or capacity is a bad idea.

I already have a SRM 27 interstate deep cycle battery of 88AH but itís not enough for our 94 SMV . If we camp more than one night and do not drive, we are out of fridge.

So I was planning to add extra capacity.

Can I do it safely with a switch (see diagram attached)? So that I can use primarily the new larger capacity battery but still keep the second one as backup for when we stay in the while longer? What type of switch do I need? I guess a pretty strong one.

Even crazier, with this system do you see an issue with having 2 different type of battery? For instance if I decide to add a lithium as the new one instead of wet. And last question if I choose lithium, do I need to change my battery isolator, and shore converter?

Sorry if the question feel naive - Iím a newbie in electricity 😆
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Old 07-29-2020, 06:45 AM   #2
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Do you have any solar charging as part of your system? how are you charging batteries now? How old is your 88ah battery? what other draws besides fridge? Let's start w these and understanding total electrical needs/usage.

Please consider adding your location to your profile, thxs!

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Old 07-29-2020, 08:28 AM   #3
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Done, I addded my location ( Boston).

So here is more details in picture of my current system and what I'm trying to achieve.

i dont know how old exactly is my 88AH battery, but it can power the fridge and LED for one night but that's about it.
Current battery: I get 12.8V fully charged
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Old 07-29-2020, 08:34 AM   #4
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I forgot on the appliance list the water pump for the faucet.
And potentially I'd like to rewire the stereo and factory amp from the car battery to the house battery.
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Old 07-29-2020, 09:07 AM   #5
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Blue Sea makes a switch for the job, I doesn't sound like your current use is high so the Blue Sea Systems 300 Amp m-Series Battery Switches should do the job. The 3 selector switch #6008 should do the job

You could also use the 4 selector switch #6007 would also add the ability to combine.

Do not mix lithium with existing battery, and yes you would need upgrades to you existing electrical system to go to lithium.

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Old 07-29-2020, 10:21 PM   #6
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Your charge rates are not going to be the same, the capacity is not the same, and most likely more internal resistance on the aged battery. Those could lead to a catastrophic failure or your old battery due to increased heat and/or reduce the life of the new battery. It would probably be best to sell the old battery and get a new higher capacity battery or a set of the same batteries that have close birth dates with the capacity you need.

My .02
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Old 07-30-2020, 10:13 AM   #7
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As has been mentioned mixing different types and / or age batteries will prove to be far less than ideal. If cost is a concern, I would recommend getting two new 6v deep cycles and connecting them in series for 12V. They are available at places like Costco for under $100 ea, and you will end up with around 220Ah's capacity. That should allow you a couple days or more of refer run time, depending on what else is drawing on them. Assuming you are running the engine to charge them, adding a battery selector switch will require you to monitor the charge on the selected battery, and when fully charged, you will have to manually switch to the other battery. That's not idea while driving, and it will take longer to get both up to full charge. If you end up with two new 12v's, of the same type, using a battery switch with the ability to combine will work fine assuming you remember to combine them while driving then separate them once stopped.

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