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So after my inverter Cut off, I concluded the batteries were no good, as soon as the inverter the total voltage (of the 2 Trojan 6v batteries in series) dropped to 2v and when switched off slowly rose back to around 10v. So I bought 2 brand new 90ah 12v batteries to achieve 180ah. Previously with the trojans I had 250ah. But 180ah should be enough.

My question is that on the internet a lot of people suggest against using 2 12v batteries in parallel, please can anybody suggest why? And a possible solution.

If I do go ahead with this, what would be the best way to charge the 2 batteries, (whilst the are connected to each other, or disconnect them from each other and then charge.

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    \$\begingroup\$ This is the third time you have posted an off-topic usage question about the same issue with powering the snow cone machine in your food truck. As with the first, you need to to be talking to people with practical experience in your industry, not asking questions from first principles here. \$\endgroup\$ – Chris Stratton Jul 28 at 1:10
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    \$\begingroup\$ To have only 2V on 6V batteries you MUST have deep deep deep discharged them to well below their sensible or safe end point. When using 12V batteries CONSULT THE MANUFACTURER'S DATA SHEET RE absolute minimum voltage. ALSO - to get decent life from lead acid you must only discharge them by a small fraction of their rated capacity. Again. the manufacturer should advise but say 20-30% discharge to allow a REMAINING 70-80% of full capacity is needed for long life. Batteries will ideally be deep-discharge rated and constructed. \$\endgroup\$ – Russell McMahon Jul 28 at 2:50
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Two things. If your truck wiring is 12V and you intend to charge the batteries off the truck, you have to connect them in parallel for the same reason your now-knackered 6V batteries were connected in series: to be compatible with the truck 12V charging system.

Also, your inverter probably needs 12V, though it might support 24. Regardless, the truck only provides 12V (I assume) so that dictates how you connect the batteries.

To charge them, you should connect the parallel-tied batteries to the main battery in your vehicle, with a self-resetting fuse and a disconnect.

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It is common practice in boats and RVs to connect batteries in parallel, but all batteries should be the same make, type, and age. Problems can occur when mixing batteries of different type or history.

On my boat I have four 6 volt "Golf Cart" batteries (similar to your Trojan 105s) in series/parallel to give me a 450 Ah, 12 Volt battery bank.

If you are running an inverter with these batteries, you should use "deep cycle" types, not automotive starting batteries, as the deep cycle types can take more frequent deep discharges without damage than a starting battery can.

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The success of running cells in series or parallel depends on how well they share voltage or current respectively. This means both the Ah capacity and ESR of each 2V cell must be well matched. The ESR can be gauged by using and acid specific gravity tester in each cell hole and should be periodically monitored/recorded over the life and State of Charge of the battery. It should be matched within 0.1% for ideal performance and 1% mismatch can lead to weak cells which can result in over or undercharge in extreme loads of charge currents before the cutoff.

When you have a heavy load such as a starter motor or A/C or refrigeration unit, the measure of stress is the load DCR/battery ESR. e.g. a cold engine is rated for CCA when the voltage drops from 12.5 to 7.5V for 30 seconds allowing a maximum current as of its CCA rating. Thus 750A @ 7.5V implies a load of 10 mOhm.

But for continuous use, this is too low. You can measure your compressor DCR and estimate or test your battery bank ESR or look at the voltage sag when it starts and determines how that stresses imbalance in your cells. What are your results?

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