Community Forums

melvin2345 · 03-02-2021, 11:34 AM

I have two 6v 225ah under the step, but I want to add 2 more. There is a storage compartment on either side of the doorway. The one to the rear of the door is a half size, and has some plumbing run through the top of it. The one to the front of the door is larger and has tanks behind a hanging flap. I also have an inverter that I want to hook up in one of the compartments (separate from the batteries). given that I'll need to add some venting, which compartment would be better suited for two batteries, and more importantly, what is the best way to reinforce the bin to hold 130lbs of batteries? I'll get some pictures tonight if that makes things easier.

Here is a crude mock up of what I'm thinking.

Krbjmpr · 03-02-2021, 02:10 PM

The batteries under step will discharge faster than batteries in compartment if connected as per your drawing. Shorter cables have less resistance and will carry greater portion of load.

I would keep batteries in series like you have, but have the jumper same length on both sets. Move coach negative to batteries in compartment, keep batteries under step connected to coach positive. Use shortest cable as practical but same length to connect compartment batteries in parallel with step batteries.

This will force the current to flow through long cables equally. You will lose a bit of power due to resistance, but batteries will discharge at same rate. Inverter is connected at same point as coach connections.

Compartment reinforcements. Easiest way is to lay 2x2 across compartment floor. Still keeps total weight on compartment only though. If it were mine, I would procure 1in as 3/16" wall aluminum tubing, anchor to bottom exterior of compartment, drill appropriate hole where needed to attach all thread going to frame member above compartment.

Just a suggestion... You might look at upgrading your existing batteries to Lithium Iron Phosphate. 200AH rated LIFE battery has 200AH available, not 100AH for similar rated lead acid for maximum life expectancy. Third of the weight as Lead Acid as well.

In other words, 1 12v LIFE rated at 100AH and 22 pounds is same capacity as (2) GC2 batteries rated at 225AH and 130 pounds. You would already be doubling your capacity, extra battery not needed.

melvin2345 · 03-02-2021, 03:01 PM

Quote:

Originally Posted by Krbjmpr

Move coach negative to batteries in compartment, keep batteries under step connected to coach positive.

I'll have to see if that's doable. I do try and keep load points at opposite ends of the bank (RV or home), but not sure if it will reach. Ideally, I would attach the inverter the same way (opposite ends of the bank). I'll keep cable length as short as I can, also planned on using 2/0 gauge as well. Should help with the resistance issue some.

Quote:

Just a suggestion... You might look at upgrading your existing batteries to Lithium Iron Phosphate. 200AH rated LIFE battery has 200AH available, not 100AH for similar rated lead acid for maximum life expectancy. Third of the weight as Lead Acid as well.

True, but my wife already took a fair amount of convincing to let me spend $850 on batteries. I'm not sure $2,000 would go over too well

twinboat · 03-02-2021, 03:40 PM

Don't worry about batteries discharging unevenly. If wired correctly, its a non issue.

If you use 2-0 cables as your diagram shows, they will all charge and discharge evenly.

The voltage drop in 2 feet of 2-0 cable is minuscule and probably not measurable with most meters.

Even if they get unbalanced during heavy dischage, they will equalize with in seconds of the load dropping off. Batteries naturally equalize their voltage.

Same with charging, batteries charge by voltage. Once the voltage is reached they don't accept anymore amps. If one set is lower, it will continue to charge, accepting amps until the voltage is reached.

Your alternator charges the chassis battery and then your more distant house batteries thru one positive cable, and the chassis steel as negetive, all of the time.

Use proper size cables to limit voltage drop and all will be fine.

As far as using 1/2 of your flooded batteries capacity, thats a myth.

You can safely use 80% of their capacity if you want to. Each time you do, you use up a full cycle instead if a half cycle.

Batteries have a set amount of cycles, most about 1000 cycles at 50% discharge. They offer up 500 cycles at 80% discharge.

An occasional full discharge of 80% capacity is an insignificant lose of life.

One other point. If you discharge a lithium down to 10%, it shuts off and your left in the dark. Discharge a lead acid battery to 50% and you still have a cushion before the light go out.