Off-Grid Engineering Dual Battery System Q&A

Well, yeah, I'm that DaveInDenver. And there are reasons to wire things in certain ways so that batteries equally charge and discharge, so I wasn't saying it's right or wrong. Just wondering why.

Where you take power and where the ground/return is relative to the charging and load does make a difference. Ultimately how you put together the circuit depends on a few factors, so you can't always say there is one right (or wrong) way.

Conceptually this diagram shows the nuances but even it assumes that both batteries always charge from and power the same load. If you have different loads on each battery the 'right' way wouldn't necessarily be this either. I /think/ what you were suggesting is similar to the wrong way, but do correct me if I've misunderstood.




So the reason I ask is that if you were to put a second battery ('B' in the diagram) designed to parallel the main then you may want to unground the main battery ('A') and move the starter/winch/aux ground to the 'B' one, if you leave the loads on the 'A' one anyway. In effect flipping the power and ground connections from the diagram.

 

Thanks for your input here @DaveInDenver

You are correct, what I was suggesting was more similar to the "wrong way" in your diagram. I was simply pulling from my experience in designing/ building these systems for multiple different vehicle types throughout the years... but at the end of the day, I am not an electrical engineer, and probably will never have the level of knowledge you do in this arena.

The "wrong way" in that diagram is how every dual battery system on the market, including mine, is set up... with the addition of an extra fuse box attached to battery "b" and auxiliary loads drawn from there.

I suppose removing the body ground from the main battery and instead, attaching it to the auxiliary battery would accomplish what you are suggesting as closely as possible. Please, correct me if Im wrong.

This diagram assumes that both batteries are in parallel all the time and not isolated. It also assumes that you want to draw a load and charge both batteries evenly. In a traditional vehicle dual battery system, such as mine, the goal is to keep one battery completely charged while drawing a load off of the other. Then, when the charging source is reintroduced, replenish the auxiliary battery (while also charging the starting battery). Since the alternator and starter traditionally remain connected to the main battery (and the main battery is usually full), there is no perfect way to make sure that the charge is sufficient to replenish the auxiliary battery.

This is why I recommend isolating the two batteries and charging them independently from time to time.

 

This is what I meant by saying the right way in the diagram is if you have a bank of batteries always connected. Having two batteries in parallel during charging but isolated during discharge is a different situation.

Indeed. We are mostly just worried about having a backup or an aux battery. The wiring in the diagram assumes you have two batteries in a bank always charging and discharging from/to the same places. It's also important to realize the length of the cables can't be ignored. Having the batteries right next to each other (like I have) means they are within tenths of volts of each other so the paralleling is different than having one in the engine bay and one on your trailer, which I suspect is the point of the diagram.

That is a good thing to do. Vehicle charging systems don't fully recharge. On a starting battery we get away with it because starting is actually not really that difficult on a battery. Well, normal starting, sitting there cranking on a cold engine or if it doesn't want to catch is hard on it. But normal crank for a second or two is actually not using much energy and is not going to be a deep discharge. However a house battery that runs a fridge for days is a very deep discharge and the vehicle charging system needs to be augmented with a shore charge.

To give some example. Odyssey rates their batteries for 400 cycles. This means taking a fully charged battery to 0% state of charge, which in the case of an Odyssey is 12.8V open circuit down to 10.02V open circuit. That's the range of voltages that represents consuming the full amp-hours available. But normally we don't do this. Say we start at 12.8V and draw down to 12V, taking 35% of the available amp-hours (e.g. capacity). When you do that 3 times (roughly) you've used one cycle of the battery's life.

What Odyssey found was charging up to 14.2V ends up with the battery failing after about 50 cycles instead of 400. When they charged up to 14.7V their sample batteries lasted 250 to 350 cycles. Their test was harsh, drawing the battery down to 10.02V each cycle and hitting with a 16 hour charging. My point is that our vehicle charging systems only run at ~14.4V and are somewhere in the middle of ideal and bad, so using a good shore charger periodically is necessary to get a good life from batteries. If you can do it every time that's ideal but I found that once a month or so is sufficient to keep them in good shape.

 

Awesome. A bunch of great info here.

 

Yes, I think that would be one solution, effectively flipping the 'right' diagram circuit.

I think the question would be undercharging the remote battery in the 'wrong' diagram and that could happen regardless of the loads you put on it. The voltage at the end of the remote battery is going to be lower than the main battery due to the loop resistance of the wires. Any current flowing through loop will lower the voltage across the battery and the problem would be if there's a lot of current flowing the drop will be relatively large.

To give some numbers. Let's say your remote battery is 10 feet from the main. That means you have a loop of 20 feet of cable (out and back, it's a loop). Using 2 AWG cable would mean that loop is 0.003 ohms. If the battery wants 35 amps (since you've run it down pretty low) that would mean the remote battery is 0.105 volts lower than the main. Since like I mention 14.4V is already marginal, 14.3V isn't going to help anything. At lower currents the drop would be lower, at higher currents the drop gets worse. At 150A (perhaps a self jump) the difference would be 0.45V between the two batteries.

With the bank configured the 'right' way both batteries are always going to see the same voltage regardless of what current is flowing. The presumption is that as the main battery (which the alternator is wired to see) dips the charging system will compensate and increase the voltage. That just means both get charged to the same level, whatever that might be.

 

Copy that. When placing a battery remotely, it is always best to oversize the wire to minimize voltage drop. But as you suggested, relocating the ground is not a bad idea either. I suppose that would be the cliff notes. Thanks Dave!

 

@HeliMedic are the new main batter holders ready? Want to order one to complete my setup.

 

Well... there is at least one done

But seriously, I should have a batch of them done in the next week. Let me figure out pricing and I will email you directly. Sound good?

 

Sounds good.

 

Outstanding info.

I have the "B" battery in the bed and the loop is ~15 feet of 1/0. Not sure how that will impact the voltage drop numbers you refferenced but I suspect there will still be some. I'm thinking I will try grounding the the "B" battery instead of the "A" battery to see if I can see any changes.

 

A 15' length of 1/0 AWG is 1.4745 mΩ (about 0.0015 ohms).

Will be interesting to hear your results.

 

So, reading through all of this info from @DaveInDenver, should I move my ground to the second battery, or will it matter in this particular scenario? With @HeliMedic's side by side trays, I can't see there being much voltage drop over the 6" of cable connecting the two to make a difference? Though I only have minimal electrical knowledge.

Also, another question for you @DaveInDenver, have you had any problems running 2x 50lb batteries in the stock spot? Trying to figure out what batteries I should be buying. Not a big fan of optima

 

While the information provided here was extraordinarily useful to know, I do not think you will have an issue with where my system currently places your chassis ground. The size of cable and the short distances provided, ensure minimal resistance.

Couple that with the fact that there are multiple dual battery systems on the market with hundreds of thousands of users that run this configuration without moving the chassis ground... and my only conclusion is that it is a solidly acceptable design.

As far as the batteries are concerned: I have personally run two Odyssey batteries, side by side, in the factory location for many years and have never seen sheet metal fatigue or cracking. It is simply something I must make people aware of. 120 lbs of battery in one location is a lot of weight. But, I believe that the battery tray's base plate spreads out the weight sufficiently.

Also, Optima batteries have gotten a pretty bad rap lately. Personally, I haven't seen a lot of failures. But... that doesn't mean their quality hasn't declined.

Check out the new Diehard Advanced Gold AGM. I think it may be the replacement for the old Platinum series. I've been running them In my truck for a while and have been impressed. Their numbers put them right in between Odyssey and Optima, performance wise.

 

I would look at the Diehard batteries but they aren't available in Canada as far as I've found. The batteries I'm really looking hard at are the NorthStar NSB-AGM35 which are a lot cheaper than Odyssey, but with near equal stats. http://saskbattery.com/ecommerce_493.htm

 

Those are great batteries. There are a lot of people here running them. I do not think you will be disappointed.

 

The Optima is made by Johnson Controls, which I believe makes the Truestart OEM Toyota batteries and those have a pretty good reputation. I've personally never had an issue with Optima but they do seem to be hit or miss.

 

Hey Matt, I'm interested in purchasing a complete Dual Battery system from you for my 15 Tacoma but I'm looking under the hood where Battery 2 will go and there's a small plastic and metal box wired into the harness and stood off the interior fender wall about 2" by a bracket. Will this be in the way? I've looked at some of the pic's and I see a similar looking device attached to the firewall in some of the pics. Hopefully you've seen this before and it's not a big deal but I'd appreciate any comments/advice.

 

He covered this previously I believe, it gets bolted onto the second battery mount, basically un bolt, rotate, and screw onto the battery tray.

 

Thanks. I should have kept reading... That explains the similar looking box wedged into the space between the new battery mount and the fire wall.