Dual Battery Setups! Let's see them! Multiple Batteries Thread!

anyone ever mount one behind the rear seats?

 

I am currently running two

$360 Optima D27F YellowTop Starting and Deep Cycle Battery

Optima D27F yellowtop batteries. The starting battery is going. Is it necessary for both batteries in a dual battery setup to be the same? I am having trouble finding a replacement battery.

 

Others have successfully mixed different types of optima batteries (yellow/red, yellow/blue, etc...), or different AGM batteries. I think the "type" of battery is more important than the rating.

 

I'm going to order a 27f from Optima. Thanks

 

Mine is behind the rear seat

 

Depends on some extent on your multi-battery set-up. Are they always parallel or do they disconnect at some point (such as when the engine is off)?

Generally speaking you need to match batteries very closely when they are configured as a bank, meaning always connected together.

You can get lax if they are disconnected under load and only remain parallel when charging. It's still better during charging if they're fairly close in condition and really should be the same chemistry (e.g. both AGM or flooded), but it's not absolutely critical.

There's some wiggle room here because the set point voltages and times on the combiner come into play. IOW, depending on how smart the relay is factors into how much you can get away with. Wouldn't change the charging side necessarily because that's also a function of the charger itself (e.g. alternator) and the battery charging profiles.

 

I'm using the Blue Sea setup. I going to stay with the same battery. Going to order from Optima. Thanks.

 

The Blue Sea ML-ACR? That's a solid combiner (also what I use BTW). It's definitely safest to stick with the same type batteries and then only the condition of each comes into play.

 

With regards to the type of battery, as long as there the same type (acid, agm, gel etc) you should be good to go. The sizes of each doesn't matter.

Looking at the blue sea ml-acr and hoping I don't have to switch batteries as they are of different sizes. Off the top of my head they are the same type.

 

As long as the batteries are of the same type (e.g. the flooded, AGM, gel, etc.) and reasonably close in condition then the capacity (amp-hour, CCA, etc.) won't matter. Each battery will consume and provide current to/from the circuit as necessary and will coexist without a problem. It's dissimilar voltages that are the concern.

There is a limit to capacity but the concern is related to the charging system. It's possible to have too much battery compared to alternator or charger. The stock alternators will be fine with a second battery for the most part, but just be aware that there is a limit where your demand may exceed the alternator's capacity. So be realistic about what you can and can't do.

 

Can you expand upon the alternator limit? I am about to install a dual battery system with Optima D35 yellow tops, I have a winch and other accessories, do you think I need a larger alternator? My taco didn't come with the factory tow package so I don't have the larger alternator. I won't really be towing but I will be hauling a decent amount of gear and two dirtbikes quite frequently. I am getting the side-by-side system from OGE and I bought the Renogy solar panel to go with it.

 

There may be some advice to be found for RVs and boats on the amount of charging you need compared to battery. All I can offer is my opinion and experience, which is limited to very simple electrical systems. I run a fridge, winch, ham radio and have chosen to keep the stock system just about completely stock. Point is YMMV.

What you have to think through is how much charging you have available and the details on how it's made available. Our stock alternators are either 90A, 100A or 130A depending on what options you have. For the sake of argument lets just say we are using the upgraded towing and offroad 130A for the time being. This number is the rated capacity of the alternator but that doesn't mean this is what you get in all conditions. In fact this amount of current is generated only in a very small window of engine RPM and temperature. If the alternator is spinning slower than the rated RPM or the temperature of it is warmer then the output is reduced. This can be very significant. At idle the alternator might be half it's rating.

So you need to run the numbers for how much energy you will take from the batteries, both normally and under a worst case scenario. Then assume how much the alternator will generate and you'll then know how long it will take to get back to 100%.

Let's say you eat up 40 amp-hours of energy over a day or two running a fridge. If your average RPM is maybe 1500 while driving on the trail and that puts (in theory) the alternator output at 80 amps then you'll need at least 30 minutes to return the bulk of the energy. It will actually take longer than this to reach full charge but for the sake of argument you get the point.

The main reason for this is the truck itself uses power to run the ignition, lights, radio, etc. So even if you have 130A at the alternator only the excess that isn't used by the truck will be used to charge the batteries. I honestly couldn't say how much this is. The truck itself is probably 10A or 20A but headlights will eat a lot of energy, A/C or heat (the fan) would, too. My guess is that at night the truck probably doesn't have much excess current to give, so factor this in.

Now if you suddenly need to winch yourself you don't want the alternator trying to also charge the aux battery. Same with running lights or a radio. You just need to know what the various draws will be and think through scenarios and probabilities of thing happening. IOW, list the things that are essential, important, optional, etc. When you run through the list you'll come up with anticipated needs that you compare against available power.

You /need/ the truck to run and start. You would probably want to have a fully charged main battery under all conditions. I personally left the stock system intact, which has a constant small draw to the ECU and stuff. So there is a limit to how long I can leave the truck without starting or charging. I know from experience that this is actually several weeks, but it's not forever. So as long as I don't leave lights on I'll be fine.

My fridge draws about 2 amps when running and I know that when it's really hot outside it will run as much as 50% duty cycle. That means it's running 5 minutes, sitting idle 5 minutes, within any 10 minute period. So that means it's using about 1 amp-hour in hot weather. Since I run a 65 A-hr Odyssey I can in theory run 65 hours between charges. In reality you can't really get all the capacity from a battery since that means running it to 0%, called a full discharge. This is usually measured at 10V on the terminals. Besides this being bad for a battery it's also not practical since my Engel shuts off below 11V anyway. In reality I can get more like 45 to 50 A-hr before nothing will run anyway. My rule of thumb is 36 hours before I start thinking about charging the aux battery. With larger batteries you can go longer obviously.

Since the ML-ACR will prioritize it takes some time before the aux battery is even brought online. Normally this is only a minute or two since starting doesn't really take much energy and the battery is usually at 95% or whatever. Once the parallel batteries are connected then the clock is running.

KEY POINT - Our stock systems can handle a second battery without much worry just as long as you realize that it will take some uninterrupted running to get the batteries both (or all) back to fully charged.

Toyota selected roughly a 1:1 ratio of max alternator to battery (a 27F being about 100 A-hr). In my case I have about 130 A-hr of battery so I haven't moved it very far. If you leave the stock battery in place and add a big deep cycle you might go from 1:1 to 1:2, a 130A alternator with a 100 A-hr main and a 100 A-hr aux. That's still not excessive or too badly imbalanced IMHO. There's a point where your expected load does exceed alternator, but that's probably going to be with an inverter or lots of lights that may be running when the engine is running. That takes away current that could be charging.

Where you start to get into what-if would be having a run down aux battery and a sustained winch session that also draws down the main battery. You could end up with two dead batteries and a real PITA if you stall or something. The likelihood of this is extremely small but it's not impossible. That's why people chose to add solar so that a second battery doesn't just run down but can still serve redundancy to back up a main for winching or if it dies in the field.

BTW, part of the reason I prefer to have the winch on the main battery is because the aux battery in my truck is likely to be discharged and I don't want the alternator to fight with the winch if both want lots of current. I'd let the aux battery stay discharged if I had to winch first thing in the morning. Also a winch is more like a starter so if someday I want to put in a real deep cycle I wouldn't want to have to rewire things. Plus I like being able to isolate the aux (back-up) from the main should winching cause a failure perhaps to the alternator or wiring. I can decide to shut off the fridge and force the ML-ACR into isolate, cold beer being only slightly less critical to survival, to preserve the charge on it.

If you have 100W of solar that can generate, say, 6A of charging current then you can significantly extend your times between starts. But you have to watch average charging here, too. That 6A will only occur at noon with bright sun and cool temperatures. Over the course of a whole 24 hours a solar charging system will only generate during daylight, so automatically you get about 50% knocked off and if it's hot the panel efficiency goes down, at low incident angles it goes down.

You can probably assume a 100W panel will give you more like 2 or 3 amp-hours during the day and so you get more like an average of 1 or less amp over a 24 hour period (e.g. returns 24 A-hr, which is probably a high estimate TBH). That means if you're lucky it's going to offset a fridge running. But even if it can only keep up with 1/2 the daily fridge consumption that's a serious advantage over nothing if you want to leave it parked for a day or two.

 

Awesome, great response @DaveInDenver

Thank you!

 

The solar part got me thinking so I put together a hasty spreadsheet to demonstrate how solar helps. It's not exact or anything since the formulas don't really act right when the battery fully tops during the day. But you can see the difference between having it and not at the end of the 24 hour period. There are a whole lot of simplifications and assumption in this... FWIW, it's based on a 6A charging current and a 2A load with various duty cycles and efficiencies.

 

Good info from Dave. When you install a dual battery system just plan on getting a good charger and give it an over night charge once in a while or after a long camping trip and you'll greatly increase the life of your batteries. Our alternators really aren't enough to maintain two batteries long term

 

Awesome information. Really the only demands that will be placed on the second battery is a fridge, the odd led lights at night, and a inverter the odd time too. However, I was running the second battery alone on a 100w solar panel, no help from the alternator. All in all I feel like the alternator should be able to keep up.

I will double check to make sure the second batter (cheap agm) didn't get ruined while I was using it (hoewever after 5-6 months of sitting it still says 12.6 volts).

Was going to run the ctek unit but for the price, I can do a whole blue sea ml-acr unit with fuses, and 1/0 cable. Already have the solar controller so...

 

Yeah, adding an aux battery for a fridge is essentially a trailer electrical system, so I see that within the expected capability for our trucks. That we add combiners and attempt to isolate main and aux systems is probably less taxing than plugging in a RV camper or something.

That's slightly lower than you'd expect ideally for a good AGM. In case anyone wonders, you should see 12.8V up to perhaps 13.0V open circuit on an AGM after doing a full charge and letting it rest about 12 hours. But 12.6V wouldn't indicate a failed battery either so since it has been sitting self discharging for a few month you may just need to give it a good charge.

 

The largest single power draw you're going to get is from a winch. A 9000 pound winch can pull 320-350 amps on a full load pull (depending on winch model). However, it will most likely only be dong this for a few minutes, and not all at once. Even if you have the 130Amp alternator it will still pull at least 220amps from the battery. Still that is a lot of work for a battery, and most of the time is not a problem. However, I would recommend putting a winch on an auxiliary battery vs the main battery. I really like the idea of keeping the stock wiring as stock as possible and running most if not all of any accessories from a second battery.

 

IMO, the best thing you can do is to invest in a smart charger like a ctek. Using it will extend the life of both of your batteries. Today's smart chargers do some cool things and are not simple chargers any longer.

This is the one I am using with my AGMs. Price is a reasonable investment:

CTEK (56-864) MUS4.3 12 Volt Fully Automatic 8 Step Battery Charger,Black

https://www.amazon.com/dp/B006G14FK8

 

The locked rotor current of a 8000 lbs winch is more than 400 amps. My XD9000 has a max current of 478 amps at rated pull on the first layer of the drum. At 2000 lbs on the first layer the current is much lower, 161 amps, so even at light pulls the battery is working.