Our (so far) successful dual battery system...

I wasn't sure if I should share the 3rd iteration of our dual battery set-up, but I have received a few emails of folks planning their PanAm drive and I think maybe it is time to share the story of our set-up and some of the issues we had.

We have been living (mostly) out of our Landcruiser since July of 2009 and here are our 12v drainers:
  • Engel 45 fridge
  • iPad
  • 2 iPhones
  • 2 Kindles
  • LED lights
  • (at beginning of trip we had an Endless Breeze fan, sold after a couple of years)
That's it. In other words the fridge and the Endless Breeze are the main battery killers...

We started out with a very cheap, simple and efficient set-up very similar to the first post by Evldave (here). 2 AGM Batteries, a dedicated starter and a deep cycle "house battery" separated by a solenoid, no solar. For most this is a great set up, but anyone embarking on an extended trip (outside of the US) and with repeated multi day remote camping: not so good... If you plan on driving after every 3 days of camping for a few hours, you will be fine because the house battery will remain kinda healthy. In our case: after about 8 months of travel we had completely killed our very expensive 105 amp hour house battery from so many complete drains... yep, that happens, no matter how good your battery is if you drain it lower than 12v (we regularly took it to 11.5v while remote camping) it will die quickly.

So here we are in Mexico with a dead battery under warranty, but nowhere to get it replaced and to make things worse our solenoid had quit working (it was keeping the 2 batteries connected at all times) no idea why. Sadly due to location and circumstances we ditched the battery, bought a cheap-ish wet cell deep cycle battery at a marine supply place and installed a marine manual battery selector switch -as in: manually select 1, 2, all or no batteries- dead simple system as long as you remember to isolate and connect the batteries depending on your needs :) I forgot... a lot... We had a new battery, problem is we were still draining it on a regular basis. So what next? Solar.

Try buying solar in Mexico! Not cheap and not quality... We bought a Sunpower 90 watt panel and a Morningstar charge controller and luckily our friends at BajaRack helped us by sending us the panel from Bajarack HQ San Diego to Bajarack factory Ensenada to San Miguel de Allende (where we were camping at the time)... we thought everything would be good. Why not? Solar would help us keep the battery full, right? Wrong. It definitely extends your remote camping by a few days, but not indefinitely. There are always cloudy days, the panel gets dirty, you don't follow the sun with the panel for maximizing output... tons of reasons why it ain't as good as they say. After another year or so we had killed another battery, but this time I was lucky enough to find a friendly solar power equipment distributor and he gave me a great deal on an AGM 110 amp hour battery...

I realize I am rambling a bit, but bear with me, here is our third, current and hopefully last(?) iteration of our electrical set up:
I started studying and trying to understand 12v systems and I realized that the alternator does not completely "top off" the house battery and it does not provide optimal charging for our AGM deep cycle battery... solution? A DC to DC charger! C-Tek makes just such a beast: D250S, it ain't as cheap (~$244 at Amazon) or as small and waterproof as I'd like, but it has several things I do like: It is a battery isolator, a 5 step intelligent charger and an MPPT solar charge controller in one, it even has a temp gauge to optimize the charging of your house battery based on its temperature. I consider it a ridiculously simple to install albeit a tad expensive dual battery/solar set-up. Since we started using the D250S I have definitely seen improved performance of our battery, mostly because the output of the solar panel and the alternator are being used in an optimal way.

We also added another member to our battery health and welfare team: an AC battery charger. Why? Because we do camp at campgrounds, restaurant parking lots or friend's houses during our travels and what better way to extend the life of the battery than fully charging it and then not using it while there is access to AC power? So we went a bit overkill and bought a Ctek 56-674 US25000 charger (~$191 at Amazon), a 25 amp, 8 stage charger that has the nifty feature of providing "Supply" power to the house. In other words once you finish charging the battery you can switch the unit to "Supply" mode and it serves as a 12v power supply for the house while saving the house battery cycles for a later time. The truth is this unit is a bit large and if I were to do it again i'd get the smaller (and cheaper) Ctek multi 7002 (~$90 at Amazon) 7 amp charger. The unit is much smaller, and still has the "Supply" mode (the 7 amps would still charge the battery well, just slower).

So far we are doing quite well... the current house battery is well over a year old and still holds a charge like a champ and we have had no issues related to our dual battery setup...

IMHO negatives with the CTEK products are only their giant size (hard to instal in a cramped engine bay) and they are not waterproof enough.

So in conclusion what would I get for my rig if I were starting my trip all over again:


  • For the starter: a high quality, reliable starter battery (we have the one we started with: a Sears DieHard Platinum)
  • for the house: the largest AGM (true) deep cycle battery I could fit in my engine bay
  • a CTEK D250S 12v to 12v charger, isolator, MPPT solar charge controller
  • a dedicated AC intelligent battery charger (we are happy with the CTEK 56-674 US2500, but the CTEK Multi US 7002 should be good
  • a reliable solar panel of the highest output you can afford (and that can be handled by the charge controller)
  • a quality 100ft extension cord (a must) for AC charger (we have been known to camp at a beach restaurant and use their power)

Whatever you do do not skimp on installation quality, keep in mind voltage drop in a 12v setting (keep cable lengths short), use 4awg wire and preferably quality crimp ends, use fuses where indicated and don't skimp there either... remember if you will be in El Salvador (or anywhere on the PanAm) when your crappy quality stuff goes out, replacing it will be a PITA and super expensive.

Remember if you plan on driving every 2 to 3 days, this stuff is really not all that necessary save yourself some ca$h and go for evldave's set up it is perfect (link) or go for the dwh method of buying cheap batteries abusing the hell out of them and replacing regularly (once a year, maybe once every 6 to 10 months depending on abuse?) (Now if dwh approves of this post, I think we are good to go)
 

lysol

Explorer
The only thing I don't like about CTEK's products are they don't offer a builtin "Connect" to connect both batteries. This would be helpful in a jump start yourself situation without having to bust out the jumper cables.
 

dwh

Tail-End Charlie
  • a reliable solar panel of the highest output you can afford (and that can be handled by the charge controller)
And/or the biggest that you can fit. :)


if you plan on driving every 2 to 3 days, this stuff is really not all that necessary save yourself some ca$h and go for evldave's set up it is perfect (link) or go for the dwh method of buying cheap batteries abusing the hell out of them and replacing regularly (once a year, maybe once every 6 to 10 months depending on abuse?) (Now if dwh approves of this post, I think we are good to go)
Most excellent post my man! Party on!

bill-and-ted.jpg

(Usually 1 year to 1.5 years till I have to replace a cheap battery repeatedly hammered into the ground. But I do charge 'em up regularly with the genny, so that probably helps. Also, I too have the evldave cheap solenoid setup - hey, it's cheap, and it does help so why not?)

(And don't ask about the weird double quote on "Remember"...I deleted it a few times, and it kept coming back, so I figured it must be an omen of some sort.)
 

dwh

Tail-End Charlie
The only thing I don't like about CTEK's products are they don't offer a builtin "Connect" to connect both batteries. This would be helpful in a jump start yourself situation without having to bust out the jumper cables.
It can't do that because it's not a solenoid - it's a DC-DC charger. Otherwise known as a DC-DC voltage converter. It's not supposed to have power run though it backwards.
 

1leglance

2007 Expedition Trophy Champion, Overland Certifie
great write up from actual use and worth a ton to those of us looking to travel like you guys someday.

thanks also for the model numbers as refernce so I can upgrade my setup
 
The only thing I don't like about CTEK's products are they don't offer a builtin "Connect" to connect both batteries. This would be helpful in a jump start yourself situation without having to bust out the jumper cables.
I understand your thought, but never thought about it. Why? Because we carry jumper cables anyway (have used them all over America to jump others :smiley_drive:), but in the more than 4 years we have owned our starter battery it has NEVER gone under 12.5v and we have been known to camp at remote locations for weeks at a time. So, no need to jump start ourselves yet (probably just jinxed myself).
 

Dazed

New member
Did you end up using the marine manual battery selector switch in your final setup as well?

Scratch that, should have read the D250S manual. Answer is No, not needed.
 
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DiploStrat

Expedition Leader
Some Pedantic Notes

Since I, too, have spent about a year studying these issues, please permit a few, pedantic comments. They may help put the problems and solutions in context.

-- The first question is how big a load do you want to support? The Engel fridge draws between .7A and 2.5A. If we assume that once it is loaded, it will stay on, then, depending on the temperature, you need something on the order of 35Ah to 45Ah, depending on the temperature. Add in 0.5A for everything else and you are right at needing 50Ah per day. Given the 50% rule, this demands a battery bank of no less than 100Ah. (N.B. AGM batteries will happily tolerate a greater discharge, if they are recharged immediately. What they don't want is to be left deeply discharged for days.)

-- Given a battery bank of only 100Ah and the probability of a full 50% discharge everyday, it is critical that the batteries be FULLY recharged every day. The quick rule of thumb is that a 50Ah discharge will require between
60Ah and 75Ah. (Corrected, sorry.) So, for safety sake, guesstimate that you need a charging system that can supply 150Ah per day.

-- AGM batteries want a charging voltage of around 14.4v and open lead acids want a bit more, say 14.8v. This is at 70F. The voltage needs to be higher at lower temperatures.

-- AGM batteries want a minimum charge rate of 25A per 100Ah of battery bank and can take a rate of up to 500A per 100 Ah. (Lifeline battery spec.)

-- For the best longevity, you will want a modern, multi-step profile - grossly: Bulk/Boost, followed by Accept/Absorb, and finally, Float/Maintain. (We'll ignore Equalize/Desulphate for the moment.)

So, if we do the numbers, it looks like this:

Most overlanders who want to use a fridge, fan, lights, etc. should have a battery bank of no less than 200Ah and be able to achieve a charge rate of no less than 50A. So how do you get this?

First, stick a voltmeter on your alternator when it is running. If you get a voltage around 13v and lower than 14v, then you are going to have to raise the voltage up to the target of 14.4v. There are a couple of ways to do this:

-- The simplest is to simply boost your alternator's working voltage a bit, say less than 1v. Companies like Mechman sell a range of products which may (or may not) do the trick for you.) http://store.mechmanhighoutputalternators.com/voltage-control-modules/ This is probably all you need for a tuck that produces over 13.5v.

-- The next, more expensive option, is a Battery to Battery charger, or B2B. Sterling ( http://sterling-power-usa.com/sterlingpowerusabatterytobatterycharger.aspx) and CTEK (http://smartercharger.com/products/batterychargers/ctek-d250s-dual/) sell these in the U.S. as do Redarc in Austalia. Most of these beasts work by using a diode to pull down the voltage on your starter battery, causing your alternator to produce more current. The B2B then bleeds off this current and boosts the voltage to the proper multi-step charge profile for your battery. (CTEK denies using a diode, but the concept is the same.) Sterling offers waterproof and more powerful units and CTEK offers one with integrated solar control. The biggest problem is low output, typically only 20A to 50A, but, compared to most solenoid kits, this is huge. I know of one overlander using a pair of the biggest Sterling B2B units on his Unimog and he professes to be VERY happy with the results.)

-- Perhaps the best option for a low voltage truck is the Sterling Alternator to Battery charger, now able to handle the output of up to 400A of alternators. This is a big beast, but it performs as advertised: http://sterling-power-usa.com/alternatortobatterycharger.aspx

BUT, if you have a more modern truck, like my 2013 Chevrolet, then the rules change. You will probably find that your truck already has a multi stage charger built it and runs at voltages up to and over 15v. At this point, an external regulator is not required. All you need is:

-- BIG wires. The dirty secret is you don't have a 6v battery or a 12v battery; you have a bunch of 2v batteries. (Count the vents on the top.) A 6v battery is merely three 2v cells in one case; a 12v battery, six. Once you understand this you understand that the connection cell to cell must be as big and as short as possible. If you want to link multiple batteries together, so as to increase the voltage or the capacity, or both, then you need to assure that you do the same. And obviously, if your battery is distant from your alternator, as in the back of your truck, then you are talking about a big cable indeed. For a typical vehicle, under 25 feet in length, then you can guesstimate that you can pass about 75A down a 1/0 cable with a loss of no more than 0.5v.

-- A big solenoid/relay. You need one rated at at least 200A continuous. N.B., there is a lot of trash on the market. Relays of this type, especially if marine rated, will cost well over $100.

-- A controller. The simplest is simply using your ignition key. Off or start - batteries separated. Key on - batteries combined. Better is some form of smart controller, from Blue Sea, Magnum, Balmar/SmartBridge, or others. In simplest terms, when these beasts detect a charge at either battery, they close the relay and share the charge. When they detect that either battery has dropped below "full" charge, they open the relay.

Why is bidirectional charging so important? Many overland vehicles spend a lot of time parked; either between trips or while wild camping. A bidirectional charge system protects your starter battery from running down. This is easy to do with a relay/solenoid based system. With a diode based system you will want something like this: http://www.lslproducts.net/TLSPage.html

As noted in the original post, most of us are unlikely to ever drive long enough to achieve a full charge in a given day. Indeed, you should probably only expect your truck alternator to provide a bulk charge. So where do you get the rest, the last two stages? As the original poster noted, solar and shore power. Many are inclined to dismiss solar charge kits as being too small. Possibly, but they have the advantage of being very patient. The general rule of thumb is that you want 100w of solar power for every 100Ah of battery bank and you can expect, under perfect conditions, to harvest between 20 and 30AH per 100w per day.

So, how do these numbers work? Looking at the original post:

The camper battery is a 100Ah AGM. He needs about 50Ah per day consumption and about 75Ah per day of charge.

-- His CTEK can provide up to 20Ah when the engine is running. If he drives four hours, he should get 75AH or more. The charge rate of 20A is lower than wanted, but probably close enough.

-- Assuming a 100w panel and a summer day, he could get up to 120Ah, call it 100Ah for safety.
OOPS! Make that: Assuming a 100w panel and a summer day, he could get up to 5Ah, call it 25Ah for safety.

-- Shore power through his CTEK. The unit he has is specced at 25A, so it should be fine. He might be able to get away with a smaller unit, but I would not want to.

Bottom line, for a 1987 vehicle, sounds like he has a pretty good set up! The numbers are a bit close, but he should get years of service out of his camper battery if he takes the least bit of care.

Last notes:

-- Charge rate is highly dependent on voltage differential - Thus to charge a "12 volt" battery you need a charging voltage of over 14 volts, otherwise the battery will not charge or will charge only very slowly. This is critical and is the main reason that you hear people insist that "You can't get a charge from the truck alternator." A 13v alternator connected to a diode isolator and long skinny wires and you can drive forever without a charge.

-- Charge rate slows as the battery comes up to charge. Naturally. You start with a fully discharged battery at 12v and a charge voltage of 14v. Do the percentage of difference. But now, as the battery approaches full charge (12.7v) the differential is much less, so it is going to take longer than you think.

-- Charging is a chemical reaction; you are not simply pouring electricity into a bucket. This is why you need hours of "acceptance" charge. And this is why solar and shore power are so critical to completing the equation.

What about a generator? Depends on your loads and needs. I don't have one on my truck, but if you do want one, then you want it to feed a modern, multi-stage charger for it to drive. (I use a 2800w Magnum.) But generators are another religious war! :)
 
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BigSwede

The Credible Hulk
The Engel fridge draws between .7A and 2.5A. If we assume that once it is loaded, it will stay on, then, depending on the temperature, you need something on the order of 35Ah to 45Ah, depending on the temperature.
I think it is a bit pessimistic to say it will "stay on" indefinitely, it will cycle off quite a bit in my experience. In fact, once the contents are down to temperature, their thermal mass will help keep the fridge from cycling as much.

Engel has told me my MR040 will average about 1 amp draw. Maybe they are optimistic, but then again I don't live in Arizona.
 

DiploStrat

Expedition Leader
I think it is a bit pessimistic to say it will "stay on" indefinitely, it will cycle off quite a bit in my experience. In fact, once the contents are down to temperature, their thermal mass will help keep the fridge from cycling as much.

Engel has told me my MR040 will average about 1 amp draw. Maybe they are optimistic, but then again I don't live in Arizona.
What I meant was that they would not switch it off as long as it has food in it, but you are correct, the compressor will cycle. The hotter the weather, the longer the compressor will run. We are saying the same thing. Don Luis is the one who reported that he was taking the battery down to 11.5v routinely. Whether that is from the fridge or from other loads, that is really hammering the battery.

As a rule of thumb, you don't want your camper battery to drop below 12v, ever. (Or at least not for very long.) It is amazing how fast 1A adds up. 24 hours x 1A = 24Ah. With a 100Ah battery, that represents 50% of your available power.
 

BigSwede

The Credible Hulk
Good point. I would hope my new $220 battery (Sears Platinum 31) would last more than a year or two.
 
I do not find your post pedantic DiploStrat, I found it very informative.

What I meant was that they would not switch it off as long as it has food in it, but you are correct, the compressor will cycle. The hotter the weather, the longer the compressor will run. We are saying the same thing. Don Luis is the one who reported that he was taking the battery down to 11.5v routinely. Whether that is from the fridge or from other loads, that is really hammering the battery.

As a rule of thumb, you don't want your camper battery to drop below 12v, ever. (Or at least not for very long.) It is amazing how fast 1A adds up. 24 hours x 1A = 24Ah. With a 100Ah battery, that represents 50% of your available power.
I stated on the OP that we were dropping the battery below 11.5v while remote camping for several days at a time and before we enjoyed the luxury of the current set-up (though I admit I could use more jucie from a larger panel or a second panel). That is why I wrote the post, I get e-mails from people designing their "expedition" vehicle on a regular basis and most people "think" they understand the electrical needs they have (much like I did when I drove south). I still don't "get" electricity all that well in all honesty, but I think I got my setup right :)
 

Dan Grec

Expedition Leader
  • Engel 45 fridge
Hey Luis, Great post !

Since you seem active right now, I'll ask you here.

How do you like the Engel 45 size-wise? I'm absolutely going to run an Engel myself, and I'm thinking either the 27, 35 or the 45.
I'm trying to keep absolutely everything in my build on the smaller/ligher side as total weight is going to be an issue
With the 45, how many days worth of food/drinks for two people can you fit in there?

Have you ever thought the 35 would have been big enough, or do you actually wish you had a little more space?

Interestingly, the rater power needs for all of them is 0.7-2.5 amps (though I imagine the 27 would cycle off more often..) It's not until you drop down to the 17 it lists 0.6-2.3A
And the overall outside dimensions are actually not that huge of a difference between the 27,35 and 45.
The 45 weighs almost double the 27, however.

Thanks!

-Dan
 
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