Will I have charging problems? (solar, 2 batteries, auto-bridging)

[4x4junkie]

I'm not so sure that's so...

You have to remember the absorption (acceptance) charging time does not vary tremendously regardless of the battery configuration (and again, it may actually be extended somewhat by a deeper discharge). If you have to first bulk-charge the batteries for a longer time before you can even get into the acceptance charge stage, I don't see how you can put all the Ah back quicker...

Lets say your vehicle's alternator is capable of 100A current:

A 50% discharged 100Ah battery (setup-A) will take that 100A of current for maybe 30-35 minutes until it reaches approx 80% state-of-charge, after which point the alternator will hit it's voltage set-point and the battery will then assume absorptive charging, which usually takes a minimum of 3-4 hours to complete (sometimes even longer).

Two 25%-discharged 100Ah batteries (setup-B, 200Ah total), will take the 100A from the alt (50A each) for approx 10-15 minutes to reach 80% SoC, then will assume absorptive charging. In this case the two batteries were not as deeply discharged as the one in setup-A so the absorptive charge may actually be completed in a little quicker time, say 2-3 hours. Also remember the charge current throughout the entire absorb stage will be double that of setup-A due to the fact you're charging two batteries at once (and is what I think makes possible for setup-B to stick more total AH back into the batteries more quickly).

So it works out to a min of 3½ hours needed for setup A to fully recover, and 2 hours 10 mins for setup B to recover. Even if we were to assume the absorb times to be the same in both cases, you're still 20 mins ahead with setup-B.

 

[4RunAmok]

Unless you've wired your dual battery system in perfect parallel, you will drain the aux battery before it begins to draw from the main battery, electricity taking the path of the least resistance.

The way most dual battery systems are wired, you are drawing power and ground for your aux accessories directly from the aux battery, and you have a ground connection to the frame, or maybe back to the front battery.

But unless you are drawing power from the aux battery, and ground from the main battery, you'll just drain the rear battery before the main battery feels any real load.

In this pic, the battery with the load wires attached will drain first.

Whereas wired in this method, batteries will share the load equally, and charge equally.

 

[DiploStrat]

Paralleling the Diagonals

4Run makes a hugely important point. So, to discuss it backwards:

-- Always wire your camper battery array "diagonally." This evens out the draw across all of the batteries. Chris Gibson goes on at length here: http://www.smartgauge.co.uk/batt_con.html

-- In a relay paralleling system, your camper battery is going to be, well, in parallel. Thus your starter load and alternator charge will hit the starter battery first and your camper load and solar/shore charge will typically hit you camper battery first. (We will ignore, for the moment, various extra charging circuits.)

This means that the cables between the starter battery and the camper battery must be as large as you can afford so that a charge or discharge to EITHER battery will be shared as quickly as possible. The speed with which a pair of batteries seeks the same voltage depends on:

-- The voltage differential. The greater the difference, the faster the flow, and,

-- The resistance of the cable. The lower the resistance, the faster the flow.

This is why your charge rate drops off as the battery voltage rises and why the absorb/accept stage takes sooooooo long.

In the morning, when my batteries are down some 150Ah from full, they will pull 150A from the alternator with no problem. Great! So I can achieve full recharge in one hour, two max? Nope.

-- There is a real world lag in the system as the alternator charges the starter battery and the starter battery, in turn, discharges into the camper battery. Think, if you will, of filling a cup which then overflows into a bowl.

-- There is as much as a 0.5 volt drop down 20 feet of cable.

-- Finally, as the camper battery voltage rises, the voltage differential drops and with it, the charge rate. This is one reason that the solar kit, which is wired directly to the camper battery is so effective in completing the charge.

SOOOO, what to do?

-- If you have a modern alternator/regulator system, running at 14 volts and above - go with a relay.

-- If you REALLY must have the fastest rate and can afford the costs, etc., go with a separate alternator and regulator dedicated to the camper battery. Make sure that this alternator senses the camper battery, not the starter battery.

-- If you have small battery bank, the various battery to battery chargers work well as they can boost the voltage to compensate for cable loss. You are limited to about 50A max, per unit. (Rob Blackwell got good, if expensive results with a pair of B2B's.)

-- If you have a low voltage system, 13 volts or so, a Sterling Alternator to Alternator charger works well. It is, however, a large and expensive bit of gear. (Don't ask how I know this!) If your charging system goes above 15.5v, however, it will alarm and shut down. Won't hurt anything, but it was designed before charging voltage went so high. Part of its magic is that it puts a sense wire directly on the camper battery and adjusts its voltage to assure that the correct voltage arrives directly where required. In this real world, this helps compensate for wire losses and seems to be worth about 25A of charging flow.

 

[unseenone]

Interesting stuff, I am looking to do more or less the same thing. I have a Traxide system, a house/winch battery. The model is an SC80. As the house battery voltage drops, it isolates it from the primary battery. My main concern is draining the house battery. The Primary battery is an Interstate Megatron Plus H8. The house battery is a AGM Yellow top.

I'm still a bit confused, with regard to hooking solar charge controller directly to the AGM battery (my preference) to keep it topped up during periods when it is sitting, and the fridge, etc. are left on. Any harm in doing that? Any problem I wonder with the mixed batteries. I would not want to overcharge either.

 

[007]

Ideally, your two batteries would be identical type, size and age. Hooking up solar to your house battery is fine, it would also be fine to hook it to the starter battery because your batteries spend the majority of life combined with the sc80. If the cable run is long, you should probably hook the solar up to the house battery so that it gets a high voltage charge, like the starter battery gets from being close to the alt.

Where did you purchase the Traxide?

 

[unseenone]

Thanks for the advice. I got it from Traxide directly. Unfortunately he does not do dealers, which seriously impedes growth in my opinion. Anyway, if you are interested I'll get you the fellows e-mail address. It's an extremely popular item in Australia, with caravans and Land Rovers in particular. The model I got, was specifically engineered for the LR3, with the factory battery and a yellow top in mind. Anyway, I haven't had any problems with it since I got it. They've since come out with a 160amp model, which I might like to upgrade to, if any folks want to work on an order, I would piggy back an upgraded controller.

The other suggestion worth considering is ordering it without wire, as shipping costs can add up, and you can source that locally. I don't want to sidetrack this thread, but feel free to drop me a PM for details, or we can start a thread if folks are interested in a group buy. I'm happy to try and coordinate, or pass the info the along. The web site is here http://traxide.com.au/ Keep in mind the pricing is probably in Aud$ All their isolators are supplied with a 5 year Parts and Labour warranty and a Lifetime Repair Guaranty. (though shipping is a hassle).. He's a nice guy and very helpful.

 

[007]

The group buy is a great idea! I am still trying to figure out if I want the Traxide usi-160

Or this: http://www.intervolt.com/products/electronic-battery-isolator/ebi-series.aspx

The Intervolt EBI Pro has a few advantages, the voltage for combining and isolating can be adjusted in .01V increments, and its waterproof.

I'll let you know what I choose, and maybe we can look into a group buy.

 

[unseenone]

Looks interested, but jeeze, another Australian company... I'll wait to hear from you regarding pricing, and you should ask them about AGM batteries as well. Most people ignore that factor, because the charging characteristics are significantly different from a standard Lead Acid Battery. I know the Traxide took that into consideration, however I can't speak to how it follows the AGM charge specifications.

I'd love to hear about any suggestions for a small solar system, whether components or a complete kit.

In inquired at Traxide regarding solar, and got an answer back already. Here's the response..
With one of my kits with the Anderson plug at the rear, while connected to a caravan with solar or a battery charger connected to the house batteries in the caravan, you will also charge the batteries in the tow vehicle.
You can use up to 20 watts of unregulated solar or unlimited regulated solar, connected to the rear Anderson and you will charge all the batteries in the tow vehicle.
You can also plug a battery charger into the rear anderson plug and charge and/or maintain all the batteries in the tow vehicle.
When using either solar or a battery charger, set the solar regulator or battery charger is set on the AGM setting and you will have no problems charging any battery in the vehicle.
BTW, the rear Anderson plug can be used to power high current devices like inverters or air compressors or even a small boat winch.