Dual Battery System in Motor Vehicles

....and canal boats.

The deep draw batteries are better for applications where you're taking a lot of pawer - either with the alternator off or more than the alternator can supply.

Their downside is that generally they can't supply the CCA (Cold Cranking Amps) needed to start the engine - which is where the standard lead-acids score more strongly.

We're currently looking into this at work as we want to retrofit a 1-2 kW (80 - 160 A)drawing system on a commercial vehicle. Finding alternator/battery combinations to meet the requirements at lowest cost is proving challenging...

You could do with checking how much current your Lan Cruiser pulls with all the auxillaries on (lights, aircon, radio, wipers etc) and see how much "headroom" you have left to charge the leisure battery.

The diode will work well and will slightly reduce the max voltage to the second battery, which is good for it and will allow it to live far longer.....

Me being who I am, I may even use two diodes in series as I don't like my Leisure batteries going much above 13.2 volts while charging.....

You still need some sort of isolation between the batteries for when the engine is not running as otherwise the car battery will try and re-charge the leisure batteries via the diode(s).....May I suggest a heavy duty relay that is driven, via a fuse from the output of the alternator (the connection for the bulb that indicates when the alternator has no output, will be ground when charging, so connecting between bat + and this signal will pull a relay in when charging) so that the two batteries are only connected when the engine is actually running (not before starting as someone else suggested!) and supplying a charge.

Using the oil pressure sensor, if one is fitted, could also be done with a minor amount of interfacing I feel.

Leisure batteries can handle being deeply discharged much better than normal car batteries.

Sounds very similar to the circuit we came up with - we don't want the vehicle starting off the second battery!

Thanks for confirming our thoughts. Now, I'm off to root out that cat...

I'm going back over 30 years of lost memory, but:

My 28 foot sailboat had a dual battery system. There was a switch to select one of the batteries as "primary" for engine starting with the "non-primary" supplying the boat's 12V system. Both batteries would be charged as necessary if the engine was running.

In addition I had a 110V shore power hookup that had a converter to isolate the batteries and take over any needed 12V duties. The shore power also energized my 110V system when it was hooked up. Some of my appliances were dual 12/110V that sensed appropriately.

I have absolutely no memory of the details of the wiring or components but this is pretty standard stuff for live aboard boats. I would guess any marine supplier could provide details or possibly google boat electrical systems for more info.

Hooker

Thanks Hooker...that's useful info.

Typically how long would you want the freezer online? Is it a reefer or an actual freezer? What type of climate would be typical? How often would you open the freezer?

What you need is a battery isolator, which is basically a pair of diodes used as one-way check valves. Output from the alternator goes to the input of the isolator, and there is an output to each battery. Current can flow from the isolator to each battery, but cannot flow from a battery to the isolator. Each battery can be discharged without affecting the other. The existing cranking battery will be used to start the engine. Use a deep-cycle battery for the freezer. You can also runs lights, radio, tv, etc off the deep-cycle battery without having to worry about running down the cranking battery.

This arrangement is very common in motor homes and recreational boats. Checked online and found 95 amp versions for under $45 US, 120 amp units for under $100 US.

Looks like everyone (myself included) except English Rose missed your 2nd question.

Cranking batteries are designed to delivery high current for brief periods, but have a tendency to break down internally when allowed to discharge over an extended period. Deep-cycle batteries are designed to tolerate prolonged discharges, but cannot deliver the high current levels required to start engine. There are also some dual-use batteries available, but they are a compromise of both functions for the benefit of a single battery that does both fairly well.

The dual battery set-up with one for cranking and one for "house" loads is the ideal solution, especially if you plan on being in the back-country where jump-starts are hard to come by (unless you happen to park near a currant bush!!!)

Actually, I had dual deep cycle marine batteries that were used interchangeably for cranking and general power supply.

Never had a problem until one broke loose in rough seas and shorted out the system. It was my fault for not adequately having the cables tied down.

Now that was a pricey replacement/repair, even for the time.

Hooker

I think the cranking amps problem of leisure batteries can be mainly eliminated by sizing the leisure battery correctly, I would guess at double the capacity, but I am open to be corrected on that point.

The biggest (joke!) problem is simply trying to get it fitted in the small space of the original battery......

I never saw the exact application information but I have 2 cents for you.

I doubt you're planning on any real extensive stays out in the woods living in a Land Cruiser. I'm assuming you just want something that will offer you a long weekends worth of modern refrigeration. The answers above are all good if you want to convert your truck to a system capable of a live aboard boat or motor home. If on the other hand you just want something that isn't going to take over your truck and is completely portable I offer the following idea. There's a company called Sun Danzer that produces small refrigerator/freezer units that draw as little as 77 watts in 24 hours. They are 12-24V DC. The reason they can make this claim is they use vacuum panels for insulation. The 77 watt claim is if you never open the door! So how you often you open the door will impact that number. You might want to look at their products if you haven't already bought a freezer.

If you did get that type of unit and you only plan on using it 2-3 days I personally wouldn't do the dual battery conversion. I would buy a good sized deep cycle battery. It's true that they aren't designed to turn a car over. They are designed for a steady and prolonged discharge. They are easily the battery of choice for battery banks on "off the grid" solar and wind storage. They mainly differ from your car battery in that they have heavy thick plates. Other than that they are very similar in chemistry and physical size. If you did want the weekend advantage I would start off with the reefer cold before I left. I'd put it in the back of the truck along with the fully charged deep cycle battery (secured of course). Depending on how many times you opened the reefer and what temperature the ambient was (the website for Sun Danzer has ambient charts and watt draws) it would last you several days. You wouldn't have to modify your trucks electrical system and you could take the reefer in and out, as well as keep it after you part with your truck. If you did modify the truck you'll have to keep the reefer close to it or in it. With the portable you can take in a tent or cabin, whatever.

Good luck!