Replacing start batteries with deep cycle ones

[lostagain]

Wanting to increase my boon docking capability.

I have 4 golf cart 6V batts for a house bank. 2 Gr 31s for starting, where the 8Ds used to be. They connect and disconnect to each other with an automatic switch, which can be overruled to the connected position. If I removed the Gr 31 starts, I would have room for 4, or even 6 golf cart deep cycle batts in the start batt compartment. That would double, or more, my house bank capacity. I am sure that the  house bank would start the main engine just fine. If I sucked it down too low for starting, I could start the generator (it has it's own dedicated start battery) to recharge the bank enough to start the engine. I start the generator anyway when I see 50% SOC or less. I have solar too, but that is a subject for another post.

Thoughts, advice ?

JC

[richard5933]

I would never use the house battery bank as a starting battery and vis-versa. Totally different types of batteries. Start batteries are designed to dump a huge current flow in a hurry, and house batteries are designed to provide steady current flow over a long period of time. There are mix-use batteries (as in marine batteries) but they are a compromise for both purposes. You're already going to be stressing out those type 31 batteries if you ever get into cold weather or have a difficult start.

What I did was to get 6v batteries with much more capacity than a 'standard' T-105. I'm running four L16g batteries from Trojan, which provides 780 Ah @12v. There are a few different batteries that provide almost as much and still fit into the footprint of the T-105, providing that you have a few inches extra height to spare.

There are various ways to skin this cat, but scrapping the start batteries wouldn't be my first choice.

[buswarrior]

There were engineers who used to  dabble here that said they'd just make one big bank of deeep cycle, if they messed with bus conversions again.

Large bank won't be harmed by the start current, disabling the bus thru careless power management was punishment for being so stupid to do so. Wasting the money, space and weight for a set of start batteries that barely do anything offended the priorities of efficiency.

Do your math, how many amp hours does the starting event typically consume, what percentage of the bank capacity is that calculation?

Do it your way, know that smart peeps have considered doing it.

Happy coaching!
Buswarrior

[lostagain]

Yes I have read that some people do away with the start batteries, and only have a large house bank. I never start my engine cold. I plug in the block heaters (2) to warm it up before starting in the morning if it is cold out, like 40F or less. I run the generator anyway in the morning for breakfast. The engine then starts easily and the starter doesn't draw much. Same later in the day: I just show it the key and it starts, lol.
So I wonder what harm it would really do to a deep cycle bank?

JC

[Geoff]

Is your engine start 12 or 24v?  24v would require a series parallel switch.

[sledhead]

I have though about the same thing .

I have 4 x 12 volt start batteries right above the 6 x 6 volt house batteries on a slide .

that would make 10 x 6 volt batteries ! at 225 amps per set or
1125 amps . this is on a 12 volt start and house system

now saying this it does take a lot to start the cat but with 1125 amps there should be lots of power ?

J C  go for it and let me know how it works

thanks   dave 

[richard5933]

Are Ah of total capacity in a deep cycle the same as CCA in a start battery?

[buswarrior]

AH and CCA are different measures.

The oomph of thin plates in a start battery will more than be made available by a large bank of thick plate deep cycles.

Way too much false religion gets passed around about batteries.

We are not typical battery users, many horror stories are based on minimalism abuses, battery capacity too small for the job at hand.

If power use and replenishment hangs in the balance, the batteries are being worked hard, and will bite back.

Large battery bank has lots of advantages as far as re-charge rates, if you can muster the amperage. Some with big solar, need a bigger bank, or they have too high a re-charge rate.

Shed the false prophets, you can unlock a bunch of battery money, and make that lead useful to you.

Happy coaching!
Buswarrior

[lostagain]

I will go for it. And let you all know how it works.

JC

[Jeremy]

Also to make the point that "deep cycle" is a much abused term which really has no defined meaning. Manufacturers can, and do, stick deep cycle labels on anything they want to sell as a 'leisure' battery, even if in fact it's not constructed very differently from a start battery, complete with a start battery's short lifespan. On the other hand a "proper" deep-cycle battery - for instance an industrial traction battery designed for a thousands of charge cycles - will weigh very considerably more, have a much lower output rating, and will last almost indefinitely.

As with all discussions of this sort there is really no satisfactory answer other than to understand how to properly interpret the specs of the batteries you're comparing, and then sit down and do the math. Personally my instinct is that, if you're talking about proper deep cycle batteries, you're going to need a lot more of them than you think to be able to replicate the short-term output capability of a purpose-designed start battery

Jeremy

[eagle19952]

my starts have been my house for 15 years...DEKA Marine AGM.
I have had as many as 4. but generally only 2 8D.
Gen has isolated battery, toad has jumper cables.
And no i don't boondock for weeks, just off pole for a few days  (but a few times a week) and have no qualms about running my gen off quiet hours.
never lost my ice cream never got stranded.

[neoneddy]

I've considered similar. 

Like others have said, when your parallel 6 or 8 deep cycle batteries, they outperform the best start batteries.

Go for it. Just make sure you have some fall back option for starting a generator.

[bevans6]

The difference between start and deep cycle batteries for starting purposes comes when the battery is minimally sized for the application.  OEM's like to size a battery to the application, no more and no less, and that is good engineering.  Two Gp 31's starting a cold 8v-71 is an example of this "enough but no extra" sizing.  What that means is the start battery, with lots of thin plates, delivers the several hundred amps of starting current by ripping the charge off the surface of the plates, and then replenishes it fast by charging hot from an alternator.  If you substitute a deep cycle battery of the same size and capacity of the start battery, it's fewer thick plates won't deliver the starting current and won't accept the fast and sudden charge after the engine has started.  Hence, the "don't use deep cycle batteries for starting" mantra.  For us, the deal changes dramatically.  We take a vastly oversized, huge deep cycle house bank (huge in comparison to correctly engineered start banks) and draw a quite small percentage of capacity to start the engine.  Then, the inrush of current to recharge after starting is a lot less, and is easily accepted by the large bank of batteries.  More than one manufacturer (but Deka is an example) recommends deep cycle batteries for exactly this purpose.  The only reason I wouldn't do it is redundancy.  Having two dedicated, separable banks means you can lose one (as I did on a trip once), separate it out with switches and run everything off the other bank until you can replace batteries.  I had a start battery fail on a trip. 

[Jim Blackwood]

I like redundancy, especially when it comes to starting the main engine. Electricity is leaky stuff and has an affinity for smoke so any little bit of smoke allowed in or around the coach and the next thing you know your electricity has all leaked out to follow it. When that happens it sure is a good idea to have two battery systems with the ability to bridge them together. Separate battery on the genset? Great idea. But Murphy hides around those things too. Solar? Fantastic, but... And battery chargers have been known to fail. The excuse of not duplicating is a fallacy. If you need more house batteries it means you are discharging them to where they would have trouble starting the engine. Do that with a combined system and, well you will have trouble starting the engine.

In the end it's all about being able to start the main engine. As you begin to whittle away at the options you have for doing that the odds of not being able to increase proportionally, so what it comes down to is, are you a gambling man? After all, it isn't like you can jump start it off a Yugo.

Jim

[lostagain]

Thanks Brian, good explanation as usual...

How often would one bank fail, so you needed redundancy? Not often. The bigger risk would be to overdraw the bank and not be able to start the engine. But as I stated above, I am careful not to suck down below 50% SOC. And if that happened, the generator has it's own dedicated battery.

JC