Using two batteries in parallel

DaveInDenver

Middle Income Semi-Redneck
What do you mean by DC? The OEM Truestart 27F that I had was rated for 710 CCA, 165 minutes, 90 A-hr. So each PC1400 I put in have very roughly similar specs but is slightly smaller and lighter (so my combined capacity is significantly greater while only being about 25 lbs heavier). Each PC1400 is a lot smaller than a group 27. The 25/35 pair is an inch here and there larger than the single that they replaced. They are shoehorned in where the original tray was.

Using the clutch defeat to start in gear isn't completely draining the truck, it's usually 5 or 10 seconds of full cranking. My starter is rated I think 1.1kW. It's about being capable of doing it for a while and still retaining a backup. I could size one large battery but then I don't have redundancy.

Nevermind, deep cycle. Duh.

I do have a group 27 Deka (badged West Marine 8A27M), 92-hr, 810 CCA, 175 reserve. It's only used very occasionally to run the fridge on long trips and serves as a UPS for my radios at home 99% of the time. It's a dual purpose, not a true cycle either. I don't know if the Truestart was going to be happy with deep cycle loads. These Odysseys seem spec'd to me to be like Yellow Top, a starting battery that isn't going to be hurt by some deep cycling.
 
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rruff

Explorer
These Odysseys seem spec'd to me to be like Yellow Top, a starting battery that isn't going to be hurt by some deep cycling.

Based on the little I've read the DCs are ok for starting loads, though the max amps will be a little less than the starting batteries. For longer term loads and deep cycling the the DCs are much better. You can get a 100ah sealed AGM battery for <$200 shipped.

If you are going to have DC storage and loads, it looks like it would make a lot of sense to just have one DC system do everything rather than isolate it and have another system for starting and other high amp loads.
 

DaveInDenver

Middle Income Semi-Redneck
A battery capable of starting isn't a true deep cycle, it's a dual purpose. Pretty much all batteries now are dual purpose because they are expected to do more than just starting with all the persistent loads from computers in cars. Things like security systems are always on waiting for a keyless fob to unlock it.

Starting and true deep cycles are physically constructed differently even though they use the same materials. What makes or breaks a dual purpose is how well it's balanced to it's task, but they are usually not ideal at either. What it amounts to is how much compromise is made to a deep cycle so that starting with it isn't damaging or to a starting so that repeated deep discharges and parasitic loads don't ruin the plates.

For long term value the best option is using a cheap starter and a cheap marine deep cycle, each is best at what it's expected to do. I chose a different path, essentially redundancy with two of the same type. In this case deciding between one large dual purpose or two smaller was just about reducing risk of having none. Cost-wise one large dual purpose makes a lot of sense since you can run the fridge longer in theory. That along with a jump pack just in case is completely valid.
 
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Red90

Adventurer
"Can I use my deep-cycle battery as a starting battery?"

Yes. Used my yellow top many years like that. No problems.

A Yellow top is a "dual-purpose" battery designed for both starting and deep cycle. Using a single purpose deep cycle for starting or other high amperage loads is not a good idea.
 

rruff

Explorer
A battery capable of starting isn't a true deep cycle, it's a dual purpose.

Not so. A battery designed for deep cycle can start a car just fine. Post #10 quote from Trojan and many others on the internet. The weight for the max amps will be higher, it will charge more slowly, and it will cost more, so it wouldn't make sense to use it if you don't need deep cycle. But if you already have multiple DC batteries for running "house" loads you certainly don't need a separate starting battery.
 

DaveInDenver

Middle Income Semi-Redneck
Fair enough and you're right, AGM has blurred the distinction such as that Trojan battery. Traditionally a deep cycle type would have thick, harder plates while a starting battery several thin plates with lots of surface area. The way each works during the reaction would mean deep cycling a starting battery would end up with disintegrating plates quickly. Vice versa using a true deep cycle would at least not be very efficient at getting current out due to a fairly high internal resistance. Techniques and marketing have been muddled so it's hard to know without actually cutting open and looking. All we can do is follow the manufacturer's guidelines and if it fails to meet those explore the warranty situation. Even the telecom batteries have started to move to AGM and aren't even dedicated deep cycle anymore.
 
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DiscoDavis

Explorer
Update to this thread and original question.

The 2 batteries are manually keyed together now. The aux battery is wired to a voltmeter in the dash so it can always show house voltage when parked, and shows the mean voltage when keyed to the starter battery. Need to throw some fuses on each battery lead at some point but space is tight. Has worked well now since March.

Thanks for advice all
 

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