Yet another battery questions thread

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Ex-Calif

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BTW, that doesn't look like a marine deep cycle battery, it looks to be a group 24 starting battery. I realize it says deep cycle, it is the smallest size, group 31 is where you want to be, or there abouts.

There are a couple clues indicating "marine" battery - One is the name "Aqua" and two is the wing nut stud posts.

Looking at the voltage trace I don't see this battery being pushed hard at all. Unless I am missing something the voltage never got below like 11.8V.

Push it to about 11.0V a couple times and I think you will be fine setting somewhere near there as you bottom line "target."
 

PaulBates

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Again great feedback, thank you all.
Good questions about the battery box, here it is.
You can see the width with the current battery centered. There's also foam to snug the depth of the current battery. If it turns out to be too small based on how I could upgrade, it doesn't look difficult to replace it
 

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Boat Bum

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There are a couple clues indicating "marine" battery - One is the name "Aqua" and two is the wing nut stud posts.
I agree it's a marine battery, just not a marine deep cycle in the true sense.

@Gary, I have purchased dedicated marine starting batteries but almost always use the deep cycle to start b/c that's how the switch is set and I don't want to crawl down in the bilge to switch.
 

Gary RV_Wizard

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The battery "50% rule" is a more a memory-reminder convenience than science. Various lab tests have shown the lead-acid batteries do lose cycles as the depth-of-discharge (DOD) increases, but there is no major drop-off at 50% or any other point beyond about 10%. It's pretty much a steady decline, so the cost of going to 60% or more isn't out of proportion to the extra amp-hours used. The attached charts show tests of typical 12v lead-acid batteries. What they show is that the only way to really maximize battery life is to not use it at all (less than 10% DOD).
 

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Tom

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The battery "50% rule" is a more a memory-reminder convenience than science. Various lab tests have shown the lead-acid batteries do lose cycles as the depth-of-discharge (DOD) increases, but there is no major drop-off at 50% or any other point beyond about 10%. It's pretty much a steady decline, so the cost of going to 60% or more isn't out of proportion to the extra amp-hours used. The attached charts show tests of typical 12v lead-acid batteries. What they show is that the only way to really maximize battery life is to not use it at all (less than 10% DOD).
Just curious, what's the source of those charts Gary?
 

Pedro Dog

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Here is the Trojan battery website information regarding lifecycle. I have East Penn Deka in my trailer but their chart is similar.

Note on chart that the T-105 family is represented by the red line.

At 50% DoD you get about 1200 cycles. At 60% about 1000 cycles. At 70% around 800 cycles


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Pedro Dog

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Using the above chart for the t-105 I crunched some numbers. Note that the T-105 is rated at 20H discharge rate of 225Ah.

50% of 225Ah = 112.5Ah multiplied by 1200 cycles you get 135,000Ah lifetime per the chart

60% of 225Ah = 135Ah multiplied by 1000 cycles you get 135,000Ah lifetime per the chart

70% of 225Ah = 157.5Ah multiplied by 800 cycles you get 126,000Ah lifetime per the chart

Please note that these numbers represent what happens if you discharge to the 60% or 70% DoD ALL OF THE CYCLES. If you only do it once in a while the impact on lifetime cycles will be much less.

I didn't get into what happens if you only go to 30% or 40% DoD - too many numbers. But the difference between 50% to 60% DoD is 0% less power over the lifetime of the battery. The difference between 50% and 70% is 6.7%.

If your camping style is that you boon dock and regularly (not always) go the 70% DoD, you can expect about 1000 cycles. This is 100 per year for 10 years.
 
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Pedro Dog

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Here is the East Penn Deka chart. They sell their batteries under a number of designations. Mine are from the GC10 family and sold by Duracell as GC2s (215Ah). They are marketing them as Deka Solar but if you look at the chart they list them as GC10 and GC15.

Notice that the number of cycles for 50% DoD is 1000 vs the Trojan T105 1200 cycles but the curve is generally very similar to the Trojans. So the Trojans seem like they have a longer life but they are also more expensive.


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Pedro Dog

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Last comment about these life cycle charts is that both East Penn and Trojan only chart cycles to DoD to 80%. I don't know why, but I take it as a hint to not go beyond 80% DoD.
 

Mark_K5LXP

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It's my opinion that the 80% point is used not because the batteries can't or shouldn't go to 100%, but to minimize the chances of bad customer perceptions if batteries go south. Not for DoD reasons but the amplified effects of instrumentation accuracy when "riding the line" at the very edge of performance. You can have a 10% error at 80% DoD and have little effect, but a 10% error at 100% takes you over the cliff. Even I add a little bit of margin at the bottom end to account for my ability to accurately determine 100%, because once you cross that line I do believe there can be an increased impact on service life. I have a discharge chart for Trojan batteries that shows the 100% point. I can't attribute the source of this specifically, I have a *lot* of files in my battery folder after 30 years. My contacts at Trojan and US Battery have since moved on or retired, so I've lost my connections there for any "inside information". Battery Ah are spec'd to the 100% DoD point, not 80% so the spec itself is based on 100% DoD. What's "recommended" is likely just as much a marketing limit as an engineering one.

Here's a delivered Ah chart for T105's to 100% DoD. Not included is the reduction in Ah curve near end of life (because Trojan wouldn't give me that data) but that doesn't affect the response curve, only the absolute Ah number.

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From here it's an academic exercise to plug in the cycle life and Ah data from any battery spec sheet and see the net result. For the sample above the 50% Rule optimum point is actually 30%, and this moves around a bit depending on the battery. It's such a minimal gain there is limited return for bothering to pursue it, which is another fallacy of the 50% Rule of pursuing "optimum". A point can be made that these numbers are under controlled test conditions and that in the field "your mileage may vary" due to the myriad of variables that influence cycles and capacity. It's impossible to quantify these so you just go with the lab data for at least a means of comparison. There is no indication of performance or life degradation anywhere between 20% to 100% DoD. There very well may be a particular battery that does take a turn at a particular DoD point. But that's not all batteries and I would offer even most of them, so the 50% Rule falls apart right there. At best for the sample above it would be a "30% suggestion". There can be a use strategy of picking a specific DoD point. RV folks often use a DoD point as a power reserve threshold, so that they know their fridge or CPAP machine won't give up when they need it, and that's perfectly fine. Another is a well defined and controlled application like off grid solar. There you literally are cycling your battery every single day and you are optimizing the number of cycles to a given number of Ah, to maximize $ per Ah for a given bank size. That is a very narrow application requiring a very deliberate and well controlled set of operating conditions that is only possible in fixed installations- nowhere near what an RV battery will see in real life. So stick a fork in it, in RV service the battery is a consumable like tires and oil. You buy them to run your stuff and any notion of "maximizing" battery life through DoD is rearranging deck chairs on the titanic and amounts to a distraction. The focus should be on having enough Ah to carry you through the duration you need, then proper charging and maintenance. Do these and they'll serve you reliably for their practical service life. If people paid as much attention to maintaining their batteries as they do anguishing over tire pressures and oil changes, I doubt much of this battery minutia would ever be discussed.

Mark B.
Albuquerque, NM
 

Gary RV_Wizard

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Just curious, what's the source of those charts Gary?
I've forgotten, Tom. I've collected excerpts from various 12v power and battery websites over the last 20+ years. I thought the "expected Cycles" chart came from Battery University, but I don't see it there now.

There are plenty of graphs available and they vary somewhat depending on the test parameters and deep cycle vs hybrid designs, but nearly all of them show a modest slope in the vicinity of the 50% DOD point. Not the cliff that internet meme would have you believe.
 

Ex-Calif

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Last comment about these life cycle charts is that both East Penn and Trojan only chart cycles to DoD to 80%. I don't know why, but I take it as a hint to not go beyond 80% DoD.

You and I are in agreement with battery usage. I even posted similar math in another thread. I'm done debating batteries - LOL... Glad you can take up the mantle.

Last time I got into a battery life debate I posted the Trojan website warning about deeper than 80% DoD. It seems to be a pretty hard number with them. Might have something to do with warranty exposure.

All the battery curve charts are similar and it boils down to where the curve in the leg is. I am a little skeptical of "home made" curves and when possible I only post the ones from the manufacturer's site.

The remaining debate is on life cycle cost by calendar. My opinion is that we earn our bucks via calendar so we should convert our battery cost to calendar. This means each user needs to estimate their cycles per month and average DoD.

I say this because in my past life with liveaboard boaters 8 year old trojans were common as the liveaboard sailboaters are way more anal about keeping their batteries charged. It's worth it because off grid boaters usually have an 8 Trojan bank and it's pricey to replace. The consensus was to try and keep batteries above 50% DoD and shoot for 30-40%
 

PaulBates

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I’m pleased and gratified with the range of response from technical to practical and it gives me a framework to go forward. I also greatly appreciate the civility of discussing differences on this forum.

Based on some of the practical and technical responses, I’m ok pushing this battery a little more than I have. Someone made the point that I don’t have a lot to lose while finding that $/performance/calendar balance, as I think about the options for replacement.

Three nights were possible for me using the heater some and about an hour plug in to my idling car.. but probably that recharge was unnecessary

In my favor its a small rig, a fridge that can run on gas and a heater fan that didn’t use as much as I thought. We don’t have a tv and use Bose portable speaker for music and my ipad for rainy tv watching... we’re not big power users.

Maybe I can survive on one battery. If not, there are some great alternative approaches to 2 batteries here.

Again, thank you all for depth of response and openness, acceptance of difference, I greatly appreciate it
 

Ex-Calif

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I have too much battery power, said no one ever.
Same might be "unsaid" for solar power...

I know you jest but for a popup or small teardrop or something you can have overkill. i.e. hauling around $$ worth of batteries that only get depleted by 20% 5 times a year...

Mark's a proponent of batteries "timing" out due to age as another consideration.
 

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