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Old 11-08-2019, 09:35 PM   #1
GSWatson
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Default Dual-Purpose Batteries for high-draw applications?

While looking through the thread on suggested microwave/inverter combos, there was a brief touch on battery construction types and their suitability for different types of demands. Most of us have deep cycle batteries for our coach batteries, as the majority of the service that those see is for long, low draw items like lights and charging phones, etc. With the advent of electric cooktopĎs, like induction types, Iím wondering if it makes sense to put in dual purpose AGMís instead of a standard deep cycle.

For example, in my van, I have to Lifeline GPL fours, but I use a small electric kettle to make 2 to 3 cups of coffee a day. That kettle draws just south of 100 A DC when operating. These are Lifelines, so theyíre pretty robust, but this discussion has me thinking that perhaps I will severely shorten their lifetime by this usage.

Yes, I know that lithiums can take this sort of output easily, but there is still a huge cost gap between the two. Two BattleBorns wouldíve cost me two grand, the two lifelines cost me 750.

Anyway, just looking to start a general discussion on whether dual purpose batteries might have an application in this department. I donít know if anybody builds a robust one, and Iíve often thought that they were, and possibly still are, more for things like fishing boats with trolling motors, or other non-critical applications.

Thoughts?


Cheers,
Greg
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Old 11-08-2019, 10:19 PM   #2
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Default Re: Dual-Purpose Batteries for high-draw applications?

No those Lifelines are **much** more robust and long-lived than 99.99% of anything labeled dual use.

Northstar and Odyssey are the only others that come close.

PC-2150 from the latter have a tremendous rep, actually true deep cycling but lots of punch IRL.

Of course, C-rate is relative, so just upping the total Ah capacity will drive bigger peak amps.

And going to LFP (not drop-in) will deliver whole orders of magnitude higher power density, 600-1000A no problem, from even a small bank
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Old 11-08-2019, 11:01 PM   #3
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Default Re: Dual-Purpose Batteries for high-draw applications?

It also entered my mind to have a starting battery fitting the inverter, and then eight cycles for everything else. But that adds complexity with charging, etc.Ö

Guess Iíll just wear these out and hope that the price of lithium has come down to something reasonable by thenÖ


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Greg
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Old 11-09-2019, 12:24 AM   #4
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Default Re: Dual-Purpose Batteries for high-draw applications?

My (very) loose arm-wave is that if the lead-acid (including AGMs) battery will supply the current for the job, it's OK.
Deep cycles with high internal resistance will drop their voltage under "too high" a load, so will (ahem) voluntarily drop out of the job.

So if your batteries were not able to handle the draw you're requesting, they'd probably trip off the inverter's "low voltage" circuit.

--dick
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Old 11-09-2019, 01:44 AM   #5
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Default Re: Dual-Purpose Batteries for high-draw applications?

Iíve noticed that happening lately. I got them new back in May; in the early days I could boil hot water all the way down to 50%. Now the low-voltage comes on at around 75, which makes me wonder if Iím killing them faster than normalÖ


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Old 11-09-2019, 02:28 AM   #6
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Default Re: Dual-Purpose Batteries for high-draw applications?

Quote:
Originally Posted by GSWatson View Post
Iíve noticed that happening lately. I got them new back in May; in the early days I could boil hot water all the way down to 50%. Now the low-voltage comes on at around 75, which makes me wonder if Iím killing them faster than normalÖ
You're definitely using them hard and thus they will not last as long.

"Dual" start / deep cycle is mostly just marketing. It's a cheaper battery with a shorter life because it has less lead than a true deep cycle battery. If you swapped 1:1, I'd say you'd be worse off right now in your ability to make coffee - but for less money. However, read the data sheets and compare. Overall, I think it would be a downgrade.

That said, I use them because that's how the math worked out for me for lowest overall cost. I found a no-brainer deal, and I ended up exchanging them still under warranty after about 5 years of pretty hard and frequent use - many long cold winter nights. They told me the warranty was re-set, though that sounds too good to be true and I haven't tested it.
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Old 11-09-2019, 02:45 AM   #7
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Default Re: Dual-Purpose Batteries for high-draw applications?

Average DoD% before charging has a **huge** impact on longevity, more so than high C-rates.

Just as critical to get back to 100% Full as per endAmps spec, as quickly and as often as possible.

And high amps rate, over 0.5C in Bulk if possible.

Last edited by john61ct; 11-09-2019 at 03:58 AM.
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Old 11-09-2019, 03:31 AM   #8
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Default Re: Dual-Purpose Batteries for high-draw applications?

I get back to 100% pretty much daily, through some driving (usually see about 50a through the CTEK dual, then topped off by solar.

Or, Iím at a place with sever shading, so I plug into the 18v 12a laptop charger that is hooked to the solar CC to make it think itís really sunny outside...

My normal DoD is to about 85% in the morning; each mug of coffee takes about 3%. I rarely get into the 70% unless Iím making coffee for for friends.

Iíll start a swear jar for a lithium upgrade in a couple years....


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Greg
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Old 11-09-2019, 03:20 PM   #9
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Default Re: Dual-Purpose Batteries for high-draw applications?

You may not be charging the batteries back to 100%. The fact that you are discharging the batteries with a high current isn't a big deal. Not getting them back to 100% each day is a big deal. They will sulfate and lose capacity.


Quote:
Originally Posted by GSWatson View Post
I got them new back in May; in the early days I could boil hot water all the way down to 50%. Now the low-voltage comes on at around 75, which makes me wonder if Iím killing them faster than normalÖ

I get back to 100% pretty much daily, through some driving (usually see about 50a through the CTEK dual, then topped off by solar.
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Old 11-09-2019, 03:39 PM   #10
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Default Re: Dual-Purpose Batteries for high-draw applications?

There are several good AGM battery suppliers out there, but for simplicity and peace of mind, I tend to just look at the Lifeline charts.

For higher draw items, such as serious inverters, the "marine oriented" GPL-27 and 31s are kind of a sweet spot IMHO and nearly ideal for many conversion vans. The 24Ts are not quite as good, but still quite strong performers.

Perhaps overly conservatively, I tend to design around a 500 - 600 watts / battery concept, so for a 2 kW inverter, 2000 / 500 = 4 batteries.

Battle born LiFe batteries I use similarly - About 500- 600 watts / battery as a design goal. With the various surges and non ideal situations that happen in a van, it is good to not run them at max ratings.

Using this approach, I run virtually my entire shop off of the 2x2 system in my van, including power saws, heat shrink tubing gun, drills, etc. Even coffee. Its a commercial grade 2 kW inverter and 4 x GPL-27Ts (100 amp-hr each) wired in series.

As a practical matter, a battery is a consumable, just like fuel or water. Space is a a premium in a van and sometimes it makes sense to accept the trade off of "pushing it hard / replace often" vs "space savings".
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