Way to size it up sizemore J
That's one of the most concise summaries I've read on the subject.
Thanks!
-Myles
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com] On Behalf Of James Sizemore
Sent: Monday, April 02, 2012 8:33 PM
To: electricboats@yahoogroups.com
Cc: electricboats@yahoogroups.com
Subject: Re: [Electric Boats] effective amps based on battery type?
All these number are hard to compare unless you are discharging and recharging at the same C rates. Some manufacture number here will help:
The Calb LiFePo4 cells cycle life is rated at 1000 to 90% DOD, 2000 to 80% DOD, and 3000 to 70% DOD at 4C continuous and 12C pulse.
So this make it a little easer to compare as the lowest cycle life of lithium at 4C is higher then the highest cycle life for any lead to 50% DOD, let us just use 90% DOD for LiFePo4. Now take note you will need to have a pretty good BMS with low voltage cutoff to regularly go to 90% DOD.
Now most golf cart batteries are rated at 550 to 50% DOD at these C rates.
These life time amp hour numbers don't include charging loss or discharging loses both of which are worse for lead anyways so not including them makes lead look better.
1000 cycles at 90% DOD for a 100ah cell is 90,000ah of life time storage.
VS
550 cycles at 50% DOD for a 100ah cell is 27,500ah of life time storage.
Now these number don't look as good as your numbers but 3000 cycles would be a lot of boat trips, you would have to drain you LiFePo cell to 70% once a day for 8 years strait to reach it.
Even my 1000 cycles would be 20 years of 90% discharge once a week. Or nearly 3 years once a day. A heap load of boating! Either way. Neither set would ever need to be replaced due to cycle life.
Where as the lead used once a week to 50% DOD would last 10 years, so you might have to replace them once in your boating life due to cycle life.
But let us be real here, cycle life rarely kills batteries, most batteries die of neglect. Either over discharged or for flooded lead lack of watering. So then the question becomes. How good a BMS/charger do you have. That is the real mistake most people make: skimping on the BMS and or charger. Either type of batteries cost a lot, so save a big chunk of your budget for a quantity charger/BMS. No other use of money will pay off faster no matter what type of batteries you use.
So to reiterate with lead you get 40% or less of your capacity and may have to replace the pack once in 20 years. VS Lithium which you buy once and get 90% of your capacity. As the lithium cost three times as much, if you plan to keep the boat 20 years it is pretty much a wash cost wise. So it really comes down to up front cost vs weight. If weight is import to you and you can afforded the upfront cost go lithium. If you can afford the weight and don't mind replacing your pack once go lead.
If you don't plan on keeping the boat 20 years, lead start to look good. Just buy a damn good charger and battery monitor. Because if you replace the lead three times. Lithium wins hands down!
On Apr 2, 2012, at 6:33 PM, Michael Mccomb <mccomb.michael@yahoo.com> wrote:
so would THIS be correct..... a lead acid battery can be discharged to about 60% perhaps 500 times? whereas a LiFePO4 battery can be discharged to about 70% 3000 times.... so a 100ah lead acid could provide about 30,000 amps (60% discharge 500 times) and a 100ah LiFePO4 could provide about 210,000 amps (70% discharge about 3000 times)... so the Li battery ends up supplying about 7 times the amp hours with respect to the two types respective life times???
From: Eric <ewdysar@yahoo.com>
To: electricboats@yahoogroups.com
Sent: Monday, April 2, 2012 11:41 AM
Subject: Re: [Electric Boats] effective amps based on battery type?
Hi exp30002(?)
I often use wikipedia for a quick reference, but there is a reason that universities will not allow wikipedia as a citation in any college paper. The information that you quoted is probaly true for some lithium powered devices, like laptops, cell phones or cordless drills, but is mostly incorrect for the the prismatic cells that are often used in EVs and electric boats like mine.
The LiFePO4 cells will not combust if overcharged. Deep discharge can short circuit the cell. Loose Li cells do not have any additional circuitry, they are just batteries like nicads or flooded lead-acid batteries. The safe voltage range differs by manufacturer, for example CALB prismatic cells have a narrower range than Thundersky prismatic cells. Without adding any BMS equipment, the cells have no added parasitical drain, though they will go dead eventually like every battery, lithium cells are slower than most other types (<2%/month). This means that a Li battery pack will probably be flat after years of unattended storage. Minimum charging temperatures also vary, but 0c deg for charging and -20c for discharge and storage are fairly common.
The real message here is to look at the manufacturer's specs for any battery, Lithium or other, that one is considering to confirm that it meets your operational conditions and normal usage.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, exp30002 <exp30002@...> wrote:
>
> The mathematics, and reasoning seems right. Else, I do not think the
> lithium
> batteries can be fully discharged. I think there is a small circuitry in
> each lithium
> cell to prevent recharge after a total discharge.
>
> This is what I found:
>
> http://en.wikipedia.org/wiki/Lithium-ion_battery
>
> Safety requirements
>
> If overheated or overcharged, Li-ion batteries may suffer thermal runaway
> and cell rupture.[56] In extreme cases this can lead to combustion. Deep
> discharge may short-circuit the cell, in which case recharging would be
> unsafe.[57] To reduce these risks, Lithium-ion battery packs contain
> fail-safe circuitry that shuts down the battery when its voltage is outside
> the safe range of 3–4.2 V per cell.[35][47] When stored for long periods
> the small current draw of the protection circuitry itself may drain the
> battery below its shut down voltage; normal chargers are then ineffective.
> Many types of lithium-ion cell cannot be charged safely below 0°C.[58]
>
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