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Old 07-09-2012, 13:40   #1
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Simple 12V Lithium Iron Phosphate Battery

Hi everyone,

So I've read through the 34 page thread on LiFePO4 and while it's had some great info, my application is way smaller than what is being considered, and lots of the advice seems like it might not scale down well. My situation is that I am building a Farrier 22 trimaran, so weight is pretty important to me, and if I can replace a heavy lead acid battery (a single blue top optima is the alternative) with a LiFePO4 stack that is appealing. Getting added capacity or smaller weight for the same capacity would be gravy. Charging will be through an outboard alternator and possibly a small solar panel.

Questions:

1) Is making a LiFePO4 "12V battery" equivalent as simple as buying 4x 40Ah cells and ganging them together in series? I understand that leveling the cells before use is important, but that seems pretty simple to do.

2) I am assuming the LiFePO4 cells are orientation independent, but they require some sort of banding to keep the walls from bulging in use? Is there something special about the yellow plastic cases with the ridges molded into them?

3) Safety wise, they shouldn't be any more prone to starting fires/exploding/leaking due to mishandling than the Optimas?

4) If the solar panel is not too big (think a 12-25W panel) and the outboard alternator is modest (50W) would I need to buy some sort of charging system or is just hooking them up directly ok (the panel would have its own little controller to keep it happy)? Loads would be small, I am planning on all LED lighting and the biggest appliances would be a depth sounder, autotiller and iPad charging. For at-maximum-capacity case dumping ~75W into the batteries doesn't seem like it would lead to dramatic overtemping, but maybe it's more complicated than that.

Thanks for looking at these, there is a big of an information overload and AFAIK no one is looking to use LiFePO4 as a direct swap for a single 12V lead acid battery.

Thanks,
Andy
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Old 07-09-2012, 14:03   #2
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Re: Simple 12V lithium iron phosphate battery

I have and love my single 200 aH Lithium bank.
I use it for engine start as well as all house loads.

Your proposal ought to work fine, but I'd recommend a voltage limiting regulator.
You don't need a MPPT, but a simple off/on limiting type will keep the max voltage below a damaging point. Set it to 14v and you'll be fine.
Make sure you have them balanced up front.
See the LifePo4 for a house bank thread if you haven't already.
The banding as I understand it is for big current charge and discharges.
The batteries can take up to 3C in both charge and discharge (120 amps in your scenario), and you won't be even coming close to that.
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Old 20-09-2012, 16:17   #3
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Re: Simple 12V Lithium Iron Phosphate Battery

Hi Andy,
The banding is needed if the cell voltage accidentally goes over 4volts or under 2volts. this will case the electrolyte to heat, soften the plastic cases and the gas from the overheated electrolyte will balloon the cases. The capacity and function of each cell relys on the thin plates inside being held very close together so if the cell balloons this close contact is lost and so is the capacity. Warming the case and crushing it flat again restores most of the capacity as long as the over heat was for a short period so very little electrolyte was lost.
For this reason regulated voltage control and monitoring is essential, keep any cell below 3.6v and above 2.8v so set the charging voltages and load disconnect with these in mind. i highly recommend monitoring individual cell voltages, a $28 Junsi cell logger from a hobby store is idea for this job, set the alarm voltage to the figures suggested above and you will have a warning if something goes outside those perameters, there is an external alarm port so a quick wire up across the test botton of a smoke alarm makes an excellent clearly audable alarm that you just can't miss within a 100 mtrs or so.
So, all you suggest looks fine and correct, band the batteries with rigid end plates, a cell logger and alarm and you will be right for many years, no one has killed one yet if they have stayed inside those perameters and as long as you don't wander too far or for too long otside those perameters they servive so they are fairly robust units.
For what you need one of these may suit 12V LFP
I haven't used this chemical combination but a few others here have in bigger battery banks, it's already packaged so all you would need to add is the Junsi cell logger and wiring, the wiring plugs are available from E Bay but I see EV Power that sell the above battery also stock the loggers so I'm assuming they would also sell the plug and fly leads.
Absolutely no connection with this company, haven't even bought anything from them but someone on another forum mentioned them so I thought I'd pass it on.
I have no idea about their cycle life, lots of confussing data but the factory claim is 2,000 cycles to 80% DoD I think.

T1 Terry
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Old 20-09-2012, 17:44   #4
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Re: Simple 12V Lithium Iron Phosphate Battery

Quote:
Originally Posted by T1 Terry View Post
For what you need one of these may suit 12V LFP
I haven't used this chemical combination but a few others here have in bigger battery banks, it's already packaged so all you would need to add is the Junsi cell logger and wiring, the wiring plugs are available from E Bay but I see EV Power that sell the above battery also stock the loggers so I'm assuming they would also sell the plug and fly leads.
Absolutely no connection with this company, haven't even bought anything from them but someone on another forum mentioned them so I thought I'd pass it on.
I have no idea about their cycle life, lots of confussing data but the factory claim is 2,000 cycles to 80% DoD I think.

T1 Terry
Hi Terry,

Thanks for the link, that is an interesting site (asked them already if they can ship to the US), from what they offer it sounds like there is a "solar charge controller" that from what I can tell has both under and over voltage protection, which seems like it would let me hook up the alternator as a "solar" load and not have to worry about it doing anything funky during charging, and to prevent lights accidentally left on from running the battery voltage too low.

The Junsi is just an indicator for when cells are out of balance? From looking at it it looks like it involves a lot of extra wiring, corrosion potential (it looks unsealed) and complication, if were to periodically check the cells with a multimeter and accept the risk of the battery dying due to an undetected imbalance it could be omitted? Since this is for a trailer boat odds are I wouldn't be on board when something went wrong anyway to hear an alarm going off it seems to me... I am ok with the risk of having a battery lose capacity/needing replacement just as long as it doesn't have the potential to start a fire in doing so...

Thanks,
Andy
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Old 20-09-2012, 19:05   #5
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Re: Simple 12V Lithium Iron Phosphate Battery

Update: They don't export to the US, so the search continues...
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