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Old 17-06-2014, 19:55   #1
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Specific Question about Voltage of LiFeYPO4 Cells

I'm curious what it means when a manufacturer specifies Operating Voltage: Min 2.5V Max 4.2V?

I'm not an electrical engineer, so a bit of patience is appreciated.
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Old 17-06-2014, 20:38   #2
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Re: Specific question about voltage of LiFeYPO4 cells

generally speaking without transformers or resistors ...

a nicad rechargeable battery is about 1.2 volts so you could use 3 batteries like AAA, AA, C or D type ... 3.6 volts 2 batteries are about 2.4 volts and might not work

An alkaline battery is about 1.5 volts, so you could use 2 batteries ... 3 volts

with Nickle Zinc batteries(1.6 volts), you could use 2 batteries ... 3.2
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Old 17-06-2014, 22:03   #3
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Re: Specific question about voltage of LiFeYPO4 cells

In general, that means that under 2.5v the thingy doesn't work. Over 4.2v the smoke comes out and the thingy never works again.

What's the thingy?

(I am an EE...but I do software now...so I may not know crap)
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Old 17-06-2014, 22:35   #4
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Re: Specific question about voltage of LiFeYPO4 cells

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Originally Posted by jeepbluetj View Post
In general, that means that under 2.5v the thingy doesn't work. Over 4.2v the smoke comes out and the thingy never works again.

What's the thingy?

(I am an EE...but I do software now...so I may not know crap)
So for your typical 12v house battery setup you would require 4 of these if I understand properly?

I will be working with a systems person familiar with Lithium cells, just trying to wrap my head around this.
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Old 17-06-2014, 22:36   #5
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Re: Specific question about voltage of LiFeYPO4 cells

Thats a single cell voltage.. Anything outside of that voltage will damage the cell (yes even low voltage)..

I would consider those voltages absolute maximums.. I think its pretty common to stay around 2.9V - 4.0V..
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Old 17-06-2014, 23:09   #6
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Re: Specific question about voltage of LiFeYPO4 cells

So then, for a basic house system what a re the key components required here for a modest, 200-260Ah bank (charging, battery management, etc.) and what is the maximum acceptance rate you'd advise if charging using the engine/alternator as the primary source?
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Old 18-06-2014, 04:42   #7
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Re: Specific question about voltage of LiFeYPO4 cells

Some brands highly recommend going beyond 3.8 or below 2.8. I do not know why anyone would want to risk the longevity of their cells by going beyond these
limits. The batteries should exceed the capacity within those voltages.

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Old 18-06-2014, 05:42   #8
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Re: oh ... LiFeYPO4 cells

I know nothing about LiFePO4 cells... I can't even afford them

But then what are "operating voltages", regarding any battery cell, since all cells I know deliver a voltage with a low limit where the battery is rather dead and an upper limit where the battery is fully charged.

Does a LiFePO4 cell (or battery) have such a high rate of 4.2 volts down to a low of 2.4 volts for normal operation? If so that's quite amazing.

Anyway, I misread the original heading and assumed that we were talking about a thingy, like a light or something that would actually be discharging a battery ... my bad.
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Old 18-06-2014, 07:14   #9
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Re: Specific Question about Voltage of LiFeYPO4 Cells

Yes, for a nominal 12V battery, you want four LiFePO4 cells.

3.5V per cell or 14.0V for a four cell battery is enough to fully charge the battery in several hours and safe to apply indefinitely. Any voltage substantially higher than that is unnecessary (unless you need to charge very quickly, which makes sense for an electric car, but not a boat) and potentially can damage the cells, including causing an imbalance among the cells. Play it safe and don't apply any charging voltages above 3.5V per cell.
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Old 18-06-2014, 10:13   #10
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Re: Specific Question about Voltage of LiFeYPO4 Cells

My prior response was somewhat flippant. There is a massive thread about LiFeYP04 batteries and how to treat them well (including top/bottom balance, etc)

It's worth the read - may save you thousands of $$$ on toasted batteries. Mainesail knows what he's talking about. (More than me, that's for sure)
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Old 18-06-2014, 11:02   #11
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Re: Specific Question about Voltage of LiFeYPO4 Cells

Ther are few things to "un-learn" from our lead acid (LA) habits for LiFePo (or any other lithium-based battery)

LA wants to be trickle charged and full all the time for a long life.

LI does NOT like to be hanging around fully charged. Keep the maximum charge voltage to 3.45V to 3.5V per cell (13.8V-14.0V) for 4 cells. I discharge my batteries to about 50% and disconnect EVERTHING from them whem I do not use the boat for months at a time. I hook up my starter LA battery to the house and keep that on a shore or solar charger for the bilge pump and the like.

Best to have a charger that can be programmed to charge to 13.8V-14.0V and then stops all together or drops back to 13.3V-13.4V (float) to keep up with any loads.

Tesla knows something about LI batteries (after all, they warrant the batteries for 8 years) and the charger defaults to an 80% charge. You have to set it to "range charge" if you need a 100% full battery for a long trip. After that it defaults back to an 80% charge.

Make sure you have a solenoid that cuts off ALL loads (not the chargers) when the voltage drops under 3.0V per cell (12V for a battery).

When you charge the voltage will stay under 13.5V for a long, long time and then suddenly start rising. Measure the voltage of each cell with a multi-meter while charging above 13.6V. Make sure each cell has the same voltage within 100mV. If one cell goes higher, place a cable with a 12V headlight bulb across the terminals of the high cell and drain it down for a while. This is to balance the cells at the upper end. Once balanced the cells should stay balanced for at least a year.
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Old 18-06-2014, 18:08   #12
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Re: Specific Question about Voltage of LiFeYPO4 Cells

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Originally Posted by jeepbluetj View Post
There is a massive thread about LiFeYP04 batteries and how to treat them well (including top/bottom balance, etc)

It's worth the read...
That thread is now 3700+ replies long which is why I skipped it. As I'm not doing the installation, I need enough information to know that I'm not getting taken for a ride, but don't need the entire War & Peace experience.

I started to read that thread a few weeks back but there seems to be enough diversity of opinion I hoped a new thread might attract some input from some of the more knowledgeable contributors.
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