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Old 01-03-2013, 20:03   #2176
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Would not a DC hall effect unit (like that within a clamp DC current meter) to monitor power out & return power back in) be more universal? They are available but I've not seen them used for Li-Ion control.
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Old 01-03-2013, 20:06   #2177
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There are many BMS's and cell loggers mentioned in this thread, but very few links to useful relays, solenoids, "make before break" switches. Could any of you post these links?
The solenoid most use is the Tyco EV200, Blue Seas sells the same one under a different part number. My system also uses a couple of Bosch relays available at any auto parts store. Search on those terms and they will show up. No suggestions on the make before break switches, I can see where you might want one though, and Im sure for a price someone makes it.
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Old 01-03-2013, 20:38   #2178
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Would not a DC hall effect unit (like that within a clamp DC current meter) to monitor power out & return power back in) be more universal? They are available but I've not seen them used for Li-Ion control.
Yes, Hall Effect current sensors work very well with microcontrollers and at around $10 each are much more cost-effective than ancient shunts, and unlike shunts they are totally non-invasive. I have a pair from devicecraft.com accurately monitoring both my solar and load current.

I hope to see HECSes become as ubiquitous as temperature sensors in tomorrow's charge controllers. Most controllers these days monitor only charge current, not true battery current, which is charge minus load.
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Old 01-03-2013, 20:53   #2179
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Originally Posted by Dsanduril View Post
The traditional answer to this is to wire a single "pilot" relay to the BMS/alarm contact that cannot handle much power. The alarm then only has to power the coil of one small relay. A second circuit is then run through the contacts of the pilot relay and feeds the coils of the main contactor(s).
Take a look at Crydom solid state relays.
They can be switched on with very little current and operate from 3.5 to 32 volts.
I can tell you from experience that they will activate at 1.5 volts.
The current draw is very low. I don't remember exactly, but it's somewhere around 20 ma if I remember correctly.
They can be paralleled as well.
I'm using two Crydom D1D40's in parallel to switch my home's solar array to my battery bank when the Enphase inverters go to sleep during a power outage.
These S.S. switches are easily powered by a 5 volt wall wart and some delay circuitry of my own design which includes a very small 5 volt pc mount relay.

CRYDOM D1D40 SOLID-STATE RELAY - Bing

Steve
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Old 01-03-2013, 20:59   #2180
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... Nothing I know of is equipped to stop/start charge for LFP. If multiple small cycles is better than a CV float, it needs to be done. But Im still waiting to see documented evidence of this behavior if the cells are CV floated at some reasonable number below 100% SOC. Say 90% for discussion.
The paper describes the behavior. To maximize lifetime it is an advantage to reduce both the terminal voltage and the "current processed". If it does not affect your cruising plans it is better to let the cells rest at a lower voltage. It is also an advantage to somehow allow the charge sources to power the small random demands rather than run the power in and out of the battery. The difference in life is significant.

However on a typical active yacht the charge and discharge events occur continuously and randomly so predicting the best practice is difficult. Time and experience will teach us.

In the simple system of a battery and a CV charge source there will be some small charge current. Lowering the CV simply delays its onset.

All that said ... Seems like a reasonable thing to do is set the charge controllers to the 90% +/- point before the knee. Leaving them enabled at that level will allow them to power the random small loads while also avoiding the troublesome higher average voltages. Others have said the same above.

Note to our silly cruisers in the cold latitudes: the paper also describes the charge behavior at low temperatures. It is not good.

And the authors are not impressed with the hype and lying-by-omission in the manufacturer datasheets.
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Old 02-03-2013, 00:02   #2181
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Quote:
Originally Posted by bill good
Would not a DC hall effect unit (like that within a clamp DC current meter) to monitor power out & return power back in) be more universal? They are available but I've not seen them used for Li-Ion control.
The Clean Power have a battery monitor which does that - it seems to work just fine,
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Old 02-03-2013, 11:08   #2182
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Originally Posted by senormechanico View Post
Take a look at Crydom solid state relays.
They can be switched on with very little current and operate from 3.5 to 32 volts.
I can tell you from experience that they will activate at 1.5 volts.
The current draw is very low. I don't remember exactly, but it's somewhere around 20 ma if I remember correctly.
They can be paralleled as well.
I'm using two Crydom D1D40's in parallel to switch my home's solar array to my battery bank when the Enphase inverters go to sleep during a power outage.
These S.S. switches are easily powered by a 5 volt wall wart and some delay circuitry of my own design which includes a very small 5 volt pc mount relay.
When choosing a solid state relay, be sure to consider three things. First and foremost, that it will work under DC. The vast majority of SSRs use a triac as the switch, and will work properly only with AC loads. DC SSRs use a FET (short for MOSFET) instead of a triac.

Second, the on state resistance ("RDSon") describes how much power is wasted inside the SSR alone---typically MUCH MUCH more than that of a mechanical relay. For the 40A Craydom D1D40, its RDSon is 0.05 ohm. So at 40 amps, the power wasted is a phenomenal 40*40*0.05=80 watts. That's huge! Some may even burn out (or open unexpectedly) if not properly heatsinked.

As Steve pointed out, it is often prudent to wire SSRs in parallel to reduce the power loss. Two D1D40s in parallel, each carrying 20A, only dissipate 20W each and 40W combined. Power wasted is cut in half---but at twice the cost.

SSRs are really designed only for high speed switching or intermittent or low cycle use. They would be terrible for constant-on uses such as disconnects.

Third, the cost of an SSR is inversely proportional to its RDSon. In other words, efficient ones cost way, way more than inefficient ones. The D1D40 costs over $60.

Things may be different in a few years when RDSon finally approaches the millohm range for affordable SSRs. Until then, I will continue to use relays to switch loads of more than five amps for more than 60 seconds.
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Old 02-03-2013, 12:11   #2183
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Here are my thoughts on the Genasun/CALB battery and the hogwash that they can be floated (truly floated) at 14.2V using alternators and dumb shore chargers. I would not take it as a recommendation to float any LFP at 3.55VPC. Here's why.

LFP manufacturers often oversize their cells. For example, a new 100Ah cell may provide 120Ah of initial capacity. This is done for two reasons. There's the customer "wow!" effect---he thinks he got 20% more value than he expected. But it's really just a marketing ploy, done simply to compensate for the inevitable capacity fade.

To illustrate, a cell is generally considered to have reached its end of life when its capacity fades to 80% of nominal, right? Suppose CALB's secret research of cells floated at 3.55V indicates a ten percent per year capacity fade. They warranty their cells for three years---so certainly don't want them failing in two.

Solution? By simply oversizing by 20%, the 120Ah floated cell will fade 30% to 84Ah at three years. But 84Ah is still more than 80% of 100Ah---so it even appears to "outperform" the warranty, as Genasun/CALB would be quick to point out.

Of course, the "constant polarizing electric field" eventually takes its toll---and the devil gets his due---when the cell finally fails five months later. I don't know about you, but I'd rather my cells last more than 41 months. So I'm back to some non-floating charging scheme.
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Old 02-03-2013, 13:07   #2184
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Just another fact on the solid state relay, be aware they have a diode across the FET & will pass current in the reverse unlike a normal relay.
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Old 02-03-2013, 13:59   #2185
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Just another fact on the solid state relay, be aware they have a diode across the FET & will pass current in the reverse unlike a normal relay.
I checked mine before installing them with my Fluke meter on both the diode check and ohms scales.
No diode, and they measured 0.02ohms when energized.
Even though they claim it takes 3.5 volts to energize them, they went full on at 1.5.
I suspect their spec sheet is for worst case samples.

I found them used on Ebay, $40 for the pair.

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Old 02-03-2013, 14:33   #2186
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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I checked mine before installing them with my Fluke meter on both the diode check and ohms scales.
No diode, and they measured 0.02ohms when energized.
I don't know of any Fluke ohm scale that can measure 0.02 ohm to any accuracy---especially across an energized FET.
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Old 02-03-2013, 18:26   #2187
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Steve, Did you check in both directions? I rechecked with my fluke on diodes & the ones I used do indicate the diode in the reverse with no control voltage applied. Only pointed it out as in some applications it may lead to "unexpected results".
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Old 02-03-2013, 20:06   #2188
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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I rechecked with my fluke on diodes & the ones I used do indicate the diode in the reverse with no control voltage applied. Only pointed it out as in some applications it may lead to "unexpected results".
Moot point, as the antiparallel diode (if included) is only there to dissipate flyback from inductive loads. The FET in a DC SSR is a polarized semiconductor and should only be connected with the proper polarity.
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Old 03-03-2013, 00:58   #2189
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Ok,

Now,

Who really thinKs these bats are ready for prime time?

Why are they?

Why are they NOT?

Lloyd

What is prime time, if you think they are, and what is prime time if you think they are not?
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Old 03-03-2013, 01:20   #2190
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Prime time is television terminology referring to mediocre entertainment and expensive advertising. Does it apply to LFP as house battery banks?
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