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Old 02-08-2011, 11:25   #31
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Oh BTW, AC does only 50 to 60 zeros per second, not thousands.......
Hmmm... seems I'm not immune to the odd dumb statement either :lol: (or dumb act it appears, wrote this last night and didn't hit "submit reply" still here on the computer this morning)

With the cell logger alarm circuit it can be wired for normally open or normally closed but I can't find a hysteresis setting so I'm guessing it's an alarm while the problem exists only. If I just wired a relay directly the alarm circuit to open a relay between the solar panels and the regulator as soon as the charge stopped the over voltage problem would probably stop too, resulting in the relay resetting etc
A few possible solutions come to mind, a ratcheting relay so a manual reset would be required or a timer circuit to hold the relay open for say 30 mins and the rest. In a HVC situation the load would still be active so the over voltage would soon be burnt off and if the self balancing things works like I think it will then the differential between the cells should be reduced after each single cell HVC till the problem self corrected. With the LVC but the solar still charging the 30 mins without load could be enough to carry the system through for a while longer before another single cell LVC occurred, this would at least save the stuff in the fridge/freezer.

T1 Terry
For those not up wit the jargon- HVC = High Voltage Cut-out, LCV = Low Voltage Cut-out
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Old 02-08-2011, 11:40   #32
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Originally Posted by bene505 View Post
It would drain the high cell(s) very slightly. IMHO (and I'm new to these batteries), that would be the way to go, using voltage comparitors, and only when above the knee.

Regards,
Brad
This is precisely what BMS does. It just takes too long on large house banks, so I recommend simple manual intervention when assembling a new pack. You only need to do it once and it doesn't need to be perfect, just close enough so all 4 segments are in the upper knee when charge voltage goes over 14V, to prevent BMS cutoff. Then you can run your new bank as usual and BMS will even out remaining imbalance over time.
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Old 02-08-2011, 11:45   #33
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

I recommend to wire solar ( or wind ) circuits directly to the bank, bypassing BMS cutoff relay, such that renewable charge is not lost in the LVC event. But only do this after initial balancing is done. The reason is that overcharge of balanced LiFePO4 bank with sources limited to 14.4V ( charge controllers/regulators ) is impossible, so no point in letting BMS cutoff renewable sources, ever.
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Old 02-08-2011, 17:23   #34
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Originally Posted by T1 Terry View Post
Hmmm... seems I'm not immune to the odd dumb statement either :lol:
Let me correct that once more: it's 100-120 times a second that we pass 0, so twice each period.

ciao!
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Old 03-08-2011, 17:47   #35
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

OK Here is a question on making a "hybrid" installation because the house batteries I have are expected to have more than 5yrs left. I am running 2X 200 watt each 37.5V solar panels in series to feed a MX-60. Standby current for the MX-60 is quoted as 1watt. So without doing anything much it appears I could put 20 cells in series directly feed from the panels & accept the inverter losses. Since the currents thru the input circuit involved are so low balance circuits may not even be needed. It would be like a collection dam for electrical storage with years of storage from the power being not used. How does one know which is going to be the best LYP or LFP or is this marketing only. I also think the prismatic construction could not be as reliable as a round form but I not in the position to have any experience to collect that type of data.

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Old 03-08-2011, 18:50   #36
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Solar panels output voltage can vary quite a lot depending on load and illuminence, would need it's own regulator. In a mixed system it would be better to have 2 independent systems and a batt selector switch.
If your batts are working well and u don't need more capacity then i would wait for the 5 years and the lifepo's will be a lot better then.
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Old 03-08-2011, 19:41   #37
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Ths Whimsical, The batteries are good but I could use more capacity!! So rather that add to the 12v circuit I was thinking "out side of the box". My thought was if the supply voltage was below the low voltage cutoff settings of the cells then the MTTP MX-60 would still be working for the present setup. (no need for other regulator) The batteries ahead would charge only when the voltage was high enough. Their job is to collect the excess power available.Given 40a/hr cells X 20 is 2.8kw fully charged pack (charged when necessary from shore or alternator power) would seem to give a very low thru current of the cells in a house battery case & with low discharge a very long backup. The cost of 20X40a/h is cheaper than the present 250ahr 12v batteries as well. There appears that cell balancing may only need to be done if out of balance is detected in low current applications by what is being expressed by people using lifepo4 in low current circuits. So what is really aimed at is the present 12v bank would be more the backup & that series string be the working system. That other question on LPY/LFE any one going to help here?

The number of 3.6v cells was based on the panels 37.5V X 2 = 20.8 cells for lifepo4 & maybe less for LYP?
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Old 03-08-2011, 20:58   #38
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Hi Bill,
I'll try to answer this one the best I can. LYP/LFP have a different charging voltage/ resting voltage to lead acid batteries.
Lead acid like some where around 14.4v to 14.7v or even 14.9v for flooded cells in bulk charging mode, once the this terminal voltage is reached the charger should drop back to absorption mode of around 14.4v till the charge rate drops to roughly 2amps, this is a formula of amps multiplied by ah capacity but 2 amps will do for a rough figure. Once this charge rate is reached the regulator should drop back into float at around 13.6v to 13.8v to maintain the battery charge.
LYP/LFP cells need the charge voltage to be limited, some say high cell voltage while charging won't harm but the Carnegie professor says different and I tend to believe him. With LYP/LFP cells there is no need for a boost/bulk charge rate that drops back to and absorption rate, they just want to be charged to 3.4v per cell and left at that. It takes a little higher voltage to get the 3.4v in there but not much, I use 4.45v per cell and that works out to 13.8v for a 12v nom. battery. Set the regulator to 13.8v and it will charge the cells as fast as they will take it till they are full, no need to stop at a certain amp acceptance rate like is required at higher cell voltages.
When the cells are full the terminal voltage will be 13.6v or a little higher and it will stay there till the batteries are discharged. The terminal voltage will remain higher than 13.v till the batteries are virtually completely flat.
Now for lead acid, up to 14.9v charging but standing voltage is only 12.8v fully charged and as low as 12v discharged.
To combine the 2 sets the charging voltage for lead acid would damage LYP/LFP cells, the resting voltage would result in the LYP/LFP cells trying to recharge the lead acid battery till they were completely flattened and the lead acid batteries would never assist in any loads applied as the LPY/LFP cells are completely flat before the fully charged lead acid terminal voltage is reached.

It could be achieved by using a Voltage Sensing relay between the 2 battery banks, reducing charging voltage to 13.8v and drawing load off the 2 banks separately, the LYP/LFP first as they can handle being left partly discharged where lead acid battery sulphate is left partly charged.
I guess there are those who will disagree with my figures recharging voltages and the point LYP/LFP cells will be damaged but I think the bulk of what I’ve said is correct.

As far as one cell manufacturer against another, there is a lot of it is hype/spin, if the cycle life quoted by the manufacturers can be believed the Winston LYP cells have the highest cycle life, prismatic V cylindrical….. for house battery stuff the cylindricals don’t offer anything more but are more complex to build into banks, cycle life ????? good cells like A123 are very expensive, headway have a lot of failures but are cheap. For house batteries prismatic cells are easy to work with so I tend to think they would be the better choice but there are a lot of followers for each different camp, mostly with electric vehicles though and their use is very different to house battery use.

Did that help or just add to the confusion?

T1 Terry
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Old 03-08-2011, 21:45   #39
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Bill
I see better now what u are trying to achieve. 75V would be your open circuit voltage but it will fall as u draw more current from the panels. That gives u a max of 3.75V per cell, well below the danger point and above the nominal cell voltage so they should take some current. A lot will depend on how much the panel voltage drops as u take current, do u know the MPPT voltage for your panels. I have been trying to find info on charge acceptance rates of lifepo's at other than the usually stated voltage of about 4.? . I have read several times where the resellers claim a 4 cell pack is a drop in replacement for a battery to be charged off an alternator at 14 to 14.4V so about 3.5 to 3.6V per cell and you would be right in the ball park, maybe a 21 cell pack to get u back to that range. I would only worry if that long string wasn't well balanced as u have a bigger potential to take a single cell way over the safe limit but that could be checked occasionally.
Where are u getting cells that are cheaper than L/A?
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Old 03-08-2011, 22:05   #40
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

T1 Terry
13.8V Thanks for that, never seen anyone state what is the lowest voltage they need to accept a full charge rate.
I think Bill wants leave his existing L/A as they are and use the lifepo's to charge them through the MX60. Not sure if there are wet or AGM so maybe he needs the higher voltages for equalisation.
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Old 04-08-2011, 02:29   #41
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Thanks all, I did say "thinking outside of the box" The MX-60 & the Gell 250ahr bank works OK but has limited capacity. Just figured with such low charge/discharge (max 4A) a 40ahr cell should not get out of bal & would employ data logging & tracking. The other question was opinion on which chemistry appears to be better LYP or LFP or is it more sales hype?

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Old 04-08-2011, 02:34   #42
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Do you mean using the LFP’s as a sort of capacitor and after the sun has gone down feeding the stored energy in the LFP’s back through the MX 60? I tried a similar thing with 3 X 6v lead acid batteries to make 18v nom. and feed that back through my Plasmatronic PL20 to charge the LYP pack at a nom.12v. The result was the smoke coming out of the PL20, after thinking about why this happened I realised there is a huge number of amps stored in the batteries, all trying to get through the regulator at once and it simply couldn't cope. Fortunately I stopped the test before all the smoke came out so it still works as a solar regulator. The next trick was to try feeding the 18v nom. to the 12v nom LYP pack via the 200amps shunt attached to the PL20 regulator using a length of 6mm sq conductor cable, I still have the marks in the fingers. The shunt topped out reading at 250amps and the plastic started melting off the cable after a few seconds. Still not satisfied I tried attempt using a pair of pliers to hold the cable, the amps were lower this time and dropped back to around 100 but the cable got hot, I stopped when the terminal voltage started to head over 14v. When the charging stopped the terminal voltage dropped back to 13.5v. I tried again but this time the amps were only around 100 and the cable was very hot, the terminal voltage again reached 14v, about this stage I realised the lead had melted away around the terminal bolt on the lead acid battery. I decided that this was not a good method of fast charging LiPo batteries I then went back to allowing the PL20 to just charge using the solar input at 10 amps. Here is the graph of the cell voltages http://www.cruisersforum.com/forums/attachment.php?attachmentid=30199&stc=1&d=13124464 22
The blue line is the terminal voltage read from the left scale, the multi coloured line is the cell voltages read from the right scale

T1 Terry
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Old 04-08-2011, 02:44   #43
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Hi Bill,
The LFP/LYP thing is that Winston Chung separated away from Thundersky batteries taking his patented formula that included Yttrium in the cathode with him and opened a company named Winston Batteries. The claim is the Yttrium gives better low temp performance and Winston Batteries claim the longest cycle life of all the prismatic cells with their LYP cells, 3000 cycles to 80% DoD and 5,000 cycles to 70% DoD. Who knows if these claims are true or not, even 3,000 cycles is over 8 yrs discharging the cells to 80% DoD every day, they haven't been available that long.

T1 Terry
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Old 04-08-2011, 02:53   #44
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Thanks T1 Terry & all

Bill
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Old 04-08-2011, 06:44   #45
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

LiFePO4 batteries have very small internal resistance and can absorb up to 3C charge rates if you can keep them cool enough. They will get warm at such high rate and it will shorten their life if you do it often. Best practical charge rate for every day charging is between 0.1C and 1C. Anything in this range will be happily absorbed at very high efficiency. Since IR is low, the battery is like a short circuit for the charger, so current limiting is critical ( as Terry's fingers can attest to ). Even slight difference in charger's voltage and battery voltage will result in max current flow until voltage evens out.

Direct solar panel connection requires a diode to prevent back flow of current, and there will be voltage drop across diode, so take it into account. Direct solar charge is inefficient and risky, since you need higher voltage to overcome battery voltage and it will only happen when plenty of sun is available, but then you risk overcharge if there is no BMS in place. Since battery has low IR, panel's voltage will always track battery voltage, which may not be its MPPT point, hence wasting potentially available energy. For these reasons MPPT controller makes more sense, removes the need for diode, and pays for itself long term due to better efficiency.
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