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Old 23-12-2012, 20:16   #841
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Bottom balancing and top balancing are just two ways to skin the same cat. One is not better than the other. Series charge should stop when any cell reaches 3.45V. Low voltage disconnect should occur when any cell reaches 3.10V. Check the cell voltages when either occur. If the balance is out of whack, perform whichever balance is easier at the time.
They have different results. It is more important to not over discharge a cell than overcharge one in a pack. Bottom balancing makes them the same at discharge. If you have a charger that cuts off based on the series voltage,
then you have to top balance, otherwise it overcharges the weak cell.

This looks like what Bob has, so he must top balance.
He needs to discharge the 4.0 cell down to match the others so they will all reach 3.8 together. Just attach something to the cell, like a light or fan to drain it with the charger turned off.
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Old 23-12-2012, 21:18   #842
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Of course, if one cell is significantly weaker than the others, then balancing would need to be done as often as every charge cycle. This is because balancing goes hand in glove with cell matching---the more matched the cells are (in terms of capacity), the less often they'll need rebalancing.

This is one reason why cell banding---if required---might want to wait until after delivery, so that cells can be ranked by capacity. Otherwise you're relying on the vendor to do the matching.
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Old 23-12-2012, 22:43   #843
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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Who did you buy your miniBMS through? I see a few folks selling them like CleanPowweAuto.com.....

I am thinking four of these: MiniBMS Cel Module
That's the one.
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Old 23-12-2012, 23:05   #844
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

deckofficer, I'm thinking you might want to use #2 (which I understand to be the one with the balance problem, and potentially a bit weaker overall) for outbound in your kayak and #1 (which I understand to perhaps be a bit stronger) for return.
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Old 23-12-2012, 23:35   #845
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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deckofficer, I'm thinking you might want to use #2 (which I understand to be the one with the balance problem, and potentially a bit weaker overall) for outbound in your kayak and #1 (which I understand to perhaps be a bit stronger) for return.
That is what I did with the AGMs that I used. One was 17 years old, the outbound battery, the other 1 year old, the return battery. Always had plenty of left over when pulling the kayak out. I have a meter for determining % of discharge, as most trips I'll just load one pack if under 30 nm.
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Old 24-12-2012, 11:07   #846
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

The MiniBMS is not a good choice for a house bank. The LVE and HVE can not be used at the same time high voltage event signal only happens when "key" is off, and low voltage event only happens when key on. This setup works fine for traction applications, but would leave a house bank with out high or low voltage protection depending on the "key" setting. The same company makes a home BMS. That would be a better choice.
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Old 24-12-2012, 11:23   #847
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

I used my fridge's electronics as the "key" with a .01 uf capacitor in series and a 1k resistor to ground.
Every time the fridge starts, it enables the LV disconnect to decide if it needs to act.
With all my solar and Aerogel insulation in the fridge, it never happens.
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Old 24-12-2012, 12:38   #848
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The MiniBMS is not a good choice for a house bank. The LVE and HVE can not be used at the same time high voltage event signal only happens when "key" is off, and low voltage event only happens when key on. This setup works fine for traction applications, but would leave a house bank with out high or low voltage protection depending on the "key" setting. The same company makes a home BMS. That would be a better choice.
I used adjustable voltage relay so when bank voltage drops below 13.3 volts energise ign term to tell BMS to expect LVE.
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Old 24-12-2012, 19:50   #849
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Cleanpowerauto IS the miniBMS guy, designer and builder. His name is Dimitri.

Chris
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Old 27-12-2012, 06:37   #850
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

OK so I have ordered 4 - 400Ah Winston Battery cells for the house bank on our boat. Also six jumpers so I can balance the pack in parallel. Balquon advised wiring in parallel and let the cells sit for a couple of days, then charge to 3.75V then draw down and re-charge to 4.0V and let sit for a few days.

They also advised a "charging voltage" of 14.0V for the 12V bank. I was thinking 14.2V but they suggest 14.0V will be safer and would not allow any cell to over shoot..

*I am ordering a House BMS and the cell BMS modules from Dimitri.

*I am wiring a "load" bus

*I am wiring a "charge bus"

*I want a relay for "load" & "charge" that has the lowest "load". Would prefer a normally closed latching style relay. What are you guys using?

*I am keeping my lead acid "reserve battery" but with the nominal voltage of this pack so high it precludes me from using an Echo Charger, Duo Charger or ACR type relay that normally "turn on" at 13V. I don't want the voltage drop of an isolator. I have not seen this specifically addressed here. Thoughts on multi-mixed bank management?
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Old 27-12-2012, 09:04   #851
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Re multi-bank management...

Tie the Li bank and Pb bank to a common ground. Use separate circuit breakers to connect the banks to a common positive load bus, the idea being that both breakers are never to be closed at the same time.

For charging, use an SPDT relay, with the common to a normal Pb charging source positive, N.C. to Pb pos, and N.O. to Li pos. The coil is energized whenever the Li bank voltage falls below 13.000V and de-energized when it reaches 14.000V.

Over time, adjust the 13.000V higher so that the Li bank is charging a few hours every day---but no more than that.

If you use a shunt trip breaker on the Li bank, the BMS can energize the shunt coil for the LVD.
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Old 27-12-2012, 09:15   #852
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

BTW I too also bit the bullet, ordering four 260Ah's from Balqon on Christmas Day. Hopefully we'll see UPS shipping notifications today or tomorrow!
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Old 27-12-2012, 10:36   #853
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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

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BTW I too also bit the bullet, ordering four 260Ah's from Balqon on Christmas Day. Hopefully we'll see UPS shipping notifications today or tomorrow!
They are on skeleton crew this week. I would not expect them to ship until next week..
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Old 27-12-2012, 10:50   #854

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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

It will be interesting to see how long it takes them to ship, and how old the batteries they ship actually are.
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Old 27-12-2012, 11:05   #855

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Re: LiFePO4 Batteries: Discussion Thread for Those Using Them as House Banks

Maine, what if you either add one more cell (externally) to the reserve battery, or replace the reserve battery with a "drag racing" battery, which is a 14v car battery that already is made with an extra cell to run at higher voltage? Bring up the battery instead of bringing down the voltage?
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