<div dir="ltr"><div class="gmail_default" style="font-size:large">Hello, I am not familiar with this battery, but have some frustrating experiences with LiFe PO4 batteries. Yes, you can series batteries without COM connections if they have internal BMS. </div><div class="gmail_default" style="font-size:large"><span class="gmail-gr_ gmail-gr_173 gmail-gr-alert gmail-gr_spell gmail-gr_inline_cards gmail-gr_run_anim gmail-ContextualSpelling gmail-ins-del gmail-multiReplace" id="gmail-173" style="display:inline;border-bottom:2px solid transparent;background-repeat:no-repeat">Here</span> is how: Absolutely must be of same capacity and voltage if connected in series. You said 12 volt, X kWh. These batteries must then be in series with no taps or parallel batteries. (you had mentioned maybe a 5 battery, it can not be used in the string of 48 volts cannot be paralleled with any other battery) The internal BMS is supposed to protect from overcharge and over discharge. When one of the cells (buddy set) reaches full charge the BMS stops the 'bulk' charge and bypasses the full charged cell (or buddy set) and allows a 'top balancing' current to continue to charge the remainder of cells (or buddy sets) at a slow rate. One example the charge rate of a few amps drops to 1 amp top balance current. The current must remain at this battery set top balance current until all sets of cells are at top charge voltage. This now assumes the charge controller (bulk charge controller) is not much higher voltage than the required top balance voltage. (another brand of 12-volt nom. LiFe battery has 14.1 as its top balance voltage) The charger should not be set at a much higher voltage as the BMS must bleed this current through internal resistors and will be damaged by excessive charge voltage. This is then 56.4 Volts for your series string of 4 batteries. The possibility of one battery being near discharge and others being mid charge can result in a much highr voltage in the full charge battery, thus the recommendation not to series batteries. However, I have had good luck with two 12 Volt nom. in series making a 24-volt battery set. </div><div class="gmail_default" style="font-size:large"><br></div><div class="gmail_default" style="font-size:large">What I would do. Take each battery and test voltage and record,. (LifeO cells are about 3.5 volts fully charged, so 4 cells in series are about 14 volts and discharged are 3.2 volts or 12.8 volts) Divide the resting voltage by 4 and that is your average cell charge state. If below about 12 Volts the battery may be damaged beyond repair, still try, however. If it is zero, the BMS may have shut down and a small charge may be necessary to reconnect. Connect each battery one at a time on your lab bench power supply. set charge voltage at 14.1 volts, or current limit C/4. (C=battery capacity in kWh/14.1= Amp hrs. then divide by 4, Ah/4= bench charger current limit.) Let the charger charge until a semi-stable current is reached at 14.1 volts. (This charge current is the top balance current) Keep it at this charge voltage, (I have found on another battery this to be about 1 amp.) Assuming the initial imbalance to be on the individual batteries (initial bat voltage-12.0) / (14.1 -12) = State of charge. Then watch the charge time until the charge rate drops to the stable state. This time x C/4 current is the charge deficit of the highest state cell. assume no other cell is more than 2x this deficit and charge until you have supplied this deficit at the stable charge rate. (this assumes the lowest is no more than doubled the DoD of the best cell.) After charging all batteries let stand for the temperature to stabilize as they will warm during charge. After a rest period of at least an hour measure the batteries voltages. They should all be the same if not you may have permanently damaged batteries. you could try again to balance charge at 14.1 but do not get your hopes up. When the cells are this far apart one cell is vey poor. </div><div class="gmail_default" style="font-size:large"> </div><div class="gmail_default" style="font-size:large"><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, Mar 4, 2019 at 7:23 AM Kienan Maxfield <<a href="mailto:maxfieldsolar@hotmail.com">maxfieldsolar@hotmail.com</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
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<span style="color:rgb(0,0,0);font-family:Calibri,Helvetica,sans-serif;font-size:12pt">Hello all,</span><br>
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I just got back from looking at an off-grid system that is only about 4 months old or so, and it has several problems. I have specialized in off-grid for 8 years, so most of the problems will be easy for me, but there's just one thing that is a bit perplexing
and I'm hoping someone might have some advice.</div>
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The system was "designed" by some retailer in Texas (I don't know who) and the home-owner found some local installer to put it together for him (they already charged a ridiculous price and they won't come back). The battery is a big bank of DKD LiFePO4 batteries,
each one says 100 Ah @ 12.8V. They are in series sets of 4 for 48V. I think there were 5 strings (500 ah total). There are no com wires from battery to battery or anything like that (nor did I find anywhere for com ports). I have not yet been successful in
finding any information about these batteries online. I'm not that familiar with this type of battery and I'm hoping that some of you might have some info/insight on a couple of questions about this situation. Here's a picture of one of the batteries,</div>
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<li style="font-size:12pt;color:rgb(0,0,0)"><font face="Calibri, Helvetica, sans-serif">The biggest question is if this configuration is okay. I'm not familiar with this type of batteries, but I saw at least one brand that said not to put their batteries in
series. They said that (at least with their brand) if you needed a 48V battery, you couldn't use (4) 12V batteries.</font></li><li style="font-size:12pt;color:rgb(0,0,0)"><font face="Calibri, Helvetica, sans-serif">Does anybody have any thoughts (positive or negative) or experiences with these batteries? </font></li><li style="font-size:12pt;color:rgb(0,0,0)"><font face="Calibri, Helvetica, sans-serif">Does anybody have any technical information (like a voltage/SOC curve or anything else) for these batteries?</font></li></ol>
<div><font color="#000000" face="Calibri, Helvetica, sans-serif"><span>I will be fixing the other things in his system, but when he asks what I think about his battery bank, I don't know what to tell him at this point. He also has no idea how to tell
how full his batteries are, and since it doesn't seem like his batteries have any kind of readout or anything, I'm not sure what to tell him on that either. If I could find a Voltage/SOC curve or a voltage/SOC conversion table, that would help on that front.</span></font></div>
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<div><font color="#000000" face="Calibri, Helvetica, sans-serif"><span>Thank-you,</span></font></div>
<div><font color="#000000" face="Calibri, Helvetica, sans-serif"><span>Kienan</span></font></div>
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