[RE-wrenches] DKD Lithium Batteries

Mon Mar 4 13:03:53 EST 2019

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.
Here 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.

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.

On Mon, Mar 4, 2019 at 7:23 AM Kienan Maxfield <maxfieldsolar at hotmail.com>
wrote:

> Hello all,
>
> 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.
>
> 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,
>
>
>
>
>    1. 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.
>    2. Does anybody have any thoughts (positive or negative) or
>    experiences with these batteries?
>    3. Does anybody have any technical information (like a voltage/SOC
>    curve or anything else) for these batteries?
>
> 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.
>
> Thank-you,
> Kienan
>
>
> *Maxfield Solar*
> *maxfieldsolar at hotmail.com* <maxfieldsolar at hotmail.com>
> * (801) 477-0-SUN (477-0786) (801) 631-5584 (Cell) (preferable contact
> number) *
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