LiFePO4 vs LiIon vs LiPo
Some people have commented on LiIon batteries, but the question and this answer are about Lithium Ferro Phosphate batteries / LiFePO4 which I'll abbreviate in places as LFP4. .
These are related to LiIon and LiPo batteries but have major differences.
Notable, compared to LiIon and LiPo (which are chemically similar) LFP4 has an added internal inactive matrix which the Lithium resides within. This reduces mass and volume energy densities but also makes them free from the destructive melt down modes, gives them a much longer cycle life and a much longer calendar life, lower capacity, lower maximum voltage, better temperature operating and storage range, better overall energy efficiency - and a whole of life cost below to far below that of any lead acid battery.
LFP4 (LiFePO4) 4 cell batteries do make excellent replacements for normal 12V lead acid car batteries BUT proper attention to care and feeding is necessary to achieve the very long cycle and calendar lifetimes that they are capable of. They are far more "whole of life cost effective" than any lead acid cells in (probably) any application when properly used. Wrongly used they will die an early and expensive death.
A few manufacturers of cells or batteries suited to this use are
Thundersky, Winston & SinoPoly
There are others which I can list but a search on those names will get you into the right area. All those brands started out as Thundersky but there have been partings of the way and expensive litigation is happening.
Winston make 12V 4 cell batteries with no access to the inter-battery connections. Their 40 Ah and 60 Ah sizes are claimed to be well suited to automotive use. Sinopoly an Winston both make single-cell batteries in sizes from around 10 Ah up to vastly-huge_Ah and you can strap these together as required. I am currently experimenting with 2 x SinoPoly 12v, 40Ah batteries each comprising 4 x 40 Ah single cells with strapping to produce a physical pack. A 12V, 40Ah 4 cell battery is about the size of a Ford-Prefect battery and less weight but is equal to about the largest LA battery you will see in a say 4 cylinder car.
HOWEVER
Claims from various battery makers overlap but are not identical, charging specs are suspect, claimed lifetimes vary depending on sales path and even reputable sellers disagree. The Winston battery charging specs use suspiciously high end point voltages - higher than I'd expect for LFP4 but below LiIon- almost like LiIon being run super conservatively to get good cycle life. Sellers warrant most brands of 10 Ah + cells for 5 years or 10 years use subject to various conditions and probably subject to use of charge/discharge management electronics. Criteria for adequate monitoring varies - many claim that Vmax and Vmin are enough as long as C rates of charge and discharge are within spec BUT a local supplier demands a gas-gauge type monitor and a low voltage cutoff. (I'd consider the gas-gauge excessive and the cutoff or similar protection essential)
Charging voltage. Since most alternators produce around 14V volts when charging it should be fine with LiFePO4 battery since maximum voltage for those is 3.6V which gives us little headroom up to 14.4V (4*3.6V=14.4V).
Some claim Winston LFP4 needs higher V than car systems provide. Others use them regardless.
SinoPoly OK on car voltages BUT ...
Individual cell balancing can be done with simple dissipative BMS which dumps excessive charge to resistor.
Winston cannot balance as sealed but claim superbly balanced cells are OK. Numerous dealers sell them and say this but at least one large site says not to use for deep discharge due to this inability. I'd be very wary. I made a Winston/SinoPoly initial choice for experience gaining based on this and strange Winston voltages and bought SinoPoly - despite apparently superior Winston specs.
But we don't have possibility to disconnect alternator when battery is charged,
Don't start. You must be able to manage your energy source - AND it is easy enough to do - but if your system insists on violating battery specs (which may not be the case) do not use LFP4.
so here is first Question: is it OK to hold LiFePO4 battery at floating charge with voltage which is close to maximum battery voltage? Or this will significantly decrease battery life?
Opinions vary
Small LFP4 - say < 1 Ah to few Ah MUST NOT be floated or they DO die early.
Some LFP4 large battery makers claim floating is OK.
Thou shalt not float LiIon or LiPo or else ... and while LFP4 has differences I'm uneasy.
Charging current. Some LiFePO4 cells can be configured so they can easily take all charge current produced by alternator. (assuming 70A output and 3 parallel 40152S cells) So second question: Can it overload alternator or modern alternators are smart enough to lower voltage to avoid being overloaded?
This deep-ends entirely on battery and alternator specs. Batteries have clear specs. Exceed them not. If your battery has NOT got good specs available do not buy it.
No over discharge protection. Currently have no idea how to deal with it besides not letting it to discharge completely.
This is essential. There are various ways of doing this but a LFP4 large Ah cell MUST NOT be taken under around 2.75V. If you cannot be SURE of that, do not use them. They cost far too much and are too light per volume to make good boat anchors.
Measuring the battery voltage "as received" prior to charging "is always wise"
However, this is a scam.
Battery
The battery contains 3 x 3.7V cells (nominal) rated at 1380 mAh each.
Placing 3 in series would at best give you a 11.1V x 1380 mAh battery.
IF they had been in paralle it would nominally be a 3.7V x 4140 mAh battery
So the 12V x 3000 mAh claim is spurious.
A LiPo cell has a
maximum voltage of 4.2 V (So 3 x 4.2 = 12.6 = Vmax_charged)
an average voltage over the whole discharge of ABOUT 3.6V or 3.7V / cell
and a minimum voltage of about 3V/cell.
Discharge tyo much below 3V/cell is liable to damage the cells and discharge to 2V/cell will fatally damage the cells.
So not only did they sell you fake junk, they sold you dead fake junk :-(.
There are many of these for sale on the internet at various prices.
IF you bought one from Seeed studios or Deal Extreme you could very likely get your $ back.
From elsewhere YMMV.
Dead Jim You Tube.
Ali Express $13.80
DX.com - capacity claim 1500 mAh V still suspect <- Somewhat reputable seller.
Amazon $31.80 junk
Amazon $28 whatever
Bang good $18.10 low price junk
Fasttech $16.45 even cheaper junk
:-(
ADDED 1/2016 in response to reader query.
How is it fake? I have a couple of this exact same battery, and they work fine and they each report 12.6V when fully charged, from the 3 x 4.2V cells. That's how you measure voltage in series.
Read the above explanation carefully and you should see where the claims are wrong. The 3 x 4.2 = 12.6V is indeed what he should see when fully charged - that's what you get, which is good. He gets ~= 2V which is fatally bad.
Even if the 3 cells in a 3S pack were 500 mAh, or even 50 mAh capacity, they would still measure 12.65V when fully charged, so battery voltage within range tells you very little. Battery voltage outside range tells you much.
Vbattery
over about 12.8V is very bad and
under about 9V is also bad.
under about 8.5V is possibly fatally bad
(2 x 3V marginal and 1 x 2.5V death's door).
under about 7.5V is junk.
When you place cells in series (3S here) the voltages add (as you say), but the mAh of the combination is that of the lowest capacity in the string - in this case 1,380 mAh for THE WHOLE string as they are all have the same capacity = 1380 mAh.
LiIon and LiPo cells have a mean operating voltage of around 3.6V to 3.7V and this is whyat is usually used for quoting battery voltage. Vmax is 4.2V/cell and SOME manufacturers use this voltage when specifying battery voltage, but this is a marginal practice - anyone who does it without comment is quite likely to be "cutting corners" elsewhere.
If you add batteries in parallel (3P if you did that here) (they must all be the same voltage) the mAh ratings DO add but Vout is V for one battery. So his battery could legitimately be described as
[10.8V, 1380 mAh] (at 3.6V/cell), or
[12V, 1380 mAh] (at the marketing hype 4.2V/cell) OR
[3.6v TO 4.2v, 4140 mAh]
but NOT [12V, 3000 mAh].
That is what makes it fake.
IF the cells used are in fact 1380 mAh (and they may be lower)(ask me how I know*) then capacity = 1380/3000 = 46% of claimed. That's fake, alas :-(.
IF your version has 1380 mAh cells the same applies.
The battery cam be 12V, 3000 mAh with 3 cells ONLY if it contains at least 3 x 3000 mAh cells.
If readers disagree with this or do not understand it please do comment or ask - the aim here is to learn.
- I've spent 6+ months in China on over a dozen electronics manufacturing related visits in the last decade - which has taught me a lot about what you may expect worst case. China can and does make excellent products. But, also, extreme rubbish if uncaring profit focused and/or ill informed resellers let them get away with it. Unfortunately, many do. Almost no specification or claim is trustable if the reseller does not stand behind it and has not done enough "due diligence" to be certain that their supplier is reputable and reliable.
A very large proportion of product from "no name" manufacturers or suppliers does not meet claims and/or specifications to some extent.
Best Answer
I believe your problem is in those absolutely unnecessary voltage dividers R7/R8, R11/R12. You basically adding 500 Ohm load to the charger's output and also shifting feedback voltages, messing up all the charging profiles the chip is designed to follow.
Remove R8, R12, C10 and put jumpers instead of R7, R11.
Also, what on Earth is R2 doing there? If you want a discharge path (again, absolutely unnecessary) for that tiny 10uF capacitor, put 1M there, not 22k
Finally, don't use VIAs in high current path. You have through-hole pad on VIN+. Why would you take a trace on one side only to bring it to the other with VIA if you already have same pad on the other side?
UPDATE:
One more thing. The whole premise of this post is an attempt to tweak output of the charger to accommodate the voltage loss in some (supposedly) too long or too thin charging cable. This is like attaching RV trailer to Fiat 500 and then tweaking the carburetor hoping it will go.
What if you change the cable tomorrow? What if somebody uses different gauge cable? IMHO, you are fighting self-created problem here.
UPDATE 2:
There might be, but IMHO it is not worth doing. First, the standard way to detect end of charge of lithium battery is to monitor current while in constant voltage stage. If you use correct shunt resistor and use short appropriately sized feedback traces then battery will probably be charged OK.
Second, you should focus on minimizing voltage losses in your wiring. Heavy and as short as possible PCB traces, no VIAs, good connector, heavy and short cable. See if you can find alternative to INA219 with lower than 20uA input bias. Then you might be able to share shunt, further reducing voltage drop.
Also, I'd definitely try another chip or two to see if this lower than expected voltage is a fluke or a norm for this IC.
Regarding discharge resistor - similar ICs from TI and AD with built-in power path technology often use ideal diodes internally, to minimize both forward drop and leakage current. Maybe it worth checking those out. In any case, I would definitely use larger value. Keep in mind that it essentially drains the battery when it left connected with no power applied.