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Originally Posted by RogerDat
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LiPo4 delivers full voltage until almost discharged, PbA voltage drops as discharge is taking place. Not sure how much this matters as most RV equipment will run on voltages provided by the PbA battery though most of its discharge but some devices might depend on a more constant voltage. |
I have never experienced it but I read that many rv devices will shut off at various voltages interpreted as 'too low for lead acid batteries' in order to prevent damaging these batteries.. Fridges and inverters in particular which is problematic to say the least.
Which eloquently demonstrates just how much of a problem PbA batteries can be that the electronics around them try so hard to accommodate them.
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As was of interest to me with an question of inverter installation. A flooded battery can under high draw have the delivered voltage drop enough to cause the inverter to cut out even though there are many amp hours potentially available. Just a case of drawing faster than the chemical reaction can deliver so voltage drops. Let it sit without load for a bit and voltage comes back up. LiFePO4 don't have this tendency nearly as much. Puckette effect (spelling?) is less for them.
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Precisely right.
https://en.wikipedia.org/wiki/Peukert%27s_law
In fact LiFePO4 batteries do suffer from this, but at
much higher current draws. I watched a video of a fellow using 4 Headway 3810 batteries in series being used to start a car. The 38120 is a round largish LiFePO4
battery that tends to be around 8-10 amp hours capacity, but which can discharge at as much as 20C or about 180 amps. Anyway, they watched the voltage of the battery as they started the car and it dropped from 12v down to about 7.x volts.
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I do wonder if the high tech Battery Management System is a plus and a minus. It exists in the LiFePO4 because the battery would not be very good without it. Has to work for battery to work but can fail. Typical PbA has to break to fail. Or fill up with sediment to point it shorts out.
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This is not at all true. You can build and use a LiFePO4 battery without the BMS. Works just as well as a PbA battery without a BMS. Which is to say if you discharge it too far it kills the battery. But as long as you don't do that the battery functions just fine.
A BMS is nothing more than a little micro-controller, a small computer. In a LiFePO4 it is there to prevent primarily over-discharge which is really bad for them, just as it is really bad for PbA batteries. However given that the BMS now exist, they are then provided a temperature sensor and programmed to prevent:
1) High temp charging (also bad for PbA, not dealt with)
2) Low temp charging (should be modified for PbA but usually isn't)
3) Over charging (taken care of by the charger for PbA)
4) Over discharging (taken care of poorly by the owner of PbA)
5) Over current discharge (Ignored by everyone for PbA, but bad as well)
The way that it does all these things is to watch the charge / discharge curve. In a LiFePO4 battery, the voltage starts at 2.5 volts - times 4 cells = 10v. That is 'fully discharged'. The BMS will physically interrupt the circuit, usually with MosFETs but sometimes with a relay, preventing the battery from supplying any more current on the discharge side. Once the voltage rises above 10v the BMS will reconnect the battery to the circuit and charging will commence.
As the charger starts charging the battery, the voltage rises rapidly, bending towards 'level' as the voltage reaches about 2.9-3.0v - times 4 = ~12.0v. This part of the charge / discharge curve represents about 10-20% of the total current storage of the battery. This rapid rise and bend is known as the 'knee' because it does in fact look like a bent knee.
From here the charge curve remains somewhat 'flat', between 3.1 and 3.3 volts, as it continues charging. Once the battery is about 80-90% charged the voltage will start to rise rapidly again forming another 'knee' as the voltage heads up towards 3.65v - times 4 = 14.6v.
This is fully charged. If the charger maintains this voltage, no damage is done to the battery but no current flows into the battery either. If the charger tries to increase the voltage beyond 14.6v, the BMS will physically disconnect the battery to prevent overcharging. AFAIK, no battery charger routinely goes beyond this voltage, with the possible exception of a 'desulfination' cycle. In any event, once the voltage drops back below 14.6 volts, the BMS will reconnect the battery to the circuit.
The point here is that unlike a PbA battery, the knees are very well defined and the micro-controller can easily sense the voltages where it needs to intervene to prevent battery damage., and it does so, both on the upper end and the lower end. It can also easily sense temperatures and intervene there as well.
One point worth noting here is that there is very little SOC or 'charge current' above or below the knees, and so insisting on charging above or discharging below the knee is not really of much benefit.
https://batteryuniversity.com/articl...w-temperatures
https://northeastbattery.com/most-co...tery-mistakes/
Other than chemistry, there is very little difference between LiFePO4 and Lead Acid. A battery is a battery, although each has a specific reason to exist.
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I think price is a the driving factor. And amount of use. Many people don't really gain much from having the "best" technology. Those who do half dozen camping trips a year have less use to amortize the higher cost over. Better doesn't beat good enough in those circumstances. Spending a lot less on a good battery maintainer for the inexpensive PbA battery makes more sense for those people. Like paying the premium for Snap-On tools, only makes sense if using the tool is your profession, then having a tool break costs you money, otherwise a decent set of Craftsman are the better value for the shade tree mechanic.
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Absolutely true. Good enough is good enough. The biggest problem is the high(er) entry cost even when better is needed. That and rampant ignorance about the realities of today's markets. Right now (summer 2021) LiFePO4 prices are finally dropping rapidly but awareness of that fact isn't growing. In fact many if not most RV owners are still blissfully unaware that LiFePO4 even exists.
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I think as market grows it may bring price down. Cordless tools the higher capacity newer batteries don't cost more than the ones from several years ago with fewer and smaller cells. My 12 volt B&D battery of years back and the 18 or 21 volt of today are about the same cost.
Tech has to solve a problem for the cost of said technology to be worth purchasing. Just being better isn't enough to make a decision on so threads such as this that help outline the use cases where it matters are I think very useful.
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Rvs are the main market for these 'drop in replacement' LiFePO4 batteries. I too believe that as they become more known and understood, and as they become more requested, they will grow in popularity, reducing their prices even faster.
Charsara
Scampy
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