Help me understand the theory of charging

[fitek]

I'm trying to make sense of how to run numbers for battery charging and what effect charging voltage has.

My observations are that as I increase the voltage of a power supply attached to the pack above the pack voltage, the current that goes into the batteries increases. Of course according to Ohm's law if the voltage increases, resistance stays the same, then current should increase. The internal resistance of the batteries is fixed I assume (does it vary depending on cell voltage or discharge/charge current?).

I've read in many places that the charging voltage shouldn't exceed some value-- however, usually the battery charging ends up pulling the power supply voltage down to pack voltage regardless of what I set the power supply to. After some time, the voltage does tend towards the power supply setting. If that's the case, then what meaning does the voltage I set on the power supply have so long as I make sure the pack voltage never exceeds the maximum voltage (something like 42v for my pack I think).

Put another way: if, for example, I have 3.7v lithium battery, and I would normally charge it using 4.2v, why shouldn't I apply, for example, 6volts and simply cut the charge off once the voltage increases to about 4.2v? For that matter, with a whole battery pack like my 36v12ah pack and a power supply dumping say 3 amps into the pack (the current limit on my supply), that is only a charge rate of 0.25C-- if I set the power supply to 60 volts, does it matter in any way except that after some time I would start to overcharge the pack?

 

[Anonymous]

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[317537]

I us a 72 volt bad boy on my 36v and 48v SLA's.

As far as I know the internal resistance decreases when the pack reaches its full charge. But this is not the mechanism that drives the current draw but more so as to why heat becomes a problem.

Think of a retracting dispersal container, a tube with two valves on either end (both valves fashioned for dispersing and retaining), and an expandable receptacle. You want to fill up the receptacle. Internal resistance of the receptacle has no control over how much the retracting container puts into it, my meaning, the receptacle doesn’t induct the fluid. However the internal resistance impedes the work so you can only get little bits in at a time. Internal resistance in batteries is more like pressure or current, or better still ability to supply current. A small charger with low amps (a thin tube) can only put in little bits at a time but will still fill the battery to its full voltage (pressure) potential, where a big charger (stong disperal contianer with thicker tube) can put higher volume at the same voltage (pressure). Higher voltages at lower currents squirts the energy in faster and nominal charging voltages at high current (volume) squirts larger volumes of energy more slowly energy.


You cant fill the receptacle up at a higher pressure (voltage) than you the dispersal container outputs.

At the end of the day the outside pressure of the receptacle is what matters and as long as you dont exceed the maximum it can handle it will live a long life. When it can not expand any more it begins to leak if you try put too much pressure in there, and if left to full for too long the skin weakens over a long time. The difference between constant voltages and overvolt charging takes its toll when maximum capacity is achieved.

The reason why higher voltage are not desirable is when overcharging occurs you battery (receptacle) bursts and plates pit, where as a constant voltage only weakens the battery if held at peak voltage the plates corrode more evenly. You may see at first increased density if you overcharge the battery in volts but with the thinner plates you risk eating right through them.

Maybe what I say here doesn’t make sense.

 

[317537]

I lost track.


easy answer is.

Why current is more at the beginning of a charge and tappers off?

The battery is attempting to put current back into the charger when the diode is open but wont because the higher voltage pushes through, when the batt terminal voltage equals a constant voltage feed the current stops feeding the battery anymore.

Edit:

My trusty solar panel puts out 22v. To get the most efficency out of the panels My regultor use a mosfet that load almost the full voltage of the panels to a 12v battery and when 14.4 volts is reached the regulator switches to a constant float charger.

There is no reason you cant use higher voltages as long as you switch it off or pay the price.

 

[Anonymous]

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[317537]

I dont know about lithium. But I can take a shot at it.

Any chemistry is not good when it gets warm. My 36v sla pack stays cool when charging at 72 volts, I have found there is a sweet spot where current is concerned. To little and the charge takes too long, when this happens, the top end of the charge takes too long and gassing sets in. When current is too high the batteries dont get enough charge.

You can charge SLA'S to full when you use float voltages but this takes 16 hours.

I look at the voltage rise and usually when it starts to race this is a good indication the battery is at full capacity. Try this over the whole pack and when you see the volt meter go up quickly this is when they are charged to max.

Look at the battery volts after you remove the charger after 10 mins and it usually has a voltage it likes to hang at. If you can get to that voltage at say 3.56v bulk charge of lifepo4 per cell then this is enough. The less current the longer it will take the longer they hang at 3.65 the warmer they will get. The more current you use the faster they will take and less time they should sit at 3.65.

Its a balance of how long it takes and how fast you need the pack to charge and how much you want to push it.

With my SLA's because I use 1.4 amps at 72v I go to the minium specified voltage of 14.75 to avoid gassing for long periods, with a bigger charger (more amps) I would stop it at the maximum voltage at 14.9 to get more energy into the batts. They don’t race as much towards the end of the charge when applying more current, indicating that the capacity isn’t quite as full.

If you could monitor the speed of voltage rise and tapper off the volts when it speeds up, this IMO would be ideal, contrary to what most advise.