batteriesbattery-chargingdatasheet
I am using a lead-acid battery in a solar charging system. I am writing for an ATMEGA328 to determine the state of charge of the battery. Its my understanding that this will require a sort of data sheet for the battery cross-referencing:
Charge Percentage – Voltage – Temperature – Discharge Rate
How is this information usually passed from the manufacturer to the programmer?
Iv'e found many visual charts but what I'm really looking for is some kind of data file with a 4-dimensional array.
The impact of running a 3.2V device on a 1.2V battery would be that the battery would go flat very rapidly, and the device won't function.
There are two values you need to consider: The voltage and the amperage (or current). The voltage is measured in volts (V) and the current is measured in amps (A or mA - 1A = 1,000mA)
The voltage of the device and the power source (battery) have to match. Too much voltage from the power source and you will destroy (or seriously damage) the device. Too little and it just won't operate.
The power source has to provide at least as much current as the device requires. The device will never draw more current than it needs, so it is perfectly safe to use a power source with a higher current rating without damaging the device. However, using a power source with a lower current rating that the device could risk damaging the power source - in the case of a battery it could cause the battery to rupture and a fire could be caused.
Batteries can be connected together in series to increase the voltage (+ of one battery connected to - of the next; + of that one to the - of the next etc), or in parallel to increase the current (all the + linked together and all the - linked together) or you can do a combination of the two to increase both the current and the voltage.
So, three batteries at 1.2V each connected in series would give 3.6V - a little over the rated voltage of the device, but it may be allowable - you should check the manual or data sheet for the device.
Batteries don't have a current rating as such, but instead have a "mAh" rating. That's *milliamp-hours" or the amount of current that can be given out in an hour.
So, an 800mAh battery can give 800mA over the course of an hour before it goes flat. Or it could give 400mA over 2 hours, or 200mA over 4 hours, etc. The more current that is drawn the quicker it will go flat.
To better understand voltage and current I like to try and get people to visualize a pipe with water flowing through it
The voltage is akin to the diameter of the pipe. The wider the pipe the more water can flow through it at once.
The current is akin to the speed of the water flowing through the pipe. The faster it flows the more kick it has as it squirts out the end.
The water pressure is akin to the power or wattage (which is the current multiplied by the voltage), which is like the number of liters per hour that flow through the pipe.
As far as chargers are concerned that depends on the chemistry of the battery you buy.
There are three major chemistries that fall into two groups:
Ni-MH - Nickle Metal Hydride. These are the run-of-the-mill AA and AAA rechargeable batteries you buy in the shop. Your normal AA or AAA battery charger charges these easily. Most will have a charge current and time on them, such as "16hr at 220 mA".
Li-Ion and Li-Pol - Lithium Ion and Lithium Polymer. These are the kind you get in things like your mobile phone. They are much harder to charge up and require special electronics to manage them. They must not be allowed to go completely flat or you won't be able to charge them up again. However, they are much more powerful than the Ni-MH ones.
My original answer (below was based on my incorrect assumption that Arik was suggesting using the wiring inductance in a controller to replace formally provided inductance. In fact, he is saying that in the controllers of interest there is NO formal inductance, and he was wondering if the wiring inductance served a useful role overall.
Simple PWM can be used to vary the current which a PV panel will deliver and to control bttery voltage. It can act as current limiter, constant current controller or voltage controller.
A PV panel used without an energy converting controller, suh a an MPPT controller, usually acts much like a CC (constant current) source. This is because Vmp (Voltage at max power) is > Vbattery under most sun conditions and the panel is loaded with a lower effective resistance load than is optimum. An MPPT controller increases the effective load resistance so the supply voltage can rise to the optimum value.
If you connect a PV panel to a battery then current flow will redcue as PWM duty cycle redcues. It will not be a linear reduction as vpanel will rise as load is reduced, tending to act against the current reduction from the PWM. However, in practice you can set current to any value equal to or lower than what ypu'd get with a hard connection.
If you want to limit battery voltage to a certain value then simply reducing or stopping current flow when the voltage is high enough, will work as a "constant voltage' source. current
Simple MPPT can be little more than the buck converter I outlined plus a controller. By doing no more than holding panel voltage at about 80% - 85% of Voc_panel_full sun you will get very close to true MPPT performance.
Second addition
As the question evolves, so can the answer :-).
There is no doubt that simple bang/bang on/off control is undesirable and causes undesirable battery current and voltage variations. My comments about the controller being able to control voltage are true over a long time period relative to a PWM cycle but all sorts of interesting stuff may happen over a single cycle or a small number of cycles.
Adding an inductor allows energy storage and smoothing - an existing controller MAY be able to be "improved" by just adding an inductor and flyback diode and maybe one or 2 reservoir caps depending what is there now BUT the existing control circuitry may have a fit (or not) due to the changed response. It would probably in many cases to use the existing power level hardware with L,C,D as requisite plus either new software or (possibly more easily) a new control core. An MPPT controller needs cost little more than what is there now. Pricing is often controlled by "because we can" and "because they can't" factors.
Having the series switch (probably MOSFET) in linear or resistive more would help make behaviour nicer at the expense of power dissipation in the switch. The heatsink size is uncertain as can only be seen end on but it looks substantial. If the switch is run as a resistor then it COULD be setup to operate a setady current feed to battery. If desired this could be only done in holding mode where current is low. eg in Panle V_light)load is say 17V and Vbat hold is say 12.6V and Itrickle is say 100 mA then dissipation in a FET in this mode is P = V x I = (17 - 12.6) * 0.1 = 0.44 Watts = minimal. If you could sink say 5 Watts and needed to provide current from 18V to 12V you could have I = W/V = 5/(18-12) =~~ 800 mA.
Using on/off PWM is non ideal and will lead to waveforms something similar to those shown by Arik in his second edit. The magnitude of the spikes will depend on how much capacitance is present BEFORE the switch, to a lesser extent how much capacitance after the switch, wiring resistance (and to some extent inductance) and battery characteristics and state of charge and, importantly, PWM frequency. Arik has shown the signals as step changes at switching boundaries followed by linear ramps. I would expect the step changes to be modified by effect of capacitances and linear ramps to tail off into more or less steady state flat spots as PWM on or off time became long relative to battery & PV panel settling down to steady state under the given conditions.
I do not show a capacitor on the PV panel in my outline schematic below but if there is one then the PV panel will slew more slowly and can be held near a constant voltage more closely. This would limit the more objectionable spikes and excursions shown by Arik.
Also, an ideal battery may exhibit step change and "instantaneous steady state" conditions as suggested but it is likely that in real life you get more complex responses- an oscilloscope would be your friend here.
Original answer - useful but not what was asked for.
It is extremely likely that you circuit diagram is incorrect and that the simple PWM controllers are Buck converters as shown below.
Wiring inductance could notionally be used but in practice is too small to use in sensibly practical converters. Resonant frequency is not the critical factor. At resonance Vcap would swing 'widely'. What is required is an inductor such that delta V is small during the on cycle - perhaps 1V p-p and ideally quite a lot less. Using wiring inductance would probably require MHz range switching and would be likely to produce ill defined low efficiency high RFI situations.
With a suitable controller such a circuit can provide constant current or current limiting or voltage controlled output.
D1 is usually either a Schottky diode or a synchronously controlled FET switch.
Best Answer
As mentioned already, there are a lot of factors which affect the voltage/current/state-of-charge including temperature and temperature cycling, age, number of charge-discharge cycles, storage. As far as I know, most state-of-charge monitors use some initial set points and generic values and then 'learn' the battery system they are connected to. Some good information is available from HomePower magazine here. I have also produced a basic info sheet on generic battery questions.