EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: Swake on March 20, 2022, 07:05:25 pm
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Situation is multiple vehicles and machines with very different capacity 12V lead-acid batteries (classic, not AGM/GEL or anything like that) from about 10Ah to 80Ah. These vehicles are parked outdoors and not used very frequently therefor batteries need some external help to stay in good shape and be ready for use.
There is no mains power. The power source available is a 260W solar panel, obviously only providing oompf when the sun is up. The panel is rated 30.6V/8.5A (38V open circuit).
How would you solve the need to keep the batteries charged knowing that cost is important?
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One option is to provide a separate resistor dropper, and a series diode, to each 12V battery. As an example the value of the resistor could limit the max charging current to say under C/20, so 4A max for largest battery, and possibly with resistor dissipation up to 80W (ie. a 4R7 150W commercial part), and using a 10A diode or 5A diode bridge with heatsinking. If the sum of all charging currents exceeds 8.5A then the max voltage will reduce down the constant current side of the curve at full-sun until equilibrium is reached. For less than full sun, the charging current will fall from max level. For low sun or night the charging current will fall to zero (due to the series diode). You just need to periodically check the terminal voltage of each battery to confirm it is being charged during the day, and that it slowly rises on average (that may be difficult to confirm due to temperature changes of the battery, but best down during the night well after charging has stopped as battery OCV is a better indicator), and that battery is not getting overcharged (eg. if vehicles are out and only a few batteries are loading the PV panel.
A variation on the above is to firstly charge each vehicles battery to nominal full SOC using a 'good' charger and knowing the battery type/spec. Then use a dropping resistor that just maintains overnight OCV - that reduces the risk that after some period of SOC rising the battery then receives more charging current than it really needs to stay at nominal full SOC (it's always a service life balance to not significantly over or under charge a battery - ie. the goldilocks zone). This variation may allow much lower dissipation resistors and diode current ratings to be used, and reduce the risk that just one battery is on-line being charged and receiving a charge current that is more than needed for a long period of time.
Yes you are wasting a lot of available power, but given the application and the KISS principle, it is likely a lot less risky in to the future than using step-down/switched regulators of various types and makes.
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A fixed duty buck converter gets close enough to MPPT for this purpose, with the drive signal gated if the output rises above 16V or so. Then for each output, have some sort of solid state switch connect/disconnect it from the output of the buck converter as well as a solid state switch to connect it to a voltage monitoring circuit. The basic logic would be to apply a charging current for something like 1-10 seconds (up to the maximum charging rate of the battery), then disconnect the charging current and measure the voltage after a very short delay to allow the inductive transient to die down but before the battery voltage has had time to decay, that way it's possible to compensate for the resistance of the wires and know whether to apply another charging pulse or to wait. Stagger the timing for each output so as to maximize the utilization of the available solar power.
All in all, it should be easy enough to implement based on an Arduino. For the buck converter, a 555 along with a transistor and zener circuit for an output voltage limit would be good enough.
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The OP has indicated there is no mains, and tacitly indicated they don't know how to charge a battery with PV, and likely have little experience in choosing equipment, and likely little experience in how to wire up and protect parts from the environment. The monoblocks may well be small flooded car batteries, with some already having failed, and the failure mode appears to be age and infrequent charging, with no indication that the vehicles/equipment load the battery with any idle current at all (ie. dumb equipment with power on/off switch).
Imho the OP has no chance of contemplating any electronic design or prep themselves, so proposing a solution based on uP or timer parts seems fanciful.
The OP could enquire of their local electronics/renewable shop what is available in a multi-output 12V battery charging that can take in a PV module, and add a link for forum members to comment on. One comment is that the cheap ebay/on-line solar charger modules emblazoned with MPPT are limited to a 12-20V PV input for just one 12V battery, and have no stated battery charge current limit and are often aimed at larger Ah batteries and so battery charge current could reach PV module capability if only one battery is connected, which is a risk for a 10Ah battery.
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You culd build a zener diode based voltage regulator with a current limiter.
See https://www.homemade-circuits.com/simple-voltage-regulator-circuits-using-transistor-and-zener-diode/ (https://www.homemade-circuits.com/simple-voltage-regulator-circuits-using-transistor-and-zener-diode/)
scroll down to the middle of the page in the link. Its only 3 transistors a zener diode and some resistors.
I have buldt a similar circuit with 2 transistors, a zener and two resistors as well.
The other option is to use a voltage regulator with current limiting like the L200 voltage regulator.
You just set the uotput voltage to 14,3V or what ever voltage you prefer, ajust the current limiting to the maximum current for the solar panel divided by number of batteries and then your good.
Because you have difrent sizse of batteries you might want to set the currents diffrently for each battery.
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Thanks for the ideas.
Didn't know multi-headed chargers where a thing. Found some but only mains powered and don't want to setup an inverter for this.
I'll be going the simple cheap buck converter route. Want them with a settable current limit because very different batteries.
Voltage drop over maximum length cable is (14.3V/200mA/0.75mm² cable/2x15m length) is 400mV.
Might add an ESP32 to drive the buck converters only when there is enough sun coming in. This means another local battery/buck converter is needed to power the ESP32. Advantage is that it can send status/alerts over WiFi and measure all the outputs. This would also allow to add theft detection and some other totally unnecessary things.
Will build a sort of patch panel with 6 or 8 channels into a weatherproof box for electrics stuff. Got plenty of XLR3 connectors with weather-seal left over from another project.
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Had you collated those thoughts before you made the first post?
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No.
Might not need the ESP32 at all, found several Chinesium buck/boost converters with under-voltage cut-off, unfortunately the ones found so far only support up to 30V on the input.