I know a desert rat who lives off the grid and uses a 180 watt solar panel to charge two 12 lead acid batteries in series (24 volts). The panel delivers about 5 amps at 36 volts which overcharges the batteries if not regulated. I see a LT1038, 10 amp adjustable regulator, but it appears to be obsolete. Digikey has a LM338K, 5 amp adjustable regulator (TO-3) but it may be.a little light (maybe 2 in parallel would work?). What would you recommend for a 36 volt input and a 27.2 volt output at 6 amps?
Probably not very useful in a desert. But if you can find a use for the spa re power, an opamp can progressively turn on a parallel smpsu until the bat tery voltage is limited to 27.2v by the added panel load. Ie there is no re gulator between panel & battery. Or you can do it better with MPPT & smpsu all round.
The battery terminal voltage will not rise to 36V while it's charging unles s there's a whopping big IR drop from the charging current, and 5 Amps will not do it- maybe 0.3V or so for smallish
wrote in message news: snipped-for-privacy@googlegroups.com... On Wednesday, July 27, 2016 at 8:39:49 PM UTC-4, Bill Bowden wrote:
He said he had wiped out one battery from overcharging and is now using a resistor in series to limit the current to 1 amp. The comparator idea should work with some hysteresis. I cooked up a spice model using a DPDT relay so the extra power can be switched somewhere else. R8 represents the relay. The hysteresis range can be adjusted with R6 and the center range with R1.
Version 4 SHEET 1 880 680 WIRE -16 -64 -112 -64 WIRE 80 -64 -16 -64 WIRE 144 -64 80 -64 WIRE 352 -64 144 -64 WIRE 480 -64 352 -64 WIRE 144 -48 144 -64 WIRE 352 -16 352 -64 WIRE 480 16 480 -64 WIRE -16 32 -16 -64 WIRE 288 32 144 32 WIRE -112 80 -112 -64 WIRE 480 112 480 96 WIRE 480 112 240 112 WIRE 80 160 80 -64 WIRE 144 160 144 32 WIRE 480 160 480 112 WIRE 256 192 240 192 WIRE 352 192 352 80 WIRE 352 192 336 192 WIRE -16 208 -16 112 WIRE 16 208 -16 208 WIRE 240 208 240 192 WIRE 240 208 208 208 WIRE 240 224 240 208 WIRE 352 224 352 192 WIRE 112 256 80 256 WIRE 144 256 112 256 WIRE -16 272 -16 208 WIRE 112 272 112 256 WIRE 240 320 240 304 WIRE 352 320 352 304 WIRE -16 368 -16 352 WIRE 112 368 112 352 FLAG 352 320 0 FLAG 240 320 0 FLAG 112 368 0 FLAG -16 368 0 FLAG -112 160 0 FLAG 480 224 0 SYMBOL voltage -112 64 R0 WINDOW 0 -77 59 Left 0 WINDOW 3 -253 144 Left 0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value PULSE(20 30 0 1 1 .01 2.01 3) SYMBOL npn 16 160 R0 WINDOW 0 115 327 Left 0 WINDOW 3 -47 326 Left 0 SYMATTR InstName Q1 SYMATTR Value 2N3904 SYMBOL npn 208 160 M0 WINDOW 0 85 324 Left 0 WINDOW 3 54 47 Left 0 SYMATTR InstName Q2 SYMATTR Value 2N3904 SYMBOL pnp 288 80 M180 WINDOW 0 -84 -406 Left 0 WINDOW 3 34 46 Left 0 SYMATTR InstName Q3 SYMATTR Value 2N2905A SYMBOL res 224 96 R0 WINDOW 0 -212 -63 Left 0 WINDOW 3 -31 47 Left 0 SYMATTR InstName R1 SYMATTR Value 2k SYMBOL res 128 -64 R0 WINDOW 0 -209 355 Left 0 WINDOW 3 43 62 Left 0 SYMATTR InstName R2 SYMATTR Value 2k SYMBOL res 224 208 R0 WINDOW 0 -57 -241 Left 0 WINDOW 3 38 96 Left 0 SYMATTR InstName R3 SYMATTR Value 2k SYMBOL res 96 256 R0 WINDOW 0 42 40 Left 0 WINDOW 3 43 67 Left 0 SYMATTR InstName R4 SYMATTR Value 2k SYMBOL res 464 0 R0 WINDOW 0 -263 105 Left 0 WINDOW 3 41 60 Left 0 SYMATTR InstName R5 SYMATTR Value 2k SYMBOL res 336 208 R0 WINDOW 0 -56 -50 Left 0 WINDOW 3 42 65 Left 0 SYMATTR InstName R6 SYMATTR Value 240 SYMBOL res -32 16 R0 WINDOW 0 297 259 Left 0 WINDOW 3 44 52 Left 0 SYMATTR InstName R7 SYMATTR Value 6.2k SYMBOL res -32 256 R0 WINDOW 0 410 -14 Left 0 WINDOW 3 -48 63 Left 0 SYMATTR InstName R8 SYMATTR Value 2k SYMBOL res 352 176 R90 WINDOW 0 -130 -168 VBottom 0 WINDOW 3 32 56 VTop 0 SYMATTR InstName R9 SYMATTR Value 25k SYMBOL zener 496 224 R180 WINDOW 0 299 -260 Left 0 WINDOW 3 -148 36 Left 0 SYMATTR InstName D1 SYMATTR Value BZX84C12L TEXT -264 304 Left 0 !.tran 4 TEXT 368 208 Left 0 ;Relay TEXT -8 -144 Left 0 ;24 Volt Solar Panel Charge Controller TEXT 8 -112 Left 0 ;Hysteresis Range 24 to 28.5 Volts
There is something fishy here. What is the Ampere Hour(AH) rating of each o f the batteries ? The maximum allowable charging current must be 0.1xAH. Yo ur friend could use a few more batteries in a series-oarallel combination t o distribute the current. If the batteries are simple lead acid or sealed l ead acid(SLA), overcharging is never a problem the charging current constra int is maintained. We have built large solar installations in western India *very dry and arid) and we have never had any issues with overcharging.
On Thursday, July 28, 2016 at 9:57:37 PM UTC+1, snipped-for-privacy@gmail.com wrote: ...
...
It's not quite as abrupt as that.
The inductance of the field winding causes it to act more like a hysteretic switching power supply. The regulator contacts will dither at many Hertz and the PWM will vary to control the output voltage.
There was usually a 50 ohm resistor across the contacts to absorb the back-emf when the current is turned off to protect the contacts.
Charging a lead-acid battery is a little more complicated than that. For full capacity, you have to charge the battery up to a higher voltage target (equalization), then you back off and float at a lower float voltage to avoid wrecking the thing. The thresholds change with chemistry and temperature.
Why not suggest "Larry" get a cheap charge controller off eBay? They aren't much.
It seems a shame to waste something like a third of the panel's 36V output charging 24V-worth of battery. Even short of full MPPT, a switcher might improve on that loss.
That's a good first cut but you'll need to add a few things:
protection for the controller of the circuit against high voltage if the battery is disconnected while the panel is hot- dunno what the open circui t voltage of the panel might be - 48V? 68V?
Generally relays don't like being driven by more than 110% nominal coil voltage- most have coil temperature rise of 40C at nominal- rise goes as sq uare voltage so 1.1^2=1.2 is 48oC and in you circuit at 28V drive of a 24 V coil gets (28/24)^2 x 40=55oC temp rise- might reduce the longevity sig nificantly. I would use a 12VDC relay and make the 2n2905 a current source by addition of emitter resistor.
add override momentary to start charging at less than full discharged ba tteries- your lower threshold, a bit lower than 24V comes out to almost 1.8 V/cell which is a very depleted state of charge
doesn't hurt to add indicators a) charging b) full charged and not charg ing
He doesn't need to "float" it if the battery is recharged every day. The le ad acid cell tempco per cell is -2mV/oC, his macro-threshold gets divided b y 12, will be hard to be off a few hundred mV cell from the 2.4V full charg e and it won't amount to a hill of beans. You think the cheap controller is temperature compensated? Really? Show me the thermocouple probe that comes with them.
That might make a difference if the available panel current takes an inordi nately long time to charge the batteries, if not, it makes no difference wh ether you're operating on the MPP or not. Since as things stand now the pan el overcharges and ruins the battery, there is a good chance the MPP is dou bly a non-consideration.
Yes, I'll do that and the name is Harry, not Larry.
True, but the price was right at $100 for 180 watts, or 55 cents a watt, so he can afford to throw some of it away. I think he may have another panel he hasn't hooked up yet. He claims he runs a swamp cooler from a 24 to 120 volt inverter, but I would imagine a swamp cooler would draw more than 180 watts. So, if he runs the swamp cooler 10 hours a day, I don't see how he charges the batteries at all. I might see him tomorrow and get more information.
Where does he indicate insufficient charge? If you understand what I wrote, you will note that, if the battery is too low, the switch is on. In the winter, it will stay on as long as the battery is low. How is that any different from what he has now? If he already has MPPT to handle the winter, it should not overcharge in the summer.
If the battery is overcharging, then its terminal voltage gets too high. If you then disconnect the panel (either manually or with electronics) at your desired threshold, you have stopped the overcharging. When you stop the charging, the battery voltage will decrease. Now you need to determine the low battery voltage threshold that will reconnect to the panel. When it reconnects, the the battery voltage will rise.
So, I suggest that charging begins at your low threshold and lasts for at least one minute. If battery voltage is above the high threshold after that time, discontinue charging for at least one minute. When the voltage drops to the low threshold, charge again.
You don't need to dissipate power in a linear regulator to do this. You might find an automotive relay to handle the job. If you use a relay, I suggest that you use the NC contacts for charging. That way, you energize the relay when the batteries are already full. You don't need to draw any more current when the batteries are low.
If you use a solid state switch, so much the better.
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