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12VDC or DC-AC inverter?

Discussion in 'On the Road' started by MW1CFN, Feb 18, 2018.

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  1. MW1CFN

    MW1CFN XML Subscriber QRZ Page

    I've reached one of those critical decision junctions in moving forward with a small solar PV system for a remote weekend location. Please note this is a remote shack, not an RV - there is no engine to charge from!

    I can see that, within a sensible budget, a couple of 12v batteries won't give a spectacular longevity to operations. The critical thing working against direct 12V DC operation is the relatively low volts presented to the rig, which expects nearer 14V than 12V.

    Based on this, some suggest it's better to use a DC-AC inverter, as a more stable and 'expected' supply is presented to the rig. But this chain is 12V DC to mains AC, and then a PSU back down to 13.8VDC. I guess the stability comes at a fairly considerable battery duration penalty.

    Once again, to save some money and make what I have go as far as possible, I'd be interested to hear from those operating (effectively!) from batteries on a regular basis.
    Last edited: Feb 18, 2018
  2. KV6O

    KV6O Ham Member QRZ Page

    Converting the DC to AC, then back down to DC incurs a pretty big efficiency penalty, and if you're running on batteries you want to be as efficient as possible. Besides being more costly, heavier, taking up more room, more to fail, etc... And since the radio wants the DC you're starting with, best to start there!

    Some radios deal with the voltage drop better than others, and running lower power can help. If you absolutely need 13.8V, then a boost converter is the way to get it. A boost converter is a more efficient way of getting the needed voltage, they only work in the voltage delta - boosting the voltage - but allowing what's available from the battery to pass thru directly. The price is higher and higher current draw from a battery that's already having issues, so you need to make sure you don't over discharge your batteries.

    Google "13.8 v battery booster" for more...
    AG6QR, KB4QAA and AC0GV like this.
  3. AC0GV

    AC0GV Ham Member QRZ Page

    Because this is a ham radio group, I’d say 12 VDC. Inverters can make a lot of RF noise (as can the regulators). Also, it takes energy to run the inverter, it is more efficient to use the DC direct. By the time you convert the Solar DC to AC to power the supply that converts it back to DC for the radio you will have wasted a LOT of pirate ninjas. RV people use a couple of 6 volt batteries and last three days with little sun. You will also not need to transport an inverter and power supply.
    MW1CFN likes this.
  4. KF5LJW

    KF5LJW Ham Member QRZ Page

    For ham radio you really do not want to use an Inverter due to the inherent noise problems associated with most inverters being modified sine wave. Even True Sine Wave Inverters can be noisy. On top of that is way too many conversions and the efficiency losses of conversions. Running straight DC is going to be a challenge from a noise perspective because most Solar Charge Controllers generate considerable levels of noise, it is the nature of the beast. Stay away from Chi-Com PWM controllers as they will generate a great deal of noise. Even good MPPT type controllers, you are going to have to deal with noise. I suggest either Morningstar Sun Saver or Tristar series. The other manufacture would be Midnite Solar like a Kid or Classic models. So stay away from th eInverters because it will double your noise problems. You will have your hands full dealing with the noise the charge controller

    There is another good reason to stay away from PWM controllers. Do not let the price fool you, they are more expensive than MPPT at the end of the day for two reasons.

    1. It takes a 300 watt PWM system to equal a 200 watt MPPT System. At best a PWM system is only 67% efficient, and MPPT is around 95 to 98% efficient. With PWM Output Current = Input Current. Example a 100 watt panel specs are 18 Vmp and Imp = 5.5 amps. So 5.5 amps x 12 volts = 66 watts from a 100 watt input. MPPT Output Current = Panel Wattage / Battery Voltage. So 100 watts / 12 volts = 8.3 amps.

    2. With PWM you MUST USE 12 volt battery panels to maximize their poor efficiency. A battery panel Vmp is 18 volts, and you loose 4 to 6 volts through the controller. Battery panels cost 3 to 6 times more than a panel made for grid tied system. Additionally battery panels are low wattage with 160 to 190 watts maximum. So depending on your application and required wattage, will require more panels which means more hardware. Grids tied panels go up to 300 watts for a single panel. To equal that with a PWM system would require 5 x 100 watt panels. Just in panel cost alone a 300 wat GT panel cost $300 to $400, and 500 watts of PWM battery panels will cost you $700 to $1000 and the pain does not stop there. To use 5 panels requires 5 times more racking and wiring material, plus you would have to use fusses and disconnect with 5 panels.

    Last thing is battery voltages. A 12 volt battery is not 12 volts. If your Pb battery goes down to 12 volts is a DEAD BATTERY. Pb batteries on solar charge at 14.4 volts, float at 13.6 volts, and when the sun sets the 100% SOC open circuit voltage is 12.6 volts.

    I design these systems for cellular and other markets that require solar battery systems for a living. Last thing is DO NOT USE a SLI battery (car-truck starting, lighting, ignition) They are not made to do that and if pressed into cycle service wil be toast n a couple of months. You want to use a either a true Deep Cycle battery or a Hybrid battery. Deep cycle batteries have very heavy thick plates which offer good cycle life. However their internal resistance is a bit high which limits the maximum discharge rate to around C/8, otherwise voltage sag can be a problem. Example say you have a 100 AH battery and C/8 would be 100 AH/8 H = 12.5 amps or about a 50 watt transceiver.

    Hybrid batteries try to be both Deep Cycle and SLI batteries. Hybrids are easy to spot by their marketing names like RV, Golf Cart, Marine, Leisure, Trolling, ect. A Hybrid battery has thicker heavier plates than SLI, bu tnot as thick and heavy as a true Deep Cycle. A Hybrid can deliver higher discharge rates than Deep Cycle at the expense of fewer cycles. Like antennas you cannot have it all. Two of th ebest batteries for what you want to do is a Trojan T-105 or US Battery 6-volt Golf Cart batteries.

    Last comments on batteries. Do not make the mistake most people make. Batteries have minimum and maximum charge current requirements. Generically no slower than C/12 and no faster than C/6. For Solar C/10 to C/8 is just about perfect. Example if you were to use say Trojan T-105 or US Battery US-2000 XC2 (both are 6-volt 220 AH), you want them to be charged at 20 to 25 amps. Using a MPPT controller would take a 250 to 325 watt panel.

    Good luck and 73's

    MW1CFN likes this.
  5. WA7PRC

    WA7PRC Ham Member QRZ Page

    ^ ^ ^ THIS ^ ^ ^
    There are several DC-DC boost regulators on the market. MFJ makes one (pn MFJ-4416C "Super Battery Booster", link). These type of units also provide protection to the source, to prevent over-discharge (and damage to the battery) by shutting down if the source drops below a predetermined voltage (usually, around 11V).

    Yes, they CAN. But, when properly designed and manufactured, they don't. As tested using a Spectrum Analyzer, my homebrewed DC-DC boost regulator (aka "inverter") designed to operate a laptop PC (16VDC @ 4.5A) during Field Day from a 12VDC battery bank showed virtually ZERO conducted and radiated RF trash from MF thru HF. In use, it was DEAD quiet. That's because I/O was sufficiently decoupled (aka "filtered") and the circuit was sufficiently isolated (shielded). Further, high current ground returns were combined, so there're minimal grounds bouncing up/down from each other. See for purty pictures and a schematic.
    MW1CFN likes this.
  6. KC0LDH

    KC0LDH Premium Subscriber QRZ Page

    Why not try a High voltage battery approach. Low voltage battery's, and the equipment suffers from I^2*R losses. If you look at the AH rating of a battery you get the best bang for the buck at lower discharge currents, thus the best Watt Hr rating of a Lead acid battery is High Voltage, low current. Right back to Ohms law - I^2*R. The best way to get (less loss) more efficient system is go with higher starting voltage. 48v systems are very common, even 115vdc is used a bunch in the Power industry. And I agree with the previous posts, some converters are better than others, some are noisier than others. The good is that MANY are cheap, and the larger investment is the battery/solar systems. Putting all of the Conversion equipment in a segregated Metallic enclosure should minimize (not eliminate) any birdies. 24v is better than 12v, 36v, better than 24, etc... Not that you need a 115vdc system, but if you are going to invest in multiple battery's, configure your investment for the best you can.
    MW1CFN likes this.
  7. KF5LJW

    KF5LJW Ham Member QRZ Page

    Completely true, but illogical and lack of knowledge applying good design priciples. It comes down to Peukert's Law which expresses approximately the change in capacity of rechargeable lead–acid batteries at different rates of discharge. As the rate of discharge increases, the battery's available capacity decreases. Consumer grade batteries are rated at the 20 hours discharge rate or C/20. Example a 100 AH battery C/20 rate is 100 AH/20 Hours = 5 amps. So what happens if you discharge at say 10 amps or the C/10 rate. You have to look at the manufactures data sheet to see if they even give you that information, if they do would would see that 100 AH battery discharge at C/10 drops to roughly 90 AH. Take it further and say you discharge at C/4 (25 Amps) and you have a 70 AH battery.

    So what can you do with this info. Well for one you might try a little math and look at how much current your load uses. Say a 100 amp transceiver draws 20 amps. If you want to limit discharge to C/20 would require a 400 AH battery. What voltage? Does not matter if it is 12, 24, or 48 volt. With a battery WATT HOUR CAPACITY = BATTY VOLTAGE x AMP HOURS. Just about all ham radio equipment operates at 12 volts, and 12 volts is low power. So if we used a 12 volt battery @ 400 AH, we satisfy the C/20 limit and get the most out of our battery and do not have to add another egg in the basket to break and cost us money (DC to DC converter).

    Armed with that piece of knowledge lets say we want use say 24 or 48 volt battery. What size is required to limit discharge to C/20. For 12 volt battery we know 400 AH right? Easy peazy it would take 24 volt @ 200 AH or 48 volt @ 100 AH.

    12 volt x 400 AH = 4800 wh
    24 volt x 200 Ah = 4800 wh
    48 volt x 100 AH = 4800 wh

    12 volts does have limitations. and to your point you want to limit 12 volt systems to no larger than say a 1000 watt load, 24 volts to 2000 watts, and 48 volt to 4000 watts. A 100 watt Transceiver only draws roughly 240 watts or 20 amps. At 24 would draw 10 amps and at 48 volts 5 amps. At thi slow of a power lever makes no sense to go above 12 volts. You would have to buy a DC to DC Converter. Now having said that if the OP were going to use Solar Power to run everything else in the cabin and had a sizable load of say 2 Kwh per day, then OK 24 or 48 volt has advantages in cost savings, but not at low power of a radio. I see a lot of fools that have a 100 watt panel, 10 amp charge controller, 12 volt battery 100 AH battery and a 2000 wat inverter. Those fools are playing with fire and do not know what they are doing.
    MW1CFN likes this.
  8. KF5LJW

    KF5LJW Ham Member QRZ Page

    Here are some good rules of thumb to use Solar Battery and calculations .

    10 to 500 Watt Panel system use 12 volt battery. To determine MPPT Charge Controller size in AMP = Panel Wattage / Nominal Battery Voltage. Example a 100 way panel would require 100 watts / 12 volts = 8.33 amps so go shopping for a 10 -amp controller. At 500 watts requires a 40-amp controller. To determine battery capacity C/10 is perfect. So if you use say a 100 watt panel and calculate 8.3 amps x 10 hours = 83 AH. So go shopping for a 80 to 100 AH battery. Inverter or load should be no larger than Panel Wattage. For those of you with a 100 Watt Transceiver a perfect match is a 200 to 250 watt Panel, a 15-Amp MPPT Controller (200 watt panel), or 20 Amp MPPT Controller (250 watt panel), with a pair of Golf Cart Batteries sized 150 to 250 AH respectively.

    For 500 to 1500 Watt Panel you need to move up to 24 volt battery. Same math for controller Panel Wattage / Nominal Battery Voltage. That puts you up into 20 to 65 Amp MPPT Controller and a battery range of 200 to 650 AMP Hours. Again Inverter or Load should be no larger than Panel Wattage.

    For 1500 to 4000 Watt Panel and you are in 48 volt battery territory. Same math which puts you in the range of 30 to 80 Amp MPPT Controler, and a battery capacity of 300 to 800 AH. Again Inverter or Load should be no larger than Panel Wattage.

    About the largest Charge controller you can buy is 80-Amp and an 80 amp controller will support 12,24, and 48 volt battery. A good 80 Amp Controller will cost you $600. However panel wattage is dependent on battery voltage. With an 80-Amp MPPT Controller panel wattage is limited to:

    1000 Watts @ 12 volt
    2000 Watts @ 24 volt
    4000 Watts A 48 volt.

    So as you can see selecting the right battery voltage greatly effects your wallet. Sure you can go up to 1000 watts @ 12 volt battery, but the controller will cost you $600 for an 80-Amp MPPT Controller. Change to 24 volts only requires a much less expensive 40 amp controller. Follow those simple rules an no wire larger than 6 AWG required. DO NOT USE 36-cell Battery Panels (18-volt Vmp) or a PWM Controller. Use higher voltage Grid Tied Panels of 60 to 72 cells as they cost a fraction of Battery Panels.
  9. KC0LDH

    KC0LDH Premium Subscriber QRZ Page

    I think we are saying the same thing. I also applaud the detail you have provided.

    I agree with the <6ga, 30-40A size range also. This is a practical, and economic size regardless of voltage. It's just details.

    Since we have no idea the details of the power budget, time expectations, hamshack odds and ends available, or solar situation, all are perfectly valid solutions. I was simply pointing out an approach that was not previously mentioned, or potentially more economical.

    Good luck!
  10. KF5LJW

    KF5LJW Ham Member QRZ Page

    No problem and thanks.

    True but he has some misconceptions.

    This is somewhat true if you used 12 volt batteries in parallel which is a recipe for short battery life. You would use 6 or 4 volt batteries to meet a specified capacity. Never Ever Parallel multi cell batteries. If you need say 200 AMP Hours, then buy 200 AH batteries. Not likely you can find 12 volt 200 AH batteries. They would weigh in at 150 plus pounds. Not would you use 2 x 12 volt 100 AH batteries in parallel. Smart money and long live is a pair of 6-Volt 200 AH batteries wired in SERIES. No problem there if you use the right battery for the job. As I said earlier a pair of Trojan T-105's or US Battery US 2000 XC2 would be great choices paired with a 200 to 250 watt panel.

    Then there is this:

    OK this is just plain incorrect. A 12 volt Pb battery operating at 12 volts is a DISCHARGED BATTERY. Any solar battery system charges at 14.2 to 14.4 volts, and when the battery is fully charged is 13.6 to 13.8 volts. Now after sunsets the surface charge will drain down to 12.8 to 12.6 volts on a 100% state of charge battery. If used heavily after sunset by morning can go down to 12.4 volts which is not a real problem. You would have to use 30% of the battery capacity to reach 12.4 volts. On a 12 volt 200 AH battery has 2400 watt hours, so 30% is 750 Watt hours. Not really possible to do in a night with a 100 watt transceiver unless you are using AM or FM at full power with the mic keyed for 6 solid hours. On SSB or CW would take a few days.

    I help out with our club on holiday 3-day weekends. We do not use solar as it does not gain us anything. We just use a pair of Trojan T-105's and can transmit 3 days without a recharge and could go another day or two before needing to reach 50% DOD when a recharge is needed. for a few more days on a 100 watt transceiver.

    As others have mentioned using an Inverter is just asking for trouble, and more inefficiency and expense to deal with. To operate a radio with solar battery is a big enough challenge dealing with the noise any charge controller is going to generate. No doubt about it, you will have NOISE. Easy to solve with $40 of snap-on beads on both panel and battery leads. To use an Inverter is more possible noise to deal with, plus the expense of the Inverter, 12 volt DC power, and more noise suppression. No need to do that, you already have 12 volts. Use it and KISS it. (Keep It Simple Stupid)
    MW1CFN likes this.

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