Maybe it’s obvious, but another factor to consider when deciding whether to use 24V or 48V strings is wire sizing and power loss in the wires.
At 24V all your wires will have to be sized for twice the current. I’ve actually run into problems with 10 or 12 AWG wires being too big for the compression terminals on some DIN rail modules, even using ferrules (which I highly recommend). Or if you have long DC power runs (like up a tower), the power loss in the cables can be significant. Even if the equipment can run on the lower voltage, you are wasting power (and battery capacity) heating up the wires. Power lost in wiring is I2R so twice the current is 4 times the power wasted for the same size wiring. Rule of thumb is 3 AWG sizes to double the resistance, 6 AWG sizes for 2X the diameter and 4X the resistance. So the difference between a 24V and 48V system could possibly mean the difference between 10 AWG and 16 AWG wiring. That sounds like a lot, and I’m doing the math in my head, and I’m not Andrew Yang or Bill Nye, so if I messed up please someone correct me. From: AF <af-boun...@af.afmug.com> On Behalf Of Forrest Christian (List Account) Sent: Tuesday, March 3, 2020 5:12 AM To: AnimalFarm Microwave Users Group <af@af.afmug.com> Subject: Re: [AFMUG] Dc system questions One note I forgot to add along those lines, is it is generally a good idea to keep all of the batteries in the array the same age and type. I don't think this is nearly as critical if you are float charging, but if you are doing any sort of rapid charging then this is an issue. It's also much more of an issue if you mix them in the same string, i.e. if you convert a 24V site to a 48V site, you shouldn't add fresh batteries to each of your 24v strings. On Tue, Mar 3, 2020 at 4:03 AM Mark Radabaugh <m...@amplex.net <mailto:m...@amplex.net> > wrote: Forrest covered most of it. On the question about one battery being lower than the others that is handled with an equalization charge (if your charger can do it). You can end up with different voltages across the battery string where one battery remains slightly below voltage compared to the others. A equalization charge temporarily (a couple hours) raises te charging voltage over the float voltage. When a battery is full and the charging voltage stays high it turns the extra energy into heat (or in an extreme case boils off the electrolyte). In the equalization charge the batteries that are full end up heating up a bit while the one that is lagging behind reaches a full charge. It’s not something you need to do very often on a pack that largely sits on standby. Once every couple of months is probably fine. Maybe even once a year. Mark On Mar 3, 2020, at 5:26 AM, Forrest Christian (List Account) <li...@packetflux.com <mailto:li...@packetflux.com> > wrote: Generally you want the voltage of the array to be whatever is typically drawing the most wattage. Anymore it's 48VDC since all the high power stuff is 48VDC. That way you avoid converting to that voltage. You need enough charger to run your stuff, plus enough to charge the batteries in however long you want to recover. So if you've got a 48V 10A charger and are drawing 9A with your radios, and a 100AH array (4 100AH batteries in series), you'd find that it would take 100 hours to recover. This is probably too long, plus there's some self-discharge to worry about. Note that the manufacturer's recommended charge rate is typically at most 0.3C, meaning that for a 100AH array, you want at most 30A of charge. You can get away with more, but I wouldn't push it if you want long life out of your batteries. This would charge most of the way in 3-4 hours. If you went down to 0.1C you'd probably still be ok, since that is 10 hours, but a rule of thumb is you really shouldn't go below 0.1C. Note that if those 4 batteries are in a 24VDC array, it's a 200AH array, and all of the currents double... since now you've got 2 batteries in series @24V, which is 100AH@24V, and another which is 100AH@24V. On Mon, Mar 2, 2020 at 9:27 PM Steve Jones <thatoneguyst...@gmail.com <mailto:thatoneguyst...@gmail.com> > wrote: First question, if i have say 4 12v batteries and want to double runtime by adding 4 more, will that require a bigger charger? Or can i just keep adding batteries and it just takes longer to reach a full charge? If for whatever reason, one of the batteries has less of a charge, once the rest reach full charge will it continue until its at the same level of charge? I seem to recall that i keep getting told 24 vold battery arrays are more efficient than 48v. If thats the case, do i just put a 24v to 48v power supply on it to run all my gear? -- AF mailing list AF@af.afmug.com <mailto:AF@af.afmug.com> http://af.afmug.com/mailman/listinfo/af_af.afmug.com -- - Forrest -- AF mailing list AF@af.afmug.com <mailto:AF@af.afmug.com> http://af.afmug.com/mailman/listinfo/af_af.afmug.com -- AF mailing list AF@af.afmug.com <mailto:AF@af.afmug.com> http://af.afmug.com/mailman/listinfo/af_af.afmug.com -- - Forrest
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