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1 to 10 kWh battery systems – rules of thumb

You’d think the Amateur Radio types would have Ohm’s law in their toolbelt and not be afraid to use it. Between QRZ forums and this month’s QST, it doesn’t seem so when it comes to emergency backup power systems. For medium sized solar systems with battery backup, systems in the one to ten kilowatt hour range where the battery of choice is the lead acid type commonly found at retail for cars and RV’s, there is a lot of ignorance, myth mongering, and presumption of ignorance. Here are a few rules of thumb I offered to a query on the topic.

  • Figure about a usable 15 watt hours per pound of lead acid battery for available energy capacity. This is the 50% DoD target often used as most cost effective.
  • Figure at least 1 watt of solar panel per pound of battery.
  • Plan on an energy harvest from solar panels equal to the maximum rated output for 2 hours (for 240 watt panels plan on 480 watt hours per day from the solar). You might get a factor of 3 to 5 instead of 2, maybe, but don’t plan on it.
  • Figure you’ll need 4 watts per pound of battery for a proper bulk charge. You need this recharge power at least once a week for batteries in use daily.
  • Figure any battery energy capacity measure has a 10% to 20% variability due to factors such as age, cycle to cycle variation, temperature, and use profile.
  • Note that the 20 hour energy capacity rating is at about a watt per pound of battery. Draws higher than this will result in lower available capacity (re Peukert).
  • Don’t let a 12v lead acid battery go below 12.0v as measured after it has had no significant charging or discharging for at least a half hour. (and that’s pushing it. recharge at 12.4v and keep above 12.2v if you can).
  • Out of use storage maintenance for batteries needs a device that will maintain a full charge without overcharging and will apply a sulfation inhibiting technique. Float alone just doesn’t cut it anymore. (if you have electrolyte loss or corrosion at the terminals or poor charge retention, storage maintenance charge is a good first suspect)
  • You will need a decent fuse at the battery. These are usually 100 or 200 amp fuses. This can then feed a main line to your operating table which will have a fused distribution center feeding fused power leads to the equipment (that’s three fuses between the equipment and the battery).
  • If you plan to run more than a couple of hundred watts, especially with large surges (like, say, using an inverter or cw), then then impedance of the battery feed can be something to consider.
  • Watch out for RF noise from solar charge controllers, especially MPPT types that you need so you can run higher voltage solar panels with smaller resistance loss in the wiring.
  • You may want to use a boost controller so you can feed a solid 13.8v to your rigs from battery power.
  • While you don’t need to go overboard on wire size, a bit of overkill here, especially for any run more than a couple of meters, might be worthwhile. Figure a millivolt per foot per amp for 10g wire and a resistance that differs by a factor of 2 for each 3 wire gauge size change. Try to keep voltage loss in the line below 5% for maximum loads and keep in mind that wire length is the distance out plus the distance back.
  • Don’t use frame ground as a power return lead but do watch out for ground loops.
  • Crimp connections with proper fittings and a good tool. Don’t solder. Use silicon dielectric in the fittings to help reduce corrosion.
  • If you don’t like what you get, try something different. Learn from your experience and use that learning.

There are indeed a lot of ideas and condensed knowledge in this pile. The Solar Calculator : Weather Underground will calculate maximum solar energy harvest and even provides for the particularities of a number of common solar panels. A lot of the battery stuff requires critical reading and getting past a lot of the bullhooey (such as in the QST article about ‘deep cycle battery’) to generalize what can actually be measured and useful.

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