User Tools

Site Tools


electrical:solar:synopsis_puzzle2

This page aims to distill the knowledge contained in HandyBob's RV battery charging puzzle 2 page.

Particularly useful and pithy statements are quoted directly under fair use.

Synopsis: HandyBob's RV Battery Charging Puzzle

[Note from Secessus: I do my best here to faithfuly summarize Bob's point of view and not editorialize or take issue with it.]

HandyBob and his wife are fulltimers in a 5th wheel and have no generator. He prefers quiet and and wants folks to stop using generators, or at least use them more considerately.

charging batteries is like airing up tires

“Batteries are not sponges that soak up amps. They are a lot more like tires that need to be filled up with air pressure. You need volts (pressure) to get the amps (volume) to go into the battery… Batteries self regulate the amps they will accept depending on level of charge and voltage. The amps going in will drop as the battery fills, and if the voltage is not high enough the battery will not be charged all the way up before the amps taper off.” – HandyBob

volts, volts, volts

Manufacturers give absorption voltages ranging 14.4V - 14.8V (temp compensated) and that is not too high.

Chargers do not read specific gravity, but can estimate State of Charge by holding constant voltage and watching current decrease over time.

“…the power stored in the upper range of a battery’s charge is greater, so it is very important to get a full charge.” – HandyBob
“The difference between 14.4 & 14.8 volts is not 3%. That difference is nearly 20% of the charging range (12.2 to 14.8 volts)” – HandyBob

Use short runs and oversized wires to minimize voltage drop.

battery monitors

Most OEM monitors (led displays, gauges, and idiot lights) are bad, tricking the owners into believing their batteries are fuller than they are. The monitors will read too high when charging and too low when discharging.

The battery bank must rest at least an hour before a reasonably accurate measurement can be made. A hydrometer reading is best but a multimeter will get you in the ballpark: 12.7V is full and 12.2V is 50% state of charge (the bottom of the deep cycle).

Battery bank life will be longer when it is charged fully and not discharged past 50% DoD. A full charge is also important to capacity:

“a 95% charged battery has 10% less usable power in it than a 100% charged one, since you are trying to keep it in the top 50% of its operating range (5% of full = 10% of 50%.)” – HandyBob

recording meters

Bob recommends the use of AH e-meters like Link or the Trimetric 2020/2025:

“…trying to run a battery system without a good meter is like driving a car with no fuel gauge” – HandyBob

The meters will require configuration/adjustment when after installation, as the batteries age, and when the system is altered.

It is common to find appliances use less power than their placards state:

“The label has to show what the appliance will not exceed, not the actual energy use.”

batteries

Bob recommends matched 6v golf cart batteries. He prefers Trojan (particularly the T-105) and avoids Interstate.

Get charging specs from the manufacturer and follow them; this is particular important with new or exotic chemistries. He does not use AGM or Gel types because the lower charging voltage means less energy harvested from the panels.

In multibattery systems the connecting cables should be the same length and gauge. Crimp connections well, avoiding the use of hammer/die crimpers.

converters

Most OEM converters are bad, functioning like trickle chargers. Even the smart ones charge at too low a voltage and commence float too early. They may get confused by multipoint charging.

generators

A generator with DC battery charging output is better than running the converter or auto charger on 110v.

solar

Bob runs his RV off 345W of panel and 450AH of 6v deep cycle batteries.

“Adding solar panels to an under wired system is like putting a big motor in a little car with a tiny drive train & skinny tires. You can’t use the power.” – HandyBob

When seeking advice:

“Don’t ask the guy with solar panels and a generator running behind his rig on a sunny day.” – HandyBob

“Professional” installers may not be any good.

charge controllers

Get an oversized 3-stage charge controller with an equalization function.

Bob prefers Morningstar.

MPPT boost controllers

They work but the gains are found in specific scenarios like deeply discharged batteries. The gains are more likely to be 5%-10% over PWM.

wiring

“Big wire” is cheaper than panels for a similar gain in power.

Wiring from panels to controller is important; wiring from the controller to batteries is critical.

Use stranded wire, the type (building, automotive, welding) is less important. Use UV-shielded wire for external runs.

“big wire is there to limit voltage drop, not to carry amps.” – HandyBob

inverters

Use a properly sized inverter for the job; this may mean owning more than one inverter.

MSW is fine unless you know you have something that needs PSW: “residential refrigerators, washing machines, some air purifiers, some desk top computers and Craftsman brand battery chargers.”

“Soft start” inverters work work well.

Many inverters have a high voltage disconnect (Vhvd) of 15v. Temperature compensated charge controllers can hit 15v easily.

installation considerations

Install the charge controller near the batteries.

Locate solar panels so they are not shaded.

fusing

Bob prefers fuses to automotive-style circuit breakers.

Avoid fuse holders with springs in them.

Fuse every positive wire within a few inches of the battery.

electrical/solar/synopsis_puzzle2.txt · Last modified: 2018/02/10 11:11 by frater_secessus