Last week's blog covered the batteries and 12 vdc distribution system. This week, I'll discuss how we'll keep those batteries charged.
The primary means of charging the 315 ah of batteries is the solar panels mounted on Blue's roof. Here in the southwest where solar energy is plentiful, we've had no trouble keeping ahead of the refrigerator. We're currently in Tucson where it hit 104F degrees today, and the fridge is still keeping the food cold, while the batteries haven't dipped below 94% of full charge. We have two 180 watt panels, for a total of 360 watts. I wrote a blog last winter on the solar panels I chose and how I installed them.
The output of the solar panels can't be connected directly to the batteries without risking the possibility of overcharging the batteries. Instead, the solar panels are connected to a solar charge controller, which optimizes the amount of current going into the batteries until they reach full charge. There are two types of solar charge controllers currently available: Pulse Width Modulators (PWM) and Maximum Power Point Tracking (MPPT) controllers. PWM controllers are less expensive, but less efficient as well, converting only 80% or less of the solar energy. MPPT controllers are more expensive, and also more efficient, converting 90% or more of the solar energy. On Nine of Cups, when we installed our solar panels two decades ago, the only charge controllers available were simple diverters that connected the solar panels to the the batteries until a certain maximum voltage was reached, then it diverted it to the hot water heater. It worked, but was very inefficient. This time, we chose an MPPT controller from Renogy Solar, the Commander 40 amp controller and the MT-50 Tracer Meter for displaying and operating the charge controller. The difference in the charge current provided by the MPPT controller versus the old diverter type charge controller on Cups is astounding.
While the solar power provided by our solar panels is more than we need right now, the same may not be true once we're in the Pacific northwest, so we've invested in a couple of other methods for keeping the batteries charged – a shore powered battery charger and a way to charge the house batteries using the engine alternator when we're driving.
Shore Powered Battery Charger
Right now, we don't have a shore power connector, but this is one of the projects we plan to complete during our 2 month sojourn. As mentioned in a previous blog, we decided to use a Magnum MMS-1012 Inverter/Transfer Switch/Charger. When we are on shore power, the inverter uses a transfer switch to connect the incoming 120 vac to Blue's ac circuits. It also powers up a battery charger that puts out 50 amps of charging current, utilizing a four stage charging algorithm that can either be preset to the battery chemistry or customized if desired. It also has an equalization mode.
I like to know how much current is going into or out of the house batteries and the State of Charge (SOC). On Nine of Cups, we tried to never allow the house batteries to get below 50% SOC, which prolonged their lives considerably. I plan to strive for this on Blue as well, and Magnum's battery monitor, the ME-RC50 really helps. I've adjusted the system settings to match my battery chemistry, optimum charge profile and battery bank size. I can see how many amps I'm using using, the battery voltage, as well as a host of other parameters, but quite frankly, I usually leave the battery monitor set to display the SOC, so I always know at a glance how low my batteries are getting. It is smart enough to know that batteries age with time and 're-learns' how much capacity the batteries have lost, and compensates as needed. A really nice device, IMHO.
Charging from Blue's Alternator
Most rv's and campers have a method of charging the house batteries from the vehicle's alternator. It only makes sense to charge the house batteries while charging the starter battery as we're driving down the road. Most use a battery isolator/combiner device, which isolates the house batteries from the vehicle battery until the engine is started. When the alternator begins charging the vehicle battery, the isolator/combiner connects the two sets of batteries together so that the alternator can charge both battery banks.
There are inexpensive versions which use either a relay or diodes to simply combine the battery banks, and there are more expensive 'smart' versions that actually monitor the house batteries and control the amount of charge current being delivered to the batteries.
I'm not a big fan of the less expensive battery combiners. One reason is that rarely are the two battery banks at the same state of charge, so a high current will flow from one bank to the other as they try to reach equilibrium. We originally had a diode based battery isolator/combiner on Nine of Cups, and there were many times when the house batteries were at 50% while the starter battery was almost fully charged. When the isolator combined the batteries, hundreds of amps would flow from the starter battery to the house batteries. This wasn't good for either battery bank.
Another reason I don't like the inexpensive versions is that the vehicle alternator and regulator are designed to optimize the charge profile of the vehicle starter battery. The regulator monitors the voltage and temperature of the vehicle battery and adjusts the alternator output accordingly. This charge profile, even if the two battery banks are the same chemistry type, will not be correct for the house batteries once they are combined. The house batteries are expensive – depending on the type and size, they could be thousands of dollars, and I'd like to get the longest possible life out of them. Saving a hundred dollars by buying an inexpensive battery isolator/combiner is a false economy in my opinion.
Some of the newer smart battery isolator/combiners are quite clever. They monitor the house battery bank charge status and draw current from the alternator to properly charge it. The Promariner Proisocharge (quite a mouthful) is one that looks good.
Another alternative is to use the alternator output to power an inverter, then feed the 120 vac generated by the 'engine' inverter to the Magnum Inverter/Transfer Switch/Charger. The Magnum would then properly charge the house batteries. I borrowed this idea from Dave Orten's upfitting blog, and I think it is a good one. I installed a Xantrex ProWatt SW 1000 watt inverter with a remote switch to use as the engine inverter. I made the mistake of buying the Xantrex 808-9001 PROWatt Remote Control Panel, thinking I could turn the inverter on or off using one of the auxiliary switches on the dash. This only works to turn the inverter off, however. The Xantrex 808-9001-01 is the correct module for this application, but can only be ordered from Xantrex directly. As of this writing, I don't yet have the control module, but the plan, once it is installed, is to switch on the engine inverter, once the engine has been running for a few minutes, to charge the house batteries. Since the auxiliary switches are only powered when the ignition is on, the inverter will automatically shut down when the engine is turned off - in case I forget to turn the inverter off with the switch.
I think it is a good plan. I'll update this blog after I've used it for awhile.
The next big hurdle was how and where to install all this gear, which is next week's topic.