More Battery / Solar questions

[Scotsman]

Hey solar gurus - my so-simple solar charging system is driving me nuts. It's got three components 1) a 100W solar panel 2) A Renogy Voyager 20A controller 3) A new and unused Die Hard 100Ah battery (with a reserve capacity of 200?), And of course the wires - which run outside the camper. The cables from the controller to the battery are 20" long. Oh - and I have a back-up briefcase - 80 Watt with it's own controller.

Questions:

1) I wanted to test the charging power of the solar panel with the new battery - so ran the battery down to 11.8V (It started at 13.1V - which I achieved from a 3-stage charger).
First of all - is that 11.8 Volts minimum the target below which I shouldn't go in order to preserve battery life? The battery doesn't say what the 200 of reserve capacity is - not Amps or Amp-hours or jelly beans - it just says 200 reserve. What does this reserve mean? - and how does it affect, if at all the 11.8 volt low bar, as mentioned, or the general service the battery should give me?

2) Starting to charge the battery with solar panel, the information on the monitor runs from a) Battery Voltage 2) Charging Amps 3)KWh On sunny day with partial clouds and some smoke haze - but I'd say 80% solar getting through cos I got a bit burned out there - the Amps reading fluctuated between 0.3 and 2.8. The battery has taken several hours to charge from 11.8 to 12.2. What should the ideal Amps reading be - something like 20?

3) The panel is clean and worked well for me in previous summers - I look after my solar panel, cover it in winter and the whole thing, and cables are in generally great order. The voltage readings are backed-up with a multimeter.

4) I have eliminated controller issues as I tested it with the controller from the suitcase...same results

5) I sometimes add the briefcase panel to the system simply by adding it to the relevant battery terminals - rather than trying to ad the two panels in series. So I have two controller attached by short wires to the battery terminals. I wonder if the controllers interfere or compete with each other and create internal damage on at least one of them? The voltage readings remain accurate.

Whew - that's a lot - but thanks for any advice.

 

[91CavGT]

I might be able to help some.

In regards to the power output going into your batteries, a 100 watt solar panel will never put out 20 amps of current. Your charge controller is rated to be able to handle a maximum of 20 amps but that doesn’t mean that is how much power it will put out all the time.

I’ve got a small 50 watt panel, 100 watt panel, and three big 250 watt panels. The 50 watt panel will typically put out a little over 2 amps of current in direct sun light. My 100 watt panel will occasionally hit 5 amps. One thing to note about how much power a solar panel puts out is it is VERY dependent on having the entire solar panel being in the sun. Just a small shadow from a skinny stick can drop power output GREATLY!! Then you also have to have the panel aimed exactly at the sun for good power output.

As far as wiring panels in series, I’m not a fan of doing this. The primary reason is listed in the paragraph above. If you end up with a shadow over one panel, it will take down the entire system output. If all of your panels are on the roof of your RV then this would be a good application for series wiring. With multiple panels wired in series, you can get away with much smaller wiring since series connected panels will increase the voltage while maintaining the single panel amperage. Wiring in parallel will keep the voltage the same, but the amperage increases this necessitating larger gauge wire(smaller number).

The ONLY time you can wire panels in series effectively is if the panels are all the same ratings on voltage and amperage. If both panels have the same voltage rating but different amperage then you need to wire them in parallel.


Battery voltage. I never bring my flooded lead acid or AGM batteries down to 11.8 volts, but that’s just my opinion. I try to turn things off when the battery(s) get down to the 12.3 volt area. This decreases the useable capacity, but it extends the life span of the batteries. If your battery drops down to 12.3 volts in less than a couple of hours then you are either asking the battery to do too much, or you need to add more battery capacity.

The reserve capacity on batteries is more for marketing, once again in my opinion.

A 100 amp/hr battery means that it can put out 100 amps of current over a 20 hour period. 100 amps divided by 20 hours gives you 5 amps that can be used during that time. In the grand scheme of things, that’s not much. For reference, my 10 cubic foot 12 volt fridge in my RV pulls 11 amps of current. At that rate, a single 100 amp/hr battery will only last about 7-8 hours at best.


Keep this in mind too. If your charge controller is a PWM(pulse width modulated), they can not handle anything over about 19 volts. An MPPT (multi power point tracking) can take in MUCH higher voltages than your battery bank and it converts the extra voltage into more amperage. An MPPT charge controller is what you want to get the most out of your system.

For reference, my last solar setup had 750 watts of 24 volt solar panels, 560 amp/hr 12 volt battery bank, and a 150 amp MPPT solar charge controller. In peak sunlight, for brief periods I would occasionally see 50 amps of current going into the battery bank. With that much power being produced, I could run an 8,000 BTU window unit all day and the voltage on my battery bank would stay above 13 volts the entire time. It has taken MANY years of learning to get to this point, and it’s not cheap to get good quality components. Nowadays it’s harder to find good quality components due to shortages. So prices now are considerably higher than what they were just a couple of years ago.

Hope this helps.

 

[RV Fam65]

Manual says ours is a whopping 10 watts, to “help keep the house batteries charged.” It doesn’t do squat. Probably won’t have it reinstalled following upcoming roof replacement.

 

[91CavGT]

Yeah, 10 watts is not much power at all. Personally, I would not bother with such a small panel.