solar add on and batteries

[JR51]

I currently have 2 renogy 100 watt solar panels and a 20 amp charge controller and very happy with equipment and output. Plan on doing more boondocking this year so am going to install another 100 watt panel. I know my controller can handle up to 400 watts. I have 2 -100 amp deep cycle house batteries. My question is with the increased output am I better off keeping just the 2 batts or adding another house battery. Thanks
New York Newbie

[pianotuna]

Since acceptance rates of lead acid batteries are limited to about 12.5 amps per 100 amp-hours of storage, at 85% state of charge, the theoretical maximum wattage is about 150 watts per 100 amp-hours.

In real life, with a flat fixed installation one would expect perhaps 80% of that 150 watts, under perfect charging conditions.

I'd recommend an MPPT controller with a series/parallel panel "farm" consisting of 4 100 watt or 2 200 watt units.

With the advent of non sulphating batteries such as SiO2 and Li, it is no longer necessary nor even desirable to get to 100% state of charge. 1500 cycles to 20% state of charge is over four years of daily full cycles. 2800 cycles to 50% is about 2800 cycles or about 7.5 years.

Next summer, my plan is to move to SiO2 jars. Cost is about 50% of Li and the temperature range of use for them is -40. Since I am no longer full time, the bank may well out live my ability to travel in the RV.

Another way to size the "farm" is to base it on usage. For example, my fridge uses 5.7 kwh per day. I live in a sunny place--so a 1000 watt install with 600 amp-hours of storage would make me propane independent.

If I abandon the 12 volt limitation, and jump to 48 volts, life gets simpler.

[corvettekent]

JR51 wrote:
so according to your formula if solar panels are 300 watts then 2 100 amp batts would be 1.5 watts per amp which tells me not to add another battery.

Your system will work fine with 200 amp hrs of batteries but if you feel that you need more battery capacity 300 amp hrs would just as well and you would have some reserve for a cloudy day.

[MEXICOWANDERER]

Overthinking this will surely lead to disappointment. It depends exactly where you live

THEN start calculating

Coastal fog and cloudiness along the Pacific coast for instance, may allow for a three-hour charging window. Central plains, afternoon thunderstorms may limit a charging window to a few hours. September sunlight is quite a bit shorter than summer solstice time. Use wisdom.

[pauldub]

How do you know your controller can handle up to 400 watts? 20a X 12v = 240 watts.

[BFL13]

I wish you guys would stop saying "amp" instead of "amp-hour". It suggests you are clueless and won't understand any answers you get to your questions. Since that is probably not the case, just use proper terms. Thank you.

[Flyfasteddie]

I think 300w and 200a capacity is perfect. I have 1.5 to 1 and it works terrific for me.

[JR51]

so according to your formula if solar panels are 300 watts then 2 100 amp batts would be 1.5 watts per amp which tells me not to add another battery.

[corvettekent]

I think you are looking at this wrong. To start with you should have a big enough battery bank to last a couple days in case you have a very cloudy day. Then you need enough solar to charge your battery bank.

I have 1 watt of solar for each amp of battery, some will say that you need 1.5 or 2 watts per amp.

[Lwiddis]

A possible 680 solar watts running through a 30 amp solar controller? That isn’t wise IMO. At 5.6 amps per 100 watts...38 or so possible amps. Yes, some controller manufacturers say the controller won’t be harmed but I wouldn’t try it.

[madjack60]

I had 300 watts on my other mh and added 2 mote deep cycle batterys. I had plenty of power and used the gen set rarely. I now have a newer mh and will be putting 680 watts and 2 extra batterys on it with a 30 amp controller

[Lwiddis]

“More” boondocking isn’t the criteria for an additional battery. An adequate solar system recharges fully your batteries almost every day. Adequate batteries supply all the reasonable power you need each day. With 300 solar watts and an anticipated 300 battery amp hours, you meet the minimum on panels.