Solar charger for regular AGM batteries

[TimothyPaul]

Can someone suggest approx how many watt Solar Panel I would need to charge two group 27 AGM house batteries. I would plug the solar panel directly into the house batteries during the day. - Hopefully with reasonably intelligent use management, they would last through a night….

 

[moabdds]

Timothy, I have lithium batteries instead of AGMs, but Battle Born told me their rule of thumb for their batteries is 250 watts of panels per each 100 amp hour battery. That would allow full charging on a day with normal sunshine. I actually have 1000 watts of panels (4 x 250) on my moho roof and only 3x100 AH batteries, so I'm over-paneled. It currently charges faster and gives me the option to add another 100 AH battery to my battery bank if I choose. Not sure if lithiums and AGMs charge comparably, but it might be a good place to start.

 

[MadMaxWinny]

The 250 watts : 100 amp-hours ratio is a good guideline, but the best way is to calculate how many amp-hours you are actually trying to replace for a given day (easy with a modern monitoring panel, tedious without). 250 watts = 20 amps of charge = 20% of capacity. Some people size for 5-10% (top off or Float Charge), and some size closer to 35% (for a single charge source, like boondocking). When boondocking, I personally run the genset for 30-60 minutes to do the Bulk Charging in the morning, and then let the smaller solar array top them off.

 

[fanrgs]

I have 450w of solar panels for 210Ah of LiFePO4 batteries with a 30A MPPT charger in a small Class C. MPPT chargers are at least 10% more efficient than a PWM charger that comes on most newer RVs equipped with a 200w panel and a 12v fridge. Our solar array and our RV lifestyle of "travelers, not campers" work well well because the batteriez also charge from the envine alternator. Our 5 yr-old generator has 1 hour of camping run-time and 18 hours of "periodic exercise" run-time despite having an induction cooktop, a conv-micro, a Keurig coffee maker, and all the normal 12v appliances--fridge, TV, water pump, tankless WH, lights, USB outlets for device chsrging, etc.

I had two 6v AGM batteries in our fifth wheel that worked great. But I had propane appliances and no inverter. The solar, Li batteries, and inverter make traveling so much simpler that my wife loves her tiny RV kitchen now so much more than the larger kitchen in the fiver! So, take a look at eventually converting to Li and an inverter as Li batteries are currently very cheap.

 

[creativepart]

Keep in mind that Lead Acid batteries take a much longer time to recharge than new LFP batteries.

If you do not discharge your batteries too deeply, they will certainly partially recover from just solar on a sunny day in the peak summer months. But, with typical use, if you are thinking you can keep your batteries fully recharged on just solar then you are mistaken.

You will need a generator or shore power to keep your AGM batteries charged even with minimal usage daily.

Solar is great. And, it's almost free power, but it cannot take the place of a generator/shore power.