RV battery sizing with generator backup?

**NCmountainsOffgrid** · 01-06-2019, 01:49 PM

heavy loads are exactly what the generator is for... when we are off-grid or overnighting without shore power, I use the generator whenever there is a need for the coffee maker, air conditioning, electric water heating, electric heating, and the aforementioned 'bath' items like blow dryers, curling irons, and the 'iron' for clothes. Long runs on the microwave would also fall into this category, though short runs are probably no issue. With my AGS(auto gen starter), for battery charging on low voltage, we usually allow these 'things' to happen when the Gen is already running anyway.

**Sunking** · 01-06-2019, 03:00 PM

Originally posted by PNW_Steve

I thought that I pretty much had it nailed until I I observed my Wife getting ready for work........ Blow dryer, curling iron and clothes iron..... I asked her if she's planning on using those when we are boondocking? Her response.. "Of course! ".

Leave the wife and high wattage gadgets at home. It is called roughing it for a reason. If you wife insist on luxuries, get a hotel room. Seriously you cannot have the high wattage gizmos on solar. Be sure to use a electronic battery isolator so you can use the engine alternator as your primary energy source. In fact if you drive every other day will not need solar. Size your battery for 3 day reserve and call it good. No way will you ever be able to depend on solar. as your primary source. The generator and alternator will do the heavy lifting. One hour of engine run time wil do more than your panels can do in two days, thus why if you drive every couple of days can do without solar.

**Mike90250** · 01-06-2019, 03:01 PM

Keep your plan, get a battery charger and use the generator for half hour in the AM to handle the heavy loads & charge batteries

**NCmountainsOffgrid** · 01-06-2019, 03:30 PM

even RVrs disagree on the real validity of sinking a lot of money into solar, as the real benefit is only realized when you are parked, off-grid, for an extended period. As Sunking said, since the Alternator typically also charges the House battery bank while running, there are really relatively few periods between travel, Shore power options, and required Generator usage that the solar has any real

**NCmountainsOffgrid** · 01-06-2019, 03:31 PM

time to benefit you to the degree of upfront costs...

**PNW_Steve** · 01-06-2019, 05:57 PM

Thanks for the input.

If I understand correctly, other than fuel and wear on the generator, it is a good idea to run the generator daily?

We will be boondocking for up to 3 weeks with no engine run time.

And even when we travel, I am not sure how practical it is to add an alternator. I did look into it and 24v 100amp alternators are not horribly expensive. Unfortunately brackets for mounting a second alternator on my Cummins 8.3 are made of unobtanium. I have spent hours searching and asked on other forums. The only positive result was a guy 2500 miles away offered to make me one. He needed the bus at his shop (in Georgia) and estimated $1000 labor & materials. Not what I was expecting. I did find a 12v to 24v charger but the current was limited to around 15 amps.

As far as leaving the wife, blow drier etc behind, we are going full time and selling the house. Besides, I kind of like having her around

Regarding upfront costs, I already have 10 - 300 watt panels in the garage. I can fit a max of 8 on the roof. I am considering installing six and running a Midnite Classic and Whizbang jr. I will take advantage of the charge current limiting feature to protect my batteries. I should be paneled enough to produce usable output when in less than ideal conditions (Seattle....)

To sum it up:

Get up in the AM and fire up the generator. Run the morning high power loads and Iota charger. When finished shut down generator and enjoy the rest of the day in peace.

**PNW_Steve** · 01-06-2019, 05:59 PM

Related question: I have seen links to an off-grid system calculator XLS in several older posts here (2012-2015) but all of the links are dead. Is there a current link available?

Thanks again.

**NCmountainsOffgrid** · 01-06-2019, 06:29 PM

When I speak of alternators, I'm referring to most RV motorhomes that use the vehicle's existing alternator, though probably sized larger than just for the vehicle, that can also provide simultaneous charging to the House batteries, which are the batteries for the 'HOME' usage - 12v lights, RV fridge, furnace, etc. There is also generally a BIM or BIRD device that detects the power from the Alternator, while the engine is running, and provides any 'extra' to the House batteries.
If though, your plan is not the regular 'travel' on an almost daily or every other day for most RVrs, then you may not find the alternator a needed 'extra' charging source.

but, yes, for your situation, as you explain your solar panels and prospective build and use, the number of panels you can attach to your roof are going to go a long way to the ultimate number of hours you can 'off-grid' without the need for constant use of the generator - your number and size of battery storage will be the other piece of the puzzle, though I can see that your bus might have more area(s) to mount these versus the normal RV. Of course, the more battery storage you have, the more CHARGING amperage you need, which is where the solar panels come in, but ALSO the generator's output and your Charger's output, since you would have a lot of amperage needed without having to run the generator even longer.

Generators are built to withstand a LOT of use, and for constant hours on end - don't be 'afraid' of the generator, as it is likely your BEST friend in the world of off-gridding - watch any 'Alaska' off-grid show and you'll quickly realize that they would be in a big world of hurt without theirs - and some run them 24/7 almost. They last a long, long time if maintained.

When we first got our diesel pusher, with the 6kw Onan diesel onboard generator, which pulls fuel from the same tank as the Cummins 6.7L isb engine, I hesitated to ever use it much because I wasn't confident in it. Why? who knows, but probably because I'd never really dealt with a generator, didn't understand it's real capabilities, and also didn't understand the power of my AGS(auto generator starter) and how effective it can be to automatically take care of CHARGING the batteries when get to a certain 'low' level.
It's a great off-grid power source, when you really need that full 120v high-amp output and usage for air conditioning, and large draws over a long time frame. You can certainly power your hair dryer or microwave or electric heater from your Inverter, for short burst of usage, without issue, but the reality is that if you are waking up in the morning with the need for a coffee maker, microwave, electric hot water, hair dryer, curling iron, and clothes iron - all within an hour's timeframe, you'll quickly sap your batteries and need the generator ANYWAY.

Use the generator - it's your friend. It is efficient. It is relatively quite. And, you'll need to know how to use it because there WILL be days, maybe even weeks, when you just won't have that SUN that you are so predicting on your calendar. Your generator won't care if the sun is out!

enjoy! and let us know how it goes - we're interested!

**PNW_Steve** · 01-06-2019, 06:36 PM

Originally posted by NCmountainsOffgrid

When I speak of alternators, I'm referring to most RV motorhomes that use the vehicle's existing alternator, though probably sized larger than just for the vehicle, that can also provide simultaneous charging to the House batteries, which are the batteries for the 'HOME' usage - 12v lights, RV fridge, furnace, etc. There is also generally a BIM or BIRD device that detects the power from the Alternator, while the engine is running, and provides any 'extra' to the House batteries.
If though, your plan is not the regular 'travel' on an almost daily or every other day for most RVrs, then you may not find the alternator a needed 'extra' charging source.

but, yes, for your situation, as you explain your solar panels and prospective build and use, the number of panels you can attach to your roof are going to go a long way to the ultimate number of hours you can 'off-grid' without the need for constant use of the generator - your number and size of battery storage will be the other piece of the puzzle, though I can see that your bus might have more area(s) to mount these versus the normal RV. Of course, the more battery storage you have, the more CHARGING amperage you need, which is where the solar panels come in, but ALSO the generator's output and your Charger's output, since you would have a lot of amperage needed without having to run the generator even longer.

Generators are built to withstand a LOT of use, and for constant hours on end - don't be 'afraid' of the generator, as it is likely your BEST friend in the world of off-gridding - watch any 'Alaska' off-grid show and you'll quickly realize that they would be in a big world of hurt without theirs - and some run them 24/7 almost. They last a long, long time if maintained.

When we first got our diesel pusher, with the 6kw Onan diesel onboard generator, which pulls fuel from the same tank as the Cummins 6.7L isb engine, I hesitated to ever use it much because I wasn't confident in it. Why? who knows, but probably because I'd never really dealt with a generator, didn't understand it's real capabilities, and also didn't understand the power of my AGS(auto generator starter) and how effective it can be to automatically take care of CHARGING the batteries when get to a certain 'low' level.
It's a great off-grid power source, when you really need that full 120v high-amp output and usage for air conditioning, and large draws over a long time frame. You can certainly power your hair dryer or microwave or electric heater from your Inverter, for short burst of usage, without issue, but the reality is that if you are waking up in the morning with the need for a coffee maker, microwave, electric hot water, hair dryer, curling iron, and clothes iron - all within an hour's timeframe, you'll quickly sap your batteries and need the generator ANYWAY.

Use the generator - it's your friend. It is efficient. It is relatively quite. And, you'll need to know how to use it because there WILL be days, maybe even weeks, when you just won't have that SUN that you are so predicting on your calendar. Your generator won't care if the sun is out!

enjoy! and let us know how it goes - we're interested!

The challenge comes when you try to charge a 24v house battery with a 12v alternator. Doesn't really matter the amperage rating. That is why, when I started down this road, I looked into adding a 24v alternator to charge the house batteries while driving.

As I mentioned above, I did find a 12v-24v battery charger but it was limited to 15 amps.

Regarding generator run time, I have lived off grid with no solar. Just a generator. I finally broke down and had utility power installed. It was expensive but it beat being in a wonderfully peaceful spot in the woods with the genny running 12-16 hours a day.

The root of my questions was: Am I creating any new problems if I keep the smaller battery bank and run my heavy morning loads & shore charger for 1-2 hours to lessen the load on my batteries and get a head start on the days charging.

From the responses, I think that I may be headed in the right direction.

**PNW_Steve** · 01-06-2019, 07:00 PM

Correction...

Reviewing my measurements it like I would be hard pressed to fit 8 panels. 7 will fit but configuration becomes problematic (prime...) so it looks like 6 x 300 watt panels. Likely in a 2S3P configuration. If I went to a Classic 200 I could run 3S2P but would be more subject to shading issues. In an RV shading is a variable that is hard to control.

**NCmountainsOffgrid** · 01-06-2019, 08:08 PM

you should be fine in moving forward, as your usage and lifestyle will dictate the outcome... you may find that your generator is not large enough, or too large, and that your solar is plenty, or not even close... but yes, you can certainly use the generator more for 'mornings' and 'evenings' for Charging, even with your predicted usage of some large draw items. You may have to add some in the middle of the day, too, but it'll just depend on the sun.
Your comment about the 'sound' of a generator is exactly what drives most to solar, especially RVrs who may be sitting right on top of their unit - but I've found that most recent day motorhomes have much quieter generators than the days of old, or especially as compared to the dreaded 'contractor' generators.

**Sunking** · 01-06-2019, 09:54 PM

Generator is best used very early in the mornings, like before sunrise, until you have strong sun to finish off the charge. Once you see the transition from Bulk to Adsorb is when you cut the genny off.

Or run genny with heavy loads.

Trick with genny is proper sizing. You want it large enough to charge batteries at C/8, plus enough to run loads with genny 75% loaded. Example say you have a 24 volt 50 volt charger and want to run 500 watts of load. Kva roughly = [(28 volts x 50 amps) + 500 watts] / .75 = 2.54 Kva. Go shop for a 3 Kva genny.

**PNW_Steve** · 01-07-2019, 02:02 PM

Originally posted by Sunking

Generator is best used very early in the mornings, like before sunrise, until you have strong sun to finish off the charge. Once you see the transition from Bulk to Adsorb is when you cut the genny off.

Or run genny with heavy loads.

Trick with genny is proper sizing. You want it large enough to charge batteries at C/8, plus enough to run loads with genny 75% loaded. Example say you have a 24 volt 50 volt charger and want to run 500 watts of load. Kva roughly = [(28 volts x 50 amps) + 500 watts] / .75 = 2.54 Kva. Go shop for a 3 Kva genny.

Thank you.

That is exactly what I was looking for .

RV battery sizing with generator backup?

RV battery sizing with generator backup?

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